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Title: A history of land mammals in the Western Hemisphere
Author: William Berryman Scott
Release date: January 14, 2025 [eBook #75110]
Language: English
Original publication: New York: The MacMillan Company, 1913
Credits: Peter Becker and the Online Distributed Proofreading Team at https://www.pgdp.net (This file was produced from images generously made available by The Internet Archive)
*** START OF THE PROJECT GUTENBERG EBOOK A HISTORY OF LAND MAMMALS IN THE WESTERN HEMISPHERE ***
A HISTORY OF LAND MAMMALS IN THE WESTERN HEMISPHERE
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[Illustration: FRONTISPIECE.—A Pleistocene tar-pool in southern
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A
HISTORY OF LAND MAMMALS
IN THE
WESTERN HEMISPHERE
BY
WILLIAM B. SCOTT
PH.D. (Heidelberg), HON.D.SC. (Harvard & Oxford), LL.D. (Univ.
of Pennsylvania)
BLAIR PROFESSOR OF GEOLOGY AND PALÆONTOLOGY
IN PRINCETON UNIVERSITY
_ILLUSTRATED WITH 32 PLATES AND MORE THAN 100 DRAWINGS
BY BRUCE HORSFALL_
New York
THE MACMILLAN COMPANY
1913
_All rights reserved_
COPYRIGHT, 1913,
BY THE MACMILLAN COMPANY.
Set up and electrotyped. Published November, 1913.
Norwood Press
J. S. Cushing Co.—Berwick & Smith Co.
Norwood, Mass., U.S.A.
Dedicated
TO
MY CLASSMATES
HENRY FAIRFIELD OSBORN AND FRANCIS SPEIR
IN MEMORY OF A NOTABLE SUMMER AFTERNOON IN 1876 AND IN TOKEN OF FORTY
YEARS’ UNCLOUDED FRIENDSHIP
Speak to the earth and it shall teach thee.
—JOB, xii, 8.
Can these bones live?
—EZEKIEL, xxxvii, 3.
PREFACE
One afternoon in June, 1876, three Princeton undergraduates were lying
under the trees on the canal bank, making a languid pretence of preparing
for an examination. Suddenly, one of the trio remarked: “I have been
reading an old magazine article which describes a fossil-collecting
expedition in the West; why can’t we get up something of the kind?” The
others replied, as with one voice, “We can; let’s do it.” This seemingly
idle talk was, for Osborn and myself, a momentous one, for it completely
changed the careers which, as we then believed, had been mapped out for
us. The random suggestion led directly to the first of the Princeton
palæontological expeditions, that of 1877, which took us to the “Bad
Lands” of the Bridger region in southwestern Wyoming. The fascination of
discovering and exhuming with our own hands the remains of the curious
creatures which once inhabited North America, but became extinct ages
ago, has proved an enduring delight. It was the wish to extend something
of this fascinating interest to a wider circle, that occasioned the
preparation of this book.
The western portion of North America has preserved a marvellous series
of records of the successive assemblages of animals which once dwelt
in this continent, and in southernmost South America an almost equally
complete record was made of the strange animals of that region. For
the last half-century, or more, many workers have coöperated to bring
this long-vanished world to light and to decipher and interpret the
wonderful story of mammalian development in the western hemisphere. The
task of making this history intelligible, not to say interesting, to the
layman, has been one of formidable difficulty, for it is recorded in
the successive modifications of the bones and teeth, and without some
knowledge of osteology, these records are in an unknown tongue. To meet
this need, Chapter III gives a sketch of the mammalian skeleton and
dentition, which the reader may use as the schoolboy uses a vocabulary
to translate his Latin exercise, referring to it from time to time, as
may be necessary to make clear the descriptions of the various mammalian
groups. Technical terms have been avoided as far as possible, but,
unfortunately, it is not practicable to dispense with them altogether.
The appended glossary will, it is hoped, minimize the inconvenience.
No one who has not examined it, can form any conception of the enormous
mass and variety of material, illustrating the history of American
mammals, which has already been gathered into the various museums. A full
account of this material would require many volumes, and one of the chief
problems in the preparation of this book has been that of making a proper
selection of the most instructive and illuminating portions of the long
and complicated story. Indeed, so rapid is the uninterrupted course of
discovery, that parts of the text became antiquated while in the press
and had to be rewritten. As first prepared, the work proved to be far
too long and it was necessary to excise several chapters, for it seemed
better to cover less ground than to make the entire history hurried and
superficial. The plan of treatment adopted involves a considerable amount
of repetition, but this is perhaps not a disadvantage, since the same
facts are considered from different points of view.
The facts which are here brought together have been ascertained by
many workers, and I have borrowed with the greatest freedom from my
fellow labourers in the field of palæontology. As every compiler of a
manual finds, it is not feasible to attribute the proper credit to each
discoverer. Huxley has so well explained the situation in the preface to
his “Anatomy of Vertebrated Animals,” that I may be permitted to borrow
his words: “I have intentionally refrained from burdening the text with
references; and, therefore, the reader, while he is justly entitled to
hold me responsible for any errors he may detect, will do well to give me
no credit for what may seem original, unless his knowledge is sufficient
to render him a competent judge on that head.”
A book of this character is obviously not the proper place for polemical
discussions of disputed questions. Whenever, therefore, the views
expressed differ widely from those maintained by other palæontologists,
I have attempted no more than to state, as fairly as I could, the
alternative interpretations and my own choice between them. Any other
course was forbidden by the limitations of space.
It is a pleasure to give expression to my sincere sense of gratitude
to the many friends who have helped me in an unusually laborious
undertaking. Professor Osborn and Dr. Matthew have placed at my disposal
the wonderful treasures of the American Museum of Natural History in New
York and in the most liberal manner have supplied me with photographs and
specimens for drawings, as well as with information regarding important
discoveries which have not yet been published. Dr. W. J. Holland,
Director of the Carnegie Museum in Pittsburgh, has likewise generously
provided many photographs from the noble collection under his charge,
kindly permitting the use of material still undescribed. To Professor
Charles Schuchert, of Yale University, I am also indebted for several
photographs.
The figures of existing animals are almost all from photographs taken
in the New York and London zoölogical gardens, and I desire to thank
Director Hornaday, of the Bronx Park, and Mr. Peacock, of the London
garden, for the very kind manner in which they have procured these
illustrations for my use. The photographs have been modified by painting
out the backgrounds of cages, houses, and the like, so as to give a less
artificial appearance to the surroundings.
To my colleagues at Princeton I am under great obligations for much
valuable counsel and assistance. Professor Gilbert van Ingen has prepared
the maps and diagrams and Dr. W. J. Sinclair has devoted much labour
and care to the illustrations and has also read the proofs. Both of
these friends, as also Professors C. H. Smyth and E. G. Conklin and Drs.
Farr and McComas, have read various parts of the manuscript and made
many helpful suggestions in dealing with the problems of treatment and
presentation.
For thirteen years past I have been engaged in the study of the great
collections of fossil mammals, gathered in Patagonia by the lamented Mr.
Hatcher and his colleague, Mr. Peterson, now of the Carnegie Museum.
This work made it necessary for me to visit the museums of the Argentine
Republic, which I did in 1901, and was there received with the greatest
courtesy and kindness by Dr. F. Moreno, Director, and Dr. Santiago Roth,
of the La Plata Museum, and Dr. F. Ameghino, subsequently Director of the
National Museum at Buenos Aires. To all of these gentlemen the chapters
on the ancient life of South America are much indebted, especially to
Dr. Ameghino, whose untimely death was a great loss to science. It is
earnestly to be hoped that the heroic story of his scientific career may
soon be given to the world.
Finally, I desire to thank Mr. Horsfall for the infinite pains and care
which he has expended upon the illustrations for the work, to which so
very large a part of its value is due.
While the book is primarily intended for the lay reader, I cannot but
hope that it may also be of service to many zoölogists, who have been
unable to keep abreast of the flood of palæontological discovery and yet
wish to learn something of its more significant results. How far I have
succeeded in a most difficult task must be left to the judgment of such
readers.
PRINCETON, N.J., June 1, 1913.
CONTENTS
PAGE
CHAPTER I
METHODS OF INVESTIGATION—GEOLOGICAL 1
CHAPTER II
METHODS OF INVESTIGATION—PALÆONTOLOGICAL 29
CHAPTER III
THE CLASSIFICATION OF THE MAMMALIA 50
CHAPTER IV
THE SKELETON AND TEETH OF MAMMALS 61
CHAPTER V
THE GEOGRAPHICAL DEVELOPMENT OF THE AMERICAS IN CENOZOIC TIMES 99
CHAPTER VI
THE GEOGRAPHICAL DISTRIBUTION OF MAMMALS 135
CHAPTER VII
THE SUCCESSIVE MAMMALIAN FAUNAS OF NORTH AND SOUTH AMERICA 192
CHAPTER VIII
HISTORY OF THE PERISSODACTYLA 288
CHAPTER IX
HISTORY OF THE ARTIODACTYLA 358
CHAPTER X
HISTORY OF THE PROBOSCIDEA 422
CHAPTER XI
HISTORY OF THE †AMBLYPODA AND †CONDYLARTHRA 443
CHAPTER XII
HISTORY OF THE †TOXODONTIA (OR †NOTOUNGULATA) 461
CHAPTER XIII
HISTORY OF THE †LITOPTERNA AND †ASTRAPOTHERIA 489
CHAPTER XIV
HISTORY OF THE CARNIVORA 516
CHAPTER XV
HISTORY OF THE PRIMATES 577
CHAPTER XVI
HISTORY OF THE EDENTATA 589
CHAPTER XVII
HISTORY OF THE MARSUPIALIA 624
CHAPTER XVIII
MODES OF MAMMALIAN EVOLUTION 645
GLOSSARY 665
INDEX 675
A HISTORY OF LAND MAMMALS IN THE WESTERN HEMISPHERE
CHAPTER I
METHODS OF INVESTIGATION—GEOLOGICAL
The term _Mammal_ has no exact equivalent in the true vernacular of
any modern language, the word itself, like its equivalents, the French
_Mammifère_ and the German _Säugethier_, being entirely artificial. As
a name for the class Linnæus adopted the term Mammalia, which he formed
from the Latin _mamma_ (_i.e._ teat) to designate those animals which
suckle their young; hence the abbreviated form Mammal, which has been
naturalized as an English word. “Beast,” as employed in the Bible, and
“Quadruped” are not quite the same as mammal, for they do not include the
marine forms, such as whales, dolphins, seals, walruses, or the flying
bats, and they are habitually used in contradistinction to Man, though
Man and all the forms mentioned are unquestionably mammals.
In attempting to frame a definition of the term _Mammal_, it is
impossible to avoid technicalities altogether, for it is the complete
unity of plan and structure which justifies the inclusion of all the
many forms that differ so widely in habits and appearance. _Mammals are
air-breathing vertebrates, which are warm-blooded and have a 4-chambered
heart; the body cavity is divided into pleural and abdominal chambers
by a diaphragm; except in the lowest division of the class, the young
are brought forth alive and are always suckled, the milk glands being
universal throughout the class. In the great majority of mammals the
body is clothed with hair; a character found in no other animals. In
a few mammals the skin is naked, and in still fewer there is a partial
covering of scales._ The list of characters common to all mammals, which
distinguish them from other animals, might be indefinitely extended,
for it includes all the organs and tissues of the body, the skeletal,
muscular, digestive, nervous, circulatory, and reproductive systems, but
the two or three more obvious or significant features above selected will
suffice for the purposes of definition.
While the structural plan is the same throughout the entire class,
there is among mammals a wonderful variety of form, size, appearance,
and adaptation to special habits. It is as though a musician had taken
a single theme and developed it into endless variations, preserving
an unmistakable unity through all the changes. Most mammals are
_terrestrial_, living, that is to say, not only on the land, but on the
ground, and are herbivorous in habit, subsisting chiefly or exclusively
upon vegetable substances, but there are many departures from this mode
of life. It should be explained, however, that the term _terrestrial_ is
frequently used in a more comprehensive sense for all land mammals, as
distinguished from those that are aquatic or marine. Monkeys, Squirrels,
Sloths and Opossums are examples of the numerous _arboreal_ mammals,
whose structure is modified to fit them for living and sleeping in the
trees, and in some, such as the Sloths, the modification is carried so
far that the creature is almost helpless on the ground. Another mode of
existence is the burrowing or _fossorial_, the animal living partly or
mostly, or even entirely underground, a typical instance of which is the
Mole. The Beaver, Muskrat and Otter, to mention only a few forms, are
_aquatic_ and spend most of their life in fresh waters, though perfectly
able to move about on the land. _Marine_ mammals, such as the Seals and
Whales, have a greatly modified structure which adapts them to life in
the sea.
Within the limits of each of these categories we may note that there
are many degrees of specialization or adaptation to particular modes of
life. Thus, for example, among the marine mammals, the Whales and their
allies, Porpoises, etc., are so completely adapted to a life in the
seas that they cannot come upon the land, and even stranding is fatal
to them, while the Seals frequently land and move about upon the shore.
It should further be observed that mammals of the most diverse groups
are adapted to similar modes of existence. Thus in one natural group or
_order_ of related forms, occur terrestrial, burrowing, arboreal and
aquatic members, and the converse statement is of course equally true,
that animals of similar life-habits are not necessarily related to one
another, and very frequently, in fact, are not so related. Among the
typically marine mammals, for example, there are at least three and
probably four distinct series, which have independently become adapted to
life in the sea.
* * * * *
Before attempting to set forth an outline of what has been learned
regarding the history of mammalian life in the western hemisphere, it
is essential to give the reader some conception of the manner in which
that knowledge has been obtained. Without such an understanding of the
methods employed in the investigation the reader can only blindly accept
or as blindly reject what purports to be the logical inference from
well-established evidence. How is that evidence to be discovered? and how
may trustworthy conclusions be derived from it?
The first and most obvious step is to gather all possible information
concerning the mammals of the present day, their structure (comparative
anatomy), functions (physiology), and their geographical arrangement.
This latter domain, of the geographical distribution of mammals, is one
of peculiar significance. Not only do the animals of North America differ
radically from those of Central and South America, but within the limits
of each continent are more or less well-defined areas, the animals of
which differ in a subordinate degree from those of other areas. The study
of the modern world, however, would not of itself carry us very far
toward the goal of our inquiries, which is an _explanation_, not merely
a statement, of the facts. The present order of things is the outcome
of an illimitably long sequence of events and can be understood only in
proportion to our knowledge of the past. In other words, it is necessary
to treat the problems involved in our inquiry _historically_; to trace
the evolution of the different mammalian groups from their simpler
beginnings to the more complex and highly specialized modern forms; to
determine, so far as that may be done, the place of origin of each group
and to follow out their migrations from continent to continent.
While we shall deal chiefly, almost exclusively, with the mammals of the
New World, something must be said regarding those of other continents,
for, as will be shown in the sequel, both North and South America have,
at one time or another, been connected with various land-masses of the
eastern hemisphere. By means of those land-connections, there has been
an interchange of mammals between the different continents, and each
great land-area of the recent world contains a more or less heterogeneous
assemblage of forms of very diverse places of origin. Indeed, migration
from one region to another has played a most important part in bringing
about the present distribution of living things. From what has already
been learned as to the past life of the various continents and their
shifting connections with one another, it is now feasible to analyze the
mammalian faunas of most of them and to separate the indigenous from the
immigrant elements. Among the latter may be distinguished those forms
which are the much modified descendants of ancient migrants from those
which arrived at a much later date and have undergone but little change.
To take a few examples from North America, it may be said that the Bears,
Moose, Caribou and Bison are late migrants from the Old World; that the
Virginia and Black-tailed Deer and the Prong-horned Antelope are of
Old World origin, but their ancestors came in at a far earlier period
and the modern species are greatly changed from the ancestral migrants.
The Armadillo of Texas and the Canada Porcupine are almost the only
survivors, north of Mexico, of the great migration of South American
mammals which once invaded the northern continent. On the other hand,
the raccoons and several families of rodents are instances of indigenous
types which may be traced through a long American ancestry.
Fully to comprehend the march of mammalian development, it thus becomes
necessary to reconstruct, at least in outline, the geography of the
successive epochs through which the developmental changes have taken
place, the connections and separations of land-masses, the rise of
mountain ranges, river and lake systems and the like. Equally significant
factors in the problem are climatic changes, which have had a profound
and far-reaching effect upon the evolution and geographical spread of
animals and plants, and the changes in the vegetable world must not be
ignored, for, directly or indirectly, animals are dependent upon plants.
To one who has paid no attention to questions of this kind, it might well
seem an utterly hopeless task to reconstruct the long vanished past, and
he would naturally conclude that, at best, only fanciful speculations,
with no foundation of real knowledge, could be within our reach. Happily,
such is by no means the case. Geology offers the means of a successful
attack upon these problems and, although very much remains to be done,
much has already been accomplished in elucidating the history, especially
in its later periods, with which the story of the mammals is more
particularly concerned.
It is manifestly impossible to present here a treatise upon the science
of Geology, even in outline sketch. Considerations of space are
sufficient to forbid any such attempt. Certain things must be taken for
granted, the evidence for which may be found in any modern text-book of
Geology. For example, it is entirely feasible to establish the mode of
formation of almost any rock (aside from certain problematical rocks,
which do not enter into our discussion) and to determine whether it
was laid down in the sea, or on the land, or in some body of water not
directly connected with the sea, such as a lake or river. With the aid
of the microscope, it is easy to discriminate volcanic material from
the ordinary water-borne and wind-borne sediments and, in the case of
the rocks which have solidified from the molten state, to distinguish
those masses which have cooled upon the surface from those which have
solidified deep within the earth.
While the nature and mode of formation of the rocks may thus be
postulated, it will be needful to explain at some length the character of
the evidence from which the history of the earth may be deciphered. First
of all, must be made clear the method by which the events of the earth’s
history may be arranged in chronological order, for a history without
chronology is unintelligible. The events which are most significant for
our purpose are recorded in the rocks which are called _stratified,
bedded or sedimentary_, synonymous terms. Such rocks were made mostly
from the débris of older rocks, in the form of gravel, sand or mud, and
were laid down under water, or, less extensively, spread by the action of
the wind upon a land-surface. Important members of this group of rocks
are those formed, more or less completely, from the finer fragments given
out in volcanic eruptions, carried and sorted by the wind and finally
deposited, it may be at great distances from their point of origin,
upon a land-surface, or on the bottom of some body of water. Stratified
or bedded rocks, as their name implies, are divided into more or less
parallel layers or beds, which may be many feet or only a minute fraction
of an inch in thickness. Such a division means a pause in the process of
deposition or a change in the character of the material deposited over
a given area. Owing to the operation of gravity, the layers of sediment
are spread out in a horizontal attitude, which disregard the minor
irregularities of the bottom, just as a deep snow buries the objects
which lie upon the surface.
A moment’s consideration will show that, in any series of stratified
rocks which have not been greatly disturbed from their original
horizontal position, _the order of succession or superposition of the
beds must necessarily be the chronological order of their formation_.
(Fig. 1.) Obviously, the lowest beds must have been deposited first and
therefore are the oldest of the series, while those at the top must
be the newest or youngest and the beds intermediate in position are
intermediate in age. This inference depends upon the simple principle
that each bed must have been laid down before the next succeeding
one can have been deposited upon it. While this is so clear as to be
almost self-evident, it is plain that such a mode of determining the
chronological order of the rocks of the earth’s crust can be of only
local applicability and so far as the beds may be traced in unbroken
continuity. It is of no direct assistance in correlating the events in
the history of one continent with those of another and it fails even in
comparing the distinctly separated parts of the same continent. Some
method of universal applicability must be devised before the histories of
scattered regions can be combined to form a history of the earth. Such a
universal method is to be found in the _succession of the forms of life_,
so far as that is recorded in the shape of _fossils_, or the recognizable
remains of animal and vegetable organisms preserved in the rocks.
[Illustration: FIG. 1.—Diagram section of a series of beds, illustrating
superposition. A is the oldest, B, C, D, etc., succeeding in ascending
order.]
This principle was first enunciated by William Smith, an English
engineer, near the close of the eighteenth century, who thus laid the
foundations of Historical Geology. In the diagram, Fig. 2, is reproduced
Smith’s section across England from Wales to near London, which shows
the successive strata or beds, very much tilted from their original
horizontal position by the upheaval of the sea-bed upon which they
were laid down. The section pictures the side of an imaginary gigantic
trench cut across the island and was constructed by a simple geometrical
method from the surface exposures of the beds, such as mining engineers
continually employ to map the underground extension of economically
important rocks, and shows how an enormous thickness of strata may be
studied from the surface. The older beds are exposed at the western end
of the section in Wales and, passing eastward, successively later and
later beds are encountered, the newest appearing at the eastern end. Very
many of the strata are richly fossiliferous, and thus a long succession
of fossils was obtained in the _order of their appearance_, and this
order has been found to hold good, not only in England, but throughout
the world. The order of succession of the fossils was thus in the first
instance actually ascertained from the succession of the strata in which
they are found and has been verified in innumerable sections in many
lands and is thus a matter of observed and verifiable fact, not merely
a postulate or working hypothesis. Once ascertained, however, the order
of succession of living things upon the earth may be then employed as an
independent and indispensable means of geologically dating the rocks in
which they occur.
[Illustration: FIG. 2.—William Smith’s section across the south of
England. The vertical scale is exaggerated, which makes the inclination
of the beds appear too steep.
N. B. The original drawing is in colors, which are not indicated by the
dotted strata.]
This is the _palæontological method_, which finds analogies in many
other branches of learned inquiry. The student of manuscripts discovers
that there is a development, or regular series of successive changes,
in handwriting, and from the handwriting alone can make a very close
approximation to the date of a manuscript. The order in which those
changes came about was ascertained from the comparative study of
manuscripts, the date of which could be ascertained from other evidence,
but, when once established, the changes in handwriting are used to fix
the period of undated manuscripts. Just so, the succession of fossils,
when learned from a series of superposed beds, may then be employed to
fix the geological date of strata in another region. Similarly, the
archæologist has observed that there is an evolution or development in
every sort of the work of men’s hands and therefore makes use of coins,
inscriptions, objects of art, building materials and methods, etc., to
date ancient structures. In the German town of Trier (or Trèves) on the
Moselle, the cathedral has as a nucleus a Roman structure, the date and
purpose of which had long been matters of dispute, though the general
belief was that the building had been erected under Constantine the
Great. In the course of some repairs made not very long ago, it became
necessary to cut deep into the Roman brickwork, and there, embedded
in the undisturbed mortar, was a coin of the emperor Valentinian II,
evidently dropped from the pocket of some Roman bricklayer. That coin
fixed a date older than which the building cannot be, though it may be
slightly later, and it well illustrates the service rendered by fossils
in determining geological chronology.
Other methods of making out the chronology of the earth’s history have
been proposed from time to time and all of them have their value, though
none of them renders us independent of the use of fossils, which have the
pre-eminent advantage of not recurring or repeating themselves at widely
separated intervals of time, as all physical processes and changes do. An
organism, animal or plant, that has become extinct never returns and is
not reproduced in the evolutionary process.
Great and well founded as is our confidence in fossils as fixing the
geological date of the rocks in which they occur, it must not be
forgotten that the succession of the different kinds of fossils in
time was first determined from the superposition of the containing
strata. Hence, it is always a welcome confirmation of the chronological
inferences drawn from the study of fossils, when those inferences can
be unequivocally established by the succession of the beds themselves.
For example, in the Tertiary deposits of the West are two formations or
groups of strata, called respectively the Uinta and the White River,
which had never been known to occur in the same region and whose relative
age therefore could not be determined by the method of superposition.
Each of the formations, however, has yielded a large number of
well-preserved fossil mammals, and the comparative study of these mammals
made it clear that the Uinta must be older than the White River and that
no very great lapse of time, geologically speaking, occurred between the
end of the former and the beginning of the latter. Only two or three
years ago an expedition from the American Museum of Natural History
discovered a place in Wyoming where the White River beds lie directly
upon those of the Uinta, thus fully confirming the inference as to the
relative age of these two formations which had long ago been drawn from
the comparative study of their fossil mammals.
The palæontological method of determining the geological date of the
stratified rocks is thus an indispensable means of correlating the
scattered exposures of the strata in widely separated regions and in
different continents, it may be with thousands of miles of intervening
ocean. The general principle employed is that _close similarity of
fossils in the rocks of the regions compared points to an approximately
contemporaneous date of formation of those rocks_. This principle must
not, however, be applied in an off-hand or uncritical manner, or it will
lead to serious error. In the first place, the evolutionary process is a
very slow one and geological time is inconceivably long, so that deposits
which differ by some thousands of years may yet have the same or nearly
the same fossils. The method is not one of sufficient refinement to
detect such relatively small differences. To recur to the illustration
of the development in handwriting, the palæographer can hardly do more
than determine the decade in which a manuscript was written; no one would
expect him to fix upon the exact year, still less the month, from the
study of handwriting alone. As is the month in recorded human history, so
is the millennium in the long course of the earth’s development.
[Illustration: FIG. 3.—Bluff on Beaver Creek, Fremont Co., Wyoming. The
White River beds were deposited on the worn and weathered surface of
the Uinta, the heavy, broken line marking the separation between them.
The valley was carved out long after the deposition of the White River
strata.]
In the second place, there are great differences in the contemporary life
of separate regions and such geographical differences there have always
been, so far as we can trace back the history of animals and plants. A
new organism does not originate simultaneously all over the world but,
normally at least, in a single area and spreads from that centre until
it encounters insuperable obstacles. Such spreading is a slow process
and hence it is that new forms often appear in one region much earlier
than in others and in the very process of extending their range, the
advancing species may themselves be considerably modified and reach their
new and distant homes as different species from those which originated
the movement. Extinction, likewise, seldom occurs simultaneously over
the range of a group, but now here and now there in a way that to our
ignorance appears to be arbitrary and capricious. The process may go on
until extinction is total, or may merely result in a great restriction of
the range of a given group, or may break up that range into two or more
distinct areas.
Of such incomplete extinctions many instances might be given, but one
must suffice. The camel-tribe, strange as it may appear, originated in
North America and was long confined to that continent, while at the
present day it is represented only by the llamas of South America and
the true camels of Asia, having completely vanished from its early home.
These facts and a host of similar ones make plain how necessary it is to
take geographical considerations into account in all problems that deal
with the synchronizing of the rocks of separate areas and continents.
Properly to estimate the significance of a difference in the fossils
of two regions and to determine how far it is geographical, due to a
separation in space, or geological and caused by separation in time, is
often a very difficult matter and requires a vast amount of minute and
detailed study. Once more, the principle involved is illustrated by the
study of manuscripts. Down to the time when the printing press superseded
the copyist, each of the nations of Europe had its own traditions and
its more or less independent course of handwriting development. A great
monastery, in which the work of copying manuscripts went on century after
century, became an independent geographical centre with its particular
styles. Thus the palæographer, like the geologist, is confronted by
geographical problems as well as by those of change and development in
general.
In addition to the method of geologically dating the rocks by means of
the fossils which they contain, there are other ways which may give a
greater precision to the result. Climatic changes, when demonstrable,
are of this character, for they may speedily and simultaneously affect
vast areas of the earth’s surface or even the entire world. From time to
time in the past, glacial conditions have prevailed over immense regions,
several continents at once, it may be, as in one instance in which India,
South Africa, Australia, South America were involved. The characteristic
accumulations made by the glaciers in these widely separated regions
must be contemporaneous in a sense that can rarely be predicated of the
ordinary stratified rocks. Such climatic changes as the formation and
disappearance of the ice-fields give a sharper and more definite standard
of time comparisons than do the fossils alone, and yet the fossils are
in turn needed to show which of several possible glacial periods are
actually being compared.
Again, great movements of the earth’s crust, which involve vast and
widely separated regions and bring the sea in over great areas of land,
or raise great areas into land, which had been submerged, may also yield
more precise time-measurements, because occurring within shorter periods
than do notable changes in the system of living things. Such changes in
animals and plants may be compared to the almost imperceptible movement
of the hour-hand of a clock, while the recorded climatic revolutions
and crustal movements often supply the place of the minute-hand. It is
obvious, however, that if the hour-hand be wanting, the minute-hand
alone can be of very limited use. There have been a great many vast
submergences and emergences of land in the history of the earth, and only
the fossils can give us the assurance that we are comparing the same
movement in distant continents, and not two similar movements separated
by an enormous interval of time.
It may thus fairly be admitted that it is possible to arrange the rocks
which compose the accessible parts of the earth’s crust in chronological
order and to correlate in one system the rocks of the various continents.
The terms used for the more important divisions of geological time are,
in descending order of magnitude, era, period, epoch, age or stage, and
the general scheme of the eras and periods, which is in almost uniform
use throughout the world, is given in the table, which is arranged so as
to give the succession graphically, with the most ancient rocks at the
bottom and the latest at the top.
Cenozoic era { Quaternary period
{ Tertiary period
{ Cretaceous period
Mesozoic era { Jurassic period
{ Triassic period
{ Permian period
{ Carboniferous period
Palæozoic era { Devonian period
{ Silurian period
{ Ordovician period
{ Cambrian period
Pre-Cambrian eras { Algonkian period
{ Archæan period
It must not be supposed that all the divisions of similar rank, such as
the eras, for example, were of equal length, as measured by the thickness
of the rocks assigned to those divisions. On the contrary, they must
have been of very unequal length and are of very different divisibility.
The Pre-Cambrian eras, with only two periods, were probably far longer
than all subsequent time, and all that the major divisions imply is that
they represent changes in the system of life of approximately equivalent
importance. It is impossible to give any trustworthy estimate of the
actual lengths of these divisions in years, though many attempts to do so
have been made. All that can be confidently affirmed is that geological
time, like astronomical distances, is of inconceivable vastness and its
years can be counted only in hundreds of millions.
To discuss in any intelligible manner the history of mammals, it will be
necessary to go much farther than the above table in the subdivision of
that part of geological time in which mammalian evolution ran its course.
As mammals represent the highest stage of development yet attained in the
animal world, it is only the latter part of the earth’s history which
is concerned with them; the earlier and incomparably longer portion of
that history may be passed over. Mammals are first recorded in the later
Triassic, the first of the three periods which make up the Mesozoic era.
They have also been found, though very scantily, in the other Mesozoic
periods, the Jurassic and Cretaceous, but it was the Cenozoic era that
witnessed most of the amazing course of mammalian development and
diversification, and hence the relatively minute subdivisions necessary
for the understanding of this history deal only with the Cenozoic, the
latest of the great eras.
In the subjoined table the periods and epochs are those which are in
general use throughout the world, the ages and stages are those which
apply to the western interior of North America, each region, even of the
same continent, requiring a different classification. The South American
formations are given in a separate table, as it is desirable to avoid the
appearance of an exactitude in correlation which cannot yet be attained.
CENOZOIC ERA
Quaternary period { Recent epoch
{ Pleistocene epoch = Glacial and Interglacial stages.
{ Pliocene epoch
{ Miocene epoch
Tertiary period { Oligocene epoch
{ Eocene epoch
{ Paleocene epoch
Continuing the subdivision of the Tertiary period still farther, we have
the following arrangement:
TERTIARY PERIOD (North America)
{ Upper Wanting
{ Middle Blanco age
_Pliocene_ { { Thousand Creek age
{ Lower { Snake Creek age
{ { Republican River age
{ Upper Loup Fork age
_Miocene_ { Middle Deep River age
{ Lower Arikaree age
_Oligocene_ { Upper John Day age
{ Lower White River age
{ Upper Uinta age
_Eocene_ { Middle Bridger age
{ Lower { Wind River age
{ Wasatch age
_Paleocene_ { Upper Torrejon age } Fort Union
{ Lower Puerco age }
This is a representative series of the widespread and manifold non-marine
Tertiary deposits of the Great Plains, but a much more extensive and
subdivided scheme would be needed to show with any degree of fullness
the wonderfully complete record of that portion of the continent during
the Tertiary period. A much more elaborate table will be found in
Professor Osborn’s “Age of Mammals,” p. 41. There are some differences
of practice among geologists as to this scheme of classification, though
the differences are not those of principle. No question arises concerning
the reality of the divisions, or their order of succession in time, but
merely as to the rank or relative importance which should be attributed
to some of them, and that is a very minor consideration.
Much greater difficulty and, consequently, much more radical differences
of interpretation arise when the attempt is made to correlate or
synchronize the smaller subdivisions, as found in the various continents,
with one another, because of the geographical differences in contemporary
life. Between Europe and North America there has always been a certain
proportion of mammalian forms in common, a proportion that was at
one time greater, at another less, and this community renders the
correlation of the larger divisions of the Tertiary in the two continents
comparatively easy, and even in the minor subdivisions very satisfactory
progress has been made, so that it is possible to trace in some detail
the migrations of mammals from the eastern to the western hemisphere and
_vice versa_. Such intermigrations were made possible by the land-bridges
connecting America with Europe across the Atlantic, perhaps on the line
of Greenland and Iceland, and with Asia where now is Bering Strait.
These connections were repeatedly made and repeatedly broken during the
Mesozoic and Cenozoic eras down to the latest epoch, the Pleistocene.
By comparing the fossil mammals of Europe with those of North America
for any particular division of geological time, it is practicable to
determine whether the way of intermigration was open or closed, because
separation always led to greater differences between the faunas of the
two continents through divergent evolution.
Correlation with South America is exceedingly difficult and it is in
dealing with this problem that the widest differences of opinion have
arisen among geologists. Through nearly all the earlier half of the
Tertiary period the two Americas were separated and, because of this
separation, their land mammals were utterly different. Hence, the lack
of elements common to both continents puts great obstacles in the way of
establishing definite time-relations between their geological divisions.
Only the marine mammals, whales and dolphins, were so far alike as to
offer some satisfactory basis of comparison. When, in the later Tertiary,
a land-connection was established between the two continents, migrations
of mammals from each to the other began, and thenceforward there were
always certain elements common to both, as there are to-day. In spite
of the continuous land between them, the present faunas of North and
South America are very strikingly different, South America being, with
the exception of Australia, zoölogically the most peculiar region of the
earth.
In the following table of the South American Cenozoic, the assignment of
the ages to their epochs is largely tentative, especially as regards the
more ancient divisions, and represents the views generally held by the
geologists of Europe and the United States; those of South America, on
the contrary, give an earlier date to the ages and stages and refer the
older ones to the Cretaceous instead of the Tertiary.
CENOZOIC ERA (South America)
Quaternary period { Recent epoch
{ Pleistocene epoch—Pampean Beds, Brazilian caverns
{ { Monte Hermoso age
{ Pliocene epoch { Catamarca age
{ { Paraná age
{
Tertiary period { Miocene epoch { Santa Cruz age
{ { Patagonian age
{
{ Oligocene epoch { Deseado age (_Pyrotherium_ Beds)
{ { Astraponotus Beds
{
{ Eocene epoch { Casa Mayor age (_Notostylops_ Beds)
The Pleistocene and Pliocene deposits are most widely distributed over
the Pampas of Argentina, but the former occur also in Ecuador, Brazil,
Chili, and Bolivia. The other formations cover extensive areas in
Patagonia, and some extend into Tierra del Fuego.
We have next to consider the methods by which past geographical
conditions may be ascertained, a task which, though beset with
difficulties, is very far from being a hopeless undertaking. As has
already been pointed out, it is perfectly possible for the geologist to
determine the circumstances of formation of the various kinds of rocks,
to distinguish terrestrial from aquatic accumulations and, among the
latter, to identify those which were laid down in the sea and those which
were formed in some other body of water. By platting on a map all the
marine rocks of a given geological date, an approximate estimate may be
formed as to the extension of the sea over the present land for that
particular epoch. It is obvious, however, that for those areas which
then were land and now are covered by the sea, no such direct evidence
can be obtained, and only indirect means of ascertaining the former
land-connections can be employed. It is in the treatment of this indirect
evidence that the greatest differences of opinion arise and, if two
maps of the same continent for the same epoch, by separate authors, be
compared, it will be seen that the greatest discrepancies between them
are concerning former land-connections and extensions.
The only kind of indirect evidence bearing upon ancient land-connections,
now broken by the sea, that need be considered here is that derived from
the study of animals and plants, both recent and fossil. All-important in
this connection is the principle that the same or closely similar species
do not arise independently in areas between which there is no connection.
It is not impossible that such an independent origin of organisms which
the naturalist would class as belonging to the same species may have
occasionally taken place, but, if so, it must be the rare exception to
the normal process. This principle leads necessarily to the conclusion
that the more recently and broadly two land-areas, now separated by the
sea, have been connected, the more nearly alike will be their animals
and plants. Such islands as Great Britain, Sumatra and Java must have
been connected with the adjacent mainland within a geologically recent
period, while the extreme zoölogical peculiarity of Australia can be
explained only on the assumption that its present isolation is of very
long standing. The principle applies to the case of fossils as well as to
that of modern animals, and has already been made use of, in a preceding
section, in dealing with the ancient land-connections of North America.
It was there shown that the connection of this continent with the Old
World and the interruptions of that connection are reflected and recorded
in the greater or less degree of likeness in the fossil mammals at any
particular epoch. Conversely, the very radical differences between the
fossil mammals of the two Americas imply a long-continued separation
of those two continents, and their junction in the latter half of the
Tertiary period is proved by the appearance of southern groups of mammals
in the northern continent, and of northern groups in South America.
Inasmuch as the connection between North and South America still
persists, the geology of the Isthmus of Panama should afford testimony
in confirmation of the inferences drawn from a study of the mammals. Of
course, the separating sea did not necessarily cross the site of the
present isthmus; it might have cut through some part of Central America,
but a glance at the map immediately suggests the isthmus as the place
of separation and subsequent connection. As a matter of fact, isthmian
geology is in complete accord with the evidence derived from the mammals.
Even near the summit of the hills which form the watershed between the
Atlantic and the Pacific and through which the great Culebra Cut passes,
are beds of marine Tertiary shells, showing that at that time the land
was completely submerged. This does not at all preclude the possibility
of other transverse seas at the same period; indeed, much of Central
America was probably under the sea also, but the geology of that region
is still too imperfectly known to permit positive statements.
When several different kinds of testimony, each independent of the other,
can be secured and all are found to be in harmony, the strength of the
conclusion is thereby greatly increased. Many distinct lines of evidence
support the inference that North and South America were completely
severed for a great part of the Tertiary period. This is indicated in the
clearest manner, not only by the geological structure of the Isthmus and
by the mammals, living and extinct, as already described, but also by the
fresh-water fishes, the land-shells, the reptiles and many other groups
of animals and plants.
The distribution of marine fossils may render the same sort of service
in elucidating the history of the sea as land-mammals do for the
continents, demonstrating the opening and closing of connections between
land-areas and between oceans. The sea, it is true, is one and undivided,
the continental masses being great islands in it, but, nevertheless,
the sea is divisible into zoölogical provinces, just as is the land.
Temperature, depth of water, character of the bottom, etc., are factors
that limit the range of marine organisms, as climate and physical
barriers circumscribe the spread of terrestrial animals. Professor Perrin
Smith has shown that in the Mesozoic era Bering Strait was repeatedly
opened and closed, and that each opening and closing was indicated by
the geographical relationships of the successive assemblages of marine
animals that are found in the Mesozoic rocks of California and Nevada.
When the Strait was open, the coast-line between North America and Asia
was interrupted and the North Pacific was cooled by the influx of water
from the Arctic Sea. At such times, sea-animals from the Russian and
Siberian coasts extended their range along the American side as far south
as Mexico, and no forms from the eastern and southern shores of Asia
accompanied them. On the other hand, when the Strait was closed, the
Arctic forms were shut out and the continuous coast-line and warmer water
enabled the Japanese, Indian, and even Mediterranean animals to extend
their range to the Pacific coast of North America. A comparison of the
marine fishes of the two sides of the Isthmus of Panama shows an amount
and degree of difference between the two series as might be expected from
the length of time that they have been separated by the upheaval of the
land.
In working out the geographical conditions for any particular epoch
of the earth’s history, it is possible to go much farther than merely
gaining an approximate estimate of the distribution of land and sea; many
other important facts may be gathered from a minute examination of the
rocks in combination with a genetic study of topographical forms. By this
physiographical method, as it is called, the history of several of the
great mountain-ranges has been elaborated in great detail. It is quite
practicable to give a geological date for the initial upheaval and to
determine whether one or many such series of movements have been involved
in bringing about the present state of things. Similarly, the history of
plains and plateaus, hills and valleys, lake and river systems, may be
ascertained, and for the earth’s later ages, at least, a great deal may
be learned regarding the successive forms of the land-surfaces in the
various continents. It would be very desirable to explain the methods by
which these results are reached, but this could hardly be done without
writing a treatise on physiography, for which there is no room in this
chapter. We must be permitted to make use of the results of that science
without being called upon to prove their accuracy.
No factor has a more profound effect in determining the character and
distribution of living things than climate, of which the most important
elements, for our purpose, are temperature and moisture. One of the most
surprising results of geological study is the clear proof that almost all
parts of the earth have been subjected to great vicissitudes of climate,
and a brief account of the evidence which has led to this unlooked for
result will not be out of place here.
The evidence of climatic changes is of two principal kinds, (1) that
derived from a study of the rocks themselves, and (2) that given by the
fossils of the various epochs. So far as the rocks laid down in the sea
are concerned, little has yet been ascertained regarding the climatic
conditions of their formation, but the strata which were deposited on
the land, or in some body of water other than the sea, often give the
most positive and significant information concerning the circumstances of
climate which prevailed at the time of their formation. Certain deposits,
such as gypsum and rock-salt, are accumulated only in salt lakes, which,
in turn, are demonstrative proof of an arid climate. A salt lake could
not exist in a region of normal rainfall and, from the geographical
distribution of such salt-lake deposits, it may be shown that arid
conditions have prevailed in each of the continents and, not only once,
but many times. As a rule, such aridity of climate was relatively local
in extent, but sometimes it covered vast areas. For example, in the
Permian, the last of the Palæozoic, and the Triassic, the first of the
Mesozoic periods (see Table, p. 15) nearly all the land-areas of the
northern hemisphere were affected, either simultaneously or in rapid
succession.
Until a comparatively short time ago, it was very generally believed that
the Glacial or Pleistocene epoch, which was so remarkable and conspicuous
a feature of the Quaternary period, was an isolated phenomenon, unique in
the entire history of the earth. Now, however, it has been conclusively
shown that such epochs of cold have been recurrent and that no less than
five of these have left unmistakable records in as many widely separated
periods of time.
When the hypothesis of a great “Ice Age” in the Pleistocene was first
propounded by the elder Agassiz, it was naturally received with general
incredulity, but the gradual accumulation of proofs has resulted in
such an overwhelming weight of testimony, that the glacial hypothesis
is now accepted as one of the commonplaces of Geology. The proofs
consist chiefly in the characteristic glacial accumulations, moraines
and drift-sheets, which cover such enormous areas in Europe and North
America and, on a much smaller scale, in Patagonia, and in the equally
characteristic marks of glacial wear left upon the rocks over which
the ice-sheets moved. Many years later it was proved that the Permian
period had been a time of gigantic glaciation, chiefly in the southern
hemisphere, when vast ice-caps moved slowly over parts of South America,
South Africa, Australia and even of India. The evidence is of precisely
the same nature as in the case of the Pleistocene glaciation. In not less
than three more ancient periods, the Devonian, Cambrian, and Algonkian,
proofs of glacial action have been obtained.
While the rocks themselves thus afford valuable testimony as to the
climatic conditions which prevailed at the time and place of their
formation, this testimony is fragmentary, missing for very long periods,
and must be supplemented from the information presented by the fossils.
As in all matters where fossils are involved, the evidence must be
cautiously used, for hasty inferences have often led to contradictory
and absurd conclusions. When properly employed, the fossils give a more
continuous and complete history of climatic changes than can, in the
present state of knowledge, be drawn from a study of the rocks alone.
For this purpose plants are particularly useful, because the great
groups of the vegetable kingdom are more definitely restricted in their
range by the conditions of temperature and moisture than are most of the
correspondingly large groups of animals. Not that fossil animals are
of no service in this connection; quite the contrary is true, but the
evidence from them must be treated more carefully and critically. To
illustrate the use of fossils as recording climatic changes in the past,
one or two examples may be given.
In the Cretaceous period a mild and genial climate prevailed over all
that portion of the earth whose history we know, and was, no doubt,
equally the case in the areas whose geology remains to be determined.
The same conditions extended far into the Arctic regions, and abundant
remains of a warm-temperate vegetation have been found in Greenland,
Alaska and other Arctic lands. Where now only scanty and minute dwarf
willows and birches can exist, was then a luxuriant forest growth
comprising almost all of the familiar trees of our own latitudes, a most
decisive proof that in the Cretaceous the climate of the Arctic regions
must have been much warmer than at present and that there can have been
no great accumulation of ice in the Polar seas. Conditions of similar
mildness obtained through the earlier part of the Tertiary. In the Eocene
epoch large palm-trees were growing in Wyoming and Idaho, while great
crocodiles and other warm-country reptiles abounded in the waters of the
same region.
It is of particular interest to inquire how far the fossils of Glacial
times confirm the inferences as to a great climatic change which are
derived from a study of the rocks, for this may be taken as a test-case.
Any marked discrepancy between the two would necessarily cast grave doubt
upon the value of the testimony of fossils as to climatic conditions.
The problem is one of great complexity, for the Pleistocene was not one
long epoch of unbroken cold, but was made up of Glacial and Interglacial
ages, alternations of colder and milder conditions, and some, at least,
of the Interglacial ages had a climate warmer than that of modern times.
Such great changes of temperature led to repeated migrations of the
mammals, which were driven southward before the advancing ice-sheets
and returned again when the glaciers withdrew under the influence
of ameliorating climates. Any adequate discussion of these complex
conditions is quite out of the question in this place and the facts must
be stated in simplified form, as dealing only with the times of lowered
temperature and encroaching glaciers.
The plants largely fail us here, for little is known of Glacial
vegetation, but, on the other hand, a great abundance of the fossil
remains of animal life of that date has been collected, and its testimony
is quite in harmony with that afforded by the ice-markings and the
ice-made deposits. Arctic shells in the marine deposits of England, the
valley of the Ottawa River and of Lake Champlain, Walruses on the coast
of New Jersey, Reindeer in the south of France, and Caribou in southern
New England, Musk-oxen in Kentucky and Arkansas, are only a few examples
of the copious evidence that the climate of the regions named in Glacial
times was far colder than it is to-day.
I have thus endeavored to sketch, necessarily in very meagre outlines,
the nature of the methods employed to reconstruct the past history of
the various continents and the character of the evidence upon which we
must depend. Should the reader be unconvinced and remain sceptical as to
the possibility of any such reconstruction, he must be referred to the
numerous manuals of Geology, in which these methods are set forth with a
fulness which cannot be imitated within the limits of a single chapter.
The methods are sound, consisting as they do merely in the application of
“systematized common sense” (in Huxley’s phrase) to observed facts, but
by no means all applications of them are to be trusted. Not to mention
ill-considered and uncritical work, or inverted pyramids of hypothesis
balanced upon a tiny point of fact, it should be borne in mind that
such a complicated and difficult problem as the reconstruction of past
conditions can be solved only by successive approximations to the truth,
each one partial and incomplete, but less so than the one which preceded
it.
CHAPTER II
METHODS OF INVESTIGATION—PALÆONTOLOGICAL
Palæontology is the science of ancient life, animal and vegetable, the
Zoölogy and Botany of the past, and deals with fossils. Fossils are the
recognizable remains or traces of animals or plants, which were buried in
the rocks at the time of the formation of those rocks. In a geological
sense, the term _rock_ includes loose and uncompacted materials, such as
sand and gravel, as well as solid stone. Granting the possibility of so
determining the relative dates of formation of the rocks, that the order
of succession of the fossils in time may be ascertained in general terms,
the question remains: What use, other than geological, can be made of the
fossils? In dealing with this question, attention will be directed almost
exclusively to the mammals, the group with which this book is concerned.
As a preliminary to the discussion, something should be said of the
ways in which mammals became entombed in the rocks in which we find
them. In this connection it should be remembered that, however firm and
solid those rocks may be now, they were originally layers of loose and
uncompacted material, deposited by wind or water, and that each layer
_formed in its turn the surface of the earth, until buried by fresh
accumulations upon it_, it may be to enormous depths.
One method of the entombing of land-mammals, which has frequently been
of great importance, is burial in volcanic dust and so-called ash, which
has been compacted into firm rock. During a great volcanic eruption
enormous quantities of such finely divided material are ejected from the
crater and are spread out over the surrounding country, it may be for
distances of hundreds of miles. Thus will be buried the scattered bones,
skeletons, carcasses, that happen to be lying on the surface; and if the
fine fragments are falling rapidly, many animals will be buried alive and
their skeletons preserved intact. A modern instance of this is given by
the numerous skeletons of men and domestic animals buried in the volcanic
ash which overwhelmed Pompeii in 79 A.D. Pliny the Younger, who witnessed
that first recorded eruption of Vesuvius, tells us in a letter written to
Tacitus, that far away at Misenum, west of Naples, it was often necessary
to rise and shake off the falling ashes, for fear of being buried in
them. In the Santa Cruz formation of Patagonia (see p. 124), which has
yielded such a wonderful number and variety of well-preserved fossils,
the bones are all found in volcanic dust and ash compacted into a rock,
which is usually quite soft, but may become locally very hard. The
Bridger formation of Wyoming (p. 110) and the John Day of eastern Oregon
(p. 116) are principally made up of volcanic deposits; and no doubt there
are several others among the Tertiary stages which were formed in the
same way, but have not yet received the microscopic study necessary to
determine this.
Much information concerning the mammalian life of the Pleistocene, more
especially in Europe and in Brazil (p. 211), has been derived from the
exploration of caverns. Some of these caves were the dens of carnivorous
beasts and contain multitudes of the bones of their victims, as well as
those of the destroyers themselves. Others, such as the Port Kennedy
Cave, on the Schuylkill River above Philadelphia, the Frankstown Cave in
central Pennsylvania, the Conard Fissure in Arkansas, are hardly caverns
in the ordinary sense of the word, but rather narrow fissures, into
which bones and carcasses were washed by floods, or living animals fell
from above and died without being able to escape. The bones are mostly
buried in the earth which partially or completely fills many caverns and
may be covered by a layer of stalagmite, derived from the solution and
re-deposition of the limestone of the cavern-walls, by the agency of
percolating waters.
A mode of preservation which is unfortunately rare is exemplified by
the asphaltic deposits near Los Angeles, at Rancho La Brea, which have
been very fully described by Professor J. C. Merriam of the University
of California. The asphalt has been formed by the oxidation and
solidification of petroleum, which has risen up through the Pleistocene
rocks from the oil-bearing shales below. At one stage in the conversion
of petroleum into asphalt, tar-pools of extremely viscid and adhesive
character were, and still are, formed on the surface of the ground; and
these pools were veritable traps for mammals and birds and for the beasts
and birds of prey which came to devour the struggling victims.
“The manner in which tar or asphalt pools may catch unsuspecting animals
of all kinds is abundantly illustrated at the present time in many places
in California, but nowhere more strikingly than at Rancho La Brea itself,
where animals of many kinds have frequently been so firmly entrapped that
they died before being discovered, or if found alive were extricated only
with the greatest difficulty. As seen at this locality, the tar issuing
from springs or seepages is an exceedingly sticky, tenacious substance
which is removed only with the greatest difficulty from the body of
any animal with which it may come in contact. Small mammals, birds, or
insects running into the soft tar are very quickly rendered helpless
by the gummy mass, which binds their feet, and in their struggles soon
reaches every part of the body. Around the borders of the pools the tar
slowly hardens by the evaporation of the lighter constituents until
it becomes as solid as an asphalt pavement. Between the hard and soft
portions of the mass there is a very indefinite boundary, the location of
which can often be determined only by experiment, and large mammals in
many cases run into very tenacious material in this intermediate zone,
from which they are unable to extricate themselves.”
The foregoing account refers to what may actually be observed at the
present time; in regard to the Pleistocene, Professor Merriam says:
“In the natural accumulation of remains at the tar pools through
accidental entangling of animals of all kinds, it is to be presumed that
a relatively large percentage of the individuals entombed would consist
of young animals with insufficient experience to keep them away from
the most dangerous places, or with insufficient strength to extricate
themselves. There would also be a relatively large percentage of old,
diseased, or maimed individuals that lacked strength to escape when
once entangled. In the census of remains that have been obtained up to
the present time the percentages of quite young, diseased, maimed, and
very old individuals are certainly exceptionally large.... In addition
to the natural accumulation of animal remains through the entangling
of creatures of all kinds by accidental encountering of the tar, it is
apparent from a study of the collections obtained that some extraordinary
influence must have brought carnivorous animals of all kinds into
contact with the asphalt with relatively greater frequency than other
kinds of animals. In all the collections that have been examined the
number of carnivorous mammals and birds represented is much greater than
that of the other groups.... Whenever an animal of any kind is caught
in the tar, its struggles and cries naturally attract the attention of
carnivorous mammals and birds in the immediate vicinity, and the trapped
creature acts as a most efficient lure to bring these predaceous animals
into the soft tar with it. It is not improbable that a single small
bird or mammal struggling in the tar might be the means of entrapping
several carnivores, which in turn would naturally serve to attract
still others.... In the first excavations carried on by the University
of California a bed of bones was encountered in which the number of
saber-tooth and wolf skulls together averaged twenty per cubic yard.”[1]
As the animals were thus entombed alive, it would be expected that a
large number of complete skeletons would be preserved, but this is not
the case: “connected skeletons are not common.” This scattering and
mingling of the bones were due partly to the trampling of the heavier
animals in their struggles to escape, but, in more important degree, to
the movements within the tar and asphalt.
In arid and semi-arid regions great quantities of sand and dust are
transported by the wind and deposited where the winds fail, or where
vegetation entangles and holds the dust. Any bones, skeletons or
carcasses which are lying on the surface will thus be buried, and even
living animals may be suffocated and buried by the clouds of dust.
An example of such wind-made accumulations is the Sheridan formation
(Equus Beds, see p. 131), which covers vast areas of the Great Plains
from Nebraska to Mexico and contains innumerable bones, especially of
horses. In this formation in northwestern Kansas, Professor Williston
found nine skeletons of the large peccary (_†Platygonus leptorhinus_),
lying huddled together, with their heads all pointing in the same
direction, and in the upper Miocene (p. 121) of South Dakota Mr. Gidley
discovered six skeletons of three-toed horses (_†Neohipparion whitneyi_)
crowded together, killed and buried probably by a sandstorm. Similar
illustrations might be gathered from many other parts of the world.
Swamps and bogs may, especially under certain conditions, become the
burial places of great numbers of animals, which venture into them,
become buried and are unable to extricate themselves. Especially is
this true in times of great drought, when animals are not only crazed
with thirst, but very much weakened as well, and so unable to climb out
of the clinging mud. In an oft-quoted passage, Darwin gives a vivid
description of the effects of a long drought in Argentina between the
years 1827 and 1830. “During this time so little rain fell, that the
vegetation, even to the thistles, failed; the brooks were dried up, and
the whole country assumed the appearance of a dusty high road.” “I was
informed by an eyewitness that the cattle in herds of thousands rushed
into the Paraná, and being exhausted by hunger they were unable to crawl
up the muddy banks, and thus were drowned. The arm of the river which
runs by San Pedro was so full of putrid carcasses, that the master of
a vessel told me that the smell rendered it quite impassable. Without
doubt several hundred thousand animals thus perished in the river; their
bodies when putrid were seen floating down the stream; and many in all
probability were deposited in the estuary of the Plata. All the small
rivers became highly saline, and this caused the death of vast numbers
in particular spots; for when an animal drinks of such water it does
not recover. Azara describes the fury of the wild horses on a similar
occasion, rushing into the marshes, those which arrived first being
overwhelmed and crushed by those which followed. He adds that more than
once he has seen the carcasses of upwards of a thousand wild horses thus
destroyed.... Subsequently to the drought of 1827 to 1832, a very rainy
season followed, which caused great floods. Hence it is almost certain
that some thousands of the skeletons were buried by the deposits of the
very next year.”[2]
In the arid and desolate regions of the interior of South Australia is a
series of immense dry lakes, which only occasionally contain water and
ordinarily “are shallow, mud-bottomed or salt-encrusted claypans only.”
One of these, Lake Callabonna, is of great interest as having preserved
in its soft mud many remains of ancient life, of creatures which were
mired in the clay and destroyed, as has been described by Dr. E. C.
Stirling. “There is, however, compensation for the unpromising physical
features of Lake Callabonna in the fact that its bed proves to be a
veritable necropolis of gigantic extinct marsupials and birds which
have apparently died where they lie, literally, in hundreds. The facts
that the bones of individuals are often unbroken, close together, and,
frequently, in their proper relative positions, the attitude of many of
the bodies and the character of the matrix in which they are embedded,
negative any theory that they have been carried thither by floods. The
probability is, rather, that they met their deaths by being entombed
in the effort to reach food or water, just as even now happens in dry
seasons, to hundreds of cattle which, exhausted by thirst and starvation,
are unable to extricate themselves from the boggy places that they have
entered in pursuit either of water or of the little green herbage due to
its presence. The accumulation of so many bodies in one locality points
to the fact of their assemblage around one of the last remaining oases
in the region of desiccation which succeeded an antecedent condition of
plenteous rains and abundant waters.”
It is a very general experience in collecting fossil mammals to find
that they are not evenly or uniformly distributed through the beds, but
rather occur in “pockets,” where great numbers of individuals are crowded
together, while between the “pockets” are long stretches of barren
ground. It is equally common to find the bones thickly distributed in
certain layers, or beds, and the layers above and below entirely wanting
in fossils. The reasons for this mode of occurrence have been partially
explained in the foregoing paragraphs, but the reason differs for each
particular mode of entombment. The important part played by drought
in causing such accumulation of closely crowded bodies in swamps and
mud-holes is indicated in the quotations from Darwin and Stirling; but
similar accumulations may take place on hard ground, as was observed in
central Africa by Gregory. “Here and there around a water hole we found
acres of ground white with the bones of rhinoceroses and zebra, gazelle
and antelope, jackal and hyena.... These animals had crowded around
the dwindling pools and fought for the last drops of water.”[3] Even
in normal seasons springs and water holes and the drinking places in
streams are the lurking places of beasts of prey and crocodiles, so that
great accumulations of bones are made around these spots. A succession
of unusually severe winters frequently leads to great mortality among
mammals, as happened in Patagonia in the winter of 1899, when enormous
numbers of Guanaco perished of starvation on the shore of Lake Argentine,
where they came to drink.
Bones which are exposed on the surface of the ground decay and crumble to
pieces in the course of a very few years; and if they are to be preserved
as fossils, it is necessary that they should be buried under sedimentary
or volcanic deposits. Several such modes of burial have been described
in the foregoing paragraphs, but there are other and equally important
methods, which remain to be considered.
The deposits made by rivers are often extremely rich in fossils, and
most of the Tertiary formations of the Great Plains are now ascribed to
the agency of rivers. The flood-plain of a stream, or that part of its
basin which is periodically overflowed, is gradually built up by the
layers of clay and silt thrown down by the relatively still waters of the
flooded area, and scattered bones, skeletons or carcasses that may have
been lying on the ground before the freshet are buried in the deposits.
Bones covered up in this manner frequently show the marks of teeth of
rodents or carnivores which have gnawed them when lying exposed. Deposits
made in the stream-channels, where the current was swiftest, are of
coarser materials such as gravel and sand, and these often contain the
skeletons of animals which were drowned and swept downward by the flooded
stream. When the Bison (the mistakenly so-called Buffalo) still roamed
in countless herds over the western plains, immense numbers of them were
drowned in the upper Missouri River by breaking through the ice, when
they attempted to cross at times when the ice had not attained its winter
thickness, or was weakened by melting in the spring. No doubt, the bed
of that river contains innumerable bones of the Bison. Frequently, too,
animals are caught in quicksands and, unable to escape, are buried in the
soft mass; fossil skeletons which are preserved in sandstones in an erect
or standing position are usually to be interpreted in this manner.
The sedimentary accumulations formed in lakes and ponds sometimes yield
fossil bones or skeletons in considerable numbers, which have, for the
most part, been derived from the carcasses of animals carried into the
lake by streams. A newly drowned mammal sinks to the bottom and, if
sufficient sediment be quickly deposited upon it, it may be anchored
there and fossilized as a complete skeleton. Otherwise, when distended by
the gases of putrefaction, the body will rise and float on the surface,
where it will be attacked and pulled about by crocodiles, fishes and
other predaceous creatures. As the bones are loosened in the course of
decomposition, they will drop to the bottom and be scattered, now here,
now there, over a wide area.
Land mammals are rarely found in marine rocks, or such deposits as were
made on the sea-bottom; but the remains of marine mammals, whales,
porpoises, dolphins, seals, etc. are often found in large numbers. In
principle, the method of entombment is the same as in the case of lakes,
but currents may drift to some bay or cove multitudes of carcasses of
these marine mammals. At Antwerp, in Belgium, incredible quantities of
such remains have been exposed in excavations and in all probability were
drifted by currents into a quiet and shallow bay, which was subsequently
converted into land.
While the foregoing account by no means exhausts the various methods
of accumulation and burial of the skeletons and scattered bones of
mammals, it covers the more important of these methods sufficiently for
a general understanding of the different processes. In whatever manner
the preservation may have been effected, there is great difference in
the relative abundance and completeness among the fossils of the various
kinds of mammals which were living at the same time and in the same
area. It need hardly be said, that the more abundant any species was,
the better was the chance of its being represented among the fossils;
hence, gregarious species, living in large herds, were more likely to be
preserved than those which led a solitary existence, or were individually
rare. Most of the hoofed mammals are and apparently always have been
gregarious, and are therefore much better represented among the fossils,
and are, in consequence, better known than the beasts of prey, which,
of necessity, were individually less numerous and generally solitary
in habits. Not only this, but large and medium-sized mammals, with
strong and heavy bones, were better fitted to withstand the accidents of
entombment and subsequent preservation than small creatures with delicate
and fragile skeletons. The mere dead weight of over-lying sediments often
crushes and distorts the bones, and the movements of uplift, compression,
folding and fracture, to which so many strata have been subjected, did
still further damage to the fossils. The percolating waters, which for
ages have traversed the porous rocks, often attack and dissolve the
bones, completely destroying the minute ones and greatly injuring those
which are massive and strong. In consequence of all those accidents it
frequently happens that only the teeth, the hardest and most resistant
of animal structures, and it may be the dense and solid jaw-bones, are
all that remain to testify of the former existence of some creature that
long ago vanished from the earth. Very many fossil mammals are known
exclusively from the teeth, and it is this fact which makes the exact
study of teeth so peculiarly important to the palæontologist.
In view of all these facts, it is not surprising that concerning the
history of many mammalian groups we have but scanty information, or
none at all, while in the case of others the story is wonderfully full
and detailed. The latter are, very generally, the groups which were
not only numerically abundant at all stages of their history, but also
had skeletons that were strong enough to resist destruction; while the
groups as to which there is little or no information are chiefly of
small and fragile animals, or such as were always rare. For example,
a great deal has been learned regarding the development of horses and
rhinoceroses in North America, but the history of the tapirs is very
unsatisfactorily known, because, while horses and rhinoceroses were
common, tapirs were solitary and rare. In Europe bats have been found in
the Eocene, Oligocene and Miocene, and there is no reason to suppose that
they were not equally ancient and equally abundant in America; but none
have been found in the western hemisphere in any formation older than
the Pleistocene. All things considered, the extraordinary fact is, not
that so many forms have irretrievably perished, but that so much has been
preserved, escaping all the chances of destruction.
As to the degree of preservation in fossil mammals, we have to do almost
entirely with bones and teeth. With very rare exceptions, and those
all of late geological date, the viscera, muscles, skin, hair, horns,
hoofs and claws have been completely destroyed and have vanished without
leaving a trace. In northern Siberia the gravel soil is permanently
frozen to a depth of several hundred feet and contains the intact
carcasses of elephants and rhinoceroses of Pleistocene date and notably
different from any species of these animals now in existence. Sometimes
such a carcass is disinterred from a bluff by the cutting action of a
stream and is in a state of nearly complete preservation, with hide,
hair and flesh almost as in an animal freshly killed. From these
remains it has been learned that the †Mammoth was an elephant densely
covered with hair and wool, just as he was depicted in the carvings and
cave-paintings of Pleistocene Man in Europe, where †Mammoth bones have
been abundantly found, and also that there were Siberian rhinoceroses
similarly protected against the cold. †Mammoth remains with hide and
flesh, but much less complete, have likewise been found in Alaska.
In a cavern in southern Patagonia an expedition from the La Plata Museum
discovered, with the remains of a gigantic, extinct †ground-sloth, large
pieces of the skin still covered with hair and affording most welcome
information as to the colouration of these most curious animals. The skin
had been preserved from decay by deep burial in dry dust. Mummies of
Pleistocene rodents have been found in the dry caves of Portugal, whereas
in the ordinary caves which are damp or wet, only bones are preserved.
Unfortunately, as has been said, such instances of complete preservation
are very rare, and none are known of mammals more ancient than those of
the Pleistocene epoch.
In general, it may be said that the higher the geological antiquity
of a skeleton is, the greater is the chemical alteration which it has
undergone. Bones of Pleistocene or later date have, as a rule, suffered
little change beyond the loss of more or less of their animal matter, the
amount of such loss depending chiefly upon exposure to the air. Bones
which, for thousands or tens of thousands of years, have been buried in
dense cave-earth, in an antiseptic peat-bog, or in asphalt, are often
perfectly sound and fresh when taken up. Skeletons of the antecedent
(Tertiary) period are, on the other hand, very frequently _petrified_;
that is to say, the original substance of the bones has been completely
removed and replaced by some stony material, most commonly lime or flint.
This substitution took place very gradually, molecule by molecule,
so that not only is the form of the bone or tooth most accurately
reproduced, but the internal, microscopic structure is perfectly retained
and may be studied to as great advantage as in the case of modern
animals.
While, save in the rarest instances, only the hard parts of fossil
mammals remain to testify of their structure, very important information
as to the size, form and external character of the brain may be secured
from “brain-casts,” which may be natural or artificial. The pressure
of the mud, sand or other material, in which the fossil was embedded,
filled up all openings in the skeleton and, as the brain decayed and
disappeared, its place was taken by this material, which subsequently
hardened and solidified and quite accurately reproduces the external form
and character of the brain. When a fossil skull is exposed and shattered
by weathering, the natural brain-cast often remains intact, and a great
many such specimens are in the collections. An artificial cast is made by
sawing open the cranial cavity, cleaning out the stony matrix which fills
it and then pouring liquid gelatine or plaster of Paris into the cavity.
These artificial casts are often quite as satisfactory as the natural
ones.
As has been shown above, the history of the mammals is recorded, save
in a very few instances, in terms of bones and teeth and, to the
uninitiated, it might well seem that little could be accomplished
with such materials. However, it is the task, and the perfectly
feasible task, of palæontology to make these dry bones live. It is a
current and exceedingly mischievous notion that the palæontologist can
reconstruct a vanished animal from a single bone or tooth and, in spite
of repeated slayings, this delusion still flourishes and meets one in
modern literature at every turn. No doubt, much of the scepticism with
which attempts to restore extinct animals are met by many intelligent
people is traceable to the widespread belief that such off-hand and
easy-going methods are used in the work. So far from being able to make
a trustworthy reconstruction from a few scattered bones, competent
palæontologists have been sometimes led completely astray in associating
the separated parts of the same skeleton. More than once it has happened
that the dissociated skull and feet of one and the same animal have been
assigned to entirely different groups, just because no one could have
ventured, in advance of experience, to suppose that such a skull and
teeth could belong to a creature with such feet. In all these cases (and
they are few) the error has been finally corrected by the discovery of
the skeleton with all its essential parts in their natural connection.
While the number of complete skeletons of Tertiary mammals as yet
collected is comparatively small, it is often possible to construct a
nearly complete specimen from several imperfect ones, all of which can be
positively shown to belong to the same species. Such composite skeletons
are almost as useful as those in which all the parts pertain to a single
individual, though in making the drawings it is not easy to avoid
slight errors of proportion. It must not be supposed that no successful
restoration of missing bones is practicable; on the contrary, this can
often be done very easily, but only when all the essential parts of the
skeleton are known.
Even if an unlimited number of perfect skeletons were available, of
what use would they be? A skeleton is a very different looking object
from a living animal, and how is it possible to infer the latter from
the former? Do the many restorations of extinct mammals which this book
owes to the skill of Mr. Horsfall and Mr. Knight deserve any other
consideration than that due to pleasing, graceful or grotesque fancies,
with no foundation of solid fact? To answer these questions, it is
necessary first to consider the relations of the bony structure to the
entire organism and then to discuss the principles in accordance with
which the restorations have been made.
The skeleton is far from being merely the mechanical frame-work of the
animal. Such a frame-work it is, of course, but it is much more than
that; it is the living and growing expression of the entire organism and
is modified, not only by age, but by the conditions of the environment
and accidental circumstances as well. The bones of the same individual
differ very materially in early youth, maturity and old age; so long as
the animal lives, its bones are perpetually changing, slowly it is true,
but with ready response to needs. Not only that, but dislocated bones
may and frequently do develop entirely new joints, and their internal
structure is remodelled to meet the requirements of stresses differing
in character or direction from those of normal, uninjured bones. The
general form and proportions of any mammal are determined chiefly by its
muscular system and this may be directly and confidently inferred from
its skeleton, for the muscles which are of importance in this connection
are attached to the bones and leave their indelible and unmistakable
mark upon them. In any good text-book of anatomy this extremely intimate
relation of bone and muscle is made clear; and it is shown how each
attachment of muscle, tendon and ligament is plainly indicated by rough
lines, ridges, projections or depressions, which speak a language
intelligible enough to those who have learned to interpret it. Given the
skeleton, it is no very difficult task to reconstruct the muscular system
in sufficient detail. Further, the teeth afford valuable information
as to the food, habits and appearance of the animal, for the bulk of
the viscera, a significant element in the general form, is principally
conditioned by the character of the diet.
Beasts of prey, which live by catching and devouring other animals,
have a certain likeness to one another, even though they are in no
wise related, except as all mammals are. The Thylacine, or so-called
“Tasmanian Wolf” (_Thylacynus cynocephalus_), a marsupial and related
to the opossums, is deceptively like the true wolves in appearance,
although belonging to an order (Marsupialia) almost as widely separated
from that to which the wolves belong (Carnivora) as two mammalian groups
well can be. This resemblance is as clearly indicated by the skeletons
as by the living animals themselves, though the fundamental differences
of structure which distinguish the marsupial from the carnivore are no
less clearly displayed. Large herbivorous mammals too, though referable
to very different orders, bear a strong resemblance to one another, the
characteristic differences, so far as the living animal is concerned,
appearing chiefly in the head. It was this general likeness that induced
Cuvier to form his order, “Pachydermata,” which comprised elephants,
rhinoceroses, hippopotamuses, tapirs, etc., animals that are now
distributed into no less than three separate orders; aside from the head,
all of these forms are quite distinctly similar in appearance.
Of course, the external features, such as ears, tail, skin and hair,
are most important factors in the general make-up of any mammal; and,
as to these matters, the fossils leave us largely in the lurch, save
in the all too rare cases, like the Siberian †Mammoth, in which these
external features are actually preserved. Two artists may so restore the
same animal as to result in two very different pictures, and no one can
positively decide between them; just as two modern mammals, which are
closely related and have very similar skeletons, may yet differ markedly
in outward appearance, because of the different character of the skin,
as do, for example, the Bornean and Indian rhinoceroses. Yet even in
dealing with purely external features, we are not left altogether to
conjecture. Ears of unusual size or form frequently leave some indication
of this on the skull, and the presence or absence of a proboscis can
nearly always be inferred with confidence from the character of the bones
of the nose and muzzle. The length and thickness of the tail may be
generally directly deduced from the caudal vertebræ, but whether it was
close-haired and cylindrical, or bushy, or tufted at the end, or flat and
trowel-shaped, as in the Beaver, is not determinable from the bones alone.
[Illustration: FIG. 4.—Wild sow and pigs, showing the uniform colour of
the adult and stripes of the young.]
Most uncertain of all the characters which determine outward appearance
are the hair and the pattern of colouration; the Horse and Zebra differ
much more decidedly in the living form than their skeletons would lead
one to expect, as do also the Lion, the Tiger and the Leopard. The
curious and exceptional colour-pattern of the Okapi, that remarkable
giraffe-like animal but lately discovered in the equatorial forests
of western Africa, could never have been inferred from a study of the
skeleton alone. However, even in the problem of colour-patterns there is
more to go upon than sheer guess-work, for certain definite principles
of animal colouration have been ascertained; the great difficulty lies
in the application of these principles to a particular case. It is quite
certain that the naked, hairless skin is never primitive, but always a
comparatively late acquisition and, in many mammalian orders, is not
found at all. Aside from a few domesticated animals, this type of skin
occurs only in very large herbivorous mammals living in warm climates,
such as elephants, rhinoceroses and hippopotamuses, in a few burrowers,
and in marine mammals, like the walruses, whales, porpoises, etc.
Useful hints as to the colouring of ancient and extinct forms may be
gathered from a study of series of living animals, such as lizards and
butterflies, in which the development of a definite scheme of colouration
may be followed step by step. Young animals very frequently retain more
or less distinct traces of the ancestral colouration, which disappear in
the adult, for the development of the individual is, in some respects
at least, an abbreviated and condensed recapitulation of the history
of the species. In many mammals which, in the adult condition, have a
solid body-colour, the young are striped or spotted, a strong indication
that these mammals were derived from striped or spotted ancestors. Thus,
the Wild Boar has a uniform body-colour in the full-grown stage, but
the pigs are longitudinally striped; many deer are spotted throughout
life, as in the Fallow Deer, the Axis Deer of India and others, but the
great majority of the species, including all the American forms, have
uniform colouration, while the fawns are always spotted. Lion cubs are
also spotted and the adults have a uniform tawny colour, and many such
examples might be given.
[Illustration: FIG. 5.—Fawns of the Mule Deer (_Odocoileus hemionus_).
Compare with Fig. 83, p. 167. (By permission of the N. Y. Zoölog.
Society.)]
The study of colouration among existing animals has led to the conclusion
that in mammals the primitive colour-pattern was that of stripes, either
longitudinal or transverse and more probably the former. In the second
stage these bands break up into spots, which still show the longitudinal
arrangement and may be either light on a dark ground, or dark on a light
ground. In a third stage the spots may again coalesce into stripes, the
course of which is at right angles to that of the original stripes, or
the spots may disappear, leaving a uniform body-colour, lighter or white
on the belly. These changes of colour-pattern have not proceeded at a
uniform rate in the various mammalian groups, or even within the same
group, for an all-important factor is the mode of life of the particular
animal. In general, it may be said that the scheme of colour is such as
to render its possessor inconspicuous, or even invisible, and many a
creature that seems to be very conspicuous and striking in a museum case
can hardly be seen at all when in its natural surroundings. Thus, Arctic
mammals and birds, in their winter dress, are white; desert animals are
tawny or sandy-brown; forest animals are frequently striped or spotted;
while those that live on open plains are more commonly of uniform
colouration. There are exceptions to these rules, but they hold good for
the most part. From careful comparative study of the teeth and skeletons
a clew may be gained as to the habits of animals and from the habits
something may be inferred as to the colouration.
[Illustration: FIG. 6.—_Tapirus terrestris_, 3 days old. Compare with
Fig. 137, p. 320. (By permission of W. S. Berridge, London.)]
It would, however, be misleading to claim a greater authority for these
attempts at restoring a long-vanished life than can fairly be ascribed
to them. The general form and proportions of the head, neck, body, tail,
limbs and feet may be deduced with a high degree of accuracy from the
skeleton, while the external characters of skin, hair and colouration
are largely conjectural, but not altogether imaginary. It cannot be
doubted that among the extinct mammals were many which, owing to some
uncommon growth of subcutaneous fat, or some unusual local development
of hair, were much more curious and bizarre in appearance than we can
venture to represent them. If, for example, the Camel, the Horse,
the Lion and the Right Whale were extinct and known only from their
skeletons, such restorations as we could make of them would assuredly go
astray in some particulars. The Camel would be pictured without his hump,
for there is nothing in the skeleton to suggest it; the forelock, mane
and characteristic tail of the Horse and the Lion’s mane would certainly
not be recognized; while the immense development of blubber in the head
of the Whale gives to it a very different appearance from that which the
skull would seem to indicate. Such cases are, however, exceptional and
restorations made by competent hands from complete skeletons probably
give a fair notion of the appearance of those animals when alive.
It will thus be sufficiently plain that the work of restoration is beset
with difficulties, but that there is no good ground for the uncritical
scepticism which summarily rejects the results as being purely fanciful,
or for the equally uncritical credulity which unhesitatingly accepts them
as fully and incontestably accurate. It is altogether likely that one
of the main sources of error consists in making the extinct animal too
closely resemble some existing species which is selected as a model.
Too much space has perhaps been devoted to the problem of restoring the
external form of these extinct mammals, a problem which, after all, is of
distinctly subordinate importance. The most valuable results which may be
gained from a study of these fossil mammals are the answers which they
afford to the great questions of relationship, classification and genetic
descent, and the light which they throw upon the processes of evolution
and the course of geographical arrangement. The bones and teeth afford
admirable means of tracing the gradual steps of modification by which the
modern mammals have arisen from very different ancestors and of following
their wanderings from region to region and continent to continent. It is
to these questions that most of the subsequent chapters are devoted.
CHAPTER III
THE CLASSIFICATION OF THE MAMMALIA
The terminology and nomenclature of science form a great barrier, which
only too often shuts out the educated layman from following the course
of investigation and keeping abreast of the discoveries in which he may
be particularly interested. No more frequent and heartfelt complaint
is uttered than that which decries the “scientific jargon,” and one
might be tempted to think that this jargon was a superfluous nuisance,
deliberately adopted to exclude the uninitiated and guard the secrets
of the temple from the curious intruder. As a matter of fact, however,
this terminology, though an unquestionable evil from one point of view,
is an indispensable implement of investigation and description. Ordinary
language has far too few words for the purpose and most of the words that
might be used lack the all-important quality of precision. The vernacular
names of animals and plants are notoriously inexact and, even when not
inaccurately employed, are not sufficiently refined and distinctive
for scientific use. This is pre-eminently true of the New World, where
the European settlers gave the names of the creatures with which they
had been familiar at home to the new animals which they found in the
western hemisphere. Some of these names, such as deer, wolf, fox, bear,
are accurate enough for ordinary purposes, while others are ludicrously
wrong. The bird that we call the Robin is altogether different from his
European namesake, and the great stag, or Wapiti, is commonly called
“Elk,” a name which properly belongs to the Moose. In short, it is
impossible to gain the necessary accuracy and abundance of vocabulary
without devising an artificial terminology, drawn chiefly from Greek and
Latin.
In dealing with fossils, the difficulty of nomenclature becomes
formidable indeed. The larger and more conspicuous mammals of the modern
world are more or less familiar to all educated people, and such names
as rhinoceros, hippopotamus, elephant, kangaroo, will call up a definite
and fairly accurate image of the animal in question. For the strange
creatures that vanished from the earth ages before the appearance of Man
_there are no vernacular names_ and it serves no good purpose to coin
such terms. To the layman names like _Uintatherium_ or _Smilodon_ convey
no idea whatever, and all that can be done is to attempt to give them a
meaning by illustration and description, using the name merely as a peg
upon which to hang the description.
The system of zoölogical classification which is still in use was
largely the invention of the Swedish naturalist Linnæus, who published
it shortly after the middle of the eighteenth century. As devised by
Linnæus, the scheme was intended to express ideal relationships, whereas
now it is employed to express real genetic affinities, so far as these
can be ascertained. The Linnæan system is an organized hierarchy of
groups, arranged in ascending order of comprehensiveness. In this scheme,
what may be regarded as the unit is the _species_, a concept around
which many battles have been waged and concerning which there is still
much difference of opinion and usage. Originally a term in logic, it
first received a definite meaning in Zoölogy and Botany from John Ray
(1628-1705) who applied it to indicate a group of animals, or plants,
with marked common characters and freely interbreeding. Linnæus, though
not always consistent in his expressions on the subject, regarded species
as objective realities, concrete and actual things, which it was the
naturalist’s business to discover and name, and held that they were fixed
entities which had been separately created. This belief in the fixity
and objective reality of species was almost universally held, until
the publication of Darwin’s “Origin of Species” (1859) converted the
biological world to the evolutionary faith, which declares that the only
objective reality among living things is the individual animal or plant.
According to this modern conception, a species may be defined as
signifying a “grade or rank assigned by systematists to an assemblage of
organic forms which they judge to be more closely interrelated by common
descent than they are related to forms judged to be outside the species”
(P. Chalmers Mitchell). The technical name of a species, which is either
in Latin, or in latinized form, is in two words, one of which designates
the genus (see below) and the other the particular species of that genus,
as, for example, _Equus caballus_, the species Horse, _E. przewalskii_,
the Asiatic Wild Horse, _E. asinus_, the species Ass, etc. In order to
identify a species, the genus to which it belongs must be stated, hence
the term, _binomial system_ of nomenclature, which Linnæus introduced,
becoming _trinomial_ when the name of a subspecies is added, a modern
refinement on the older method. A very large species (_i.e._ one which
is represented by great numbers of individuals), extending over a very
large area, is often divisible into groups of minor rank, as _varieties_,
_geographical races_ or _subspecies_. Taking the species as the unit
in the scheme of classification, the varieties and subspecies may be
considered as fractions.
There is great difference of usage among writers on systematic zoölogy
in the manner of applying the generally accepted concept of species,
some making their groups very much more comprehensive than others,
according as they are “lumpers” or “splitters,” to employ the slang
phrase. The difficulty lies in the fact that there are no fixed and
definite criteria, by which a given series of individuals can be surely
distinguished as a variety, a species or a genus; it is a matter for the
judgment and experience of the systematist himself. The individuals of a
species may differ quite widely among themselves, provided that they are
all connected by intergradations, and the more or less constant varieties
or subspecies are to be distinguished from the individual variants,
which are inconstant and fluctuating. No two specimens agree exactly
in every particular, but if a very large suite of them be compared, it
will be found that the great majority depart but little from the average
or norm of the species, and the wider the departure from the norm, the
fewer the individuals which are so aberrant. Taking so easily measured
a character as size, for example, and measuring several hundred or a
thousand representatives of some species, we see that a large majority
are of average size, a little more or a little less, while very large
or very small individuals are rare in proportion to the amount by which
they exceed or fall short of the norm. Subspecies or varieties are marked
by differences which are relatively constant, but not of sufficient
importance to entitle them to rank as species.
A group of the second rank is called a _genus_, which may contain few
or many species, or only a single one. In the latter case the species
is so isolated in character that it cannot properly be included in
the same genus with any other species. A large genus, one containing
numerous species, is frequently divisible into several _subgenera_, each
comprising a group of species which are more similar to one another than
they are to the other species of the genus.
The third of the main groups in ascending order is the _family_, which
ordinarily consists of a number of genera united by the possession of
certain common characters, which, at the same time, distinguish them
from other genera, though a single isolated genus may require a separate
family for its reception. Just as it is often convenient to divide a
genus into subgenera, so families containing many genera are usually
divisible into _subfamilies_, as indicative of closer relationships
within the family. The name of the family is formed from that of the
genus first described or best known, with the termination _-idæ_, while
that for the subfamily is _-inæ_. To take an example, all the genera
of cats, living and extinct, are assembled in the family Felidæ (from
the genus _Felis_) which falls naturally into two subfamilies. One of
these, the Felinæ, includes the true cats, a very homogeneous group,
both the existing and the extinct genera; the other subfamily, that of
the highly interesting series of the “Sabre-tooth Tigers,” called the
†Machairodontinæ, comprises only extinct forms.
The fourth principal rank or grade is the _order_, distinguished by some
fundamental peculiarity of structure and usually including a large number
of families. Some of the orders, however, contain but a single family, a
single genus, or even, it may be, a single species, because that species
is in important structural characters so unlike any other that it cannot
properly be put into the same order with anything else. Such isolation
invariably implies that the species or genus in question is the sole
survivor of what was once an extensive series. As in the case of the
family and the genus, it is often necessary to recognize the degrees
of closer and more remote affinity by the use of _suborders_. Existing
Artiodactyla, or even-toed hoofed animals, an enormous assemblage,
may conveniently be divided into four suborders: (1) Suina, swine and
the Hippopotamus; (2) Tylopoda, the Camel and Llama; (3) Tragulina,
“mouse-deer,” or chevrotains; (4) Pecora, or true ruminants, deer,
giraffes, antelopes, sheep, goats, oxen, etc. In nearly all of the orders
such subordinal divisions are desirable and it is frequently useful to
employ still further subdivisions, like _superfamilies_, which are groups
of allied families within the suborder, _sections_ and the like.
In the Linnæan scheme, the next group in ascending rank is the _class_,
which includes all mammals whatsoever, but the advance of knowledge has
made it necessary to interpolate several intermediate grades between the
class and the order, which, in the descending scale, are _subclass_,
_infraclass_, _cohort_, _superorder_ and others, while above the class
comes the _subkingdom_ of Vertebrata, or animals with internal skeletons,
which includes mammals, birds, reptiles, amphibians and fishes.
A word should be said as to the conventions of printing technical names.
The names of all species are, in American practice, printed in small
letters, but many Europeans write specific terms which are proper nouns
or adjectives with a capital. Generic, family and all groups of higher
rank are always written with a capital, unless used in vernacular form,
_e.g._ Artiodactyla and artiodactyls. It is also a very general custom to
give capitals to vernacular names of species, as the Mammoth, the Coyote,
the Black Bear. Genus and species are almost invariably in italics,
groups of higher rank in roman.
Such a scheme of classification as is outlined above has a decidedly
artificial air about it and yet it serves a highly useful purpose in
enabling us to express in brief and condensed form what is known or
surmised as to the mutual relationships of the great and diversified
assemblage of mammals. The scheme has been compared to the organization
of an army into company, battalion, regiment, brigade, division,
army corps, etc., and there is a certain obvious likeness; but the
differences go deeper, for an army is an assemblage of similar units,
mechanically grouped into bodies of equal size. A much closer analogy
is the genealogical or family tree, which graphically expresses the
relationships and ramifications of an ancient and widespread family,
though even this analogy may easily be pushed too far. Blood-relationship
is, in short, the underlying principle of all schemes of classification
which postulate the theory of evolution.
The system of Linnæus, as expanded and improved by modern zoölogists,
has proved itself to be admirably adapted to the study of the living
world; but it is much more difficult to apply it to the fossils, for
they introduce a third dimension, so to speak, for which the system was
not designed. This third dimension is the successive modification in
time of a genetically connected series. The cumulative effect of such
modifications is so great that only very elastic definitions will include
the earlier and later members of an unbroken series. In attempting to
apply the Linnæan system to the successive _faunas_ (_i.e._ assemblages
of animals) which have inhabited the earth, palæontologists have employed
various devices. One such method is to classify each fauna without
reference to those which precede and follow it, but this has the great
drawback of obscuring and ignoring the relationships, to express which
is the very object of classification. Another and more logical method
is to treat species and genera as though they belonged to the present
order of things, for these groups, particularly species, were relatively
short-lived, when regarded from the standpoint of geological time, and
either became so modified as to require recognition as new species and
genera, or died out without leaving descendants. Groups of higher rank,
families, orders, etc., are treated as _genetic series_ and include the
principal line or stock and such side-branches as did not ramify too
widely or depart too far from the main stem. Under the first arrangement,
the horses, a long history of which has been deciphered, would be divided
into several families; under the second, they are all included in a
single family.
One of the most interesting results of palæontological study is the
discovery that in many families, such as the horses, rhinoceroses and
camels, there are distinct series which independently passed through
parallel courses of development, the series of each family keeping a
remarkably even pace in the degree of progressive modification. Such a
minor genetic series within a family is called a _phylum_, not a very
happy selection, for the same term had been previously employed in a
much wider sense, as equivalent to the subkingdom. In both uses of the
term the underlying principle, that of genetic series, is the same; the
difference is in the comprehensiveness of meaning.
It must be admitted that no method, yet devised, of applying the Linnæan
scheme to the fossils is altogether satisfactory, and indeed it is only
the breaks and gaps in the palæontological record which makes possible
any use of the scheme. Could we obtain approximately complete series
of all the animals that have ever lived upon the earth, it would be
necessary to invent some entirely new scheme of classification in order
to express their mutual relationships.
In the present state of knowledge, classification can be made only in
a preliminary and tentative sort of way and no doubt differs widely
from that which will eventually be reached. So far as the mammals are
concerned, part of the problem would seem to be quite easy and part
altogether uncertain. Some mammalian groups appear to be well defined and
entirely natural assemblages of related forms, while others are plainly
heterogeneous and artificial, yet there is no better way of dealing with
them until their history has been ascertained. The mutual relations of
the grand groups, or orders, are still very largely obscure.
The class Mammalia is first of all divided into two subclasses of
very unequal size. Of these, the first, PROTOTHERIA, is represented
in the modern world by few forms, the so-called Duck-billed Mole
(_Ornithorhynchus paradoxus_) and Spiny Anteaters (_Echidna_) of
Australia. They are the lowest and most primitive of the mammals and
retain several structural characters of the lower vertebrates. Their
most striking characteristic is that the young are not brought forth
alive, but are hatched from eggs, as in the reptiles, birds and lower
vertebrates generally.
The second subclass, EUTHERIA, which includes all other mammals, is
again divided into two very unequal groups or infraclasses. One of
these, DIDELPHIA, contains but a single order, the Marsupialia, or
pouched mammals, now in existence, and is also very primitive in many
respects, though far more advanced than the Prototheria. The young,
though born alive, are brought forth in a very immature state and, with
the exception of one genus (_Perameles_) the fœtus is not attached by
a special structure, the placenta, to the womb of the mother. Like the
Prototheria, the Marsupials, which were once spread all over the world,
are at present almost entirely confined to Australia and the adjoining
islands, the Opossums of North and South America, and one small genus
(_Cænolestes_) in the latter continent being the exceptions to this
rule of distribution. The second and vastly larger infraclass, the
MONODELPHIA, is characterized by the _placenta_, a special growth, partly
of fœtal and partly of maternal origin, by means of which the unborn
young are attached to the mother and nourished during the fœtal period;
they are born in a relatively mature state and are generally able to walk
immediately after birth and resemble their parents in nearly all respects.
The vast assemblage of placental mammals, which range over all the
continents, are divided into numerous orders, most of which appear to be
natural groups of truly related forms, while some are but doubtfully so
and others again are clearly unnatural and arbitrary. As has already been
pointed out, the mutual relationships of these orders, as expressed in
groups of higher than ordinal rank, offer a much more difficult problem,
chiefly because our knowledge of the history of mammals is most deficient
just where that history is most important and significant, namely, in its
earlier portion. In many instances, the evolution of genera and families
may be followed out within the limits of the order in a very convincing
way, but very rarely can the origin of an order be demonstrated. When the
history began to be full and detailed, the orders had nearly all been
established, and, until the steps of their divergence and differentiation
can be followed out, their mutual relationships can be discussed only
from the standpoint of their likenesses and differences. In the valuation
of these, there is much room for difference of opinion, and such
difference is not lacking. On the other hand, concerning the number and
limits of the orders themselves there is very general agreement.
In the following table only the major groups are included and those which
are extinct are marked with a dagger (†). The scheme is almost identical
with that given in Professor Osborn’s “Age of Mammals,” the few points
in which I should prefer a somewhat different arrangement being waived
in the interests of uniformity and avoidance of confusion. A few changes
are, however, made in matters which I regard as too important to ignore.
I. SUBCLASS PROTOTHERIA. Egg-laying Mammals.
1. ORDER =†PROTODONTA=.
2. ORDER =MONOTREMATA=, _e.g._ the Duck-billed Mole and Spiny
Anteaters.
II. SUBCLASS EUTHERIA. Viviparous Mammals.
_A._ INFRACLASS DIDELPHIA. Pouched Mammals.
1. ORDER =†TRICONODONTA=.
2. ORDER =MARSUPIALIA=.
_a._ SUBORDER =Polyprotodonta=. Opossums, carnivorous and
insectivorous Marsupials.
_b._ SUBORDER =Diprotodonta=. Herbivorous Marsupials; Kangaroos,
etc.
_c._ SUBORDER =†Allotheria=.
_B._ INFRACLASS MONODELPHIA. Placental Mammals.
_AA._ COHORT UNGUICULATA. Clawed Mammals.
1. ORDER =†TRITUBERCULATA=.
2. ORDER =INSECTIVORA=. Insect-eating Mammals.
_a._ SUBORDER =Lipotyphla=, _e.g._ Moles, Hedgehogs, Shrews, etc.
_b._ SUBORDER =†Hyopsodonta=.
_c._ SUBORDER =†Proglires=.
_d._ =Suborder= =Menotyphla=, _e.g._ Tree and Jumping Shrews.
3. ORDER =†TILLODONTIA=.
4. ORDER =DERMOPTERA=. The Flying Lemur.
5. ORDER =CHIROPTERA=. Bats.
6. ORDER =CARNIVORA=. Beasts of Prey.
_a._ SUBORDER =†Creodonta=. Primitive Flesh-eaters.
_b._ SUBORDER =Fissipedia=. Wolves, Bears, Weasels, Cats, etc.
_c._ SUBORDER =Pinnipedia=. Marine Carnivores—Seals and Walruses.
7. ORDER =RODENTIA=. Gnawing Mammals.
_a._ SUBORDER =Duplicidentata=, _e.g._ Hares, Rabbits, Pikas.
_b._ SUBORDER =Simplicidentata=, _e.g._ Squirrels, Marmots,
Beavers, Rats, Mice, Porcupines, etc.
8. ORDER =†TÆNIODONTIA=.
9. ORDER =EDENTATA=.
_a._ SUBORDER =Pilosa=. Hairy Edentates, _e.g._ Sloths,
Anteaters, etc.
_b._ SUBORDER =Loricata=. Armoured Edentates, _e.g._ Armadillos,
†Glyptodonts.
10. ORDER =PHOLIDOTA=. Scaly Anteaters or Pangolins.
11. ORDER =TUBULIDENTATA=. The Aard Vark.
_BB._ COHORT PRIMATES. Mammals with nails.
12. ORDER =PRIMATES=.
_a._ SUBORDER =Lemuroidea=. Lemurs.
_b._ SUBORDER =Anthropoidea=. Monkeys, Apes, Man.
_CC._ COHORT UNGULATA. Hoofed Mammals.
13. ORDER =†CONDYLARTHRA=.
14. ORDER =†AMBLYPODA=.
15. ORDER =ARTIODACTYLA=. Even-toed Hoofed Mammals.
_a._ SUBORDER =†Artiodactyla Primitiva=.
_b._ SUBORDER =Suina=. Swine, Peccary, Hippopotamus.
_c._ SUBORDER =Tylopoda=. Camels, Llama, Guanaco.
_d._ SUBORDER =Tragulina=. Mouse-deer or Chevrotains.
_e._ SUBORDER =Pecora=, _e.g._ Deer, Antelopes, Sheep, Oxen, etc.
16. ORDER =PERISSODACTYLA=. Odd-toed Hoofed Mammals.
_a._ SUBORDER =Chelodactyla=, _e.g._ Horses, Tapirs,
Rhinoceroses, etc.
_b._ SUBORDER =†Ancylopoda.= †Chalicotheres.
17. ORDER =PROBOSCIDEA=. Elephants and †Mastodons.
18. ORDER =†BARYTHERIA=.
19. ORDER =†EMBRITHOPODA=.
20. ORDER =SIRENIA=. Sea-cows and Dugongs.
21. ORDER =HYRACOIDEA=. Conies.
22. ORDER =†TOXODONTIA=.
_a._ SUBORDER =†Toxodonta=.
_b._ SUBORDER =†Typotheria=.
_c._ SUBORDER =†Entelonychia=.
_d._ SUBORDER =†Pyrotheria=.
23. ORDER =†ASTRAPOTHERIA=.
24. ORDER =†LITOPTERNA=.
_DD._ COHORT CETACEA. Whales, Dolphins, Porpoises.
25. ORDER =†ZEUGLODONTIA=.
26. ORDER =ODONTOCETI=. Toothed Whales, Dolphins, Porpoises.
27. ORDER =MYSTACOCETI=. Whalebone Whales.
CHAPTER IV
THE SKELETON AND TEETH OF MAMMALS
With very rare exceptions, and those only of the latest geological period
(Quaternary), the fossil remains of mammals consist only of bones and
teeth. The evolutionary changes, so far as these are preserved, are
recorded therefore in terms of dental and skeletal modifications. To
render these changes intelligible, it is necessary to give some account
of the mammalian skeleton and teeth, with no more use of technical
language than is unavoidable; ordinary speech does not furnish a
sufficient number of terms, nor are most of these sufficiently precise.
With the aid of the figures, the reader may easily gain a knowledge of
the skeleton which is quite adequate for the discussion of fossil series,
which will follow in the subsequent chapters.
I. THE SKELETON
I. The most obvious distinction of the skeletal parts is into _axial_
and _appendicular_ portions, the former comprising the skull, backbone
or _vertebral column_, ribs and breast-bone or _sternum_, and the latter
including the limb-girdles, limbs and feet. In the axial skeleton
only the ribs and certain bones of the skull are paired, but in the
appendicular all the bones are in pairs, for the right and left sides
respectively.
[Illustration: FIG. 7.—Skull of Wolf (_Canis occidentalis_). _P.Mx._,
premaxillary. _Mx._, maxillary. _Na._, nasal. _L._, lachrymal. _Ma._,
malar or jugal. _Fr._, frontal. _Pa._, parietal. _Sq._, squamosal.
_Zyg._, zygomatic process of squamosal. _O.S._, orbitosphenoid. _Pl._,
palatine. _M._, mandible. _cor._, coronoid process of mandible. _m.c._,
condyle of mandible. _ang._, angular process of mandible. _p.g._,
postglenoid process of squamosal. _Ty._, tympanic (auditory bulla).
_mas._, mastoid. _p.oc._, paroccipital process. _con._, occipital
condyle. _Ex.O._, exoccipital. _S.O._, supraoccipital.]
The _skull_ is a highly complex structure, made up of many parts, most
of which are immovably fixed together, and performing many functions of
supreme importance. In the first place, it affords secure lodgement and
protection for the brain and higher organs of sense, those of smell,
sight and hearing, and second, it carries the teeth and, by its movable
jaws, enables these to bite, to take in and masticate food. The portion
of the skull which carries the brain, eyes and ears, is called the
_cranium_, and the portion in front of this is the face, the boundary
between the two being an oblique line drawn immediately in front of the
eye-socket (Fig. 7). A great deal of the endless variety in the form of
the skull of different mammals depends upon the differing proportions
of cranium and face. In the human skull, for example, the cranium is
enormously developed and forms a great dome, while the face is shortened
almost to the limit of possibility; the skull of the Horse, on the other
hand, goes to nearly the opposite extreme of elongation of the facial and
shortening of the cranial region. The posterior surface of the skull,
or _occiput_, is made up of four bones, which in most adult mammals are
fused into a single _occipital_ bone. At the base of the occiput is
a large opening, the _foramen magnum_, through which the spinal cord
passes to its junction with the brain; and on each side of the opening
is a large, smooth, oval prominence, the _occipital condyles_, by means
of which the skull is articulated with the neck. The _paroccipital
processes_ are bony styles of varying length, which are given off, one
on each side external to the condyles. The boundary of the occiput
is marked by a ridge, the _occipital crest_, which varies greatly in
prominence, but is very well marked in the more primitive forms and tends
to disappear in the more highly specialized ones. The roof and much of
the sides of the cranium are formed by two pairs of large bones, the
_parietals_ behind and the _frontals_ in advance; along the median line
of the cranial roof, where the two parietals meet, is usually another
ridge, the _sagittal crest_, which joins the occipital crest behind.
The sagittal crest also varies greatly in prominence, being in some
mammals very high and in others entirely absent, and, like the occipital
crest, is a primitive character; as a rule, it is longest and highest in
those mammals which have the smallest brain-capacity. As pointed out by
Professor Leche, the development of the sagittal crest is conditioned
by the relative proportions of the brain-case and the jaws. Powerful
jaws and a small brain-case necessitate the presence of the crest, in
order to provide sufficient surface of attachment for the temporal
muscles, which are important in mastication, while with large brain-case
and weak jaws the crest is superfluous. Though the brain-case proper
may be quite small, yet it may have its surface enormously increased
by great thickening of the cranial bones, as is true of elephants and
rhinoceroses, and in them sufficient surface for attachment is afforded
to the muscles without the development of a crest.
[Illustration: FIG. 8.—Skull of Wolf, top view. _P.Mx._, premaxillary.
_Na._, nasal. _Ma._, malar or jugal. _L._, lachrymal. _Fr._, frontal.
_Sq._, squamosal. _Pa._, parietal. _S.O._, supraoccipital.]
[Illustration: FIG. 9.—Skull of Wolf, view of base. _P.Mx._,
premaxillary. _Mx._, palatine process of maxillary. _Pl._, palatine.
_Fr._, frontal. _Pt._, parietal. _Ma._, malar or jugal. _Sq._, glenoid
cavity of squamosal. _B.S._, basisphenoid. _B.O._, basioccipital. _Ty._,
tympanic (auditory bulla). _p.oc._, paroccipital process. _con._,
occipital condyle. _S.O._, supraoccipital.]
The structure of these cranial bones, more particularly of the parietals,
is subject to important changes; in most mammals they are of moderate
thickness and have dense layers, or “tables,” forming the outer and
inner surfaces and, between these, a layer of spongy bone. In many large
mammals, however, especially those which have heavy horns or tusks, the
cranial bones become enormously thick and the spongy layer is converted
into a series of communicating chambers, or _sinuses_, the partitions
between which serve as braces, thus making the bone very strong in
proportion to its weight. Sinuses are very generally present in the
frontals and communicate by small openings with the nasal passage, even
in genera of moderate size and without horns or tusks. The frontals
form the roof of the eye-sockets, or _orbits_, and usually there is
a projection from each frontal, which marks the hinder border of the
orbit and is therefore called the _postorbital_ process. The roof of the
facial region is made by the _nasals_, which are commonly long and narrow
bones, but vary greatly in form and proportions in different mammals;
in those which have a proboscis, like tapirs and elephants, or a much
inflated snout, such as the Moose (_Alce_) or the Saiga Antelope (_Saiga
tatarica_) the nasals are always very much shortened and otherwise
modified in form.
The anterior end of the skull is formed by a pair of rather small bones,
the _premaxillaries_, which carry the incisor teeth; they bound the sides
of the nasal opening, or _anterior nares_, reaching to the nasals, when
the latter are of ordinary length; they also form the front end of the
hard or bony palate, which divides the nasal passage from the mouth.
The _maxillaries_, or upper jaw-bones, make up nearly all of the facial
region on each side and send inward to the median line from each side a
bony plate which together constitute the greater part of the hard palate;
the remainder of the upper teeth are implanted in the maxillaries. A
varying proportion of the hinder part of the hard palate is formed by the
_palatines_, which also enclose the _posterior nares_, the opening by
which the nasal passage enters the back part of the mouth. The maxillary
of each side extends back to the orbit, which it bounds anteriorly and
in the antero-superior border of which is the usually small _lachrymal_.
The inferior, and more or less of the anterior, border of the orbit is
made by the cheek-bone (_malar_ or _jugal_) which may or may not have a
postorbital process extending up toward that of the frontal; when the
two processes meet, the orbit is completely encircled by bone, but only
in monkeys, apes and Man is there a bony plate given off from the inner
side of the postorbital processes, which extends to the cranial wall and
converts the orbit into a funnel-shaped cavity. For most of its length,
the jugal projects freely outward from the side of the skull and extends
posteriorly beneath a similar bar of bone, the _zygomatic process_ of
the _squamosal_. This process and the jugal together constitute the
_zygomatic arch_, which on each side of the skull stands out more or
less boldly, and the size and thickness of which are subject to great
variation in different mammals, the massiveness of the arch being
proportional to the power of the jaws. One of the principal muscles of
mastication (the _masseter_) is attached to the zygomatic arch.
The squamosal itself is a large plate, which makes up a great part of
the side-wall of the cranium and articulates above with the frontal and
parietal; it also supports the lower jaw, the articular surface for which
is called the _glenoid cavity_. The lower jaw is held in place by the
_postglenoid process_, which is a projection, usually a transverse ridge,
behind the cavity. Back of the postglenoid process is the entrance to
the middle ear, the _auditory meatus_, which may be merely an irregular
hole, or a more or less elongated tube. The meatus is an opening into
the _tympanic_, a bone which at birth is a mere ring and in some mammals
remains permanently in that condition, but as a rule develops into a
swollen, olive-shaped _auditory bulla_, which sometimes reaches enormous
proportions, especially in nocturnal mammals. The labyrinth of the
internal ear is contained in the _periotic_, a very dense bone which is
concealed in the interior of the cranium, but in many mammals a portion
of it, the _mastoid_, is exposed on the surface between the squamosal and
occipital.
The lower jaw-bone (_inferior maxillary_, or _mandible_) is the only
freely movable element of the skull; it consists of two halves which
meet anteriorly at the chin in a contact of greater or less length,
called the _symphysis_. In nearly all young mammals and in many adult
forms the two halves of the lower jaw are separate and are held together
at the symphysis only by ligaments, while in others, as in Man, they
are indistinguishably fused to form a single bone. Each half consists
of two portions, a horizontal part or _ramus_ and an _ascending ramus_
or vertical part; the former supports all of the lower teeth, and its
length, depth and thickness are very largely conditioned by the number
and size of those teeth. The ascending ramus is a broad, rather thin
plate, divided at the upper end into two portions, the hinder one of
which terminates in the _condyle_, a rounded, usually semicylindrical
projection, which fits into the glenoid cavity of the squamosal. The
anterior portion of the ascending ramus ends above in the _coronoid
process_, which serves for the insertion of the temporal muscle, the
upper portion of which is attached to the walls of the cranium and thus,
when the muscle is contracted, the jaws are firmly closed; the coronoid
process passes inside of the zygomatic arch. The lower jaw is therefore
a lever of the third order, in which the power is applied between the
weight (_i.e._ the food, the resistance of which is to be overcome) and
the fulcrum, which is the condyle. At the postero-inferior end of the
ascending ramus is the _angle_, the form of which is characteristically
modified in the various mammalian orders and is thus employed for
purposes of classification.
The _hyoid arch_ is a U-shaped series of small and slender bones, with an
unpaired element closing the arch below; each vertical arm of the U is
attached to the tympanic of its own side and the whole forms a flexible
support for the tongue, but with no freely movable joint like that
between the lower jaw and the squamosal.
The mammalian skull in its primitive form may be thought of as a tube
divided into two parts, of which the hinder one is the brain-chamber, or
cranial cavity, and the forward one the nasal chamber or passage. With
the growth of the brain and consequent enlargement of the cranium, this
tubular character is lost; and various modifications of the teeth, jaws
and facial region, the development of horns and tusks, bring about the
many changes which the skull has undergone.
This brief sketch of the skull-structure is very incomplete, several
of its elements having been altogether omitted and only those parts
described which are needful in working out the history and descent of the
various mammalian groups.
The second portion of the axial skeleton is the backbone, or _vertebral
column_, which is made up of a number of separate bones called
_vertebræ_. These are so articulated together as to permit the necessary
amount of flexibility and yet retain the indispensable degree of
strength. The function of the backbone is a twofold one: (1) to afford
a firm support to the body and give points of attachment to the limbs,
and (2) to carry the spinal cord, or great central axis of the nervous
system, in such a manner that it shall be protected against injury, a
matter of absolutely vital necessity.
While the vertebræ differ greatly in form and appearance in the various
regions of the neck, body and tail, in adaptation to the various degrees
of mobility and strength which are required of them, yet they are all
constituted upon the same easily recognizable plan. The principal mass
of bone in each vertebra is the body, or _centrum_, which is typically a
cylinder, or modification of that form, and the two ends of the cylinder
are the _faces_, by which the successive vertebræ are in contact with one
another. In the living animal, however, the successive centra are not in
actual contact, but are separated by disks of _cartilage_ (gristle) which
greatly add to the elasticity of the column. From the upper surface of
the centrum arises an arch of bone, the _neural arch_, enclosing with
the centrum the _neural canal_, through which runs the spinal cord. As
already mentioned, the protection of the spinal cord is essential to the
life of the animal, yet this protection must be combined with a certain
flexibility, both lateral and vertical. Mere contact of the centra, even
though these be held in place by ligaments, would not give the column
strength to endure, without dislocation, the great muscular stresses
which are put upon it. Additional means of articulation between the
successive vertebræ are therefore provided, and these vary in size, form
and position in different regions of the backbone, in nice adjustment
to the amount of motion and degree of strength needed at any particular
part of the column. Of these additional means of articulation, which are
called the _zygapophyses_, each vertebra has two pairs, an anterior and
a posterior pair, placed upon the neural arch. From the summit of the
arch arises the _neural spine_, a more or less nearly straight rod or
plate of bone, which may be enormously long or extremely short, massive
or slender, in accordance with the muscular attachments which must be
provided for. Finally, should be mentioned the _transverse processes_,
rod-like or plate-like projections of bone, which arise, one on each
side of the vertebra, usually from the centrum, less commonly from the
neural arch; these also differ greatly in form and size in the various
regions of the column. Anatomists distinguish several other processes of
the vertebra, but for our purpose it is not necessary to take these into
consideration.
[Illustration: FIG. 10.—First dorsal vertebra of Wolf from the front.
_cn._, centrum. _r._, facet for the head of the rib. _r′._, facet for
the tubercle of the rib. _tr._, transverse process. _pr.z._, anterior
zygapophyses. _n.sp._, neural spine.]
Five different regions of the backbone may be distinguished, in each of
which the vertebræ are modified in a characteristic way. There is (1) the
_cervical_ region, or neck, the vertebræ of which, among mammals (with
only one or two exceptions) are always seven in number, however long or
short the neck may be; the immoderately long neck of the Giraffe has no
more and the almost invisible neck of the Whale has no less, and thus
the elongation of the neck is accomplished by lengthening the individual
vertebræ and not by increasing their number. (2) Those vertebræ to which
ribs are attached are named _dorsal_ or _thoracic_ and can always be
recognized by the pits or articular facets which receive the heads of the
ribs. (3) Behind the dorsal is the _lumbar_ region, or that of the loins,
made up of a number of vertebræ which carry no ribs. The dorso-lumbars
are known collectively as the _trunk-vertebræ_ and are generally quite
constant in number for a given group of mammals, though often differently
divided between the two regions in different members of the same
group. In the Artiodactyla, for example, there are very constantly 19
trunk-vertebræ, but the Hippopotamus has 15 dorsals and 4 lumbars, the
Reindeer (_Rangifer_) 14 D., 5 L., the Ox (_Bos taurus_) 13 D., 6 L.,
the Camel (_Camelus dromedarius_) 12 D. and 7 L. (4) Next follows the
_sacrum_, which consists of a varying number of coalesced vertebræ. The
number of sacral vertebræ varies from 2 to 13, but is usually from 3 to
5. (5) Finally, there are the _caudal_ vertebræ, or those of the tail,
which are extremely variable in number and size, depending upon the
length and thickness of the tail.
We must next consider briefly some of the structural features which
characterize the vertebræ of the different regions. (1) The length of
the neck varies greatly in different mammals and, up to a certain point,
flexibility increases with length, but, as the number of 7 cervicals is
almost universally constant among mammals and the lengthening of the
neck is accomplished by an elongation of the individual vertebræ, a
point is eventually reached, where greater length is accompanied by a
diminution of mobility. For instance, in the Giraffe the movements of the
neck are rather stiff and awkward, in striking contrast to the graceful
flexibility of the Swan’s neck, which has 23 vertebræ, more than three
times as many.
[Illustration: FIG. 11.—Atlas of Wolf, anterior end and left side.
_cot._, anterior cotyles. _n.c._, neural canal. _n.a._, neural arch.
_tr._, transverse process. _v.a._, posterior opening of the canal for the
vertebral artery.]
The first two cervical vertebræ are especially and peculiarly modified,
in order to support the skull and give to it the necessary degree of
mobility upon the neck. The first vertebra, or _atlas_, is hardly more
than a ring of bone with a pair of oval, cuplike depressions (_anterior
cotyles_) upon the anterior face (superior in Man) into which are fitted
the occipital condyles of the skull. By the rolling of the condyles
upon the atlas is effected the nodding movement of the head, upward
and downward, but not from side to side; this latter movement is
accomplished by the partial rotation of skull and atlas together upon
the second vertebra in a manner presently to be explained. On the hinder
aspect are two articular surfaces (_posterior cotyles_) in shape like
the anterior pair, but very much less concave, which are in contact
with corresponding surfaces on the second vertebra. The neural arch of
the atlas is broad and low and the neural canal is apparently much too
large for the spinal cord, but, in fact, only a part of the circular
opening belongs to the neural canal. In life, the opening is divided by
a transverse ligament into an upper portion, the true neural canal, and
a lower portion, which lodges a projection from the second vertebra.
The atlas usually has no neural spine and never a prominent one; the
transverse processes are broad, wing-like plates and each is perforated
by a small canal, which transmits the vertebral artery.
[Illustration: FIG. 12.—Axis of Wolf, left side. _od.p._ odontoid
process. _cot._, anterior cotyles. _n.a._, neural arch. _n.sp._, neural
spine. _pt.z._, posterior zygapophyses. _tr._, transverse process.
_v.a′._, anterior opening of canal for the vertebral artery. _v.a″._,
posterior opening of the same.]
The second vertebra, or _axis_, is a little more like the ordinary
vertebra, having a definite and usually elongate centrum, on the anterior
end of which are the two articular surfaces for the atlas. Between these
is a prominent projection, the _odontoid process_, which fits into the
ring of the atlas and has a special articulation with the lower bar of
that ring. In most mammals the odontoid process is a bluntly conical peg,
varying merely in length and thickness, but in many long-necked forms
the peg is converted into a semicylindrical spout, convex on the lower
side and concave above. The neural spine of the axis is almost always a
relatively large, hatchet-shaped plate, which is most developed in the
carnivorous forms, and the transverse processes are commonly slender rods.
The five succeeding cervical vertebræ are much alike, though each one has
a certain individuality, by which its place in the series may readily
be determined. The centrum has a convex anterior and concave posterior
face, which in long-necked animals form regular “ball and socket” joints;
neural spines are frequently wanting and, when present, are almost always
short and slender; the zygapophyses are very prominent and are carried
on projections which extend before and behind the neural arch; the
transverse processes are long, thin plates and, except in the seventh
cervical, are usually pierced by the canal for the vertebral artery, but
in a few forms (_e.g._ the camels) this canal pierces the neural arch.
(2) The dorsal or thoracic vertebræ have more or less cylindrical centra,
with nearly flat faces, and on the centra, for the most part at their
ends, are the concave facets for the rib-heads. The transverse processes
are short and rod-like and most of them articulate with the tubercles of
the ribs. The zygapophyses are smaller than in the cervical region, less
prominent and less oblique; the anterior pair, on the front of the neural
arch, face upward and the posterior pair downward. The neural spines are
very much longer than those of the neck and those of the anterior dorsals
are often of relatively enormous length, diminishing toward the hinder
part of the region.
[Illustration: FIG. 13.—Fifth cervical vertebra of Wolf, left side.
_tr._, transverse process. _v.a″._, posterior opening of canal for
the vertebral artery. _pr.z._ and _pt.z._, anterior and posterior
zygapophyses. _n.sp._, neural spine.]
[Illustration: FIG. 14.—First dorsal vertebra of Wolf, left side. _c._,
centrum. _r._, anterior rib-facet. _r″._, posterior rib-facet. _tr._,
transverse process. _pr.z_. _pt.z._, anterior and posterior zygapophyses.
_n.sp._, neural spine.]
(3) The lumbar vertebræ are almost always heavier and larger than those
of the dorsal region; they carry no ribs and their neural spines and
transverse processes are broad and plate-like and the latter are far
larger and more prominent than those of the dorsals. As an especial
degree of strength is frequently called for in the loins, together with
a greater flexibility than is needed in the dorsal region, the modes of
articulation between the successive vertebræ are more complex, sometimes,
as in the Edentata, most elaborately so. Taking the dorso-lumbars, or
trunk-vertebræ, as a single series, we may note that in a few mammals
(_e.g._ the elephants) all the neural spines have a backward slope, but
in the great majority of forms this backward inclination ceases near the
hinder end of the dorsal region, where there is one vertebra with erect
spine, while behind this point the spines slope forward.
[Illustration: FIG. 15.—Third lumbar vertebra of Wolf, front end and left
side. _tr._, transverse process. _cn._, centrum. _pr.z._ and _pt.z._,
anterior and posterior zygapophyses. _n.sp._, neural spine.]
(4) The sacral vertebræ, varying from 2 to 13 in number, are fused
together solidly into one piece, the combined centra forming a heavy mass
and the neural canals a continuous tube, while the neural spines are
united into a ridge. As a rule, only the first two vertebræ of the sacrum
are in contact with the hip-bones, to support which they have developed
special processes, the remainder of the mass projecting freely backward.
[Illustration: FIG. 16.—Sacrum of Wolf, upper side. _I_, _II_, _III_,
first, second and third sacral vertebræ. _pl._, surface for attachment to
hip-bone.]
[Illustration: FIG. 17.—Caudal vertebræ of Wolf, from anterior and middle
parts of the tail. Letters as in Fig. 15.]
(5) The caudal vertebræ vary greatly, in accordance with the length and
thickness of the tail. In an animal with well-developed tail several
of the anterior caudals have the parts and processes of a typical
vertebra, centrum, neural arch and spine, zygapophyses and transverse
processes. Posteriorly, these gradually diminish, until only the centrum
is left, with low knobs or ridges, which are the remnants of the various
processes. A varying number of long, cylindrical centra, diminishing
backward in length and diameter, complete the caudal region and the
vertebral column. In some mammals, _chevron bones_ are attached to
the under side of the anterior and middle caudals; these are forked,
Y-shaped bones, which form a canal for the transmission of the great
blood-vessels of the tail.
[Illustration: FIG. 18.—Ribs of Wolf from anterior and middle parts of
the thorax. _cp._, head, _t._, tubercle.]
The _ribs_, which are movably attached to the backbone, together with
the dorsal vertebræ and breast-bone, compose the _thorax_, or chest. The
articulation with the vertebræ is by means of a rounded head; in most
cases the head has two distinct facets, the pit being formed half on
the hinder border of one dorsal vertebra and half on the front border
of the next succeeding one, but posteriorly the pit is often shifted,
so as to be on a single vertebra. A second articulation is by means of
the _tubercle_, a smooth projecting facet on the convexity of the rib’s
curvature and near the head; the tubercle articulates with the transverse
process of its vertebra. The ribs, in general, are curved bars of bone,
which in small mammals generally and in the clawed orders are slender
and rod-like, while in the hoofed mammals they are broader, thinner and
more plate-like, especially the anterior ones. The number of pairs of
ribs is most commonly 13, but ranges among existing mammals from 9 in
certain whales to 24 in the Two-toed Sloth (_Cholœpus didactylus_). The
complex curvature of the ribs, outward and backward, is such that, when
they are drawn forward (in Man upward) by muscular action, the cavity of
the thorax is enlarged and air is drawn into the lungs, and when they are
allowed to fall back, the cavity is diminished and the air expelled.
Below, a varying number of the ribs are connected by the cartilages in
which they terminate with the breast-bone (_sternum_); sometimes these
cartilages are ossified and then form the _sternal ribs_, but there is
always a flexible joint between the latter and the true ribs. In certain
edentates, notably the anteaters and the extinct †ground-sloths, these
sternal ribs, at their lower ends, are provided with head and tubercle,
for articulation with the sternum.
The _sternum_, or breast-bone, is made up of a number of distinct
segments, usually broad and flat, but often cylindrical, which may
unite, but far more commonly remain separate throughout life. The
number, size and form of these segments often give useful characters in
classification. The first segment, or _manubrium_, has quite a different
shape from the succeeding ones and is considerably longer.
[Illustration: FIG. 19.—Sternum and rib-cartilages of Wolf, lower side.
_P.S._, manubrium. _X.S._, xiphisternum.]
II. The appendicular skeleton consists of the limb-girdles and the bones
of the limbs and feet. The limb-girdles are the means of attaching the
movable limbs to the body, so as to combine the necessary mobility with
strength. The anterior, or _shoulder-girdle_, has no direct articulation
with the vertebral column, but is held in place by muscles; it is made up
of the shoulder-blade and collar-bone, though very many mammals have lost
the latter.
[Illustration: FIG. 20.—Left scapula of Wolf. _gl._, glenoid cavity.
_c._, coracoid. _ac._, acromion. _sp._ spine.]
[Illustration: FIG. 21.—Left scapula of Horse. This figure is much more
reduced than Fig. 20.]
[Illustration: FIG. 22.—Left scapula of Man in position of walking on all
fours. Letters as in Fig. 20.]
The shoulder-blade, or _scapula_, is a broad, thin, plate-like bone,
which contracts below to a much narrower neck, ending in a concave
articular surface, the _glenoid cavity_, for the head of the upper
arm-bone, the two together making the shoulder-joint. On the outer side
the blade is divided into two parts by a prominent ridge, the _spine_,
which typically ends below in a more or less conspicuous projection,
the _acromion_, which may, however, be absent, its prominence being,
generally speaking, correlated with the presence of the collar bone. A
hook-like process, the _coracoid_, rises from the antero-internal side of
the glenoid cavity and varies greatly in size in the different groups of
mammals; though it usually appears to be merely a process of the scapula,
with which it is indistinguishably fused, yet its development shows it to
be a separate element and in the lowest mammals (Prototheria), as in the
reptiles and lower vertebrates generally, it is a large and important
part of the shoulder-girdle and articulates with the sternum.
The collar-bone, or _clavicle_, is a complexly curved bar, which,
when present and fully developed, extends from the forward end of the
sternum to the acromion, the projecting lower end of the scapular spine,
supporting and strengthening the shoulder-joint. In many mammalian
orders, notably all existing hoofed animals, the clavicle has become
superfluous and is lost more or less completely; it may be said, in
general, that the clavicle is developed in proportion to the freedom
of motion of the shoulder-joint and to the power of rotation of the
hand upon the arm. In arboreal animals, such as monkeys, in which the
hand rotates freely and the arm moves in any direction on the shoulder,
the clavicle is large and fully developed, as it also is in Man. Many
burrowing mammals (_e.g._ the moles) have very stout clavicles.
[Illustration: FIG. 23.—Left clavicle of Man, front side.]
[Illustration: FIG. 24.—Left hip-bone of Wolf. _Il._, ilium. _Is._,
ischium. _P._, pubis. _ac._, acetabulum.]
The posterior, or _pelvic_, girdle is composed on each side of a very
large, irregularly shaped bone, which is firmly attached to one or more
of the coalesced vertebræ which form the sacrum and thus affords a solid
support to the hind leg. Each half of the _pelvis_, or hip-bone, is made
up of three elements, called respectively the _ilium_, _ischium_ and
_pubis_, which are separate in the very young animal, indistinguishably
fused in the adult. The three elements unite in a deep, hemispherical
pit, the _acetabulum_, which receives the head of the thigh-bone, a
perfect example of the “ball and socket joint.” In the inferior median
line the two pubes meet and may become coalesced, in a _symphysis_,
the length of which differs in various mammals. The pelvis and sacrum
together form a short, wide tube, the diameter of which is normally
greater in the female skeleton than in the male.
The limbs are each divided into three segments, which in the anterior
extremity are the arm, fore-arm and hand (or fore foot) and in the
posterior extremity are the thigh, leg and foot (or hind foot), and there
is a general correspondence between the structure of these segments in
the fore and hind legs, however great the superficial difference. The
bones of the limbs, as distinguished from those of the feet, are the
_long bones_ and, except in a few very large and heavy mammals, are
essentially hollow cylinders, thus affording the maximum strength for a
given weight of bone; the cavity of a long bone contains the marrow and
hence is called the _medullary cavity_. In the young mammal each of the
long bones consists of three parts, the _shaft_, which makes up much the
greater part of the length, and at each end a bony cap, the _epiphysis_.
Growth takes place by the intercalation of new material between the shaft
and the epiphyses; when the three parts unite, growth ceases and the
animal is adult.
[Illustration: FIG. 25.—Left humerus of Wolf, from the front and outer
sides, the latter somewhat oblique. _h._, head. _int.t._, internal
tuberosity. _ext.t._, external tuberosity. _bc._, bicipital groove.
_dt._, deltoid ridge. _sh._, shaft. _s._, supinator ridge. _int. epi._,
internal epicondyle. _s.f._, anconeal foramen. _tr._, trochlea. _tr′._,
trochlea, posterior side. _ext. epi._, external epicondyle. _a.f._,
anconeal fossa.]
[Illustration: FIG. 26.—Left humerus of Horse, front side. _i.t._,
internal tuberosity. _ex.t._, external tuberosity. _bc._, outer part of
bicipital groove. _dt._, deltoid ridge. _s._, supinator ridge. _tr._,
trochlea.]
[Illustration: FIG. 27.—Left humerus of Man, front side. Letters as in
Fig. 25.]
The superior segment of the fore limb has a single bone, the _humerus_,
the upper end of which is the rounded, convex _head_, which fits into
the glenoid cavity of the shoulder-blade, forming the joint of the
shoulder; in front of the head are two prominent and sometimes very
large projections for muscular attachment, the _external_ and _internal
tuberosities_, separated by a groove, in which play the two tendons of
the biceps muscle and is therefore called the _bicipital groove_. In a
few mammals, such as the Horse, Camel and Giraffe, the groove is divided
into two by a median tubercle or ridge. From the external tuberosity
there generally passes down the front face of the shaft a rough and
sometimes very prominent ridge, the _deltoid crest_, to which is attached
the powerful _deltoid_ muscle. At the lower end of the humerus is the
_trochlea_, an irregular half-cylinder, for articulation with the two
bones of the fore-arm and varying in form according to the relative
sizes of those bones. On each side of the trochlea is frequently a
rough prominence, the _epicondyle_, and above the inner one is, in many
mammals, a perforation, the _epicondylar foramen_, for the passage of a
nerve. Extending up the shaft from the outer epicondyle is a rough crest,
the _supinator ridge_, to which is attached one of the muscles that
rotate the hand and is conspicuously developed in those mammals which
have the power of more or less free rotation and especially in burrowers.
On the posterior face of the humerus, just above the trochlea, is a
large, deep pit, the _anconeal fossa_.
[Illustration: FIG. 28.—Left fore-arm bones of Wolf, front side. _R._,
radius. _U._, ulna. _ol._, olecranon. _h._, head of radius.]
[Illustration: FIG. 29.—Left fore-arm bones of Man, front side. Letters
as in Fig. 28. The small object at the right of each figure is the head
of the radius, seen from above.]
The two bones of the fore-arm, the _radius_ and _ulna_, are, in most
mammals, entirely separate from each other, but in certain of the more
highly specialized hoofed animals are immovably coössified. Primitively,
the two bones were of nearly equal size, but in most of the mammalian
orders there is a more or less well-defined tendency for the radius to
enlarge at the expense of the ulna. These bones are normally crossed, the
radius being external at the upper end and passing in front of the ulna
to the inner side of the arm. The radius varies considerably in form in
accordance with the uses to which the hand is put; if the capacity of
rotation is retained, the upper end, or head, of the radius is small,
circular or disk-like, covering little of the humeral trochlea, but when
the head of the radius is broadened to cover the whole width of the
humerus, then all power of rotation is lost. (Cf. Figs. 28 and 29.) As a
rule, the radius broadens downward and covers two-thirds or more of the
breadth of the wrist-bones.
[Illustration: FIG. 30.—Coössified bones of left fore-arm of Horse, front
side. For most of its length, the ulna is concealed by the radius.]
[Illustration: FIG. 31.—Left fore-arm bones of the Tapir (_Tapirus
terrestris_). _R._, radius. _U._, ulna. _h._, head of radius. _h′._,
sigmoid notch of ulna. _ol._, olecranon. N.B. This figure is on a much
larger scale than Fig. 30.]
The ulna is longer than the radius, its upper end being extended into
a heavy process, the _olecranon_, or _anconeal process_, into which is
inserted the tendon of the great triceps muscle, the contraction of
which straightens the arm; this process is the bony projection at the
back of the elbow-joint. Below the olecranon is a semicircular articular
concavity, which embraces the humeral trochlea and its upper angle fits
into the anconeal fossa of the humerus. The ulna contracts and grows more
slender downwards and its lower end covers but one of the wrist-bones.
While in the more primitive mammals, and in those which retain the power
of rotating the hand, the ulna has nearly or quite the same thickness
as the radius, it is often much more slender and in the more highly
specialized of the hoofed animals, such as the horses, camels and true
ruminants, the radius carries the entire weight and the ulna has become
very slender, more or less of its middle portion is lost and the two
ends are coössified with the radius, so that the fore-arm appears to have
but a single bone. The reverse process of enlarging the ulna and reducing
the radius is very rare and practically confined to the elephant tribe.
[Illustration: FIG. 32.—Left manus of Wolf, front side. _SL._,
scapho-lunar. _Py._, pyramidal. _Pis._, pisiform. _Tm._, trapezium.
_Td._, trapezoid. _M._, magnum. _U._, unciform. _Mc. I-V_, first to fifth
metacarpals. _Ph. 1_, first phalanx. _Ph. 2_, second phalanx. _Ung._,
ungual phalanx. _I_, first digit, or pollex. _II-V_, second to fifth
digits.]
[Illustration: FIG. 33.—Left manus of Man. _S._, scaphoid. _L._,
lunar. _Py._, pyramidal (pisiform not shown). _Tm._, trapezium. _Td._,
trapezoid. _M._, magnum. _Un._, unciform.]
The fore foot, or hand, for which the term _manus_ may be conveniently
employed, is divisible into three parts, corresponding in ourselves
to the wrist, back and palm of the hand, and the fingers. The bones
of the wrist constitute the _carpus_, those of the back and palm the
_metacarpus_, and those of the fingers the _phalanges_.
The carpus consists primitively of nine distinct bones, though one of
these, as will be shown later, is not a true carpal. These bones are
of a rounded, subangular shape, closely appressed together, with very
little movement between them, and are arranged in two transverse rows.
The upper row contains four bones, which enumerating from the inner side
are the _scaphoid_, _lunar_, _pyramidal_ (or cuneiform) and _pisiform_.
The scaphoid and lunar support the radius, while the ulna rests upon the
pyramidal. The pisiform, though very constantly present, is not a true
carpal, but an ossification in the tendon of one of the flexor muscles,
which close the fingers; it projects more or less prominently backward
and articulates with the ulna and pyramidal. The second row is also
made up of four bones, which, from within outward, are the _trapezium_,
_trapezoid_, _magnum_ and _unciform_. The relations of the two rows
vary much in different mammals and the arrangement may be serial or
alternating; thus, the scaphoid rests upon the trapezium and trapezoid
and usually covers part of the magnum; the lunar may rest upon the magnum
only, but very much more frequently is equally supported by the magnum
and unciform and the pyramidal by the latter only. The ninth carpal is
the _central_, which, when present and distinct, is a small bone, wedged
in between the two rows. Few existing mammals have a separate central,
which, though present in the embryo, has coalesced with the scaphoid in
the great majority of forms. In the more advanced and differentiated
mammals the number of carpals may be considerably reduced by the
coössification of certain elements or the complete suppression and loss
of others. In all existing Carnivora and a few other mammals the scaphoid
and lunar are united in a compound element, the _scapho-lunar_ (or, more
accurately, the scapho-lunar-central); hoofed animals with a diminished
number of toes generally lose the trapezium, and other combinations
occur. The second row of carpals carries the metacarpals, and primitively
the trapezium, trapezoid and magnum are attached each to one metacarpal
and the unciform has two.
The metacarpus consists typically of five members, a number which is
never exceeded in any normal terrestrial mammal; the members are numbered
from the inner side, beginning with the thumb or _pollex_, from I to V.
Many mammals have fewer than five metacarpals, which may number four,
three, two or only one; the third is never lost, but any or all of the
others may be suppressed, and functionless rudiments of them may long
persist as splints or nodules. The metacarpals are elongate, relatively
slender and of more or less cylindrical shape; but the form varies
considerably in different groups, according to the way in which the
hand is used. When employed for grasping, as in many arboreal animals
and pre-eminently in Man, the pollex is frequently opposable to the
other fingers and enjoys much freedom of motion. In the camels and true
ruminants the third and fourth metacarpals are coössified to form a
_cannon-bone_ (_see_ Fig. 43, p. 91), but the marrow cavities and the
joints for the phalanges remain separate.
The phalanges in land mammals never exceed three in each digit, except
the pollex, which, when present and fully developed, has but two. The
phalanges are usually slender in proportion to their length, but in very
heavy hoofed animals they are short and massive. The terminal joint is
the _ungual phalanx_, which carries the nail, claw, or hoof, its shape
varying accordingly.
[Illustration: FIG. 34.—Left femur of Wolf, front side. _h._, head.
_gt.tr._, great trochanter. _tr. 2_, second trochanter. _int. con._,
internal condyle. _r.g._, rotular groove, _ext. con._, external condyle.]
[Illustration: FIG. 35.—Left femur of Horse. _tr. 3_, third trochanter.
Other letters as in Fig. 34, than which this drawing is very much more
reduced.]
The hind leg is constituted in very much the same manner as the fore,
but with certain well-marked and constant differences. The thigh-bone,
or _femur_, is usually the longest and stoutest of the limb-bones and
in very large animals may be extremely massive. At the upper end is the
hemispherical _head_, which is set upon a distinct _neck_ and projects
inward and upward, fitting into the acetabulum of the hip-bone. Nearly
all land mammals have a small pit on the head of the femur, in which is
inserted one end of the _round ligament_, while the other end is attached
in a corresponding depression in the floor of the acetabulum. This
ligament helps to hold the thigh-bone firmly in place and yet allows the
necessary freedom of movement. On the outer side of the upper end of
the femur is a large, roughened protuberance, which often rises higher
than the head and is called the _great trochanter_; another, the _second_
or _lesser trochanter_, is a small, more or less conical prominence on
the inner side of the shaft, below the head. These two processes are
well-nigh universal among land mammals; and in a few of the orders occurs
the _third trochanter_, which arises from the outer side of the shaft,
usually at or above the middle of its length. Though comparatively rare
in the modern world, the third trochanter is an important feature, and
the early members of most, if not all, of the mammalian orders possessed
it. The shaft of the femur is elongate and, except in certain very
bulky mammals, of nearly cylindrical shape. The lower end of the bone
is thick and heavy and bears on the posterior side two large, rounded
prominences, the _condyles_, which articulate with the shin-bone to form
the knee-joint. On the anterior side is a broad, shallow groove, the
_rotular groove_, in which glides the _patella_, or knee-cap. The patella
is a large ossification, of varying shape, in the tendon common to the
four great extensor muscles of the thigh, the action of which is to
straighten the leg.
[Illustration: FIG. 36.—Left femur of Wolf, inside of lower end. _ext.
con._, external condyle. _int. con._, internal condyle. _r.g._, rotular
groove. Above, are two views of the left patella, anterior and internal
sides.]
The lower leg, like the fore-arm, has two bones, which, however, are
always parallel, never crossed, and have no power of rotation. Of these,
the inner one is the shin-bone, or _tibia_, which is always the larger
and alone enters into the knee-joint. The external bone is the _fibula_,
which is almost entirely suppressed in certain highly specialized forms,
such as the horses and ruminants, the tibia carrying the whole weight.
The upper end of the tibia is enlarged and extends over that of the
fibula; it has two slightly concave surfaces for articulation with the
condyles of the femur, the approximate edges of which are raised into a
bifid _spine_. The upper part of the shaft is triangular, with one edge
directed forward, and the superior end of this edge is roughened and
thickened to form the _cnemial crest_, to which is attached the patellar
ligament. The middle portion of the shaft is rounded and the lower end
broad and usually divided by a ridge into two grooves or concavities
for the ankle-bone; from the inner side of this end projects downward
a tongue-like process, the _internal malleolus_, which prevents inward
dislocation of the ankle.
The fibula is relatively stoutest in the less advanced mammals and is
usually straight and slender, with enlarged ends, the lower one forming
the _external malleolus_, which serves to prevent outward dislocation
of the ankle. In many forms the fibula is coössified with the tibia at
both ends, and in the most highly specialized hoofed animals, such as the
horses, camels and true ruminants, the bone has apparently disappeared.
The young animal, however, shows that the ends of the fibula have been
retained and the shaft completely lost; the upper end is in the adult
firmly fused with the tibia and, in the horses, the lower end is also,
but this remains separate in the ruminants and camels, forming the
_malleolar bone_, which is wedged in between the tibia and the heel-bone.
Because of its importance in holding the ankle-bone in place, this lower
end of the fibula is never lost in any land mammal.
[Illustration: FIG. 37.—Bones of left lower leg of Wolf, front side.
_T._, tibia. _F._, fibula. _sp._ spine of tibia. _cn._ cnemial crest.
_i.m._, internal malleolus. _e.m._, external malleolus.]
[Illustration: FIG. 38.—Bones of left lower leg of Horse (much more
reduced). _cn._ cnemial crest. _F._, lower end of fibula, coössified with
tibia. Other letters as in Fig. 37.]
[Illustration: FIG. 39.—Bones of lower leg, left side, of Tapir. _T._,
tibia. _F._, fibula. _sp._, spine of tibia. _cn._, cnemial crest. _i.m._,
internal malleolus. _e.m._, external malleolus. N.B. This figure is on a
much larger scale than Fig. 38.]
[Illustration: FIG. 40.—Left pes of Wolf, front side. _Cal._, calcaneum.
_As._, astragalus. _N._, navicular. _Ch._, cuboid. _Cn. 1_, _Cn. 2_,
_Cn. 3_, internal, middle and external cuneiforms. _Mt. I_, rudimentary
first metatarsal. _Mt. II-V_, second to fifth metatarsals. _Ph. 1_,
first phalanx. _Ph. 2_, second phalanx. _Ung._, ungual phalanx. _I_,
rudimentary hallux. _II-V_, second to fifth digits.]
[Illustration: FIG. 41.—Left pes of Man. Note the large size of _Mt. I_,
the metatarsal of the first digit, or hallux. Letters as in Fig. 40,
except _Cb._, cuboid.]
The hind foot, or _pes_, like the manus, is clearly divisible into
three parts, the bones of which are called respectively the _tarsus_,
_metatarsus_ and _phalanges_, and the correspondence in structure between
manus and pes is close and obvious. The tarsus consists typically of
seven bones, which are tightly packed and rarely permit any movement
between them. The upper row of the tarsus consists of two bones, which
are peculiarly modified to form the ankle-joint and heel; on the inner
side is the ankle-bone, or _astragalus_, the shape of which is highly
characteristic of the various mammalian orders. The upper or posterior
portion of the astragalus, according to the position of the foot, is a
pulley which glides upon the lower end of the tibia and is held firmly in
place by the internal and the external malleolus. Below the pulley-like
surface the astragalus usually contracts to a narrow neck, which ends in
a flat or convex head. The astragalus is supported behind (or beneath) by
the heel-bone, or _calcaneum_, which is elongate and extends well above
(or behind) the remainder of the tarsus; it frequently has a distinct
articulation with the fibula, but more commonly is not in contact with
that bone. The astragalus rests upon the _navicular_, which is moulded
to fit its head and corresponds in position to the central of the carpus,
but, unlike that carpal, it is a very important element and is never
suppressed or lost in any land mammal. The navicular, in turn, rests
upon three bones of the second row, which are called respectively the
_internal_, _middle_ and _external cuneiform_, which correspond to the
trapezium, trapezoid and magnum of the carpus and to which are attached
the three inner metatarsals, one to each. Finally, the _cuboid_, the
external element of the second row, is a large bone, which supports the
calcaneum and often part of the astragalus and to which the fourth and
fifth metatarsals are attached; it is the equivalent of the unciform
in the manus. The number of tarsals is more constant than that of the
carpals, but some suppressions and coössifications do occur.
The long bones of the pes constitute the metatarsus, which is the
counterpart of the metacarpus. There are never more than five metatarsals
in any normal mammal, but there may be any number less than five, down
to a single one. In form and size the metatarsals of any given mammal
are usually so like the metacarpals, that it requires some experience
to distinguish them, but when either manus or pes is especially adapted
to some particular kind of work, there may be very decided differences
between metatarsals and metacarpals. For example, the burrowing forefoot
of the moles is very different from the hind foot, which has undergone
but little modification, and even more striking is the difference between
the wing of a bat and its foot. Many other instances of a less extreme
divergence might be enumerated, but when manus and pes are used only for
locomotion, as in nearly all hoofed animals and many other mammals, the
metacarpals and metatarsals are very similar. When there is a difference
in number, it is the general rule that there are fewer metatarsals; an
instance of this is found in the tapirs, which have four toes in the
front foot and three in the hind. Forms which have a cannon-bone in the
manus have it also in the pes, and some, like the peccaries and the
jumping rodents called jerboas, have it only in the pes. The first (or
inner) metatarsal, that of the great toe, or _hallux_, is sometimes
opposable to the others, as in the monkeys, apes and lemurs.
The word _metapodial_ is a useful general term which includes both
metacarpals and metatarsals. A metapodial with its phalanges is a
_digit_, a term often employed because of the ambiguity which arises in
the use of the words “fingers” and “toes,” and is applicable to both fore
and hind feet.
Normally, the phalanges of the pes are so like those of the manus as
to require no particular description; and only when the two pairs of
extremities are specialized for entirely different functions, is there
any notable divergence between the phalanges of manus and pes.
[Illustration: FIG. 42.—Left pes of Black Bear (_Ursus americanus_),
showing the plantigrade gait. _T._, tibia. _F._, fibula. _Cal._,
calcaneum. _As._, astragalus. _N._, navicular. _Cn. 3_, external
cuneiform. _Cb._, cuboid. _Mt. V._, fifth metatarsal.]
[Illustration: FIG. 43.—Left pes of Patagonian Deer (_Hippocamelus
bisulcus_), showing the unguligrade gait. _T._, tibia. _F._, lower end of
fibula (malleolar bone). _Cal._, calcaneum. _As._, astragalus. _N.Cb._,
coössified cuboid and navicular. _Mt. III_, _Mt. IV_, cannon-bone,
formed by the coössification of the third and fourth metatarsals. _V._,
Rudimentary fifth digit.]
Before leaving the subject of the skeleton, it will be well to explain
the terms used in describing the gait and manner of using the feet.
When the entire sole of the foot is in contact with the ground and
weight is thrown upon the heel-bone, or calcaneum, the gait is said to
be _plantigrade_ and is exemplified in Man, bears, raccoons and many
other mammals. The Dog is _digitigrade_, that is to say, the feet in the
standing position are nearly erect and the wrist and heel are raised high
above the ground; the weight is borne upon ball-like pads, one under
the phalanges of each functional digit and one under the metapodials.
The digitigrade gait is found not only in all the dogs and cats, but in
many other Carnivora and in the camels and llamas, as well. Transitions
between the plantigrade and digitigrade gait are so numerous and
gradual, that such terms as _semi-plantigrade_ and _semi-digitigrade_
are sometimes necessary. An animal is said to be _unguligrade_ when the
weight is carried entirely upon the hoofs and is used only of hoofed
animals; examples are the horses, pigs, deer, antelopes, oxen, etc.
The so-called “knee” of a horse is really his wrist and the “hock” is
the heel, so that the feet make nearly half the apparent length of the
legs. Certain very large and massive animals, such as the rhinoceroses
and elephants, are unguligrade in a modified sense; the foot is a
heavy column, seemingly a part of the leg, and the weight is borne
upon a great pad of elastic tissue, with the hoofs disposed around its
periphery. A very peculiar mode of locomotion is exemplified by certain
of the Edentata, in the forefoot of the existing Ant Bear (_Myrmecophaga
jubata_) and in both extremities of some of the later representatives of
the extinct †ground-sloths, or †Gravigrada. Here the weight is carried
upon the outer edge of the foot, the palm and sole being turned inward.
No term has been suggested for this very exceptional gait, which is a
modified form of plantigradism.
II. THE TEETH
It was pointed out in Chapter II (p. 38) that very often the teeth are
all that remains to us of extinct genera and species of mammals, and it
may be further noted that the teeth are very characteristic and often
suffice to fix the systematic position of a genus. Since, therefore,
the teeth play such an uncommonly important part as fossils and are so
pre-eminently useful to the palæontologist, it is necessary to give some
general account of them.
Among the mammals the teeth display a very great variety of size and
form in accordance with the manner in which they are used. Primarily,
the function of the teeth is to seize and masticate food, and the kind
of food habitually eaten by any animal is well indicated by the form of
its teeth. The beasts of prey have teeth adapted for shearing flesh and
crushing bones; plant-feeders have teeth fitted for cropping plants and
triturating vegetable tissues; insect-eaters have teeth with numerous
sharp-pointed cusps, or it may be, no teeth at all, swallowing without
mastication the insects which they capture, etc. Among animals that have
similar diet there is very great difference in the form and elaborateness
of the grinding apparatus and it is often possible to follow out the
steps of evolutionary change, by which, from simple beginnings, a high
degree of complexity has been attained. In addition to the uses of the
teeth as organs of mastication, they frequently serve as weapons of
offence or defence. In the flesh-eaters which capture living prey they
are formidable offensive weapons, and the fangs of the Lion or the Wolf
are instances familiar to every one; but the tusks of the elephants or
the hippopotamuses have nothing to do with the taking of prey. Several
Old World deer, which have no antlers or very small ones, possess
scimitar-like upper tusks, with which they are able to defend themselves
very effectually.
In the lower vertebrates, such as reptiles and fishes, the number of
teeth is usually indefinite and they continue to be shed and replaced,
as needed, throughout life; but in each species of mammal, aside from
abnormalities, the number is fixed and constant. Mammalian teeth are very
generally divisible into four categories: (1) the _incisors_, or front
teeth, which in the upper jaw are inserted in the premaxillary bones,
(2) the _canines_, or eye-teeth, which are never more than one on each
side of each jaw, or four in all, (3) the _premolars_, called in Man the
bicuspids, the anterior grinding teeth which have predecessors in the
milk-series and (4) the _molars_, the posterior grinding teeth which have
no such predecessors.
[Illustration: FIG. 44.—Dentition of Wolf, left side. _i. 3_, third
incisor. _C._, canine. _p. 1_, first premolar. _p. 4_, fourth premolar.
_m. 1_, first molar.]
It is customary and convenient to express the numbers and kinds of teeth
of a given mammalian species by means of a “dental formula”; for example,
in Man the formula is: _i_ 2/2, _c_ 1/1, _p_ 2/2, _m_ 3/3, × 2 = 32;
the reason for the multiplication by two is that the figures deal only
with one side of the mouth and must be doubled to give the sum total.
Just because, however, the two sides are alike, it is usual to take the
doubling for granted. Written out in full, the formula means that Man has
two incisors, one canine, two premolars and three molars on each side of
each jaw, the horizontal line indicating the division between upper and
lower teeth. The number of teeth is frequently not the same in the upper
and lower jaws; for instance, the formula for the Sheep is: _i_ 0/3, _c_
0/1, _p_ 3/3, _m_ 3/3, × 2 = 32; the total is the same as in Man, but
the arrangement is entirely different. The meaning is that in the Sheep
there are no upper incisors or canines, but three incisors and a canine
are present in each half of the lower jaw, with three premolars and
three molars on each side above and below. The Dog gives still another
formula: _i_ 3/3, _c_ 1/1, _p_ 4/4, _m_ 2/3, × 2 = 42. What is called
the typical formula for the higher terrestrial mammals above the grade
of the marsupials and which is but rarely exceeded, is _i_ 3/3, _c_ 1/1,
_p_ 4/4, _m_ 3/3, × 2 = 44, though most existing mammals have fewer teeth
than this. Compared with the typical formula, the Dog has lost but two
teeth, the third upper molar on each side, while Man and the Sheep have
each lost twelve.
As every one knows from his own experience, mammals normally have two
sets of teeth, the first, temporary, or milk-dentition, in the young
animal, and the second, or permanent dentition, in the adult. The
milk-dentition, when fully developed, consists of incisors, canines and
premolars, which usually agree in number, though often not in form, with
the permanent teeth which replace them in the adult. The milk-teeth are
frequently more conservative than the permanent ones and retain ancestral
characters which have disappeared in the adult series, thus affording
welcome information as to lines of descent and steps of evolutionary
change. While there can be little doubt that the development of more than
one dentition, or set of teeth, is the primitive condition among mammals
and was derived by inheritance from their lower vertebrate ancestors, in
which there was an indefinite succession of teeth; yet there are many
mammals in which the milk-dentition is greatly reduced or altogether
lost. In some, the milk-teeth are shed and replaced before birth, in
others only the germs of the milk-teeth are formed and never cut the gum,
while in others again all traces of the temporary series have vanished.
This complete loss of the milk-teeth, like the presence of a great
number of simple and similar teeth in the dolphins and porpoises, or the
total absence of teeth, as in the anteaters and whalebone whales, is a
secondary and derivative condition, never a primitive one.
[Illustration: FIG. 44_A_.—First upper molar, right side of Deer
(_Odocoileus_). On the left, the masticating surface; heavy black
line, enamel. On the right, external side, showing crown and roots.
_Brachyodont._]
[Illustration: FIG. 45.—First upper molar, left side, of a fossil horse
(_Equus sp._). On the right, external side. On the left, the grinding
surface, showing two stages of wear. Heavy black line, enamel; white,
dentine; shaded, cement. _Hypsodont_, roots not yet formed.]
The structure of mammalian teeth varies greatly, from the simplest
slender cones to enormous and highly complicated apparatus, and, in
order to comprehend the significance of these differences, we must
look a little more closely into the materials of which the teeth are
constructed and the manner in which those materials are combined. In
all primitive mammals and in many of the higher and more advanced ones
(including Man) a tooth is composed of the _crown_, or portion which is
exposed above the gum, and the _roots_, one or more in number, by means
of which the tooth is firmly inserted in the jaw-bone; the roots are at
least partly formed before the tooth comes into use. Such a tooth is said
to be short or low-crowned, or _brachyodont_. In many plant-feeders, such
as horses, cattle, elephants, beavers, etc., the teeth continue to grow
in height for a long time and do not form roots until late in life, or
perhaps not at all. Such teeth are said to be long- or high-crowned, or
_hypsodont_, and in very many instances the development of brachyodont
into hypsodont teeth may be followed through every step of the change.
The advantage of the change is obvious in lengthening the animal’s
life, especially in those which feed upon abrasive substances, like
grass, for the growth of the teeth long continues to make up for the
loss through wear. Serious trouble has often been caused for captive
elephants by giving them too soft food, so that the growth of the teeth
is not properly balanced by abrasion. Still another category of teeth is
the _rootless_, which are of simple form, like those of an armadillo,
and grow throughout life, never forming roots. The chisel-like, or
_scalpriform_ incisors of the rodents do not cease to grow while the
animal lives; they are kept of constant length by continual use, and the
arrangement of harder and softer tissue is such that the sharp edge is
maintained; through accident or malformation it sometimes happens that
the upper and lower teeth fail to meet, then the continued growth causes
them to form curved hoops in the mouth, locking the jaws and bringing
death by starvation to the unfortunate animal.
[Illustration: FIG. 46.—Dentition of Beaver (_Castor canadensis_). _m.
3_, last molar. _p. 4_, last premolar. _i._, scalpriform incisors; enamel
face black, dentine in vertical lines.]
The typical mammalian tooth is composed of three kinds of tissue,
all differing in structure and hardness and called respectively (1)
dentine, (2) enamel, (3) cement. (1) The _dentine_, or ivory, is the
indispensable tissue of the tooth; the other kinds may be absent, but
never the dentine. Chemically, it is like bone, but the microscope shows
that its structure is quite different from that of true bone, being
composed of an immense number of fine tubules, which radiate from the
“pulp-cavity,” or chamber which contains the blood-vessels and nerves,
these entering the tooth through the canals of the roots. The tubules
of the dentine lodge excessively fine fibrillæ of the nerve and that is
why the cutting into a live tooth is so painful an operation. (2) The
_enamel_, which is the hardest of all animal tissues, has a polished and
shining appearance and is arranged in a mosaic of microscopic prisms,
closely packed together, which in most mammals are solid, but in the
marsupials, with some exceptions, are tubular. The enamel normally covers
the entire crown of the tooth, but does not extend upon the roots, where
its superior hardness would be of no advantage. In several instances,
always as a secondary specialization, the enamel does not cover the whole
crown, but is arranged in vertical bands, it may be on one side only, or
at intervals around the tooth. The scalpriform incisors of the rodents,
already alluded to, have the enamel band on the front face of the tooth;
the softer dentine behind wears away more rapidly, keeping the cutting
surface bevelled, like the edge of a chisel, while the hard enamel forms
the sharp edge. In some instances the enamel is absent altogether and the
teeth are composed entirely of dentine, as in the elephant tusk. In all
the Edentata, such as sloths and armadillos, both living and extinct,
that have any teeth at all, the teeth have no enamel, but in some of the
fossil forms the place of the missing enamel is taken by a harder dentine
and thus the effect of differential hardness is secured.
[Illustration: FIG. 47.—Section through a lower molar of the Indian
Elephant (_Elephas maximus_). Enamel, heavy black; dentine, white;
cement, horizontal lines.]
(3) The _cement_ is simply bone, both chemically and in microscopic
structure; it is not quite so hard as dentine, but it is less affected by
the fluids of the mouth and the juices of the food. In the brachyodont
or low-crowned tooth, such as a human molar, the cement merely forms a
sheath over the roots and does not appear upon the crown, but in many
hypsodont teeth, those of horses and elephants, for example, the cement
completely encases the entire tooth in a thick layer, filling up all the
depressions and irregularities of the enamel surface and making a freshly
erupted and unworn tooth look like a shapeless lump. When the cement and
the enamel covering are partially worn through, the masticating surface
is made up of three distinct substances, each having a different degree
of hardness and thus, through unequal wear, the grinding surface is
always kept rough and therefore efficient. Not all hypsodont teeth have
the cement covering, but in such teeth the differing degrees of hardness
of enamel and dentine suffice to keep a rough surface, though not so
effectively.
CHAPTER V
THE GEOGRAPHICAL DEVELOPMENT OF THE AMERICAS IN CENOZOIC TIMES
I. TERTIARY PERIOD
In the interior regions of western North America the transition from the
Mesozoic to the Cenozoic was so gradual that there is great difficulty
in drawing the line between them and therefore, as might be expected,
there is much difference of opinion as to just where that line should be
drawn. From one point of view, the matter is of no great consequence;
but from another, it is of the utmost importance, for, unless the events
in different continents can be approximately synchronized, it will often
prove a hopeless undertaking to trace the course of migration of the
various mammalian groups and determine their place of origin and primary
home. Until a definitive answer can be given to the question as to when
the Cenozoic era began, many significant points must be left in doubt,
and much remains to be done in the geology of the Far West before that
definitive solution can be reached.
1. _Paleocene Epoch_
So far as North America is concerned, the best available evidence points
to the conclusion that we should regard the Fort Union, Puerco and
Torrejon as the most ancient of the Cenozoic formations (see Table, p.
17), though retaining so many features of Mesozoic life that a separate
division of the Tertiary, the Paleocene epoch, is made for them. Such
a separation is not the common practice in this country, where it is
more usual to employ the terms “Lowest” or “Basal” Eocene. In my
judgment, however, the balance of advantage is in favour of giving to
this so-called Basal Eocene a rank equivalent to that of the four other
universally recognized and admitted epochs of the Tertiary period. No
marine rocks of Paleocene date have yet been found in North America,
which indicates that the continent was at least as extensive as it is
now. The very scanty development of deposits representing this epoch
in Europe renders the comparison with the fossils of the Old World
unsatisfactory and hence leads to uncertainty, when it is attempted to
determine the land-connections of the time. During the Mesozoic era the
shallow Bering Sea had repeatedly been elevated into a land joining North
America with Asia and had as often been depressed, so as to separate
the continents and allow the waters of the Arctic Ocean to mingle with
those of the Pacific. A like alternation of junction and separation went
on during the Tertiary and Quaternary periods and, by a comparison of
the fossil mammals of Europe and America for any particular division of
geological time, it is almost always feasible to say whether the two
continents were connected, or altogether separated. This statement does
not imply that the proportion of common elements in the two faunas during
epochs of continental connection was a constant one at all times, for
that was by no means true. Mere land-connections or separations are not
the only factors which limit the spread of terrestrial animals; if they
were, the community of forms between North and South America would be
much greater than it actually is. Climatic barriers are of almost equal
importance in determining animal distribution, and changes of climate may
greatly alter the conditions of migration between connected continents.
As the connections between North America and the Old World were probably
in high latitudes, where the seas are narrow, changes of climate produced
a greater effect upon migration than they could have done had the
land-bridges been in the tropical or warm temperate zones. That these
vicissitudes of climate really did occur and are not mere guesses to
bolster up a tottering hypothesis, there is abundant evidence to prove.
In the Paleocene, or most ancient epoch of the Tertiary period, the
geographical condition of North America was approximately as follows: The
continent had attained nearly its modern outlines and on the Atlantic and
Pacific coasts probably extended farther seaward than it does to-day.
Florida, however, and perhaps a narrow strip of the northern Gulf coast
were still submerged, the Gulf of Mexico opening broadly into the
Atlantic. It is very probable that the continent was connected with the
Old World by a land occupying the site of Bering Sea and perhaps also
by way of Greenland and the North Atlantic; and there is some evidence,
though not altogether convincing, that it was also joined to South
America. The great mountain ranges were largely what they now are, though
subsequent upheavals greatly modified the Rocky Mountains, Sierra Nevada
and the ranges of the Pacific coast, while the lofty St. Elias Alps
of Alaska were not in existence. The region of high plateaus, between
the Rockies and the Sierras, was much less elevated than it is now.
The Appalachians, which were of far more ancient date than the western
ranges, had been worn down by ages of weathering and stream-erosion into
a low-lying, almost featureless plain, with some scattered peaks rising
from it here and there, of which the mountains of western North Carolina
were the highest. In general, it may be said that while the _average_
height of the continent above the sea-level may have been as great or
greater than at present, yet the inequalities of surface appear to have
been less marked, and both along the Atlantic coast and in the interior
were vast stretches of plains.
The Paleocene formations of the western interior are of non-marine or
continental origin. In northwestern New Mexico is the typical area
of the _Puerco_ and _Torrejon_, a series of beds 800 to 1000 feet in
thickness and for the most part quite barren of fossils, but there are
two horizons, one near the top and the other near the bottom of the
series, which have yielded a very considerable number of fossil mammals,
and of these the lower is the Puerco, the upper the Torrejon. The _Fort
Union_ is quite different in character and is composed of great areas
of sandstone and clay rocks, with a maximum thickness of 2000 feet,
in eastern Wyoming, South Dakota, Montana and the adjoining parts of
Canada. The modes of formation of these beds have not yet been fully
determined; that they may have been partly laid down in shallow lakes is
indicated by the masses of fresh-water shells in certain localities. In
others are preserved multitudes of leaves, which have given a very full
conception of the plants of the time, and great swamps and bogs have left
the traces of their presence in beds of lignite, or imperfectly formed
coal. Deposits made on the flood-plains of rivers and wind accumulations
are probably also represented. “Vast stretches of subtropical and more
hardy trees were interspersed with swamps where the vegetation was rank
and accumulated rapidly enough to form great beds of lignite. Here were
bogs in which bog iron was formed. Amid the glades of these forests
there wandered swamp turtles, alligators, and large lizards of the
characteristic genus _Champsosaurus_” (Osborn, p. 100).
Fort Union mammals are relatively rare and most of those that have been
found are very fragmentary; they are amply sufficient, however, to
demonstrate the Paleocene date of the beds and to make it probable that
they include both the Puerco and the Torrejon faunas.
The climate, as shown by the plants, was much milder and more uniform
than that of the Recent epoch, though some indication of climatic zones
may already be noted. The vegetation was essentially modern in character;
nearly all our modern types of forest-trees, such as willows, poplars,
sycamores, oaks, elms, maples, walnuts and many others, were abundantly
represented in the vast forests which would seem to have covered nearly
the entire continent from ocean to ocean and extended north into Alaska
and Greenland, where no such vegetation is possible under present
conditions. Numerous conifers were mingled with the deciduous trees,
but we do not find exclusively coniferous forests. Palms, though not
extending into Greenland, flourished magnificently far to the north of
their present range. On the other hand, the Paleocene flora of England
points to a merely temperate climate, while that of the succeeding Eocene
was subtropical.
_South America._—Nothing is definitely known concerning the condition of
Central America and the West Indies and very little as to South America.
As no marine rocks of Paleocene date have been found in any of these
regions, it may be inferred that all the existing land areas were then
above the sea, and there is some evidence that South America was much
more extended in certain directions than now. From the character and
distribution of modern plants, fresh-water fishes, land and fresh-water
shells, there is strong reason to believe that in late Mesozoic times a
land-bridge connected Brazil with equatorial Africa and this connection
may have continued into the Paleocene, though it is only fair to observe
that some highly competent authorities deny the reality of this bridge.
There is also evidence, though incomplete, of a connection between South
America and Australia by way of the Antarctic continent, and it is
clear that that polar region could not have had the rigorous climate of
the present time. In the upper part of the Cretaceous, the last of the
Mesozoic periods, there was a possibility of migration, however indirect,
between every continent and every other, for the huge land reptiles
called Dinosaurs have been found in the non-marine Cretaceous rocks of
every continent, which could not have been the case, had any of the great
land areas been isolated. There is no known reason to assume that the
land-bridges were essentially different in the Paleocene.
2. _Eocene Epoch_
_North America._—The Eocene witnessed quite extensive geographical
changes, though but little is known of it in Central or South America, or
the West Indies. Along the Atlantic and Gulf coasts of the United States
there was an extensive submergence of the coastal plain, the sea covering
the southern half of New Jersey and extending thence to the southwestward
in an ever broadening band, through the South Atlantic and Gulf states.
Northern Florida was under water and the Gulf extended as a narrow sound,
known as the “Mississippi Embayment,” up the valley of that river to
southern Illinois and westward into Texas. The Embayment was present
in the Cretaceous and again in the Eocene, but it is not known whether
it persisted through the Paleocene; probably it did not, as the whole
Atlantic coast region appears to have stood at a higher level then than
now. While the condition of Mexico and Central America during the Eocene
is not known in any save the vaguest manner, it is evident that there
was then a broad communication between the Atlantic and the Pacific,
completely severing North and South America, though the place of this
transverse sea has not been fixed. On the Pacific side, a long, narrow
arm of the sea occupied what is now the great valley of California,
extending north into Oregon and Washington. It will be noted that in
North America the Eocene sea was almost confined to the neighbourhood of
the present coast-lines, nowhere penetrating very far inland, except in
the Mississippi Embayment, and thus differing widely from the condition
of Europe at that epoch, where much of what is now land was submerged.
The greatly expanded Mediterranean covered most of southern Europe, where
the great mountain ranges, the Pyrenees, Alps, etc., had not yet been
formed. Very important, from the point of view of American geography,
is the fact that Europe was completely separated from Asia by a narrow
strait or sea, which ran down the eastern side of the Ural Mountains
from the Arctic Ocean and joined the enlarged Mediterranean. During the
existence of this Ural Sea any land connection of North America with
Europe must necessarily have been by means of a North Atlantic bridge,
or by one across the Arctic Sea, since communication with Asia by way of
Alaska would not have reached eastern Europe.
[Illustration: FIG. 48.—Map of North America during the Eocene epoch.
The present limits of the continent are shown in outline; white areas =
land; horizontal lines = sea; dotted areas = non-marine deposits; black
circles with white dots = active volcanoes. (After Schuchert.)]
Any such general statement of geographical conditions during the Eocene
as the foregoing sketch, cannot but be to some extent misleading, because
it brings together, as contemporary, arrangements which were, in some
cases at least, separated by considerable intervals of time and which
were subject to continual change. Along nearly all coasts the position of
the sea was quite different in the latter part of the epoch from what it
had been in the earlier portion. On the north side of the Gulf of Mexico,
for example, the sea retreated from time to time, and the successive
divisions of the Eocene rocks are so arranged that the later ones are
farther to the south. Limitations of space, however, forbid the attempt
to follow out these minor changes.
[Illustration: FIG. 49.—Bad Lands of the lower Eocene. Wasatch stage.
Big Horn Basin, Wyo. (Photograph by Sinclair.)]
In the western interior are found extensive non-marine or continental
deposits of Eocene date, which must be considered more in detail, because
of the highly important bearing which they have upon mammalian history.
With the exception of a few small areas in Colorado, these deposits are
all situated in the plateau region west of the Rocky Mountains, and were
made of the débris of older rocks washed down by rain and rivers and
deposited in broad basins. Some of them are the sediments of shallow or
temporary lakes, and one series, at least, is made up of volcanic ash and
dust showered upon the land, or into water of no great depth. The oldest
of these Eocene stages, known as the _Wasatch_ (see Table, p. 17) covers
a very large region, though in a discontinuous manner; the principal area
begins in New Mexico, where it lies over the Torrejon, of the Paleocene,
and extends far to the north through western Colorado and eastern Utah
to the Uinta Mountains, around the eastern end of which it passes in a
narrow band and then expands again over southwestern Wyoming. A second
area is in the Big Horn Basin of northwestern Wyoming and southern
Montana, and probably two small areas in southern Colorado are of the
same date. The Wasatch beds are richly fossiliferous and have yielded a
most interesting and important series of mammals, which were far more
advanced than those of the Paleocene; and, at first sight, the student
is tempted to believe that they must be of very much later date. A more
critical examination shows that this appearance of a great lapse of time
between the Paleocene and the Wasatch is deceptive; the more advanced and
characteristic of the Wasatch mammals were obviously not the descendants
of ancestors in the North American Paleocene, but were altogether
newcomers to this continent, immigrants from some region which cannot
yet be identified. On the other hand, a considerable number of the old,
indigenous types still persisted, and these, when compared with their
Paleocene ancestors, are found not to have changed so much as to require
a very great length of time, geologically speaking, for the degree of
development involved. This is the earliest recorded one of the great
waves of mammalian migration which invaded North America down almost to
our own time.
The same wave of migration extended to Europe, and that there was a broad
and easy way of communication between that continent and North America
is plain, for the similarity between the Wasatch mammals and those of
the corresponding formation in France, the _Sparnacian_, is remarkably
close. At no subsequent time were the mammalian faunas of North America
and Europe so nearly identical as during the Wasatch-Sparnacian age,
which is especially remarkable when the discrepancy is noted between the
vast stretches of the Wasatch (150,000 square miles) and the very limited
areas in France.
If, as is probable, the Ural Sea was in existence at that time, the
land-connection with Europe must have been across the North Atlantic,
most likely from Greenland eastward. At the present time a land-bridge
in such high latitudes would be of little service in bringing about a
similarity of mammals in the two continents, for the severity of the
Arctic climate would be as effective a barrier against the intermigration
of all save the Arctic mammals as the ocean itself; but in the mild and
genial Eocene climate the latitude of the bridge was of small consequence.
The second of the Eocene stages, the _Wind River—Green River_, is found
in two very different phases. The Wind River phase occupies the basin
of that stream, north of the Wind River Mountains in central Wyoming,
and in the Big Horn Basin of the same state it very extensively overlies
the Wasatch, and in this phase the sediments are very like those of the
latter, flood-plain and wind accumulations. A widely distant area of
this stage occurs in the Huerfano Cañon in Colorado. The Wind River beds
contain numerous mammals which were clearly sequential to those of the
Wasatch, of which they were the more or less modified descendants. With
two possible exceptions, there were no new immigrants and the connection
with the Old World may have been already severed, as it assuredly was in
the succeeding age, the Bridger, though divergent development had not yet
had time to produce the very striking differences in the mammals of North
America from those of Europe, which characterized the Bridger.
The Green River phase is a thick body of finely laminated “paper shales,”
which seem to have been deposited in a very shallow lake and occupy
some 5000 square miles of the Green River valley in southern Wyoming
and northern Utah, where they overlie the Wasatch, just as do the Wind
River beds in the Big Horn Basin. These fine-grained and thinly laminated
shales have preserved, often in beautiful perfection, countless remains
of plants, insects and fishes, but no traces of mammals, other than
footprints, have been found.
The third of the Eocene stages of the interior is the _Bridger_ of
southern Wyoming and northeastern Utah, where it lies upon the Green
River shales, but overlaps these shales both eastward and westward,
extending out upon the Wasatch. The Bridger beds are largely made up
of volcanic ash and dust deposited partly upon the land and partly in
shallow or temporary lakes. The frequency with which the remains of
fishes, crocodiles and fresh-water shells are found indicates deposition
in water, and the large crystals of gypsum which are abundant in certain
localities show that the water became salt, at least occasionally. From
the immense mass of volcanic débris, it is evident that volcanic activity
broke out at this time on a much greater scale than had been known in
that region since the Cretaceous period. Two different horizons, or
substages, are distinguishable in the Bridger, lower and upper, each of
which has its distinct mammalian fauna, though the two are very closely
allied. Their difference from the contemporary mammals of Europe is very
great, hardly any genera being common to the two continents. So striking
a difference indubitably points to a severance of the land-connection,
a severance which, as was shown above, probably took place during the
Wind River stage, for its effects would not be immediately apparent; time
would be required for the operation of divergent evolution, the fauna of
each continent developing along its own lines, to make itself so strongly
felt. Had the connection never been renewed, North America, on the one
hand, and Eurasia on the other, would to-day be utterly different from
the zoölogical point of view, instead of containing, as they do, a great
many identical or closely similar animals of all classes, a likeness due
to subsequent migrations.
The fourth and last of the stages referred to the Eocene is the _Uinta_,
the geological position of which is the subject of much debate; almost
as good reasons can be brought forward for placing it in the Oligocene
as in the Eocene, so nearly is it on the boundary line between those two
epochs. The Uinta is found in the Green River valley of northeastern
Utah and northwestern Colorado, where it lies upon the upper Bridger
and is the latest of the important Tertiary formations to be found in
the plateau region west of the Rocky Mountains. It is probable that the
separation of North America from the Old World still continued, for, as a
whole, the Uinta fauna is totally different from that of the upper Eocene
of Europe. There were, however, a few doubtful forms, which may prove to
be the outposts of a renewed invasion.
The Eocene climate was decidedly warmer than the present one, and
subtropical conditions extended over the whole United States and perhaps
far into Canada. On the other hand, signs of increasing aridity in
the western part of the continent are not wanting, and that must have
resulted in a great shrinkage of the forests and increase of the open
plains. The vegetation was essentially the same as in the Paleocene, when
it had already attained a modern character, the differences from the
present being chiefly in regard to geographical distribution. Large palms
were then flourishing in Wyoming and Idaho, and another indication of a
warm climate is furnished by the large crocodiles which abounded in all
of the Eocene stages.
So far as North America was concerned, the Eocene epoch was brought to
a close by extensive movements of the earth’s crust, which more or less
affected the entire continent and were registered both on the sea-coasts
and in the mountain ranges of the interior. Upheaval added a narrow belt
of land along the Atlantic and Gulf coasts and the Mississippi Embayment
was nearly closed. On the Pacific side the sea withdrew from the great
valley of California and Oregon, and in the interior the plateau region
was elevated by a great disturbance, which also increased the height of
the western mountains.
Our knowledge of Eocene land-mammals in North America is almost wholly
derived from the formations of the western United States, but it may
be inferred from the uniform climatic conditions that there were no
very great geographical differences among the animals. This inference
is confirmed by the discovery of a Bridger genus, very fragmentary but
identifiable, in the marine Eocene of New Jersey.
_South America._—No Eocene rocks, marine or continental, are known in
the West Indies or Central America, but the latter region has been so
imperfectly explored that no great importance can be attached to this
fact. North and South America were separated completely, as is proved by
the entire dissimilarity of their mammalian faunas, but the position of
the transverse sea or strait cannot be determined. There is much reason
to believe that the Greater Antilles were connected into a single large
land, which has been called “Antillia” and may have been joined to the
mainland of Central America. Certain marine rocks in Patagonia and Chili
have been referred to the Eocene by South American geologists, but the
reference is almost certainly erroneous, the rocks in question being
much more probably Miocene. The Andes, probably throughout their length
and certainly in their southern half, stood at a much lower level than
they do now, and, no doubt, were rising, either slowly and steadily, or
periodically and more rapidly, throughout the whole Tertiary period. At
all events, their present height in the south is due to movements in
the Pliocene or later. Continental deposits of Eocene date have been
discovered only in northern Patagonia (Casa Mayor) where they occupy
depressions in the worn and eroded surfaces of the Cretaceous rocks; the
mode of their formation has not been carefully studied.
There is great uncertainty as to the status of the land-bridge which,
it is believed, in the Cretaceous period connected South America with
Africa. Some of the evidence goes to show that the connection persisted
throughout the Eocene epoch, but the testimony is that of fragmentary
and therefore imperfectly understood fossils and is far from being
unequivocal. The connection with Antarctica probably continued.
3. _Oligocene Epoch_
_North America._—The Oligocene, or third of the Tertiary epochs, was a
time of great significance in the history of the American mammals and
of great geographical changes in the West Indian and Central American
regions, but in North America proper the changes were not so widespread.
On the Atlantic coast the marine Oligocene is but scantily displayed
except in the Florida peninsula, where it is found in a thickness of some
2000 feet, but it is well developed along the north shore of the Gulf of
Mexico, where the coast-line followed that of the Eocene, only a little
farther to the south, marking the retreat of the sea at the end of the
Eocene. The Gulf Stream entered the Atlantic over the site of northern
Florida and flowed northward nearer the coast than it does to-day, in
consequence of which warm-water conditions extended far to the north and
West Indian shells flourished on the New Jersey coast. In the middle
Oligocene part of northern Florida was elevated into an island and the
water over much of the remainder of the peninsula became shallower, but
this did not greatly alter the course of the Gulf Stream. The Pacific
encroached upon the western shore of Oregon and British Columbia and very
extensively upon that of Alaska, where strata no less than 10,000 feet
thick are assigned to this epoch.
In the western interior Oligocene formations are among the most important
and widely spread of the continental Tertiaries and are divisible into
two principal stages and each of these again into three substages.
Of these, the older or _White River_ stage covers a vast region in
northeastern Colorado, western Nebraska, eastern Wyoming and southern
South Dakota, with separate areas in the Black Hills, North Dakota and
the Northwest Territory of Canada. The deposits are believed to be
chiefly of fluviatile origin, and many of the ancient stream-channels,
some of great size, may still be traced, filled with the consolidated
sands and gravels of the old rivers. The country was very flat and the
divides between the streams very low, so that in seasons of flood great
regions were converted into shallow, temporary lakes, in which were
deposited the finer silt and mud, but were dry for most of the year. The
volcanic activity which had gone on so impressively in the Bridger Eocene
was renewed in White River times, as is proved by thick beds of pure
volcanic ash, which must have been carried long distances by the wind,
for they occur far from any volcanic vent.
[Illustration: FIG. 50.—Map of North America in the upper Oligocene.
Explanation as in Fig. 48. (After Schuchert.)]
The White River fauna is more completely known than that of any other
Tertiary formation of this continent. The first discovery of these
fossils was made more than 70 years ago and since then oft-repeated
expeditions have brought to light an astonishing number and variety of
mammals. Not only are these beds remarkable for the immense quantity
of material which they have yielded, but also for its completeness and
beauty of preservation, a most unusual number of skeletons having been
obtained. The mammals demonstrate that the land-connection with the Old
World had been re-established, for many European genera, which could not
have been derived from an American ancestry, are found in the White River
beds. At the same time, there was no such proportion of forms common to
both continents as there had been in the Wasatch-Sparnacian stage of
the lower Eocene, each having many genera and even families which did
not extend their range into the other. The reason for this remarkable
and, at first sight, inexplicable difference between the lower Eocene
and the lower Oligocene is probably to be found in climatic changes, in
consequence of which relatively fewer genera were able to take advantage
of the reopened connection, which lay far to the north. The White River
mammals, like those of the Recent epoch, are thus divisible into two
groups or elements, one set indigenous and descended from ancestors
which are found in the American Eocene, and the other composed of late
immigrants from the Old World. Migrants from North America likewise made
their way to Europe.
The upper continental Oligocene of the interior has received the peculiar
appellation of the _John Day_, from the river of that name in eastern
Oregon, a large part of which was buried to a depth of 3000 or 4000 feet
in stratified volcanic ash and tuff. This great mass of finely divided
volcanic material was derived from the craters of the Cascade Mountains
to the westward; a long-continued series of eruptions would be needed
to form such thick accumulations at such a distance from the sources of
supply. The John Day evidently succeeded the White River very closely
in time, but is marked by the disappearance of almost all the European
migrants. This fact, together with the absence of any new immigrant
genera, is evidence that the connection had again been broken and it was
not renewed until after a considerable lapse of time.
There are many reasons for believing that the Oligocene climate marked
the beginning of the very long and gradual process of refrigeration
which culminated in the glacial conditions of the Pleistocene epoch, but
the change was slight and probably chiefly affected the far north. The
climate, however, remained notably warmer than the present one of the
same extra-tropical latitudes, as is abundantly proved by the fossils.
The Atlantic coast, as noted above, was bathed in warm waters, the plants
of the Alaskan Oligocene point to temperate conditions and the vegetation
of Europe was subtropical, palms growing in the north of Germany. The
change which was distinctly to be noted in the Great Plains region of
North America was probably due rather to the elevation and increased
altitude of the western interior than to general climatic alteration.
Crocodiles are very rare indeed in the White River beds and those that
have been found all belong to dwarf species, while none are known from
the John Day. Unfortunately, hardly anything has been ascertained
concerning the Oligocene vegetation of the region, but the reptiles
indicate diminished warmth.
_South America._—Marine Oligocene strata have great extent around the
Gulf of Mexico and the Caribbean Sea, and the distribution of these shows
that Antillia was broken up by great submergences, the islands of the
Greater Antilles being much smaller than they are to-day. The greater
part of Central America and the Isthmus were under water, a broad sea,
broken only by scattered islands, separating North and South America.
Very little is known of the Oligocene in the latter continent save a
non-marine formation in northern Patagonia, the _Deseado_ stage (or
Pyrotherium Beds), which, like the Eocene of the same region, occupies
depressions in the worn and irregular surface of the Cretaceous rocks.
The attribution of the Deseado to the Oligocene is open to some doubt,
because of the entire absence in its mammalian fauna of any elements
which are also found in the northern hemisphere. Hence, there are no
means of direct comparison.
4. _Miocene Epoch_
_North America._—The Atlantic and Gulf coasts, which had been raised
in the Oligocene, were again depressed, almost restoring the Eocene
coast-line, the chief differences being the presence of the Florida
islands and the nearly complete closing of the Mississippi Embayment.
There was a remarkable change in the marine fauna from that of Oligocene
times; a cool current flowed southward along the coast and entered the
Gulf of Mexico through the strait between the Florida island and the
mainland, bringing the northern animals with it and driving out the
tropical forms. This complete faunal change, which might fairly be called
a revolution, was the most sudden and striking in the Tertiary history of
the continent.
On the Pacific coast also there was a depression, which caused a renewed
transgression of the sea. The Coast Range formed a chain of reefs and
islands in the Miocene sea, which again filled the great valley of
California, except in the northern part of what is now the Sacramento
Valley, where there was an accumulation of continental deposits. The
immense thickness (5000 to 7000 feet) of the California Miocene is
largely made up of volcanic material, which testifies to the great
activity of the vents. In the Sierras, the height of which was increased
in the upper Miocene, there was also a great display of vulcanism,
recorded in the lava-flows and tuffs of the time. In the region of
Lower California and northwestern Mexico considerable changes of the
coast-line took place during the Miocene; in the earlier half of the
epoch the Gulf of California was much shorter and narrower than it is
to-day and the peninsula was broadly united with the mainland to the east
as well as to the north. A wide submergence marked the upper Miocene,
reducing the peninsula to a long, narrow island and enlarging the gulf
considerably beyond its present limits, flooding an extensive area in
northwestern Mexico and sending a small bay into southeastern California.
There were great disturbances in the course of the epoch, for in the
Santa Cruz Mountains near San Francisco the lower Miocene strata were
crumpled into folds, before those of the upper Miocene were deposited
upon them. British Columbia, Washington and Oregon were invaded by the
sea, which extended up the valley of the Columbia River and its southern
tributary, the Willamette, though here the beds are far thinner than
those of California. Much of Alaska, both on the north and west coasts
and in the valley of the Yukon, was submerged, and the land-connection
with Asia appears to have been broken. This is made probable not only
by the submergence of the Alaskan coast, but also by the fact that the
marine animals of the California coasts and shoal waters, which could
not migrate across the ocean, were quite unlike the contemporary forms
of the eastern Asiatic shore, which would hardly have been the case, had
a continuous coast-line united the two continents. On the other hand,
there was a renewed connection with Europe, as is shown by the appearance
of Old World land-mammals, beginning scantily in the lower and becoming
numerous in the middle Miocene. This connection, it will be remembered,
had been interrupted during the upper Oligocene. Many students of the
problem have maintained that the land-bridge was by way of the West
Indies and the Mediterranean lands, but such a bridge would not account
for the facts of mammalian distribution, which would seem to require its
location in the far north.
[Illustration: FIG. 51.—Map of North America in the upper Miocene.
Explanation as in Fig. 48. (Modified from Schuchert.)]
Several distinct lines of evidence go to prove that the junction of
the Americas dates from the Miocene, possibly from the beginning of
it. The absence of Atlantic species from the Pacific Miocene is an
indication that the passage from ocean to ocean had been closed, and
this is confirmed by the geology of the Central American and Isthmian
region. In the middle Miocene of Oregon and Nebraska have been found
remains, which are unfortunately too incomplete for altogether convincing
identification, but which can be interpreted only as belonging to the
extinct and most characteristically South American group of edentates,
the †ground-sloths or †Gravigrada; if this reference is correct, the fact
of the junction cannot be questioned.
Continental deposits of Miocene date, chiefly accumulations made by
rivers and the wind, cover vast areas of the western interior, though but
rarely to any considerable depth. These have been divided into several
stages and have received various names; the lower Miocene, known as the
_Arikaree_, _Harrison_ or _Rosebud_, overlies the White River in South
Dakota, western Nebraska and eastern Wyoming, with smaller areas in
Montana and Colorado. In the deposits of this stage there are no mammals
of indisputably Old World type, though a few which I consider to be such
are a probable indication of renewed connection with Europe. The middle
Miocene is found typically in central Montana, where it is called the
_Deep River_ (or _Smith River_) stage, but occurs also in numerous small,
scattered and widely separated areas in Oregon, Wyoming, Colorado and
Texas, with local names in these different states. It is most likely
that these middle Miocene formations are not strictly contemporaneous in
the geological sense, but rather form a closely connected and successive
series. The mammals of the Deep River stage leave no doubt that the way
of migration from the Old World was again open.
The _Loup Fork_, or upper Miocene, itself susceptible of further
subdivision, is by far the most extensive of the Miocene formations and
covers much of the Great Plains region, in separate areas, from South
Dakota far into Mexico. Perhaps also referable to the upper Miocene is
a small, but very interesting formation, the _Florissant_, which is in
the South Park of Colorado; it was made by very fine volcanic material
showered into a small and shallow lake. The finely laminated papery
shales of the Florissant have preserved countless plants and insects
and many fishes, and these throw very welcome light upon the vegetation
and climatic conditions of the epoch and afford an interesting contrast
to the fauna and flora of the Green River shales of the lower or middle
Eocene. That the Florissant shales are Miocene, no one questions, but
their isolated position and the fact that they have yielded no mammals
make it somewhat doubtful whether they belong in the middle or later part
of the epoch.
In the western portion of the continent vulcanism was displayed on a
grand scale during the Miocene. Mention has already been made of the
quantity of volcanic material in the marine Miocene of California and
also in the lavas and tuffs of the Sierras. The magnificent cones,
such as Mts. Hood and Tacoma, which are the glory of the Cascades,
are believed to date from this time. In Idaho and eastern Oregon and
Washington are the immense lava-fields of the Columbia River, which are,
partly at least, of Miocene date and were chiefly extruded through great
fissures, the lava flooding the valleys and plains in a fiery sea of
molten rock. In Oregon these lavas rest upon the upper Oligocene (John
Day stage) and middle Miocene beds are deposited upon them, which fixes
their date sufficiently. In the Yellowstone Park was piled up a huge mass
of volcanic products, lava-flows and beds of ash and tuff, to a thickness
of several thousand feet. The ash-beds have preserved the petrified
forests, with their tree-trunks still standing one above another; one
locality in the Park shows seven such forests, each one killed and buried
by a great discharge of ash and then a new forest established and growing
upon the surface of the accumulation. In the tuffs are leaf-impressions
which permit identification of the plants.
In the latter part of the Miocene and at its close there were important
crustal movements, which affected all the Pacific coast mountain ranges,
though this epoch was no such time of mountain making in America as it
was in the Old World. The principal elevation of the Coast Range in
California and Oregon was due to these movements, and the Sierras and the
plateaus of Utah and Arizona were increased in height. On the Atlantic
side the Florida island was joined to the mainland and thus the present
shape of the continent was almost exactly gained.
The Miocene climate of North America, as indicated by the plants of
Florissant, the Yellowstone Park and Oregon, was distinctly milder than
at present, a southern vegetation of warm-temperate character extending
to Montana and perhaps much farther north, but it was not so warm as it
had been in the Eocene, and palms are not found in any of the localities
mentioned, nor do crocodiles occur in any of the northern Miocene
formations. In Europe the climate of the early Miocene was considerably
warmer than in North America, the vegetation of central and western
Europe being very much like that of modern India. This difference between
the two sides of the Atlantic was probably due, in large part, to the
manner in which Europe was broken and intersected by arms and gulfs of
the warm southern sea. In the latter half of the epoch, however, the
climate became colder, the subtropical flora giving way to a distinctly
temperate one.
_South America._—In Central America, where marine Oligocene beds are
of great extent, no Miocene is known, and on the Isthmus Oligocene is
the latest marine formation, except a narrow fringe of Pleistocene
on the Caribbean coast. These facts and others already cited lead to
the conclusion that in the Miocene the connection of the Americas was
complete and that the Isthmus was considerably broader than at present,
extending nearly to Jamaica. The condition of the Greater Antilles is but
vaguely understood, but they were involved in the general elevation of
the Caribbean region and were at least as large as they are now and may
have been considerably larger, and Cuba was perhaps joined to Central
America, as Hayti probably was.
In South America proper nearly the whole of Patagonia was submerged
by the transgression of a shallow, epicontinental sea, in which were
accumulated the beds of the _Patagonian_ stage, containing an exceedingly
rich and varied assemblage of marine fossils, an assemblage which has
very little in common with the contemporary formations of the northern
hemisphere. It is this lack of elements common to the northern faunas
which has led to the long debate concerning the geological date of the
Patagonian formation, the South American geologists very generally
referring it to the Eocene. However, the occurrence of genera of
Cetaceans (whales and dolphins), which are also found in the Miocene
of Maryland and Virginia, is very strong evidence that the proper date
of the Patagonian is Miocene. A continuous coast-line, or at least an
unbroken continuity of shoal-water conditions, seems necessary to account
for the similarity of the Patagonian fossils with those of New Zealand
and Australia, and that this connection was by way of the Antarctic
continent is indicated by the occurrence of similar fossils in the South
Shetland Islands, an Antarctic group. On the Chilian coast the _Navidad_
formation, which is believed to be approximately contemporaneous with
the Patagonian, has so different a fauna as to point to some kind of a
barrier between the Atlantic and the Pacific, and this barrier, Dr. von
Ihering holds, was the land-extension from South America to Antarctica.
After some oscillations of retreat and advance, the sea withdrew from
Patagonia and the terrestrial accumulations of the _Santa Cruz_ stage
were formed. These beds are partly composed of river-deposits, but
chiefly of more or less consolidated volcanic ash or tuff, and have
yielded a surprising number of beautifully preserved mammals. No other
assemblage of South American Tertiary Mammalia is so well known and
understood as the Santa Cruz fauna, and the very large number of all but
complete skeletons which have been found strongly suggests that many of
the animals were buried alive in the showers of volcanic ash. The Santa
Cruz fauna is completely and radically different from any of the North
American assemblages, and at that time no immigrant from the north had
penetrated so far as Patagonia.
In the upper Miocene the Andes stood at a much lower level than they
do now; fossil plants, some of them collected at a great height in the
mountains, are the remains of a luxuriant and purely tropical flora
nearly identical with the vegetation of the modern forests of Bolivia
and Brazil. Such a vegetation could not exist at the altitudes where the
fossils occur and these demonstrate a great elevation of the mountains
since those leaves were embedded. The same mild climatic conditions which
prevailed in the northern hemisphere during the Miocene must also have
characterized Patagonia, subtropical shells extending far to the south of
their present range.
Whatever may have been true of the land-bridge connecting South America
with Africa during the early Tertiary epochs, it must have been
submerged in the Miocene, otherwise there would not have been the open
pathway for the Cetacea of Patagonia to reach the Atlantic coast of North
America and _vice versa_.
5. _Pliocene Epoch_
_North America._—The Pliocene of North America is not nearly so well
displayed or so satisfactorily known as the preceding Tertiary epochs,
and only of comparatively late years has it been recognized at all upon
the Atlantic coast. The Atlantic and Gulf shores had very nearly their
present outlines, but with some notable differences. It would seem that
the northeastern portion of the continent stood at a higher level than
it does now, north Greenland being joined with the islands of the Arctic
archipelago and Newfoundland with Labrador, the Gulf of St. Lawrence
then being land. From Nova Scotia to southern New Jersey the coast-line
was many miles to the east and south of its present position, but the
sea encroached here and there upon the shores of Virginia, the Carolinas
and Georgia, and southern Florida was mostly under water, as was also a
narrow strip of the Gulf coast from Florida to Texas and along the east
of Mexico. On the Pacific side of the continent the marine Pliocene is
far thicker and more important than on the east coast and in California
is largely made up of volcanic materials. Quite extensive disturbances
in this region had marked the close of the Miocene, the strata of
which in the Coast Range had been violently compressed and folded. An
elevation of the land had caused the sea to withdraw from the central
valley of California and had restored Lower California to its peninsular
conditions, reducing the gulf to the narrow limits which it had had in
the lower Miocene and extending southern Mexico to the west and south.
British Columbia and southeastern Alaska stood at higher than their
present levels and the countless islands of that region were part of
the mainland. Bering Strait was closed, for at least a great part of
the epoch, and, as a continuous shore-line was thus formed and a way of
migration opened, the marine fauna of California and Japan became closely
similar.
[Illustration: FIG. 52.—Map of North America during the Pliocene epoch,
Bering Strait open. Explanation as in Fig. 48. (Modified from Schuchert.)]
In the interior, the Pliocene continental formations and faunas followed
so gradually upon those of the Miocene, that there is great doubt as to
where the line between them should be drawn. These interior formations
are mostly of small extent and are very widely scattered, and much
remains to be learned regarding the mammals of the epoch. In northern
Kansas are the _Republican River_ beds, which are so doubtfully Pliocene,
that they may almost equally well be called uppermost Miocene. Other
lower Pliocene stages, representing various divisions of time, are the
_Alachua_ of northern Florida, the _Snake Creek_ of western Nebraska,
the _Thousand Creek_ and _Virgin Valley_ of northwestern Nevada and the
_Rattlesnake_ of Oregon. Probably middle Pliocene is the _Blanco_ of
northwestern Texas, a valley cut in the middle and lower Miocene rocks
and filled in with Pliocene deposits. Possibly upper Pliocene, or, it may
be, lowest Pleistocene, are the _Peace Creek_ of southwestern Florida and
the so-called “_Loup River_” (not Loup Fork) of western Nebraska.
The volcanic activity of the Rocky Mountain and Pacific coast regions,
which was so remarkable in the Miocene, continued into and perhaps
through the Pliocene. The great outflow of light-coloured lava which
built up the central plateau of the Yellowstone Park is referred to the
Pliocene, and some of the enormous fissure-eruptions which formed the
vast Columbia River fields of black basaltic lava were probably Pliocene,
as some were demonstrably Miocene. Both of these epochs were remarkable
for volcanic activity in the western part of the continent.
The Pliocene climate, as may be inferred from the plants and marine
shells, was colder than that of the Miocene, and refrigeration was
progressive, as is shown by the proportion of Arctic shells in the
Pliocene beds of the east coast of England, rising from 5 per cent in
the oldest to more than 60 per cent in the latest beds. In the Arctic
regions the cold must have been severe, at least during the latter half
of the epoch, for in the succeeding Pleistocene we find an Arctic fauna
already fully adapted to the extreme severity of present day polar
conditions and time was necessary for such an adaptation. In the western
interior the climate was not only colder, but also drier than it had
been in the Miocene, the desiccation which had begun in the latter epoch
becoming progressively more and more marked.
_South America._—The Greater Antilles were larger than at present and
Cuba was much extended, especially to the southeastward, and was probably
connected with the mainland, not as one would naturally expect, with
Yucatan, but with Central America; this island, it is most likely, was
cut off from Hayti. The Isthmian region was considerably broader than
it is now and afforded a more convenient highway of intermigration.
Costa Rica was invaded by a Pliocene gulf, but it is not yet clear
whether this persisted for the whole or only a part of the epoch. In the
Argentine province of Entrerios is a formation, the _Paraná_, which is
most probably Pliocene, though it may be upper Miocene. This formation
is largely marine and shows that the present Rio de la Plata was then a
gulf from the Atlantic. A few northern hemisphere mammals in the Paraná
beds show that the migration had advanced far into South America. A large
part of Patagonia was again submerged beneath the sea, which extended
to the Andes in places, but just how general the submergence was, it
is impossible to say, for the _Cape Fairweather_ formation has been
largely carried away by erosion and only fragments of it remain. Along
the foothills of the Andes these beds are upturned and raised several
thousand feet above the sea-level, a proof that the final upheaval of
the southern mountains took place at some time later than the early
Pliocene. Continental formations of Pliocene date are largely developed
in Argentina; the _Araucanian_ stage is in two substages, one in the
province of Catamarca, where the beds are much indurated and were
involved in the Andean uplift, the other, of unconsolidated materials, is
at Monte Hermoso near Bahia Blanca on the Atlantic coast. The very small
proportion of northern animals in the Araucanian beds is surprising, but
not more so than the almost complete absence of South American types in
the upper Miocene and lower Pliocene of the United States. Intermigration
between the two Americas would seem to have been a much slower and more
difficult process than between North America and the Old World, and the
reason for the difference is probably the greater climatic barriers
involved in a migration along the lines of longitude. Upper Pliocene is
found in the Tarija Valley of Bolivia and probably also in Ecuador, in
both of which areas the proportion of northern animals was very greatly
increased.
II. QUATERNARY PERIOD
The Quaternary period was a time of remarkable geographical and climatic
changes, which had the profoundest and most far-reaching effects,
partly by migration and partly by extinction, upon the distribution of
animals and plants, effects which are naturally more obvious than those
of earlier geological events, just because they were the latest. It is
customary to divide the period into two epochs, (1) the _Pleistocene_ or
_Glacial_, and (2) the _Recent_, which continues to the present day.
1. _Pleistocene Epoch_
When Louis Agassiz first suggested (1840) the idea of a time,
comparatively recent in the geological sense, when northern and central
Europe was buried under immense sheets of slowly moving ice, like
the “ice-cap” of modern Greenland, the conception was received with
incredulity. Nearly thirty years passed before this startling theory
gained the general acceptance of geologists, but now it is one of the
commonplaces of the science, for no other hypothesis so well explains
the complicated phenomena of Pleistocene geology. One great obstacle
to the acceptance of the glacial theory was the supposed fact that the
Pleistocene glaciation was something quite unique in the history of
the earth, a violent aberration in the development of climates. Now,
however, we have every reason to believe that at least three other and
very ancient periods had witnessed similar climatic changes and that
“ice-ages” were recurrent phenomena. This is not the place to discuss or
even to summarize the evidence which has convinced nearly all geologists
of the reality of Pleistocene glacial conditions on a vast scale in Asia,
Europe and, above all, in North America. The reader who may wish to
examine this evidence will find an admirable presentation of it in Vol.
III of the “Geology” of Professors Chamberlin and Salisbury.
_North America._—There has long been a difference of opinion among
students of the Pleistocene as to whether the glaciation was single,
or several times renewed. That there were many advances and retreats
of the ice, is not denied; the question is, whether there were truly
interglacial stages, when the ice altogether disappeared from the
continent and the climate was greatly ameliorated. The present tendency
among American and European geologists is decidedly in favour of
accepting several distinct glacial stages (Chamberlin and Salisbury
admit six of these) separated by interglacial stages, and for this there
are very strong reasons. While it is out of the question to present the
evidence for this conclusion here, one or two significant facts may be
noted. On the north shore of Lake Ontario, near Toronto, are certain
water-made deposits, which rest upon one sheet of glacial drift and are
overlaid by another. The fossils of the aqueous sediments are in two
series, upper and lower, of which the older and lower contains plants and
insects indicative of a climate considerably warmer than that of the same
region to-day and corresponding to the temperature of modern Virginia. In
the upper and newer beds the fossils show the return of cold conditions,
much like those of southern Labrador, and this was followed by the
reëstablishment of the ice, as recorded in the upper sheet of drift.
Even far to the north, on the Hudson’s Bay slope, an interglacial forest
is embedded between two glacial drift-sheets. In Iowa and South Dakota
numerous mammals of temperate character occur in interglacial beds.
At the time of their greatest extension, the glaciers covered North
America down to latitude 40° N., though the great terminal moraine,
which marks the ice-front and has been traced across the continent from
Nantucket to British Columbia, describes a very sinuous line. The ice
was not a homogeneous sheet, moving southward as a whole, but flowed in
all directions away from several, probably four, centres of accumulation
and dispersal. At the same time, the western mountain ranges had a far
greater snow-supply than at present, and great glaciers flowed down all
the valleys of the Rocky Mountains as far south as New Mexico and in the
Sierras to southern California, while the Wasatch, Uinta and Cascade
ranges and those of British Columbia and Alaska were heavily glaciated,
but, strange to say, the lowlands of Alaska were free from ice. During
the periods of greatest cold the rain-belt was displaced far to the south
of its normal position, bringing a heavy precipitation to regions which
are now extremely arid. In the Great Basin were formed two very large
lakes; on the east side, rising high upon the flanks of the Wasatch
Mountains, was Lake Bonneville, the shrunken and pygmy remnant of which
is the Salt Lake of Utah, and on the west side, in Nevada, was Lake
Lahontan. Lake Bonneville, which was nearly two-thirds the size of Lake
Superior, discharged northward into the Snake River, a tributary of the
Columbia, but Lahontan had no outlet. Each of these lakes had two periods
of expansion, with a time of complete desiccation between them.
Over the Great Plains the principal Pleistocene formation is that known
as the _Sheridan_, or, from the abundance of horse-remains which are
entombed in it, the _Equus Beds_. These beds extend as a mantle of
wind-drifted and compacted dust from South Dakota to Texas and in places
contain multitudes of fossil bones; they correspond to one of the early
interglacial stages and in South Dakota pass underneath a glacial moraine.
The upheaval which came at or near the end of the Pliocene had raised the
continent, or at least its northeastern portion, to a height considerably
greater than it has at present, and this must have facilitated the
gathering of great masses of snow; but before the end of the Pleistocene
a subsidence of the same region brought about important geographical
changes. The depression, which lowered the coast at the mouth of the
Hudson about 70 feet below its present level, increased northward to 600
feet or more in the St. Lawrence Valley and allowed the sea to invade
that valley and enter Lake Ontario. From this gulf ran two long, narrow
bays, one far up the valley of the Ottawa and the other into the basin
of Lake Champlain. The raised beaches, containing marine shells and the
bones of whales, seals and walruses, give eloquent testimony of those
vanished seas. The recovery from this depression and the rise of the
continent to its present level inaugurated the Recent epoch.
When the ice had finally disappeared, it left behind it great sheets
of drift, which completely changed the surface of the country and
revolutionized the systems of drainage by filling up the old valleys,
only the largest streams being able to regain their former courses.
Hundreds of buried valleys have been disclosed by the borings for oil and
gas in the Middle West, and these, when mapped, show a system of drainage
very different from that of modern times. Innumerable lakes, large and
small, were formed in depressions and rock-basins and behind morainic
dams, the contrast between the glaciated and non-glaciated regions in
regard to the number of lakes in each being very striking.
On the west coast events were quite different; marine Pleistocene beds
in two stages are found in southern California. The upheavals late in
the Pleistocene, or at its close, were far greater than on the Atlantic
side, 4000 feet in southeastern Alaska, 200 feet on the coast of Oregon
and rising again to 3000 feet in southern California; all the western
mountain ranges and plateaus were increased in height by these movements.
The volcanoes continued to be very active, as may be seen from the
lava-sheets and streams in Alaska, all the Pacific states, Arizona and
New Mexico.
_South America._—No such vast ice-sheets were formed in the southern
hemisphere as in the northern. Patagonia was the only part of South
America to be extensively covered with ice and there traces of three
glaciations have been observed, of which the first was the greatest and
reached to the Atlantic coast, and there were great ice-masses on the
coast of southern Chili. Mountain glaciers existed throughout the length
of the Andes across the Equator to 11° N. lat., the elevation increasing
northward to the tropics. The surface of the great Argentine plain of the
Pampas between 30° and 40° S. lat. is covered with a vast mantle, largely
of wind-accumulated dust, the _Pampean_, which is the sepulchre of an
astonishing number of great and strange beasts. The Pampean formation
corresponds in a general way to the Sheridan or Equus Beds of North
America, but involves a much greater lapse of time, beginning earlier,
possibly in the late Pliocene, and apparently lasting through the entire
Pleistocene. While largely of æolian origin, the Pampean seems to be in
part made of delta deposits laid down by rivers. One striking difference
between the Pampean, on the one hand, and the Sheridan and the loess of
the Mississippi Valley and of Europe, on the other, is that the former is
in many places much more consolidated and stony, which gives it a false
appearance of antiquity. Another and very rich source of Pleistocene
mammals is found in the limestone caves of eastern Brazil, which have
yielded an incredible quantity of such material, but not in such a
remarkably perfect state of preservation as the skeletons of the Pampean.
Very little is known of the Pleistocene in the West Indies, though
probably to this date should be assigned the notable oscillations of
level which are recorded in the raised sea-terraces of Cuba and other
islands. The Windward groups were joined, at least in part, to the
continent and large extinct rodents reached Antigua, which would not be
possible under present conditions. The Isthmus of Panama was 200 feet or
more higher than it is now and correspondingly wider, but was depressed
to a lower than the present level, and finally raised to the height it
now has. Marine beds, of presumably Pleistocene date and certainly not
older, extend from the Caribbean shore to Gatun, some seven miles, and
are nowhere more than a few feet above sea-level.
The question of Pleistocene climates is a very vexed one and is far
from having received a definitive answer. Limitations of space forbid a
discussion of the problem here and I shall therefore merely state the
conclusions which seem best supported by the evidence so far available.
Such immense accumulations of ice might be due either to greatly
increased snow-fall, or to a general lowering of the temperature. The
balance of testimony is in favour of the latter factor and no great
refrigeration is required. Professor Penck has calculated that a
reduction of 6° or 7° in the average yearly temperature would restore
glacial conditions in Europe. Even the tropics were affected by the
change, as is shown not only by the glaciation of the Andes, but also by
Mt. Kenya, which is almost on the Equator in eastern Africa and still
has glaciers. The presumably Pleistocene ice covered the whole mountain
like a cap, descending 5400 feet below the present glacier limit. It was
pointed out above that the interglacial stages had greatly ameliorated
climatic conditions and that, in some of them at least, the climate was
warmer than it is to-day in the same localities. The cause of these
astonishing fluctuations and of the climatic changes in general, to which
Geology bears witness, still remains an altogether insoluble mystery.
CHAPTER VI
THE GEOGRAPHICAL DISTRIBUTION OF MAMMALS
To every one who has paid the slightest attention to the subject, it is a
familiar fact that different parts of the earth have different animals;
school-children learn from their geographies that kangaroos are found
in Australia, the Hippopotamus in Africa, the Tiger in southern Asia,
armadillos and llamas in South America. These examples are all taken from
distant lands, yet the zoölogical difference between two given land-areas
is by no means proportional to the distance between them. An Englishman
landing in Japan finds himself surrounded by animals and plants very
like and often identical with those which he left at home, while the
narrow Strait of Lombok, east of Java, separates two profoundly different
regions. In crossing Mexico from east to west, the traveller meets very
different animals in closely adjacent areas; and, at first sight, the
arrangement of animals appears to be so capricious as to admit of no
formulation in general laws.
In pre-Darwinian times, when it was the almost universal belief that
each species had been separately created and was exactly fitted to the
region which it inhabits, no explanation of the geographical arrangement
of animals was possible, but the acceptance of the theory of evolution
demanded that such an explanation should be found. A failure to devise
any rational and satisfactory account of the geography of animal life
would be a fatal weakness in the evolutionary theory, hence the facts of
distribution were subjected to a renewed and searching analysis as one
of the best means of critically testing the new doctrine. Not that the
subject had received no attention before the publication of Darwin’s
book; on the contrary, it had attracted much interest as a study of
facts, and this study was one of the principal avenues by which Darwin
approached his great generalization. In his autobiographical fragment
he tells us: “I had been deeply impressed by discovering in the Pampean
formation great fossil animals covered with armour like that on the
existing armadillos; secondly, by the manner in which closely allied
animals replace one another in proceeding southward over the Continent;
and third, by the South American character of most of the productions of
the Galapagos archipelago and more especially by the manner in which they
differ slightly in each island of the group.”
Obviously, before attempting to explain the facts of the geographical
distribution of mammals, we must first ascertain what those facts are.
The following brief sketch of the terms used in describing geographical
arrangement is summarized from Mr. Wallace’s “Island Life.”
Though with fluctuating boundaries and subject to slow secular changes, a
mammalian species is limited to a fairly definite area, which may be of
immense or very restricted extent, and throughout which it may be found
in greater or less abundance. Many species, however, are not distributed
continuously over the areas which they inhabit, but occur only in
suitable _stations_ adapted to their habits and mode of life. Thus, some
will be found only where there are trees, others in the neighbourhood of
water, others only on open plains, etc. A _specific area_ is then the
whole extent of country within which the species may be found, while the
_stations_ are the limited districts contained in the area which are
exactly suited to the habits of the species in question; these stations
may be hundreds of miles apart, as in the case of mountain-tops, or they
may be close together. A marsh-living species, for example, will occur
in all the marshes throughout its area, whether these be many or few,
near together or widely scattered; for such a species marshes are its
stations.
_Generic areas_ differ in character according as the genus is large,
that is, comprising many species, or small and having but few species,
or, it may be, a single one. The species, as a rule, occupy each its own
area, and the areas may be entirely distinct, or they may be contiguous
and more or less extensively overlapping, though it seldom happens that
two or more species of the same genus inhabit exactly the same area.
Often some physical feature, such as a range of high mountains, a great
river, the edge of a forest, plain or desert, exactly defines the limits
of species of the same genus. The Amazon, for example, acts as such a
boundary to many species. It was to this change of related species from
one area to another that Darwin referred in the passage quoted above,
saying that he had been deeply impressed “by the manner in which closely
allied animals replace one another in proceeding southward over the
Continent [_i.e._ South America].” On the other hand, the overlapping of
areas may be very extensive, and one species of great range may cover the
whole area of another and much more besides.
A remarkable example of the widely separated areas of species belonging
to the same genus is that of the tapirs. Of this genus there are two
or three species in Central and South America and one inhabiting the
Malay Peninsula and Borneo, almost as wide a separation as the size of
the earth permits. Discontinuous distribution of this character can be
explained in terms of the evolutionary theory only in one of two ways.
Either (1) the American and Asiatic species developed independently of
one another from different ancestors, or (2) the regions intervening
between these widely separated areas once formed a continuous land,
occupied by species of the genus which have become extinct. From all that
we know concerning the operation of the evolutionary process, the first
alternative may be set aside as altogether improbable, and, even had we
no information concerning the history of the tapirs and their former
distribution, the second explanation would be chosen as incomparably the
more likely. As a matter of fact, we have definite knowledge that tapirs
once ranged all over Europe and North America and doubtless over northern
Asia, as well, and, further, that North America was joined to Asia by a
land occupying the place of the shallow Bering Sea, at a time when the
tapirs were able to take advantage of this means of passing from one
continent to the other. Such appears to be the invariable explanation of
discontinuous distribution, though we may not always be able to give so
clear a proof of it.
The genera of a family are distributed in much the same fashion as the
species of a genus, but, as a rule, much more widely. While no genus of
terrestrial mammals is cosmopolitan (_i.e._ universally distributed),
at least as genera are defined and limited by most modern systematists,
certain families are represented in every continent. If the extremely
peculiar and isolated Australian continent be excepted, the number
of such cosmopolitan families is considerable and wide separation
between the genera is frequent. Of the camel family, for instance, one
genus, that of the true Camel (_Camelus_), is confined to the northern
hemisphere and the Old World, the other (_Lama_), comprising the Llama,
Guanaco, etc., is found only in the southern hemisphere and the New
World. Less extreme instances of the discontinuous distribution of a
family are common enough.
The principles of distribution are the same when applied to families and
orders. Most of the mammalian orders are very widely distributed and many
are cosmopolitan, except for Australia, though some are confined to one
or two continents. The monotremes are limited to Australia and Tasmania,
the marsupials to Australia and the Americas, the edentates to the
latter, the elephants and hyracoids to Africa and Asia. Carnivores and
rodents, on the contrary, are found in every continent, even Australia.
We have next to inquire what is the nature of the obstacles or barriers
that prevent the indefinite spread of terrestrial mammals, so that the
mammalian fauna of the whole earth, and even of a single continent,
is not uniform, but highly variegated. The rate of multiplication of
animals is so rapid that, under normal conditions, the animal population
is always pressing hard upon the means of subsistence and every species
that is increasing in numbers must constantly extend its range in search
of food. Every species would increase indefinitely, if there were no
countervailing checks. Were all the young to survive and breed in their
turn, “even large and slow-breeding mammals, which only have one at a
birth, but continue to breed from eight to ten successive years, may
increase from a single pair to 10,000,000 in forty years” (Wallace).
Evidently, a species must spread from its place of origin until stopped
by insuperable obstacles, the most obvious of which are wide seas. A few
land mammals are not only excellent swimmers, but will cross straits
without hesitation, as the Guanaco has been seen to swim the Straits
of Magellan; for the great majority, however, a very few miles of sea
form an impassable barrier. As was shown above, a broad or deep river is
sufficient to limit many species, as the Santa Cruz River in Patagonia
marks the southern boundary of the armadillos.
Important geographical changes, such as the joining of lands that before
were separate, or the dividing of continuous lands by transgressions
and incursions of the sea, must necessarily have a profound effect upon
the distribution of land mammals. Separated land-areas, however similar
may have been their faunas at the time of separation, will, through the
operation of the divergent evolutionary process, grow more unlike in
proportion to the length of time that the separation continues. Regions
which have been severed within a short time (in the geological sense of a
short time) are zoölogically very similar or even identical, while those
that have long been isolated are correspondingly peculiar. Attention has
already been called, in another connection, to the contrasted cases of
such great continental islands as Great Britain, Java, Sumatra, etc.,
on the one hand, and Australia, on the other. The continental islands,
which have but lately been detached from the neighbouring main lands,
are hardly more peculiar zoölogically than equal areas of the adjoining
continents, while the long-continued isolation of Australia has made it
the most peculiar region of the earth. Climatic changes, which, as we
saw in Chapter I, have indubitably taken place many times, have also had
a great effect in shifting the distribution of mammals, which in its
present form is the outcome of a very long series of geographical and
climatic changes, on the one hand, and of adaptive changes in the animals
themselves, on the other.
Of almost equal importance as a barrier is climate and especially
temperature. Not that similar climates can produce similar forms in
separate areas. Regions of almost exactly similar climate in Australia,
Africa and South America have totally different faunas, but, _within
continuous land-areas_, the most effective of barriers is temperature.
This acts differently in the case of limiting the northward spread
of southern forms and the southward spread of northern species. Dr.
Merriam’s long study of this problem has led him to the conclusion
that southern species are bounded on the north by the temperature of
the breeding season, in which the total quantity of heat must reach a
certain minimum, while “animals and plants are restricted in southward
distribution by the mean temperature of a brief period covering the
hottest part of the year.” On the Pacific coast there is a remarkable
mingling in the same areas of species which, east of the high mountains,
are distributed in sharply separated zones. This is explained by the mild
and equable climate of the coastal belt, where the hottest season of the
year does not reach the limiting maximum for the northern species, while
the total quantity of heat in the breeding season is sufficient to enable
southern species to thrive and maintain themselves.
Dr. Merriam thus sums up the effects of climatic factors upon
distribution: “Humidity and other secondary causes determine the presence
or absence of particular species in particular localities within their
appropriate zones, but temperature pre-determines the possibilities of
distribution; it fixes the limits beyond which species cannot pass.”
“Concurrently with these changes in vegetation from the south northward
occur equally marked differences in the mammals, birds, reptiles,
and insects. Among mammals the tapirs, monkeys, armadillos, nasuas,
peccaries, and opossums of Central America and Mexico are replaced to the
northward by wood-rats, marmots, chipmunks, foxes, rabbits, short-tailed
field-mice of several genera, shrews, wild-cats, lynxes, short-tailed
porcupines, elk, moose, reindeer, sables, fishers, wolverines, lemmings,
musk-oxen, and polar bears.”
Dr. J. A. Allen has reached closely similar conclusions. “Of strictly
climatic influences, temperature is by far the most important, although
moisture plays an influential part. Where a low temperature prevails
life, both animal and vegetable, is represented by comparatively few
forms; under a high temperature it is characterized by great diversity
and luxuriance. Within the Arctic Circle the species of both animals
and plants are not only few, but they are widely distributed, being for
the most part everywhere the same. Under the tropics they are a hundred
fold more numerous and of comparatively restricted distribution.” “The
influence of temperature is perhaps most strikingly displayed in the
distribution of life upon the slopes of a high mountain, especially if
situated near the tropics. While its base may be clothed with palms
and luxuriant tropical vegetation, its summit may be snow-capped
and barren.... The animal life becomes likewise correspondingly
changed, tropical forms of mammals, birds, and insects of the lower
slopes gradually giving place to such as are characteristic of arctic
latitudes.” “The effect of humidity upon plant life is thus obvious,
but it is equally potent, though less evident, upon animal life. Many
animals ... are so fitted for a forest life, as regards both food and
shelter, that their very existence depends upon such surroundings....
Thus moisture alone may determine the character of life over extensive
regions.”
While climate is thus the most important of the barriers which determine
distribution in continuous land-areas, the absence of any particular
species from a given region is no proof that the climate is unsuitable
to that species. This is sufficiently shown by the manner in which
animals introduced into a new country often run wild and multiply to an
incredible extent, as the rabbits have done in Australia, the Mongoose in
Jamaica, horses on our western plains, horses and cattle on the Pampas of
Argentina, etc.
Topographical features, such as great mountain-ranges and plateaus,
also limit many species, not only by the difficulty of crossing them,
but also by the effect which they have upon temperature and moisture.
For this reason long ranges of mountains and table-lands may carry a
northern fauna very far to the south of its ordinary range, as do the
mountain-systems of North America in a very conspicuous manner. The
great Mexican plateau is zoölogically a part of North America, while the
low coastal lands as far as southeastern Texas have Central American
affinities.
A different kind of obstacle to the spread of a species into a new
area may be the pre-occupation of that area by another species. The
pre-occupier may be one that plays so similar a part in the economy of
nature as to leave no opportunity for the newcomer to establish itself.
On the other hand, the obstructing form may be an active enemy and of
a totally different character from the intruder, as in the case of the
Tse-tse Fly in parts of Africa. The bite of the fly is fatal to horses
and oxen, so that these mammals are unable to enter the fly-infested
regions. Many times in the course of the Tertiary period various mammals
reached North America from the south or from the Old World, which
were unable to gain a permanent foothold and speedily died out. At
this distance of time it is seldom, if ever, possible to explain why a
species which succeeded in reaching this continent could not maintain
itself, though the most probable assumption is that the forms already in
possession of the land were an insuperable obstacle to the intruders.
The rate of dispersal of a species into new areas may be fast or slow,
according as the conditions are more or less favourable. Newly introduced
insect-pests, like the Gypsy and the Brown-tailed Moths in New England,
often spread with portentous rapidity; and introduced mammals have
frequently taken possession of vast areas in a surprisingly short time.
One of the most remarkable of these cases is cited by Darwin. “In the
time of Sarmiento (1580) these Indians had bows and arrows, now long
since disused; they then also possessed some horses. This is a very
curious fact, showing the extraordinarily rapid multiplication of horses
in South America. The horse was first landed at Buenos Ayres in 1537
and the colony being then for a time deserted, the horse ran wild; in
1580, only forty-three years afterwards, we hear of them at the Strait
of Magellan!” (“Voyage of a Naturalist,” pp. 232-233.) In this example,
something must be allowed for human agency, but even so, it is very
surprising.
In the case of lands newly raised above the sea and connecting formerly
separated areas, it is necessary that they should first be taken
possession of by vegetation, before they can become passable by animals,
for the migration of mammals from continent to continent is an entirely
distinct phenomenon from the annual migration of birds. The latter,
though a fact familiar to every one, is an unexplained mystery, and it is
somewhat unfortunate that the same term should be used for the completely
different process of the spread of mammals into newly opened land. This
spread is purely unconscious and is due to the pressure of increasing
numbers upon the means of subsistence, each new generation ranging
farther and farther from the original home of the species and continuing
so to extend until some insuperable obstacle is encountered. When a
sea-barrier is removed by upheaval and the newly formed land rendered
habitable for mammals through the invasion of plants, the interrupted
process is resumed and an interchange of species between the areas thus
connected is brought about. The interchange is, however, always an
incomplete one, certain forms not being able so to extend their range,
because of climatic differences, pre-occupation or some such barrier.
It is customary to give a graphic expression to the facts of animal
distribution by dividing the land surface of the earth into districts
which are characterized by their faunas. It is not possible to construct
a geographical scheme which will be equally satisfactory for all classes
of animals, because the geological date of most rapid development and
diffusion was so different in the various classes. The geographical and
climatic conditions which favoured a particular geographical arrangement
of one class had been so completely altered that the class coming in
later could not attain a similar distribution. For this reason, land
mammals are chosen as affording the best criteria; their adaptability is
such that they are found all over the earth, their dispersal is primarily
dependent upon the arrangement and connections of the continental
land-masses, modified by the topographical and climatic conditions, and
they, with the birds, are the latest of the vertebrate classes to assume
a dominating importance. Their history is the most fully known and falls
within the best understood portion of the earth’s history, making it
possible to follow their migrations with a precision which is seldom
feasible for the other classes of animals, and thus to correlate the
successive physical and organic changes. A particularly great advantage
which mammals possess for this purpose is that the mutual relationships
of the various kinds are better understood than in the case of most
other groups of animals. It is true that we shall find a great many
unsolved problems, upon which the most divergent opinions are held, but
the main outlines of the scheme are quite generally agreed upon.
Many plans for the zoölogical division of the continental areas have been
proposed by various writers on the subject, some differing very radically
from others. It would be useless and tedious to review even the more
important of the many proposals and suggestions which have been made in
the last half-century; and we may, with advantage, adopt an eclectic
scheme which has been slowly reached by successive approximations to a
satisfactory arrangement.
Just as in political geography it is found necessary to recognize
divisions of different rank and scope, like nation, state, county,
township, the facts of zoölogical geography require divisions of
different orders of importance. Thus, in descending order, the terms
_realm_, _region_, _subregion_, _province_, etc. are commonly employed,
but unfortunately they are often used loosely and even interchangeably;
yet it is desirable to attach a more or less precise significance to each
and more terms are needed for an accurate expression of the many complex
facts.
The extreme zoölogical peculiarity of Australia is recognized by making
that continent and its adjoining islands one of the great primary
divisions, of which the other includes all the rest of the world; the
former is characterized by its almost exclusively marsupial fauna, while
the other continents are inhabited by the Monodelphia or placental
mammals. Aside from Australia, by far the most isolated and peculiar
region of the earth is South America, and this fact is expressed by
constituting it into a _realm_, or division of the second order, and to
this realm is given the name _Neogæa_. The remaining continents, North
America, Europe, Asia and Africa, make up the other realm, _Arctogæa_,
in which there is an unmistakable general likeness among the mammals.
The three continents of the Old World form a vast, connected land-mass,
and the final separation of North America from this great complex is an
event of geologically recent date. For reasons that will be made clear
in the course of the history, the junction of the two Americas has had
comparatively little effect upon the zoölogy of the northern continent,
except in its tropical portion. It is obvious from a glance at the
map, that the great zoölogical divisions are of very unequal size, but
the arrangement is made on the basis of degrees of difference in the
mammalian faunas. These degrees of difference are, in turn, an expression
of length of separation or of the difficulty of communication between
connected lands.
The following table gives the major divisions of the earth apart from
Australia:
I. NEOGÆIC REALM. _Neotropical Region._—South and Central America,
lowlands of Mexico, the West Indies.
II. ARCTOGÆIC REALM. { 1. _Malagasy Region._—Madagascar.
{ 2. _Ethiopian Region._—Africa south of the
{ Sahara Desert.
{ 3. _Oriental Region._—Southern peninsulas of
{ Asia, Malay Archipelago.
{ 4. _Holarctic Region._—N. Africa, Europe, Asia,
{ (except southern part), boreal N. America.
{ 5. _Sonoran Region._—Remainder of N. America
{ (except lowlands of Mexico).
North America, as is expressed by this scheme, is zoölogically composite;
the northern half, including nearly all of Canada, belongs to the vast
Holarctic Region, which also comprises Europe, Africa north of the
Sahara and Asia north of the Himalaya Mountains. The remainder of the
continent, exclusive of the Mexican coastal lowlands, is set off as the
Sonoran Region. Inasmuch as we have here to do with broadly continuous
land-areas, not demarcated by great physical features, and as the genera
and species of mammals differ greatly in regard to their ability to
withstand a wide range of climatic variations, it is not to be expected
that the boundaries between the regions which make up North America
should be very sharply drawn. It is not surprising, therefore, to find
a transition zone, extending all across the continent, in which the
Holarctic and Sonoran faunas mingle, or that Central America should,
in considerable measure, be transitional to South America, though
zoölogically a part of the latter.
[Illustration: FIG. 53.—Zoölogical Divisions of North America. (After
Merriam.)]
Dr. Merriam’s arrangement, which deals only with North America
without reference to the Old World, divides the land into a series of
transcontinental zones, which he calls the Arctic, Boreal, Upper and
Lower Sonoran and Tropical. These zones have very irregular and sinuous
boundaries, which follow lines of equal temperature (isothermal lines)
during the breeding season, May, June and July, the tortuous boundaries
being conditioned by topographical features, which deflect the isothermal
lines.
[Illustration: FIG. 54.—Polar Bear (_Thalarctus maritimus_).—By
permission of the N.Y. Zoölog. Soc.]
The Arctic zone is part of a circumpolar area, which is very much the
same in North America, Asia and Europe; and in any of these continents
the fauna differs much more from that of the contiguous zone to the south
than from the Arctic fauna of another continent. There are some local
differences, but the characteristic mammals of this Arctic zone are the
Polar Bear, Arctic Fox, Musk Ox, Barren-ground Caribou, Lemming, Arctic
Hare, and a marmot. Most, if not all, of these forms are of Old World
origin.
[Illustration: FIG. 55.—Musk Ox (_Ovibos wardi_) female; the males have
much larger horns.—By permission of the N.Y. Zoölog. Soc.]
[Illustration: FIG. 56.—Arctic Fox (_Vulpes lagopus_) in winter dress.—By
permission of the N.Y. Zoölog. Soc.]
[Illustration: FIG. 57.—Arctic Fox in summer dress.—By permission of the
N.Y. Zoölog. Soc.]
The American portion of the great Holarctic region is called by Mr.
Lydekker, who uses Wallace’s term, the “Canadian subregion,” and by
Dr. Merriam the “Boreal region.” Not that there is any difference of
principle involved in this varying nomenclature, for Dr. Merriam says:
“It so happens that the Boreal element in America resembles that of
Eurasia so closely that in the judgment of many eminent authorities
the two constitute a single primary region—a view in which I heartily
concur.” The Canadian or Boreal subregion of the Holarctic is the great
belt of coniferous forest, which extends obliquely across North America
from Alaska to New England; its frontier with the Arctic zone is the
northern limit of trees and it is divided from the Transition zone
approximately by the line of latitude 45° N., though with a sinuous
course, and it is carried far to the south by the wooded heights of the
Appalachian, Rocky and Sierra Nevada Mountains, and along the Pacific
coast, the mixed character of which has already been explained; it
extends almost to San Francisco. The subregion is further divisible into
northern and southern belts, called the Hudsonian and Canadian faunas,
the limit between them approximately following the isothermal line of
57° F. The mammals of this subregion are largely of Old World origin,
many of them coming in with the great immigrations of the Pliocene and
Pleistocene epochs; but there are also native American elements and even
one genus of South American origin, the Short-tailed or Canada Porcupine
(_Erethizon_).
[Illustration: FIG. 58.—Canada Porcupine (_Erethizon dorsatus_).—By
permission of the N.Y. Zoölog. Soc.]
[Illustration: FIG. 59.—Woodchuck or Marmot (_Marmota monax_).—By
permission of the N.Y. Zoölog. Soc.]
[Illustration: FIG. 60.—Mink (_Lutreola vison_).—By permission of the
N.Y. Zoölog. Soc.]
In considering the mammals of this subregion, it should be remembered
that they are not uniformly distributed throughout even one subdivision,
but in a scattering way and in accordance with their habits and stations,
and also in accordance with a gradual change to the south, following the
changing temperature. The Muskrat will not be found far from water or the
Porcupine from woods. Especially characteristic of the Canadian subregion
are the Old World types of deer, none of which range farther south than
the Transition zone. The Wapiti, erroneously called the Elk (_Cervus
canadensis_), is very closely allied to the European Stag (_C. elaphus_)
and still more closely to the Stag of the Thian Shan in Central Asia (_C.
eustephanus_). So great is the resemblance, that some naturalists would
refer all three forms to a single species. The Moose (_Alce americanus_),
which should be called the Elk, is so near to the Scandinavian Elk (_A.
machlis_) that it is hardly distinguishable as a separate species, and
the Woodland Caribou (_Rangifer caribou_) is the American representative
of the Lapland Reindeer (_R. tarandus_). The so-called Rocky Mountain
Goat (_Oreamnos montanus_), a peculiar and aberrant form of the Chamois
subfamily of the Antelopes, is confined to the subregion. The Mountain
Sheep (_Ovis montana_, _O. dalli_) are represented by three or four
species, one of which extends into the Sonoran region, as does also the
Bison, wrongly called Buffalo (_Bison bison_), which is nearly allied to
the European _B. bonasus_. In Cæsar’s time the European Bison (German,
Wisent) ranged through Germany and is described in his account of the
Hercynian Forest; but the advance of civilization has almost exterminated
it, only a few small herds being maintained by the most rigid protection
in Russia and in the Carpathian Mountains. Of the Carnivora, the weasels,
martens, Fisher, Mink and Ermine are Boreal, as are the Wolverene
(_Gulo_) and the Grey Wolf (_Canis_), the three last-named extending also
into the Arctic zone. Essentially Boreal, though reaching and entering
the Sonoran, are the bears (_Ursus_), the red foxes (_Vulpes_), the
otters (_Lutra_) and the Old World shrews (_Sorex_), while the Star-nose
Mole (_Condylura_) and the mole-shrews (_Urotrichus_) do not extend
south of the Transition zone. Probable intruders from the south into the
Boreal subregion are the pumas, or “mountain lions,” which just enter
the subregion, the Canada Lynx (_Lynx rufus_) and one species of skunks
(_Mephitis_), the Raccoon (_Procyon lotor_), Badger (_Taxidea americana_)
and the American deer (_Odocoileus_). A large number of rodents are
characteristically Boreal: marmots, or woodchucks (_Marmota_), the
Sewellel (_Aplodontia rufa_), lemmings (_Myodes_), Jumping Mouse
(_Zapus_), the Canada Porcupine (_Erethizon dorsatus_) and the pikas,
“tailless or whistling hares” (_Ochotona_). Boreal rodents that enter the
Sonoran are the chipmunks (_Tamias_), beavers (_Castor_), meadow-mice
(_Microtus_), the Muskrat (_Fiber zibethicus_). The white-footed mice
(_Sitomys_) and the wood-rats (_Neotoma_) are southern rodents that reach
or enter the Boreal.
Between the Boreal subregion and the Sonoran region is the Transition
zone, which follows all the complex windings of the boundary lines. It
covers most of New England, New York, Pennsylvania and southern Ontario;
passing through southern Michigan and Wisconsin, it bends northward over
Minnesota and covers most of North Dakota, Manitoba and the plains of
the Saskatchewan, then turns abruptly southward and includes eastern
Montana and parts of South Dakota and Nebraska. Crossing Wyoming, it
follows around the northern edge of the Great Basin to the plains of the
Columbia. The three great mountain-systems carry the zone far to the
south and arms of it extend along the Appalachians to northern Georgia,
along the Rockies to New Mexico, and it follows the Sierras to southern
California. “The Transition zone, as its name indicates, is a zone of
overlapping Boreal and Sonoran types. Many Boreal genera and species here
reach the extreme southern limits of their distribution and many Sonoran
genera and species their northern limits. But a single mammalian genus
(_Synaptomys_) [one of the field mice] is restricted to the Transition
zone.... A number of species, however, seem to be nearly or quite
confined to this zone” (Merriam).
[Illustration: FIG. 61.—Upper figure, European Bison (_Bison bonasus_).
Lower figure, American Bison (_B. bison_).—By permission of the N.Y.
Zoölog. Soc.]
[Illustration: FIG. 62.—Wolverene (_Gulo luscus_).—By permission of the
N.Y. Zoölog. Soc.]
[Illustration: FIG. 63.—Wapiti or “Elk” (_Cervus canadensis_).—By
permission of the N.Y. Zoölog. Soc.]
[Illustration: FIG. 64.—Alaska Brown Bear (_Ursus middendorfi_).—By
permission of the N.Y. Zoölog. Soc.]
[Illustration: FIG. 65.—Moose (_Alce americanus_). Young male with
undeveloped antlers.—By permission of the N.Y. Zoölog. Soc.]
[Illustration: FIG. 66.—Beaver (_Castor canadensis_).—By permission of
the N.Y. Zoölog. Soc.]
[Illustration: FIG. 67.—Woodland Caribou (_Rangifer caribou_).—By
permission of the N.Y. Zoölog. Soc.]
[Illustration: FIG. 68.—Red Fox (_Vulpus fulvus_).—By permission of the
N.Y. Zoölog. Soc.]
[Illustration: FIG. 69.—Rocky Mountain “Goat” (_Oreamnos montanus_).—By
permission of the N.Y. Zoölog. Soc.]
[Illustration: FIG. 70.—Ermine (_Mustela erminea_).—By permission of the
N.Y. Zoölog. Soc.]
[Illustration: FIG. 71.—Timber or Grey Wolf (_Canis nubilis_).—By
permission of the N.Y. Zoölog. Soc.]
[Illustration: FIG. 72.—Boreal Mammals. _A._ Black-footed Ferret
(_Mustela nigripes_). _B._ Otter (_Lutra canadensis_). _C._ Jumping Mouse
(_Zapus hudsonius_).—_A_ and _B_ by permission of the N.Y. Zoölog. Soc.
_C_, by permission of W. S. Berridge, London.]
[Illustration: FIG. 73.—Opossum (_Didelphis marsupialis_).—By permission
of the N.Y. Zoölog. Soc.]
The most characteristic portion of North America, zoölogically speaking,
is the Sonoran region of Dr. Merriam, the Warm Temperate of Dr. Allen.
It crosses the continent from ocean to ocean, its northern boundary
following for most of the way the 43d parallel of latitude, but over the
Great Plains and Great Basin, on each side of the Rocky Mountains and
the high plateaus, it extends to lat. 48°. On the south, it takes in the
greater part of Mexico, covering all of the table-land of that country,
the lowlands of which belong to the South American or Neotropical
region. The Sonoran is invaded from the north by the long branches
from the Boreal and Transition zones, which follow the three great
mountain-systems in the manner already explained, and the Mexican plateau
permits the similar invasion of Neotropical territory by the Sonoran
fauna. Characteristic Sonoran genera, none of which extend into the
Boreal, are the opossums (_Didelphis_), in the southern part a peccary
(_Tagassu_) or “Wild Texas Pig,” representative of a family of swine
quite different from the true pigs of the Old World, and an armadillo
(_Tatu_). A very isolated form is the Prong-horned Antelope (_Antilocapra
americana_); there are several species of the typically American deer
(_Odocoileus_) which differ in important respects from those of the
eastern hemisphere, and the Bison was very abundant until exterminated by
Man. Bison, antelope and deer also reach or extend into the Boreal zone,
but the former, or Wood Bison, is probably a different species from the
plains animal.
[Illustration: FIG. 74.—Prong-horned Antelope (_Antilocapra
americana_).—By permission of the N.Y. Zoölog. Soc.]
[Illustration: FIG. 75.—Kangaroo-Rat (_Dipodomys philippii_).—By
permission of the N.Y. Zoölog. Soc.]
[Illustration: FIG. 76.—Thirteen-lined Spermophile (_Spermophilus
tredecimlineatus_).—By permission of the N.Y. Zoölog. Soc.]
The grey foxes (_Urocyon_), Coyote (_Canis latrans_), large Timber
Wolf (_Canis occidentalis_), the Caxomistle (_Bassariscus_), the Coati
(_Nasua_), Raccoon (_Procyon_), Badger (_Taxidea_), three genera of
skunks, pumas, several species of lynx and some bears (_Ursus_) represent
the Carnivora, though one species each of raccoon, skunk, badger, puma
and lynx range into the Boreal. The American types of shrews (_Blarina_)
and moles (_Scalops_ and _Scapanus_) are characteristic of the Sonoran,
though partially shared with the Boreal. A great many peculiar rodents
inhabit the Sonoran; cotton-rats (_Sigmodon_), pocket-gophers (_Geomys_,
etc.), several genera of the beautiful little kangaroo-rats (_Dipodomys_,
etc.); while the prairie-dogs (_Cynomys_), the white-footed mice
(_Sitomys_), wood-rats (_Neotoma_) and one genus of pocket-gophers
(_Thomomys_) are chiefly Sonoran, but have Boreal representatives.
The flying squirrels (_Sciuropterus_), true squirrels (_Sciurus_),
ground-squirrels (_Spermophilus_), rabbits (_Lepus_), wolves (_Canis_)
and otters (_Lutra_) have a very wide range through both the Boreal and
Sonoran, but have many more species in the latter region.
[Illustration: FIG. 77.—Grey Squirrel (_Sciurus carolinensis_).—By
permission of the N.Y. Zoölog. Soc.]
The Sonoran region may be divided into the upper and lower Sonoran zones,
which are demarcated by temperature and are of transcontinental extent.
Each of these zones may, in turn, be subdivided into _arid_ and _humid_
provinces, but our purpose does not necessitate entering into such
refinements.
[Illustration: FIG. 78.—Grey Fox (_Urocyon virginianus_).—By permission
of the N.Y. Zoölog. Soc.]
[Illustration: FIG. 79.—Prairie Wolf or Coyote (_Canis latrans_).—By
permission of the N.Y. Zoölog. Soc.]
[Illustration: FIG. 80.—Raccoon (_Procyon lotor_).—By permission of the
N.Y. Zoölog. Soc.]
[Illustration: FIG. 81.—Virginia Deer (_Odocoileus virginianus_).—By
permission of the N.Y. Zoölog. Soc.]
[Illustration: FIG. 82.—Skunk (_Mephitis mephitis_).—By permission of the
N.Y. Zoölog. Soc.]
[Illustration: FIG. 83.—Mule Deer (_Odocoileus hemionus_).—By permission
of the N.Y. Zoölog. Soc.]
[Illustration: FIG. 84.—Badger (_Taxidea americana_).—By permission of
the N.Y. Zoölog. Soc.]
[Illustration: FIG. 85.—Puma or Mountain Lion (_Felis concolor_).—By
permission of the N.Y. Zoölog. Soc.]
[Illustration: FIG. 86.—Lynx (_Lynx rufus_).—By permission of the N.Y.
Zoölog. Soc.]
[Illustration: FIG. 87.—Prairie-Dog (_Cynomys ludovicianus_).—By
permission of the N.Y. Zoölog. Soc.]
The Neotropical, which is the only region of the Neogæic realm, comprises
the West Indian islands, all of Central and South America and the
lowlands of Mexico, extending a short distance into southeastern Texas.
Of its four subregions, the most typical is (1) the _Brazilian_, which
includes not only Brazil, but all of South America east of the Andes and
as far south as Paraguay, and is a vast area of tropical forests. (2)
The _Chilian_ subregion takes in the west coast, the high Andes and the
southern end of the continent, south of the Brazilian subregion; it is
a country chiefly of open plains and high mountains, and a few deserts,
of which South America has less than any other continent, except Europe,
which has none. (3) The _Central American_ subregion reaches from the
Isthmus of Panama to Mexico, the lowlands of which are included and even
a small portion of southeastern Texas. (4) The _West Indian_ subregion
includes all the islands of that archipelago, except Trinidad, which is a
fragment of the continent, detached at a comparatively recent date; the
southern extremity of Florida also belongs to this subregion.
The two subregions into which continental South America is divided are
not altogether satisfactory and will doubtless require change when
the distribution of South American mammals has been more accurately
determined.
[Illustration: FIG. 88.—Map of the Neotropical region. (After Wallace.)
Mexico inaccurate; cf. Fig. 53, p. 147.]
[Illustration: FIG. 89.—Fox-like Wolf (_Cerdocyon gracilis_).—By
permission of W. S. Berridge, London.]
“Richness combined with isolation is the predominant feature of
Neotropical Zoölogy, and no other region can approach it in the number
of its peculiar family and generic types” (Wallace). Just as North
America has received many immigrants from the Old World, so it has sent
many migrants into South America, materially changing the character of
the Neotropical mammalian fauna, but these intruders may be readily
identified and almost seem to be out of place in their new surroundings.
Not all of these northern migrants were able to maintain their footing
in the southern continent and several became extinct during and at the
close of the Pleistocene epoch, as was even more markedly the case with
the southern forms which invaded the northern continent.
[Illustration: FIG. 90.—Spectacled Bear (_Tremarctos ornatus_).—By
permission of the N.Y. Zoölog. Soc.]
There are two families of monkeys in the forested areas of South America,
both very different from those of the Old World. One of these families,
the marmosets (Hapalidæ), differs from all other monkeys in several
particulars, most obvious of which are the long claws on the feet and the
non-opposable thumb. The second family (Cebidæ) comprises forms which are
superficially much more like those of the eastern hemisphere, but many of
them have prehensile tails, which are used as efficient grasping organs.
Insectivora are entirely absent from the South American continent, but
some shrews (_Blarina_) have entered Central America from the north and
a very curious genus is represented by one species in Cuba (_Solenodon
cubanus_) and another in Hayti (_S. paradoxus_). These remarkable animals
are, strange to relate, most nearly allied to the tenrecs (_Centetes_) of
Madagascar and by some authorities are placed in the same family.
[Illustration: FIG. 91.—_Solenodon cubanus._—By permission of the N.Y.
Zoölog. Soc.]
[Illustration: FIG. 92.—Argentine Skunk (_Conepatus gibsoni_).—By
permission of W. S. Berridge, London.]
[Illustration: FIG. 93.—Little Skunk (_Spilogale putorius_).—By
permission of W. S. Berridge, London.]
The Carnivora are quite numerous and varied and rather peculiar, but
they all belong to northern families and are the more or less modified
descendants of northern immigrants. The dogs (Canidæ) belong to
genera not represented elsewhere and form a considerable assemblage
of interesting types. There are no true wolves or foxes, but several
species of fox-like wolves (_Cerdocyon_), with bushy tails, are common,
especially in the plains regions. The Bush-Dog (_Icticyon venaticus_),
a small, short-legged animal, is very peculiar. The musteline or weasel
family (Mustelidæ) is rather scantily represented. There are no badgers
and but few skunks (_Spilogale_ and _Conepatus_); weasels are absent,
but their place is taken by the Grison (_Galera vittata_) and Tayra
(_Tayra tayra_) and in the far south _Lyncodon patagonicus_. These
animals are peculiar in having a lighter colouration on the back than
on the belly. There are two or three species of otter (_Lutra_). The
raccoons (_Procyon_) have a very wide range in South America, as in the
northern continent, and the curious, long-snouted coatis (_Nasua_), which
just enter the Sonoran region, are typically Neotropical. The Spectacled
Bear (_Tremarctos ornatus_) is the only member of the family that occurs
in South America and is confined to the highlands of Peru and Chili. The
cat family is quite numerously represented; the Jaguar (_Felis onca_),
which ranges from Texas to Patagonia, is a large spotted cat, rivalling
the Leopard in size and ferocity; the Ocelot (_F. pardalis_, Arkansas to
Paraguay) is smaller and streaked and blotched rather than spotted. The
pumas differ little from those of North America, and there are many small
cats, spotted, clouded and of solid colour, but no lynxes, which are
essentially northern types.
[Illustration: FIG. 94.—Tayra (_Tayra tayra_).—By permission of W. S.
Berridge, London.]
[Illustration: FIG. 95.—Kinkajou (_Potos caudivolvulus_), Central
America.—By permission of W. S. Berridge, London.]
[Illustration: FIG. 96.—Ocelot (_Felis pardalis_).—By permission of the
N.Y. Zoölog. Soc.]
[Illustration: FIG. 97.—Jaguar (_Felis onca_).—By permission of the N.Y.
Zoölog. Soc.]
[Illustration: FIG. 98.—Collared Peccary (_Tagassu tajacu_).—By
permission of the N.Y. Zoölog. Soc.]
Hoofed animals are not numerously represented in South America. The
only existing Perissodactyla of the western hemisphere are the tapirs
(_Tapirus_) of Central and tropical South America, a very remarkable
contrast to the ancient faunas, especially of the northern continent, as
will be shown in the sequel. The Artiodactyla are more varied, though
very scanty in comparison with those of the Old World; even North
America, which has but a poor representation of these animals, is much
richer than the southern continent, where, indeed, all the hoofed animals
are the descendants of comparatively recent immigrants from the north
and none are truly autochthonous. Members of three different artiodactyl
suborders occur in the Neotropical region; the peccaries (_Tagassu_)
extend through Central and South America to Paraguay, though also
entering the Sonoran region in Texas. Most interesting are the members of
the camel family, which are very distinct from the true Camel of Asia.
Tierra del Fuego and the Patagonian plains support great herds of the
Guanaco (_Lama huanacus_), which extends along the Andes to Ecuador and
Peru, where it is associated with the Vicuña (_L. vicunia_), a smaller
and more slenderly built species. The Vicuña does not range south of
Bolivia. Just as the mountain systems of North America carry the Boreal
and Transition faunas through nearly the whole breadth of the Sonoran
region, so the high Andes afford a pathway by which the mammals of the
south temperate zone extend their range to the equator.
[Illustration: FIG. 99.—Vicuña (_Lama vicunia_).—By permission of the
N.Y. Zoölog. Soc.]
[Illustration: FIG. 100.—Florida Deer (_Odocoileus virginianus
osceola_).—By permission of the N.Y. Zoölog. Soc.]
[Illustration: FIG. 101.—Marsh Deer (_Blastoceros paludosus_), female.—By
permission of the N.Y. Zoölog. Soc.]
The suborder Pecora of the Artiodactyla is represented in the Neotropical
region only by the deer family (_Cervidæ_), of which there are several
genera (or subgenera), all of them North American as distinguished from
the Old World type, but some are so peculiar that they must have had a
relatively long South American ancestry. The Virginia Deer (_Odocoileus
virginianus_) of the northern United States is a comparatively large
animal, becoming much smaller in Florida and the Southwest. The type
extends through Mexico and Central America to Guiana and Peru, the
Neotropical forms being so small and having such weak antlers that
they are referred to separate species. Another type is the Marsh Deer
(_Blastoceros paludosus)_ of eastern South America, which has short,
stout antlers, each beam with two double bifurcations; there are other
species of the same genus, such as the Pampas Deer of Argentina (_B.
bezoarticus_). In the Andes of Peru and Chili and the forests of western
Patagonia are two species of a genus which bears the preposterous name of
_Hippocamelus_ and in which the antlers are simply forked. The vernacular
name of these animals is “Huemul.” Peculiarly Neotropical are the little
brockets, which hardly exceed a height of two feet at the shoulder, with
simple spike-like antlers not more than three inches long; the genus,
_Mazama_, has several species, one of which occurs as far north as the
state of Puebla in Mexico. “The smallest of all deer is the Chilian pudu
(_Pudua pudu_), a creature not much larger than a hare, with almost
rudimentary antlers” (Lydekker). Old World types of deer, such as the
Wapiti, Moose and Caribou, of the Boreal and Transition zones of North
America, are entirely absent from the Neotropical region.
[Illustration: FIG. 102.—Wood Brocket (_Mazama nemorivagus_).—By
permission of W. S. Berridge, London.]
[Illustration: FIG. 103.—Brazilian Tree Porcupine (_Coendou
prehensilis_).—By permission of the N.Y. Zoölog. Soc.]
South America has an astonishingly rich and varied assemblage of
rodents, both indigenous and immigrant, but the former are much the more
important, varied and abundant. Of the four divisions of the order,
all of which are represented, three are immigrants from the north and
the fourth is autochthonous, but this far outnumbers the other three
combined. The hares and rabbits have but very few species, one of which
occurs in Brazil and is separated by a very wide interval from the one in
Costa Rica, while the pikas are absent. Of the squirrel division, only
the true squirrels are found, and of these there are many species, the
ground-squirrels, marmots, prairie-dogs and beavers all being lacking.
In the same way the rat and mouse division is represented by a single
family. The vesper or white-footed mice (_Sitomys_) have invaded the
southern continent and a number of peculiar genera have arisen there, but
all of northern ancestry, such as the groove-toothed mice (_Rheithrodon_)
and the fish-eating rats (_Ichthyomys_). The voles, or meadow-mice, the
muskrats, jumping mice, kangaroo-rats and pocket-gophers of the northern
continent are all absent. While the immigrant suborders have thus but
one family each in South America, the case is very different with the
fourth or porcupine group, of which that continent is to-day, as it has
been for ages past, the headquarters. No less than six families and
twenty-nine genera are known, all of the genera and four of the families
being restricted to the Neotropical region. Contrast this assemblage with
the extreme scantiness of this group in North America, where but a single
genus, the Short-tailed or Canada Porcupine (_Erethizon_) represents it,
and that is a late immigrant from the south.
[Illustration: FIG. 104.—Neotropical rodents. _A._ Vizcacha
(_Viscaccia_). _B._ Paca (_Agouti paca_). _C._ Rock Cavy (_Cavia
rupestris_). _D._ Water-Hog, or Carpincho (_Hydrochærus_). _D_, by
permission of the N.Y. Zoölog. Soc. _A_, _B_, _C_, by permission of W. S.
Berridge, London.]
[Illustration: FIG. 105.—Chinchilla (_Chinchilla laniger_).—By permission
of W. S. Berridge, London.]
It would lead us too far to attempt a description of this horde of
curious and interesting rodents, so only a few of the more striking
and characteristic forms can be mentioned. There are two genera of
porcupines (_Coendou_ and _Chætomys_), both arboreal, which belong in
the same family as the North American _Erethizon_, but are distinguished
by their long, prehensile tails, which they use, as monkeys and opossums
do, for grasping and climbing. The very large family of the Octodontidæ
has 17 Neotropical genera and four others are found in Africa. The
Degu (_Octodon_) of Chili, Bolivia and Peru has the appearance of a
large rat with tufted tail; the tuco-tucos (_Ctenomys_) are extremely
abundant burrowers in Patagonia, where they honeycomb the ground over
wide areas. The spiny rats (_Echimys_ and _Loncheres_) are so called
from their appearance, not because they are related to the true rats;
they have numerous horny spikes through the fur of the back. The Coypu
(_Myocastor_) is a large, aquatic animal, remotely like the northern
Muskrat, and the Hutias (_Capromys_ and _Plagiodontia_) are arboreal
and found only in Cuba, Hayti and Jamaica. The chinchillas (_Chinchilla_
and _Lagidium_) of the Andes and the Vizcacha (_Viscaccia_) of the
Argentine plains have somewhat the appearance of hares, but with long
and bushy tails. The cavies, to which the familiar, misnamed Guinea-Pig
(_Cavia porcellus_) belongs, are a very characteristic family; besides
the true cavies, it includes the Patagonian Mara (_Dolichotis_), a large,
long-legged, long-eared, short-tailed creature, and the Water-Hog, or
Carpincho (_Hydrochærus_), an aquatic animal, as its name implies, and
much the largest of existing rodents; it occurs in the warmer regions,
south to Argentina. The heavy Paca (_Agouti_) and the slender-limbed
Agouti (_Dasyprocta_) make up another family. Altogether, this assemblage
of the porcupine-like suborder of rodents is a very remarkable one and in
no other region of the earth is anything like it to be found.
[Illustration: FIG. 106.—Hairy-rumped Agouti (_Dasyprocta
prymnolopha_).—By permission of W. S. Berridge, London.]
[Illustration: FIG. 107.—Three-toed Sloth (_Bradypus tridactylus_).—By
permission of the N.Y. Zoölog. Soc.]
With the exception of one genus of armadillos, which has invaded
Texas, the entire order of the Edentata is at present confined to the
Neotropical region, the so-called edentates of the Old World now being
removed to other orders. The Edentata, which were once far more varied
and abundant than they now are, comprise three groups of animals so
bizarre and strange that they seem more like fabulous creatures than
actual, living mammals. One group, or suborder, is that of the sloths
(Tardigrada), arboreal, shaggy animals, with short, almost monkey-like
head and no tail; their very long legs and hook-like feet make them
nearly helpless on the ground, but are very useful for hanging from the
branches of the trees, in which the creatures live. Indeed, the sloths
are the only mammals which habitually hang in a suspended position. Two
genera of sloths inhabit the tropical forests, between which the most
obvious difference is that in one (_Bradypus_) the forefoot has three
toes, and in the other (_Cholœpus_) two.
[Illustration: FIG. 108.—Two-toed Sloth (_Cholœpus didactylus_).—By
permission of W. S. Berridge, London.]
The suborder of the anteaters (Vermilingua) is more varied, and is the
only one of the order to which the term “edentate” applies strictly, for
they alone in the order are altogether toothless. The great Ant-Bear
(_Myrmecophaga jubata_), which may reach a total length of seven feet,
has an extravagantly long, slender and nearly cylindrical head, long,
shaggy, black and white hair and an immense, bushy tail; the forefeet are
armed with huge, sharp-pointed claws, which are used for tearing open
ant-hills, and when occasion arises, as formidable weapons of defence,
for the Ant-Bear can successfully repulse even the Jaguar. In walking,
the claws are curved inward and the preposterous beast rests his weight
upon the outside edges of the forefeet, while the hind feet apply the
sole to the ground, as does a bear or raccoon. The Collared Anteater
(_Tamandua_) is much smaller and mainly arboreal in habits. It has a
short-haired, black body, with a white stripe down the back, white neck
and limbs, a colour-pattern which gives to the animal the appearance of
wearing a close-fitting black jacket; the long tail, which has some cross
bars, is short-haired, very different from the extremely bushy tail of
the Ant-Bear. The little Two-toed Anteater (_Cyclopes didactylus_),
hardly larger than a rat, is exclusively arboreal and has a prehensile
tail, like so many other South American mammals. Sloths and anteaters are
forest animals and are not found west of the Andes or south of Paraguay.
[Illustration: FIG. 109.—Ant-Bear (_Myrmecophaga jubata_).—By permission
of the N.Y. Zoölog. Soc.]
[Illustration: FIG. 110.—Collared Anteater (_Tamandua tetradactyla_).—By
permission of the N.Y. Zoölog. Soc.]
[Illustration: FIG. 111.—Six-banded Armadillo (_Dasypus sexcinctus_).—By
permission of the N.Y. Zoölog. Soc.]
The third existing suborder of edentates is that of the armadillos
(Dasypoda), which have a very complete armour of bony scutes,
ossifications in the skin, covered with scales of horn. They are all more
or less burrowers in habit and omnivorous in diet, eating roots, insects,
worms, etc.; the extraordinary rapidity with which they burrow into
the ground is almost their only way of escape from pursuit, but in one
genus, _Tolypeutes_, the animal can roll itself into a ball, completely
protected by mail all around. The armadillos are much more varied than
the anteaters or sloths and have a wider geographical range, extending
from Texas to Patagonia. The head, which is long-snouted, is protected
by a shield made up of numerous horn-covered plates of bone, and the
tail is encased in a tubular sheath of more or less regular rings, each
ring of bony plates and horny scales. The body-shield, or carapace,
which covers the back and sides, consists of an anterior and posterior
buckler, in which the plates are immovably attached to one another by
their edges, and between the two is a series of movable, overlapping
bands, the number of which varies in the different genera. In the little
Pichiciago (_Chlamydophorus truncatus_) the head and back are covered
with four-sided plates of horn, the bony scutes being small and thin
and much reduced. The carapace has no bucklers, but about 20 transverse
rows of plates, and is attached along only the middle line of the back
and beneath it the body is covered with silky, white fur; the rump is
covered with a solid shield of bone, placed nearly vertically and covered
with thin scales, and is notched below for the tail, altogether a most
exceptional arrangement. Seven or more distinct genera of armadillos are
found in the Neotropical region and they display a great range in size;
the Giant Armadillo of Brazil (_Priodontes_) is a yard or more in length,
while the little _Zaëdyus_ of Patagonia is smaller than a rabbit and,
least of all, the Pichiciago is but five inches long.
[Illustration: FIG. 112.—Nine-banded Armadillo (_Tatu novemcinctus_).—By
permission of the N.Y. Zoölog. Soc.]
Two families of marsupials occur in South America. The opossums are
much more numerous and varied than in North America; three genera and a
large number of species, some not larger than mice, range through the
forested parts of the continent. Of particular interest is the little
_Cænolestes_, which has two species, with two enlarged lower front
teeth, the sole survivors of a group which is abundantly represented in
the Tertiary deposits of Patagonia.
The fauna of the Central American subregion is less rich and
characteristic than that of the Brazilian and is, to a certain extent,
transitional to that of the Sonoran region of North America, several
genera proper to the latter region extending into it, which are not known
to pass the Isthmus of Panama, such as shrews, a fox and one of the
pocket-mice. The West Indian islands are exceedingly poor in mammals,
a great contrast to the East Indian, or Malay, Archipelago; only a few
rodents, insectivores and bats occur in them.
CHAPTER VII
THE SUCCESSIVE MAMMALIAN FAUNAS OF NORTH AND SOUTH AMERICA
The natural method of telling a story is to begin at the beginning and
go on to the end, but to deal in that manner with the many different
assemblages of mammals which have in turn inhabited the western
hemisphere has the great drawback of beginning with a time when
everything was utterly strange to the modern eye. Could the reader be
carried back to the far distant days of the Paleocene epoch, he would
find himself in a completely unfamiliar world; and there is therefore a
real practical advantage in reversing the story and starting with the end
and thus proceeding gradually from the more to the less familiar. The
foregoing chapter gave a sketch of the more striking and characteristic
mammals which inhabit the Americas to-day, and we may now take a step
backward to the epoch immediately preceding our own, the Pleistocene.
As was shown in Chapter V, the Pleistocene was a time of many and great
climatic vicissitudes, periods of cold, when the northern part of the
continent was buried under great ice-sheets, alternating with far milder
periods, when the climate was much as at present, or even warmer. These
climatic changes necessitated many changes in the distribution of
animals and plants, increasing cold driving them southward, while the
return of more genial conditions permitted the northward migration of
southern forms. The effects of these changes of climate are still plainly
visible in the geographical arrangement of living beings in the northern
continents and many anomalies of distribution, otherwise inexplicable,
are thus made clear. Attention was long ago directed to the fact that
the tops of high mountains support a flora and fauna which, on the
lowlands, will be found only hundreds, or even thousands, of miles to the
northward. The plants which grow on the summits of the White Mountains
of New Hampshire recur in Labrador, but not in the intervening area; the
vegetation and animals of the high Alps are those of the Arctic regions,
and many similar instances might be cited. Hooker and Darwin were the
first to find a highly probable explanation of this curious phenomenon by
referring it to the climatic changes of the Pleistocene epoch. During the
last period of cold and glaciation, the northern plants and animals were
driven far to the south and occupied the lowlands along the ice-front and
well beyond it; when milder conditions gradually returned, the northern
forms not only retreated northward, but also ascended the mountains,
as the latter were freed from ice, and thus became cut off as isolated
colonies. The general explanation of “discontinuous distribution” (see
p. 138) is thus always the same, viz., that the intervening regions were
once occupied by the forms now so widely separated, which, for one reason
or another, have vanished from the connecting areas.
I. QUATERNARY FAUNAS
_North America._—The Quaternary faunas of North America are extremely
difficult to correlate and place in chronological order, because, for
the most part, they are found in locally restricted areas, such as
tar-pools, bogs, caverns and similar places. Professor Osborn has,
however, succeeded in making an admirable arrangement, which, though it
will doubtless be corrected and expanded by future research, represents a
most important advance. Of the general problem he says: “The study of the
mammals of the Quaternary has by no means progressed so far in America as
in Europe; it will be many years before the faunistic succession can be
worked out with such chronologic accuracy and precision as has at last
been attained by European geologists and palæontologists.” According to
Osborn’s arrangement, there are three principal successive Pleistocene
faunas, two of which appear to have coincided with interglacial stages,
and the third with the last reëstablishment of glacial conditions on a
grand scale. Regarding the details of these faunas, there still remains
much uncertainty, and consequently there will be no attempt made here to
do more than discriminate between the general Pleistocene assemblage, on
the one hand, and that of the last cold period, on the other. It must be
emphasized that we are as yet unable to assert that all of the animals
listed together were actually living at the same time.
It is probable that the Pleistocene fossils already obtained give us a
fairly adequate conception of the larger and more conspicuous mammals of
the time, but no doubt represent very incompletely the small and fragile
forms. With all its gaps, however, the record is very impressive; “the
early and mid-Pleistocene life of North America is the grandest and most
varied assemblage of the entire Cenozoic Period [_i.e._ era] of our
continent” (Osborn). There is the further advantage that the fossils
have been gathered over a very great area, extending from ocean to ocean
and from Alaska to Central America. Thus, their wide geographical range
represents nearly all parts of the continent and gives us information
concerning the mammals of the great forests, as well as of the great
plains.
Those divisions of the early and middle Pleistocene which enjoyed milder
climatic conditions had an assemblage of mammals which, from one point
of view, seems very modern, for most of the genera, and even many of the
species, which now inhabit North America, date back to that time. From
the geographical standpoint, however, this is a very strange fauna, for
it contains so many animals now utterly foreign to North America, to find
near relatives of which we should have to go to Asia or South America.
Some of these animals which now seem so exotic, such as the llamas,
camels and horses, were yet truly indigenous and were derived from a long
line of ancestors which dwelt in this continent, but are now scattered
abroad and extinct in their original home, while others were migrants
that for some unknown reason failed to maintain themselves. Others again
are everywhere extinct.
[Illustration: FIG. 113.—Some of the more characteristic Pleistocene
mammals, reduced to a uniform scale, with a pointer dog (in the frame)
to show relative sizes.—1. †Columbian Elephant (_Elephas †columbi_).
2. Giant †Ground-Sloth (_†Megalonyx jeffersoni_). 3. †Stag-Moose
(_†Cervalces scotti_). 4. †American Mastodon (_†Mastodon americanus_).
5. †Giant Beaver (_†Castoroides ohioensis_). 6. †Texas Horse (_Equus
†scotti_). 7. †Sabre-tooth Tiger (_†Smilodon californicus_).]
Most surprising, perhaps, in a North American landscape, is the presence
of the Proboscidea, of which two very distinct kinds, the †mastodons and
the true elephants, are found together. Over nearly the whole of the
United States and southern Canada, and even with sporadic occurrence in
Alaska, ranged the †American Mastodon (_†Mastodon americanus_) which was
rare in the plains, but very abundant in the forested regions, where it
persisted till a very late period and was probably known to the early
Indians. This animal, while nearly related to the true elephants, was
yet quite different from them in appearance, as will be immediately
seen on comparing 1 and 4, Fig. 113, p. 195. The most obvious external
difference was the comparative shortness of the legs in the †Mastodon,
which did not exceed and seldom attained a height of 9 ft. 6 in. at the
shoulder; the head also was lower and more flattened. The teeth were
very different from those of the elephants; the grinding teeth were much
smaller and simpler, being low-crowned and rooted and having three or
four high, transverse, enamel-covered crests, without cement. The tusks
were elephant-like except that in the male there was a single small tusk
in the lower jaw, which cannot have been visible externally; this is a
remnant of an earlier stage of development, when there were two large
tusks in the lower as well as the upper jaw. The creature was covered
with long, coarse, dun-coloured hair; such hair has been found with some
of the skeletons.
Of true elephants, the North American Pleistocene had three species.
Most interesting of these is the northern or Siberian †Mammoth (_Elephas
†primigenius_), a late immigrant from northern Asia, which came in by
way of Alaska, where Bering Land (as we may call the raised bed of Bering
Sea) connected it with Asia. The †Mammoth was abundant in Alaska, British
Columbia and all across the northern United States to the Atlantic coast.
Hardly any fossil mammal is so well known as this, for the carcasses
entombed in the frozen gravels of northern Siberia have preserved every
detail of structure. It is thus definitely known that the †Mammoth was
well adapted to a cold climate and was covered with a dense coat of wool
beneath an outer coating of long, coarse hair, while the contents of the
stomach and the partially masticated food found in the mouth show that
the animal fed upon the same vegetation as grows in northern Siberia
to-day. The grinding teeth were very high, cement-covered, and composed
of many thin plates of enamel, dentine and cement, and were closely
similar to those of the existing Indian Elephant (_E. maximus_). In size
this is the smallest of the three Pleistocene species, 9 feet at the
shoulder. The †Mammoth was not peculiar to Siberia and North America, but
extended also into Europe, where it was familiar to Palæolithic Man, as
is attested by the spirited and lifelike carvings and cave-paintings of
that date. Thus, during some part of the Pleistocene, this species ranged
around the entire northern hemisphere.
Closely related to the †Mammoth and in some cases hardly distinguishable
from it, is the †Columbian Elephant (_E. †columbi_) which, however,
attained a considerably larger size, as much as 11 feet, rivalling the
largest African elephants of the present time. The head was very high
and had a curiously peaked appearance, and the tusks in old males curved
inward, overlapping at the tips. From the likeness in teeth and skeleton
to the †Mammoth, it may be inferred, though somewhat doubtfully, that
the †Columbian Elephant was clothed with hair, but not so heavily as the
†Mammoth, which was a northern species, the Columbian form replacing
it southward, and ranging over the whole United States, including
Florida and even throughout the table-land of Mexico. The areas of the
two species overlapped along the northernmost United States, but are
elsewhere distinct.
[Illustration: FIG. 114.—Restoration of the †Columbian Elephant (_Elephas
†columbi_) from a skeleton in the American Museum of Natural History.]
A third species was the huge †Imperial Elephant (_E. †imperator_), the
largest of American forms, to which Osborn’s calculations give the almost
incredible height of 13 ft. 6 in. This great creature was characterized
not only by its enormous stature, but also by the proportionately very
large size of its grinding teeth, and was a survivor from the preceding
Pliocene epoch; it is not known to have passed beyond the middle
Pleistocene and was thus the first of the species to become extinct. In
geographical range, the †Imperial Elephant was a western form, extending
from the Pacific coast almost to the Mississippi River, east of which it
has never been found, and from Nebraska southward to the City of Mexico.
The meaning of this distribution is probably that this elephant shunned
the forests and was especially adapted to a life on the open plains. Over
most of its area the winters were severe, and this fact makes it likely
that the animal was clothed with hair, but nothing is definitely known on
this point.
[Illustration: FIG. 115.—A Horse (_Equus †scotti_) from the older
Pleistocene of Texas. Restored from a skeleton in the American Museum of
Natural History.]
Many other hoofed animals, far more than now inhabit North America, are
found in this Pleistocene fauna. The Perissodactyla were represented by
horses and tapirs, but not by rhinoceroses; it might seem superfluous
to say that there were no rhinoceroses, but, as a matter of fact, that
family had a long and varied American history and became extinct only
during or at the end of the Pliocene epoch. The horses were extremely
numerous, both individually and specifically, and ranged, apparently in
great herds, all over Mexico and the United States and even into Alaska.
All the known species (at least ten in number) belong to the genus
_Equus_, but the True Horse (_E. caballus_), to which all the domestic
breeds are referred, is not represented. The smallest known member of
the genus is the pygmy _E. †tau_ of Mexico. _E. †fraternus_, likewise a
very small species, is found especially in the southeast, but extended
as far north as Pennsylvania and west to Nebraska. On the other hand,
_E. †giganteus_ of Texas exceeded the heaviest modern draught-horses
in size and was the largest of the American species; of other Texan
forms, one (_E. †scotti_) resembled Burchell’s Zebra (_E. burchelli_)
in the proportions of head and neck, body and limbs, while another (_E.
†semiplicatus_) was more ass-like. The forest horse of the eastern states
has been named _E. †pectinatus_, an animal of moderate size. The Great
Plains must have been fairly covered with enormous herds of horses, the
countless bones and teeth of which, entombed in the Sheridan formation,
have given to it the name of “Equus beds.” The most abundant of the
plains species is _E. †complicatus_, a horse of about 14½ hands in height
(_i.e._ 4 feet 10 inches at the shoulder) which also ranged down the
Mississippi Valley nearly or quite to the Gulf of Mexico. In California
was _E. †occidentalis_, equalling _E. †complicatus_ in size, but with
much more simple teeth, and associated with it the much larger _E.
†pacificus_, which was inferior only to _E. †giganteus_ and therefore the
second largest of the American Pleistocene horses.
To one who knows nothing of the geological history of North America
it would be natural to suppose that the Pleistocene horses must
have been immigrants from the Old World, which failed to establish
themselves permanently here, since they completely disappeared before
the discovery of the continent by Europeans. This would, however, be a
mistaken inference, for North America was for long ages the chief area
of development of the equine family, which may here be traced in almost
unbroken continuity from the lower Eocene to the Pliocene. On the other
hand, it is quite possible that some of the species were immigrants.
Tapirs, which are now confined to southern Asia, Central and South
America, were not uncommon in the forested parts of eastern North America
as far north as Pennsylvania, but they have not been found west of the
Mississippi in the plains region. Two species are known, a larger and
heavier one, _Tapirus †haysii_, and a smaller one which seems to be
identical with the living _T. terrestris_ of Central and South America.
Like the horses, the tapirs had a long history of development in North
America and may well have originated here, but they withdrew from the
continent in the Pleistocene, probably yielding to the last of the
glacial advances.
There was likewise a much greater variety of Artiodactyla than North
America can boast at the present day; some were autochthonous, but, for
the most part, they were migrants from the eastern hemisphere, where the
great group of the true ruminants (Pecora) passed through the greater
part of its development and where its headquarters still are. Indigenous
were the peccaries, or American swine, which still occur from Texas south
to Brazil. In Pleistocene time they ranged over nearly all of the United
States, as far northward as Pennsylvania, and across the plains to the
Pacific coast; they were represented by two genera, now extinct, one of
which (_†Platygonus_) had crested grinding teeth and much longer legs
than the modern peccaries. Another indigenous group, strange as that may
seem, is the suborder (Tylopoda) of the camels and llamas, both of which
are represented in the North American Pleistocene, the descendants of a
very long American ancestry. Some of these tylopodans were far larger
than existing forms, and at least one species extended its range to
Alaska.
Of ultimately Old World origin, but through a considerable line of
descent in America, were the typically American deer (_Odocoileus_)
of which the Virginian and Black-tailed species are familiar modern
instances. Whether or not the Old World types, the Caribou (_Rangifer_)
and Wapiti (_Cervus canadensis_) had reached the western hemisphere,
is a matter of some doubt; if present at all, they must have been
comparatively rare. The Moose (_Alce americanus_), on the other hand,
had already appeared, but seems to have been confined to the western
half of the continent, its presence in the east being questionable. The
mistakenly named “Rocky Mountain Goat” (_Oreamnos montanus_), which is
an antelope of the chamois group, was an apparently late arrival in the
Pleistocene, while the peculiar Prong-Buck (_Antilocapra americana_),
which is very different from any of the Old World antelopes, was
present in the early part of the epoch. The descent of this remarkable
animal is still a problem, but not improbably it was derived from the
“deer-antelopes” of the Miocene and Pliocene, the last of which occurred
in the early Pleistocene. Mr. Gidley has announced the surprising
discovery in Maryland of a large antelope hardly distinguishable from the
African Eland (_Taurotragus_). Other late arrivals from the Old World
were several forms allied to the existing Musk Ox (_Ovibos_), at least
two genera of which (_†Preptoceras_ and _†Euceratherium_) have been found
in California. A surprising number of species of _Bison_ occurred in the
Pleistocene, no less than seven of which are recognized as distinct,
ranging from Florida to Alaska. It is not likely that all these species
coexisted at the same time, but we cannot yet determine their order of
succession, though the modern species, _B. bison_, was probably the
latest to arise. Most of these species were much larger than _B. bison_,
and some were gigantic, such as _B. †latifrons_, which had a spread of
horns of 6 feet and is found through the Mississippi Valley, and _B.
†crassicornis_ of Alaska.
[Illustration: FIG. 116.—Restoration of _†Preptoceras_, a musk-ox like
animal from the Californian Pleistocene. (From a skeleton in the museum
of the University of California.)]
Preying upon this great assemblage of hoofed animals was a corresponding
array of Carnivora, most of which were indigenous and derived from
American stocks, but there was a considerable migrant element also,
such as the bears and badgers. Nearly all the modern kinds of
flesh-eaters found in the North America of to-day were already here
in the Pleistocene, minks, weasels, martens, skunks, otters, badgers,
wolverenes, raccoons, foxes, wolves, coyotes, pumas, etc., etc., but
there were several others which are either now extinct or no longer
to be found in this continent. Of the extinct types much the most
striking were the several species of †sabre-tooth tigers (_†Smilodon_,
see Frontispiece) which have been found in the greater part of the
United States and no doubt ranged over the whole. These were massive,
short-tailed and rather short-legged, but very muscular and powerful,
cat-like animals, in which the upper canine teeth were converted into
great, recurved, scimitar-like tusks. These large beasts of prey, which
about equalled the Leopard in height, but were far heavier, belonged
to a group which, at one time or another, spread over nearly the whole
world and persisted much later and attained a larger size and higher
development in the western hemisphere than in the eastern. They had a
very long American ancestry, from the lower Oligocene to the end of
the Pleistocene, but the place of their origin is still unknown. In
addition to the pumas and lynxes, there were some very large true felines
(_Felis †atrox_ and _F. †imperialis_), which closely resembled the Lion
(_F. leo_) in size, appearance and structure, and have been found in
California and the Mississippi Valley; probably these great cats were
immigrants, but they may represent a native development of Miocene and
Pliocene stock; the history of the family is too imperfect for a decision
of this question.
Besides coyotes and wolves which are indistinguishable from existing
species, there were some very large wolves, now extinct, of which the
commonest and most widely distributed was _Canis †dirus_ (also called _C.
†indianensis_) so abundant in the asphalts of southern California. Bears
were not so common in the middle Pleistocene and have not been found in
the older part of that epoch, though they probably had already reached
North America from the Old World, where they originated. Their absence
from the older Pleistocene (Equus Beds) may be accounted for by the fact
that those beds contain a fauna of the open plains, while bears are
chiefly forest-living animals. An extinct type of the family is the group
of species which constitute the †short-faced bears (_†Arctotherium_),
very large and powerful creatures, with remarkably shortened jaws, which
have been found from ocean to ocean. The smaller beasts of prey, badgers,
weasels, etc., were, as intimated above, substantially the same as now.
The rodents of the Pleistocene were very nearly in their modern stage
of development, most of the genera and many of the species surviving
to present times. Just what members of the order were introduced from
the Old World, the imperfect and fragmentary history will not permit
us to say, but some interesting South American immigrants should be
noted. One of these, the Capybara or so-called Water-Hog (_Hydrochærus
capybara_), the largest of existing rodents, failed to gain a permanent
foothold, but another South American form, the Short-tailed or Canada
Porcupine (_Erethizon dorsatus_), common all over the United States in
the Pleistocene, has maintained itself to the present day. One especially
peculiar form, not derived from South America or the Old World, is the
†Giant Beaver (_†Castoroides_), one species of which, _†C. ohioensis_,
was as large as a Black Bear and occurred in the later Pleistocene, while
a smaller species (_†C. species indet._) is found in the more ancient
deposits of the epoch. In almost all respects _†Castoroides_ was simply
a gigantic beaver, but the grinding teeth were remarkably like those of
the South American Capybara (_Hydrochærus_), so much so that it has been
mistakenly referred to the same family by some authorities.
By far the strangest elements of the Pleistocene faunas were the two
suborders of gigantic edentates, the _†Gravigrada_, or †ground-sloths,
and the _†Glyptodontia_, which might well be called giant armadillos,
if that name were not already in use for a living Brazilian animal.
Both suborders are completely extinct, but they long played a very
conspicuous rôle in South America, where they originated and whence the
North American representatives migrated. The †ground-sloths were great,
unwieldy, herbivorous animals covered with long hair, and in one family
(†Mylodontidæ) there was a close-set armour of pebble-like ossicles in
the skin, not visible externally; they walked upon the outer edges of
the feet, somewhat as the Ant-Bear (_Myrmecophaga_) uses his fore paws,
and must have been very slow-moving creatures. Their enormous claws may
have served partly as weapons of defence and were doubtless used also
to drag down branches of trees and to dig roots and tubers. Apparently,
the latest of these curious animals to survive was the very large
_†Megalonyx_, which, it is interesting to note, was first discovered and
named by Thomas Jefferson. The animals of this genus were very abundant
in the forests east of the Mississippi River and on the Pacific coast,
much less common in the plains region, where they would seem to have
been confined to the wooded river valleys. The still more gigantic
_†Megatherium_, which had a body as large as that of an elephant and much
shorter, though more massive legs, was a southern animal and has not
been found above South Carolina. _†Mylodon_, smaller and lighter than
the preceding genera, would seem to have entered the continent earlier
and to have become extinct sooner; it ranged across the continent, but
was much commoner in the plains region and less so in the forested areas
than _†Megalonyx_, being no doubt better adapted to subsisting upon the
vegetation of the plains and less dependent upon trees for food.
The †Glyptodonts were undoubtedly present in the North American
Pleistocene, but the remains which have been collected so far are very
fragmentary and quite insufficient to give us a definite conception of
the number and variety of them. It will be better therefore to defer the
description of these most curious creatures until the South American
Pleistocene is dealt with, as they were incomparably more varied and
characteristic in that continent. In North America they have been found
only in Mexico and the southern United States.
The many and great climatic changes which took place in the Pleistocene
led to very extensive migrations of mammals from one part of the
continent to another, as the conditions of temperature and moisture
changed. In Interglacial stages, when the climate was much ameliorated,
southern species spread far to the north, as when the †Mastodon ranged
into Alaska, and the Manatee, or Sea-Cow, of Florida waters, came up the
coast to New Jersey, while the increasing cold of oncoming glaciation
caused a reverse movement and drove northern and even Arctic forms far
to the south. Thus, the Musk-Ox, the Caribou and the northern †Mammoth
came south beyond the Ohio and the Potomac, and the Walrus was found on
the South Atlantic coast. It is these migrations which give such a mixed
character to the Pleistocene faunas from the climatic point of view, as
it is often very difficult to correlate or synchronize the fossiliferous
deposits with the Glacial and Interglacial stages, though this has been
definitely accomplished in several very important instances.
The latest of the Pleistocene faunas is less completely known than those
of the earlier and middle portions of the epoch, for but few localities
have yet been discovered with any extensive series of fossils. As worked
out by Osborn, this fauna coincided with the last Glacial stage and was
a greatly reduced and impoverished assemblage as compared with those of
the middle and lower Pleistocene, though it is not safe to argue that all
the animals not found in this fauna were already extinct, for the known
list is still far too short to be entirely representative. The American
†Mastodon (_†Mastodon americanus_, see p. 196) was still abundant in
the forested regions and was apparently able to withstand severe winter
temperatures, as certainly was the †Mammoth (_Elephas †primigenius_, see
p. 196), which was so abundant in the coldest part of Siberia and which
extended south to the Potomac, presumably at this time. Horses were
still present in North America, though apparently in greatly diminished
numbers and variety. Tapirs have not been found, though they may have
lingered on in the southern regions. The typically North American genus
of deer (_Odocoileus_) was, of course, well represented, and Old World
types had a much more southerly distribution than at present. The Caribou
(_Rangifer caribou_) came down into Pennsylvania and Ohio, the Moose
(_Alce americanus_) into Kentucky and Kansas, and the Wapiti (_Cervus
canadensis_) is reported as far south as Florida. A very remarkable
animal is the Stag-Moose (_†Cervalces scotti_), the best preserved
skeleton of which is that in the museum of Princeton University. This was
found in a shell-marl beneath a peat-bog at Mt. Hermon, N. J., _north of
the great terminal moraine_, and therefore most probably this particular
individual dates from a time not earlier than the beginning of the final
retreat of the ice.
_†Cervalces_, as its name implies, was in some respects intermediate
between the Stag (_Cervus_) and the Moose (_Alce_); in general
proportions it most nearly resembled the latter, having a short neck,
long body and very long legs; but the skull differed in many respects
from that of the Moose, especially in parts which show that the great,
inflated snout and prehensile upper lip had no such development in the
extinct as in the living form. The antlers were unique among the known
members of the deer family, resembling those of the Moose, though much
less palmated and with the addition of great trumpet-shaped plates.
The feet were large, almost as large as in the Caribou, and the whole
structure indicates an animal well fitted to travel through deep snows
and flourish in severe winters.
[Illustration: FIG. 117.—_†Cervalces scotti_: restored from a skeleton
in the museum of Princeton University.]
Even more typically northern than the Caribou were the Musk-Oxen,
of which two genera occurred in the late Pleistocene. One of these,
_†Symbos_, is extinct and was characterized by its short horns; the
other, _Ovibos_, is the genus to which the existing species, _O.
moschatus_ and _O. wardi_, belong and is now confined to the extreme
north of the continent, the Arctic islands and Greenland. The remains of
Musk-Oxen have been found mostly along the great terminal moraine which
marks the front of the last ice-invasion, but they occurred also as
far south as Oklahoma, and in Utah they ranged far to the south of the
ice-front. Nothing could be more conclusive evidence of a climate much
colder than the modern one than the presence of Caribou and Musk-Oxen in
the United States and of the Walrus on the coast of Georgia.
The smaller animals were much as they are now, differing only in range.
The †sabre-tooth tigers, the last of a most interesting line, persisted
in the south, and an extinct genus of skunks has been discovered in
Arkansas, but otherwise the Carnivora were entirely modern in character.
Unfortunately, these smaller animals are very incompletely known, much
the richest aggregation which has yet been found being that collected by
Mr. Brown in the Conard Fissure, Arkansas. From this collection Mr. Brown
has described thirty-seven genera and fifty-one species of mammals, of
which four genera and twenty-four species are extinct. That is to say,
less than one-ninth of the genera and one-half of the species represent
extinct forms. Contrast this with the middle Pleistocene assemblage found
in the Port Kennedy cavern in eastern Pennsylvania, of sixty-four species
with at least forty extinct ones.
The foregoing sketch, brief and imperfect as it necessarily is, makes
it sufficiently plain that North America during the Pleistocene was far
richer in mammalian life than it was when the continent was first settled
by Europeans. When we make the proper allowance for the many forms which
undoubtedly remain to be discovered and for those which may have vanished
without leaving a trace behind them, the contrast becomes all the more
striking. Not only did Pleistocene North America have substantially all
the mammals that it now possesses, but it had many more. The lions and
†sabre-tooth tigers, the gigantic †short-faced bears, the tapirs and
many varieties of horses, large and small, the camels and llamas, many
species of bisons, some of enormous proportions, several forms allied to
the Musk-Ox, the elephants and †mastodons, the †giant beavers and South
American water-hogs, the huge †ground-sloths and †glyptodonts, have all
disappeared, leaving a continent, that, by contrast, is “zoölogically
impoverished.” The Pleistocene fauna was strangely mixed in character,
the free roads of migration bringing together Old World and South
American types, and mingling them with indigenous forms in a cosmopolitan
assemblage.
Turning to _South America_, we find in the pampas of Argentina a
wonderful museum of Pleistocene mammals, such as occurs nowhere else in
the known world, and this is supplemented by the very rich collections
gathered from the caverns of Brazil and from deposits of Ecuador and
Bolivia, and thus all the important regions of the continent, save the
far south, are well represented. These faunas are far stranger than the
corresponding ones of North America and differ more radically from those
of modern times, since they include a much larger proportion of extinct
types, and the extinctions have swept away not only species and genera,
but families and orders as well.
The South American Pleistocene assemblage of mammals is very clearly
divisible into two elements: (1) the immigrants from the north, which
reached the southern continent in successive waves of migration, that
have left records of themselves as early as the older Pliocene, perhaps
even the upper Miocene, and (2) the indigenous element, which had a very
long history of development in South America. To the immigrant class
belonged all of the Carnivora, which therefore resembled their North
American relatives, but were less varied in character. Of the bears,
only the huge, †short-faced kind (_†Arctotherium_, Fig. 275, p. 549)
are known, and it is not likely that true bears existed except in the
Andes, as is also the case to-day. Of the cat family, the †sabre-tooth
tigers (_†Smilodon_) were as common in South America as in North, and,
while there were no lions, there were large cats nearly allied to the
Jaguar and Puma, and smaller ones, like the Ocelot. The dogs were quite
numerously represented by species resembling closely the existing South
American fox-like wolves and the Bush-Dog (_Icticyon_) and, strange
to say, by one which seems referable to the same genus (_Cyon_) as
the Dhole of India. The weasel family (Mustelidæ) were less numerous
and varied than in the northern continent, as they still are; coatis
(_Nasua_) and raccoons (_Procyon_) were abundant and one species of the
latter was much larger than any existing one; extinct species of skunk
(_Conepatus_), tayra (_Tayra_) and otter (_Lutra_) were also present, but
the badgers, minks, martens and wolverenes were not.
[Illustration: FIG. 118.—Some of the commoner Pampean mammals, reduced
to a uniform scale, with a pointer dog (in the frame) to show the
relative sizes. 1. _†Dœdicurus clavicaudatus._ 2. _†Glyptodon clavipes_,
†glyptodonts. 3. _†Macrauchenia patachonica_, one of the †Litopterna.
4. †Pampas Horse (_†Hippidion neogæum_). 5. _†Toxodon burmeisteri_,
a †toxodont. 6. _†Megatherium americanum_. 7. _†Mylodon robustus_,
†ground-sloths.]
The hoofed animals were represented by a great variety of forms, both
immigrant and indigenous, of which the latter belonged to orders now
entirely extinct. Horses were common in all parts of the continent, where
fossils of this epoch have been obtained, and are referable to two very
distinct groups: (1) to the typical genus _Equus_, of which three species
have been described, all somewhat more primitive than the True Horse (_E.
caballus_) and, like most of the Pleistocene species of North America,
with a certain resemblance to the zebras and asses; (2) to an extinct
group of four genera, the best known of which is _†Hippidion_. The
species of this genus (which has also been reported from North America,
though upon hardly sufficient evidence) had most exceptional characters
in the skull, and the head was relatively large and clumsy, with narrow
and very high facial region. The neck was comparatively short, the limbs
heavy and the feet short. These animals can hardly have been very swift
runners. A very interesting member of this group is _†Hyperhippidium_, a
small horse found in the Andes, with remarkably short feet, well adapted
for a mountain life. The only other perissodactyls were tapirs, which
ranged down to the Argentine pampas, much farther south than now.
[Illustration: FIG. 119.—A Pampas Horse (_†Hippidion neogæum_). Restored
from a skeleton in the National Museum, Buenos Aires.]
The Artiodactyla were much more varied; there were peccaries, many
species of llamas, which then extended into Brazil, and were not
confined, as at present, to the colder portions of the continent.
There were also numerous deer, all of the South American type, and
two different antelopes have been reported, though that family has
no representatives in the southern continent now. Several species of
†mastodons have been found in Brazil, Argentina, Bolivia and elsewhere,
but none of the true elephants. Why the †mastodons were able to make
their way into South America, while the elephants were not, is one of the
puzzling questions of mammalian distribution to which no answer can be
given.
All the preceding types of hoofed animals, the horses, tapirs, peccaries,
llamas, deer, antelopes and †mastodons were migrants from the north, and
four of these, tapirs, peccaries, llamas and deer, were able to gain a
permanent footing in South America and are more or less abundant there
to-day, while the horses, antelopes and †mastodons failed to do so and
died out. In addition to these, there were the indigenous types, which
are now extinct and have never been found outside of the Neotropical
region. An extremely peculiar creature, _†Macrauchenia_, was the last
surviving member of an order, the †Litopterna, which for ages played
a very important rôle in South America. _†Macrauchenia_ was a large
animal, somewhat larger and of much heavier build than a camel, to which
it had a considerable, though entirely superficial, resemblance. The
head was relatively small and must have had quite a long proboscis; the
neck was very long, suggesting that the animal browsed upon trees, which
is also indicated by the character of the teeth; the legs were long
and stout, the feet short and each provided with three toes. Another
curious creature was _†Typotherium_, from which is named the group of the
†Typotheria, which some authorities regard as a suborder, while others
assign to it a full ordinal rank.
[Illustration: FIG. 120.—A Pampean †Litoptern (_†Macrauchenia
patachonica_). Restored from a skeleton in the Museum of La Plata.]
The †Typotheres throughout the Tertiary period were among the most
abundant and characteristic of the South American hoofed animals, and the
genus _†Typotherium_ was the last of a very long series and was an animal
of moderate size, with chisel-shaped incisor teeth so like those of the
rodents that the genus was long referred to that order. Finally, we have
_†Toxodon_, type of the order †Toxodontia, a ponderous beast, as large as
a rhinoceros, which, there is some reason to think, was largely aquatic
in its habits. The first species of this extraordinary creature was found
by Charles Darwin, who says of it: “Perhaps one of the strangest animals
ever discovered; in size it equalled an elephant or megatherium, but the
structure of its teeth, as Mr. Owen states, proves indisputably that
it was intimately related to the Gnawers [_i.e._ Rodentia] ... in many
details it is allied to the Pachydermata: judging from the position of
its eyes, ears, and nostrils, it was probably aquatic, like the Dugong
and Manatee, to which it is also allied.”[4] Modern views concerning
the relationships of _†Toxodon_ are very different from those advanced
by Darwin, but he gives a vivid picture of its diverse likenesses.
Neither _†Macrauchenia_, _†Typotherium_ nor _†Toxodon_ has been found
in the Brazilian caverns, but this is no doubt due to the accidents of
preservation, for the latter animal ranged north to Nicaragua.
[Illustration: FIG. 121.—A Pampean †Toxodont (_†Toxodon burmeisteri_).
Restored from a skeleton in the La Plata Museum.]
The rodents likewise were partially of immigrant and partially of native
stock. To the former belonged the few mice and rats and a meadow-mouse
(_Microtus_), a group not represented in present-day South America, and
a rabbit. Very much more abundant and varied were the indigenous forms,
all of which belonged to existing families and most of them to existing
genera; the tree-porcupines, cavies, agoutis, spiny-rats, vizcachas,
capybaras, coypus, etc., were abundantly represented, for the most part
by extinct species.
The monkeys were of purely Neotropical type and several modern genera,
such as _Cebus_ and _Callithrix_, and one very large extinct genus,
_†Protopithecus_, of the same family, have been found in the caverns of
Brazil, but not in the pampas of Argentina, which would seem to have been
a country of open plains.
In the South America of to-day one of the most striking and peculiar
elements of the fauna is that formed by the Edentata, the sloths,
anteaters and armadillos, and this was even more true of the same region
in Pleistocene times. Anteaters and sloths are very scantily represented,
but this is merely an accident of preservation; armadillos, on the
other hand, were very numerous both in Brazil and in Argentina, and, in
addition to many modern genera, there were several which are no longer in
existence, such as _†Chlamydotherium_, which was a huge creature almost
as large as a rhinoceros. Then there were the two extinct suborders of
the †glyptodonts (†Glyptodontia) and the †ground-sloths (†Gravigrada)
which were astonishingly abundant in Argentina and which, as was shown in
a previous page (p. 205), were also well represented in North America.
Few more fantastic-looking mammals than the †glyptodonts have ever been
found; the short, deep head, with its shield of thick, bony plates, the
huge carapace made up of innumerable plates of bone firmly united at
their edges and without the movable bands of the armadillo carapace, the
enormous tail-sheath, the short legs and massive feet with broad hoofs,
must have given these animals rather the appearance of gigantic tortoises
than of mammals. The †glyptodonts were especially numerous and varied
in the Argentine pampas, and a stately array of them is mounted in the
museums of La Plata and Buenos Aires; in length, they ranged from six to
twelve feet, including the tail. The skeleton and carapace did not differ
very greatly in appearance among the various genera, but there were great
differences in the form and size of the bony sheath enclosing the tail.
In the genus _†Glyptodon_ the sheath was composed throughout of movable
overlapping rings, with prominent spines on them; in _†Sclerocalyptus_
the hinder half of the sheath coalesced into a single piece, marked only
by the elaborate ornamentation of the horny scales, while in _†Dœdicurus_
the end had a tremendous, club-like expansion, which must have been set
with great horn-like spines. The †glyptodonts were ponderous, slow-moving
and inoffensive plant-feeders, almost invulnerable to attack, and
probably used their massive tails, which could be freely swung from side
to side, as redoubtable weapons of defence, much as the alligator uses
his tail. In comparison with the bewildering variety in South America,
the few that made their way into North America were quite insignificant.
Much the same statement applies to the †ground-sloths, and though these
ranged far more widely through the northern continent than did the
†glyptodonts, they were but few in comparison with the multitude which
inhabited alike the forests of Brazil and the plains of the south. Two
of the three genera of †ground-sloths which occur in the North American
Pleistocene, _†Megatherium_ and _†Mylodon_, are also found in South
America; and though _†Megalonyx_ has not yet been obtained there, the
family of which it is a member was represented. In size, these creatures
varied from a tapir to an elephant, though all were much shorter-legged
than any elephant; the extremely massive tail, which the larger forms
had, served to support the huge body, when erected to tear down the
branches and leaves upon which these strange creatures fed.
[Illustration: FIG. 122.—A gigantic Pampean †Ground-Sloth (_†Megatherium
americanum_). Restored from a skeleton in the Museum of La Plata.]
Opossums were extremely numerous, especially in the Brazilian caves,
where in half a cubic foot of earth 400 jaws were collected.
The Pleistocene mammalian fauna of South America was a mixture of modern
forms with ancient, vanished types similar to that which we found in
North America. The †ground-sloths and †glyptodonts, the †litopterns,
†toxodonts and †typotheres, the antelopes, horses and †mastodons have all
disappeared from the continent, or vanished altogether from the face of
the earth.
II. TERTIARY FAUNAS
1. _Pliocene_
_North America._—No part of the Cenozoic history of North America is
so imperfectly recorded and so unsatisfactorily known as that of the
Pliocene, and the later portion of that epoch is especially obscure. If
the Peace Creek formation of Florida is properly referred to the upper
Pliocene, it would show that the mammals of that time were substantially
the same as those of the early Pleistocene.
The only fauna, as yet discovered, which can be referred to the middle
Pliocene, is that of the Blanco beds of northwestern Texas, which have
yielded but a scanty list of mostly ill-preserved fossils. Obviously,
these give us a very incomplete picture of the life of that time. The
great †ground-sloths had already reached North America, and the genus
_†Megalonyx_, so common in the forested areas of Pleistocene North
America, was perhaps already in existence. The †glyptodonts were likewise
represented by one genus (_†Glyptotherium_) which was distinguished
by the simple rings of the tail-sheath. No rodents have yet been found
and only a few of the Carnivora, though a large cat, a musteline and a
large “†bear-dog” are known. There were no true elephants, but several
species of †mastodons, all of which were different from those of the
Pleistocene; and in some, grinding teeth, though still low-crowned, had
become much larger and more complex, marking a stage of advance toward
the elephantine dentition. Horses of primitive type, the feet having
three functional toes instead of one, were relatively abundant. Very
large llama-like animals were present, but nothing has been ascertained
with regard to the deer and antelopes of the time, and the only
other representative of the Artiodactyla yet recovered is a peccary,
interesting as being a species of the genus (_†Platygonus_) which became
so abundant and widespread in the Pleistocene. Scanty and incomplete as
this fauna is, it suffices to show that the middle Pliocene mammals were
much more primitive than those of the Pleistocene.
[Illustration: FIG. 123.—†Horned Gopher (_†Epigaulus hatcheri_), lower
Pliocene, Nebraska. Restored from a skeleton in the U.S. National Museum.]
The fauna of the Snake Creek formation in western Nebraska and that
of the presumably somewhat later beds of northwestern Nevada, which
are referable to the lower Pliocene, may be considered together. The
rodents, which are not very fully represented, were quite modern in
character and belonged mostly to extinct species of modern genera, such
as hares, pocket-gophers, beavers, forerunners of the †Giant Beaver,
marmots, sewellels, etc. A remnant of a more ancient world, especially
characteristic of the Miocene, is found in the remarkable burrowers,
the horned †mylagaulids which have been extinct since the lower
Pliocene. Carnivora were abundant, and members of all the families which
inhabit North America to-day have been obtained; wolves, “†bear-dogs,”
“†hyena-dogs” and forms like the Dhole of India were common. The terms
“†bear-dogs” and “†hyena-dogs” are not to be understood as implying any
relationships of these animals to bears or hyenas, but merely a certain
superficial resemblance; these were very large members of the dog family
(Canidæ), now extinct. Mustelines, large and small, are found, and
possibly some bears had already made their way from the Old World, but
this is still uncertain. †Sabre-tooth tigers and true cats, some as large
as lions and one species fairly gigantic, were likewise characteristic
of the time. There was a great wealth of horses, though the modern genus
_Equus_ was not among them; all the genera are now extinct and all were
three-toed. Several distinct phyla were represented, some progressive and
advancing toward the modern forms, others conservative and stationary.
Browsing horses with low-crowned teeth, grazing horses with prismatic,
cement-covered teeth, heavier and lighter, larger and smaller, must have
covered the plains and thronged the woods. Ancestral tapirs were present,
though far less common. A family which seems to be utterly exotic to
North America, that of the rhinoceroses, was present, and of these there
were three or four series, mostly without horns, or with a very small
horn on the tip of the snout. The extremely aberrant perissodactyls
(†Ancylopoda), in which the hoofs were converted into great claws,
perhaps persisted, but the evidence is not conclusive.
The Artiodactyla were, for the most part, totally different from those
of modern times, though several forms were ancestral to some now
living. Peccaries more primitive than the living genus were the only
representatives of the swine-like suborder; ancestral camels and llamas
were among the commonest of the hoofed animals and an extinct phylum,
that of the “†giraffe-camels” (_†Alticamelus_) continued over from the
Miocene. The giraffe-camels are so called, not because of any actual
relationships with the giraffes, but on account of certain likenesses
in the proportions of the animals compared. _†Alticamelus_ was a very
large, camel-like creature, with remarkably elongate neck and limbs
and comparatively small head, which no doubt resembled the giraffes in
browsing upon trees which were above the reach of the ordinary camels and
llamas of the time. It was the terminal member of a series, or phylum,
which branched off from the main stock in Oligocene times and pursued a
course of development which was independent of the principal series, but
curiously parallel with it.
The deer of the lower Pliocene were little, graceful creatures
(_†Blastomeryx_) which had no antlers, but the males were armed with
sabre-like upper canine tusks, so that they must have resembled the
Musk-Deer of Tibet, but were smaller and more slender. The remarkable
group of “†deer-antelopes,” now extinct, was represented by _†Merycodus_,
a dainty little creature, less than two feet high at the shoulder,
which had the antlers and general appearance of a small deer, but the
high-crowned grinding teeth which most antelopes have. True antelopes
of two different lines were also present, though they are as yet known
from little more than the bony horn-cores; of these, one is the
flat-horned and the other the twisted-horned or strepsicerine type, such
as is illustrated by the Eland and Kudu of modern Africa. The latter
may, however, be related to the peculiarly North American Prong-Buck
(_Antilocapra_) and not to the strepsicerine antelopes of the Old
World. The last survivors of an exclusively North American family, the
†oreodonts, which were wonderfully numerous and varied from the upper
Eocene onward, are found here.
The †mastodons (_†Gomphotherium_) of this formation had well-developed
tusks in the lower as well as in the upper jaw, and in one species the
chin-region or symphysis of the lower jaw was greatly prolonged, an
ancient feature.
That the South American edentates had already reached the northern
continent is sufficiently proved by remains of †ground-sloths, which
are, however, too incomplete to permit identification of the genus.
†Glyptodonts have not yet been found, but this fact does not demonstrate
that they had not accompanied the †ground-sloths in their migration,
for at no time did they range so far north as Nebraska or northwestern
Nevada, and the only mammal-bearing formation of lower Pliocene date
known in the south, the Alachua Clay of Florida, has yielded too scanty a
list of fossils to make its negative evidence at all conclusive on this
point.
The mammals of the middle and especially of the lower Pliocene were much
stranger and more primitive than might be inferred from the foregoing
brief account. Except several of the Rodentia and perhaps one or two
of the Carnivora, the _genera_ are all extinct and such familiar terms
as horses, rhinoceroses, camels, etc., can be employed only in a very
comprehensive sense, as equivalent to families.
The Pliocene of _South America_ is involved in some obscurity; not
that there is any question as to the formations, or their order of
succession, but there is much doubt as to the limits of the epoch both
above and below. The latest Pliocene fauna, that of the Tarija Valley
in Bolivia, was essentially the same as that of the Pleistocene and
contained a similarly large proportion of migrant elements from the
north, but it was evidently older and many of the species were different.
The two divisions of the Araucanian fauna, contained in the beds of
Catamarca and Monte Hermoso respectively, are very much alike and need
not be given separate consideration. In one respect these presumably
upper Pliocene faunas formed a very strong contrast to the mammalian
assemblage of the Pleistocene, and that is in the quite insignificant
part taken by the migrants from North America. Of the Carnivora there
were but two representatives, one referable to the raccoon family and
one to the dogs, while a hare and a small member of the Artiodactyla, of
indeterminate family, complete the list of northern forms, though this
list will doubtless be extended by future discovery. The peccaries, deer,
antelopes, tapirs, horses, †mastodons, cats, weasels, otters, squirrels,
mice, etc. had not reached the southern continent, or were still so
rare that remains of them have not been found. This rarity and relative
insignificance of the northern forms gave a very different aspect to the
fauna.
On the other hand, the indigenous South American groups were very fully
represented. Many kinds of opossums and a few large carnivorous types,
much like the so-called Tasmanian Wolf (_Thylacynus_), were the remnants
of a much larger assemblage of marsupials which inhabited South America
in the Miocene. Of the Edentata, there were great abundance and variety,
many large †glyptodonts and some gigantic armadillos, as well as numerous
examples of normal size; the †ground-sloths, though somewhat smaller
than those of the Pleistocene, were mostly of gigantic size, and true
or arboreal sloths (Tardigrada) have been reported. The very numerous
rodents, with the exception of the intrusive hare, all belonged to
typically South American families. Some of the rodents were gigantic
and one (_†Megamys_), a member of the Chinchilla family, was equal to a
rhinoceros in size and the largest known representative of the order.
Especially characteristic was the abundance of the cavy family (Caviidæ).
The hoofed animals, with the single known exception of the immigrant
artiodactyl, all belonged to the autochthonous orders, all of which
are extinct at the present time. Forerunners of the extraordinary
genus _†Macrauchenia_, which was one of the most conspicuous elements
of Pleistocene life, were quite common in the Pliocene and differed
from the Pampean genus chiefly in their smaller size and less advanced
specialization. We find here also the last survivors of another family of
the †Litopterna, the †proterotheres (†Proterotheriidæ), which imitated
the horses in such a surprising manner that some authorities believe
them to have been actually related to those perissodactyls. The Monte
Hermoso genus (_†Epitherium_) had feet which were wonderfully, though
but superficially, like those of the three-toed horses. The †Toxodonta
were numerous and most of them were large, ponderous animals; one genus
(_†Trigodon_) had the interesting peculiarity of a single median horn
on the forehead, much like that of a rhinoceros. Horned species were
always rare among the indigenous groups of South American ungulates, and
all that have been discovered so far belonged to the †toxodonts. The
remaining group, that of the †Typotheria, was also well represented,
both by larger and by very small forms, some no larger than a rabbit
(_†Pachyrukhos_).
The presumably lower Pliocene (perhaps upper Miocene) fauna of the
Paraná formation is as yet known only from very fragmentary material.
Representatives of the dogs, raccoons and bears have been reported, but
the identifications are doubtful; at all events, these would seem to
have been the most ancient of the northern immigrants. A considerable
number of marsupials, both opossums and large predaceous types, have been
found. The rodents were very numerous, all belonging to South American
families and some of them very large. The edentates were gigantic
†ground-sloths and †glyptodonts, with numerous armadillos of ordinary
size. The hoofed animals all belonged to the indigenous South American
orders, the predominant place being taken by the †toxodonts, some of
which were large. There were many †typotheres, both of the larger and
smaller kinds. The †Litopterna were represented both by the horse-like
†proterotheres and the long-necked †macrauchenids, the latter smaller and
less specialized than those of the Pampean.
[Illustration: FIG. 124.—Head of Horned †Toxodont (_†Trigodon gaudryi_).
Pliocene of Monte Hermoso. Restored from a skull in the Ameghino
collection.]
2. _Miocene_
_North America._—Upper Miocene beds cover extensive areas of the Great
Plains region and are scattered from Montana far into Mexico. The rich
fauna is an outgrowth and development of that of the middle Miocene, with
but few immigrant additions and, on the other hand, passes so gradually
into that of the lower Pliocene, that any line of separation between
them is very difficult to draw. The rodents, numerous as they are among
the fossils, are almost certainly very incompletely represented in the
collections; the families are almost all still in existence, but nearly
every genus is extinct, and thus the vernacular names used to designate
them must be understood in a broad sense. Hares, mice, pocket-gophers,
squirrels, marmots, beavers and the extraordinary †mylagaulids were all
abundant.
In even more strongly marked sense must the broad meaning for the
vernacular names of the other mammals be emphasized, for we have to deal
almost exclusively with extinct genera, which differed much from their
modern descendants. Many of the Carnivora have been obtained; there were
numerous dogs, some rivalling the largest of existing bears in size, true
felines and †sabre-tooth tigers, which were smaller and lighter animals
than the great beasts of the Pleistocene; weasels, martens, otters and
raccoons, but no bears. The bears, a family of Old World origin, are
not certainly known in America before the Pleistocene, but had probably
reached this continent in the Pliocene.
As is so very generally true, the commonest and best-preserved of the
fossils are those of the hoofed animals. The †mastodons were of the
four-tusked kind (_†Gomphotherium_ or _†Trilophodon_), the skull and
teeth of which differed so markedly from those of the true elephants.
The relatively small, low-crowned and simple grinding teeth were common
to all the †mastodons, but the tusks were different from those of the
larger members of the group. The upper tusks were comparatively short
and nearly straight and retained a band of enamel, while the lower tusks
were still shorter, chisel-shaped and so worn as to prove that they were
regularly used, no doubt in cropping leaves; the shortness of these lower
tusks was compensated for by the great elongation of the lower jaw. The
head was proportionately broad and low and, for Proboscidea, these were
small animals, not more than five or six feet high at the shoulder.
The body, limbs and feet had already attained substantially their
modern grade of structure, advance among the Proboscidea being chiefly
restricted to the teeth and skull.
[Illustration: FIG. 125.—_†Teleoceras fossiger_, a short-legged
rhinoceros, with small nasal horn; lower Pliocene and upper Miocene of
Nebraska. Restored from a skeleton in the American Museum of Natural
History.]
Four families of Perissodactyla were represented in the upper Miocene.
The rhinoceroses, which were very abundant, were present in considerable
variety; some were hornless, others had a single small horn on the end
of the nose. Among these rhinoceroses there was much difference in
bodily proportions, some being extremely heavy, with very short legs
and feet, and these were the commonest, while others had longer legs and
less massive bodies. Tapirs, on the contrary, would seem to have been
scantily represented; at least, they are rare among the fossils. The
extraordinary, aberrant †chalicotheres, perissodactyls with claws instead
of hoofs, still persisted, but are far better known from the lower
Miocene, in connection with which they will be described. The dominant
perissodactyl family was that of the horses, of which no less than five
genera are already known. There were some with very low-crowned teeth,
which must have fed principally by browsing upon leaves and such soft
diet; but the grazing kinds, which had high-crowned, cement-covered and
very complex grinding teeth, had come to the fore. Still retaining three
toes in each foot, with the middle toe so enlarged as to bear nearly the
entire weight, save in snow or soft ground, these eminently cursorial
animals, which had the slender limbs of a deer, must have roamed the
plains in great herds.
Still commoner were the Artiodactyla. Many species of grazing camels,
which were the predominant artiodactyl family in North America during
upper Miocene times, were the ancestors both of the true camels of
the Old World and the South American llamas. †Giraffe-camels have not
yet been found and no doubt they were much less abundant than in the
middle Miocene, but that they had not completely disappeared is shown
by their recurrence in the Pliocene. As compared with earlier ages, the
†oreodonts had begun a rapid decline and had lost notably both in numbers
and variety, but one most curious beast (_†Pronomotherium_, Fig. 197,
p. 375) marked the final step in the development of the short-faced,
proboscis-bearing series, which may be traced back to its beginnings in
the Oligocene. In this wonderful creature the skull was so short and
deep as to suggest that of a gorilla or some other great ape. No other
artiodactyls even approximate these later proboscis-bearing †oreodonts
in the altogether exceptional form of the skull. Grazing †oreodonts
(_†Merychyus_), of moderate and small size with high-crowned teeth,
were evidently quite common on the upper Miocene plains. The †hornless
deer and “†deer-antelopes” differed but little from those of the lower
Pliocene. Peccaries were fairly abundant.
[Illustration: FIG. 126.—_†Procamelus elrodi_, a large camel from the
upper Miocene. Restored from specimens in the Carnegie Museum.]
The upper Miocene fauna was especially characterized by the large number
of mammals, belonging to several different orders, which had acquired the
high-crowned, persistently growing pattern of grinding teeth. Many of the
horses, camels, ruminants and rodents displayed this structure, and, as
was first pointed out by Kowalevsky, the explanation is probably to be
found in the spread of grassy plains at the expense of the forests. On
account of the silica which they contain, the grasses are very abrasive
and rapidly wear the teeth down. In adaptation to this new source of
abundant and nutritious food, many kinds of mammals developed a form
of tooth which was fitted to compensate by growth for the loss through
abrasion.
The middle Miocene, small areas of which occur in Montana, eastern Oregon
and northeastern Colorado, has received various local names, the typical
one being the Deep River of Montana. Very probably, these scattered
areas are not strictly contemporaneous, but form a closely connected
series. That a land-connection with the eastern hemisphere existed, is
made clear by the appearance of several unmistakably Old World types of
animals and the beginnings of migration from South America are perhaps
also to be noted, though this cannot be positively stated. The evidence
for the South American connection is the finding in the middle Miocene of
Oregon of what are believed to be the earliest remains of †ground-sloths
yet discovered in North America, but the material is too scanty for
altogether certain determination.
The smaller animals are not very well represented in the middle
Miocene faunas, as conditions appear to have been unfavourable to
their preservation; something is known of them, nevertheless. The
very curious extinct family of rodents known as the †Mylagaulidæ, the
presence of which was noted in the upper Miocene and lower Pliocene,
first appeared here. These †mylagaulids, which were distantly related
to the modern Sewellel (_Aplodontia rufa_), were characterized by the
great enlargement and complication of one of the grinding teeth in
each jaw and the consequent reduction of the others. One genus of this
family, as in the Pliocene, had the peculiarity, unique among rodents, of
developing a large horn upon the nose, like a miniature rhinoceros. Among
the Carnivora, we find a great variety of dogs, large and small, all
belonging to extinct genera, as indeed is true of the other carnivores
also. True felines have been found, but as yet, none of the †sabre-tooth
series; the abundance of the latter, however, in both preceding and
succeeding formations, is sufficient proof that the discovery of them in
the middle Miocene is merely a question of time. Mustelines were present,
and especially noteworthy is the appearance of the first American otters,
immigrants from the Old World.
Of the hoofed animals, the most interesting are the Proboscidea, the
most ancient of which that are definitely determinable in America occur
in this horizon. The place of origin and ancestry of these animals were
long exasperating puzzles. Appearing suddenly in the Miocene of Europe
and North America, in which regions nothing was known that could, with
any plausibility, be regarded as ancestral to them, they might as well
have dropped from the moon, for all that could be told concerning their
history. The exploration of the Eocene and Oligocene beds of Egypt has
dispelled the mystery and shown that Africa was the original home of the
group, whence they gradually spread to every continent except Australia.
Little is known of these earliest American proboscideans, but they were
doubtless small †mastodons of the four-tusked type.
Among the Perissodactyla, the rhinoceroses were perhaps the most
conspicuous; the native American stocks of this family appear to have
mostly died out and to have been replaced by two or more phyla of
immigrants from the Old World, some of which were hornless, others had
a small horn on the tip of the nose and others again had a second and
smaller horn on the forehead. Tapirs, though unquestionably present, are
rare as fossils and not well known. Several distinct phyla of horses
may be distinguished, which were like small ponies in size, but of more
slender form; they were all three-toed, but there were marked differences
among them with regard to the degree to which the middle toe (the
third of the original five) had been enlarged to carry the whole weight
and the lateral toes (second and fourth) reduced to mere “dew-claws.”
While browsing horses, with low-crowned teeth, still persisted in large
numbers, we find also the extremely interesting beginnings of the highly
complex, cement-covered and high-crowned teeth of the grazing kinds. The
clawed †chalicotheres were present, though very little is known about
them because of the fragmentary character of the remains.
The Artiodactyla were much more varied and abundant, though they did
not rival the great assemblage of these animals found in the European
Miocene. Of the peccaries little more can be said than that they were
present in these faunas. The †oreodonts were very numerous, both
individually and generically; two stages of the proboscis-bearing kind
are found here together, the older, long-faced genus (_†Promerycochœrus_)
surviving from the Oligocene, while the newer Miocene type was
short-faced and had a moderate proboscis (see Fig. 196, p. 373). Others
had more the proportions of peccaries and still others were very small
and presumably aquatic in habits. Camels abounded, both the grazing kinds
which were ancestral to the modern forms of South America and Asia, and
the great, browsing †giraffe-camels. The †hornless deer and the antlered
†deer-antelopes were much like those of the Upper Miocene, slender and
graceful little creatures, and there were also considerably larger
ruminants (_†Dromomeryx_) with straight, simple and non-deciduous horns,
which may be called antelopes.
[Illustration: FIG. 127.—Gigantic †giraffe-camel (_†Alticamelus altus_)
from the middle Miocene of Colorado. Restored from specimens in the
American Museum of Natural History.]
The line of division between the lower Miocene and the uppermost
Oligocene is a very obscure and difficult one to draw. Personally, I
prefer to begin the Miocene with the widespread formation of the Great
Plains, which has been variously named Arikaree, Harrison, Rosebud,
etc., but this is a moot point. Concerning the lower part of these beds
Osborn says: “They may be either: (1) Upper Oligocene or (2) transitional
from Oligocene to Miocene, or (3) of pure Lower Miocene age.” The upper
division is referred to the Miocene without question by any one, but
for the purposes of this rapid sketch it will be best to treat the two
faunas together. This many-named formation, for which the term _Arikaree_
is here employed, as having priority, is found over extensive areas of
South Dakota, northern Nebraska and central Wyoming. The fauna was almost
entirely a development from that of the North American Oligocene, with
very little admixture of foreign elements, so that the land communication
with the eastern hemisphere must have been difficult. In this, as in
most of the Miocene formations, the smaller mammals are not fairly
represented, and it is evident that much remains to be learned with
regard to them; this is especially true of the upper division of this
stage.
[Illustration: FIG. 128.—Most ancient American Antelope (_†Dromomeryx
antilopina_), middle Miocene. Restored from specimens in the Carnegie
Museum and Princeton University.]
The rodents, which were fairly numerous, were directly continuous with
those of the upper Oligocene and included forms which were more or
less distantly connected with the modern hares, squirrels, beavers,
sewellels, pocket-gophers and kangaroo-rats. A few Insectivora of
doubtful reference have been found. Among the Carnivora there was also
considerable variety: dogs, large and small, were abundant, but all of
them were decidedly primitive from the modern standpoint; the cats were
represented both by the true felines, which were probably immigrants,
and by the †sabre-tooth series. There were several large and powerful
mustelines, or members of the weasel family, which were likewise
immigrants, one of which resembles in many ways the modern Wolverene
(_Gulo_). Very interesting is the beginning of the raccoon family
(Procyonidæ) or, at least, what is believed to be such, which arose from
a branch of the dogs; this most ancient of the raccoons was _†Phlaocyon_,
a small and slender animal.
The earliest traces of the Proboscidea in America have been reported
from this formation, but the fragmentary specimens are inconclusive.
The Perissodactyla are among the commonest fossils. The rhinoceroses
belonged to native stocks, including both the horned and hornless forms.
The horned genus (_†Diceratherium_) differed from all other rhinoceroses
in having a transverse _pair_ of horns on the nose, and the species
of the lower Miocene were quite small and light; the hornless genus
(_†Cænopus_) was a larger and heavier animal. Tapirs are rare as fossils
and consequently not well known. While there were several kinds of
horses, they all agreed in having short-crowned and relatively simple
grinding teeth and three-toed feet; they were smaller and of lighter,
more slender build than those of the middle Miocene. The wonderful
aberrant perissodactyls with clawed feet, the †chalicotheres (suborder
†Ancylopoda), appear to have been more abundant in the Arikaree than at
any other time in North America, though their history in this continent
extends from the middle Eocene to the lower Pliocene. _†Moropus_, the
lower Miocene genus, was as grotesque a creature as could well be
imagined and, in advance of experience, no one ever did imagine such a
beast. With rather small and somewhat horse-like head, long neck, long
fore limbs and shorter hind limbs, these extraordinary animals united
short, three-toed feet, which were armed with enormous claws. The long
persistence (to the Pleistocene of Asia) and wide geographical range
of the †chalicotheres are sufficient evidence that their very unusual
structure must have been advantageous to them, but the problem of their
habits and mode of life is still unsolved. From the character of the
teeth, the long neck and fore limbs, it may, however, be inferred that
they fed chiefly upon the leaves of trees.
[Illustration: FIG. 129.—The small, †paired-horned rhinoceros
(_†Diceratherium cooki_) of the lower Miocene. Restored from a skeleton
in the Carnegie Museum, Pittsburgh.]
[Illustration: FIG. 130.—A †chalicothere (_†Moropus elatus_) of the lower
Miocene. Restored from a skeleton in the Carnegie Museum, Pittsburgh.]
Even more numerous and varied were the Artiodactyla. Peccaries of a
primitive sort were common, and we find the last of the series of Ҡgiant
pigs,” which had been a very conspicuous group throughout the Oligocene.
The lower Miocene genus, _†Dinohyus_, was a monstrous beast, six feet
or more in height, with formidable canine tusks and a very long head
made grotesque by bony excrescences upon the skull and jaws. For a pig,
the legs were very long and the feet slender, having but two toes. The
†oreodonts were present in great numbers, both small and large forms;
except for bodily stature and modifications of the head, they all
looked very much alike; _†Merycochœrus_, with its incipient proboscis,
here made its first appearance. The last representatives of a family
(†Hypertragulidæ) of small and graceful artiodactyls are found in this
formation. One of these (_†Syndyoceras_, _see_ Fig. 215, p. 403), an
animal considerably larger than the existing Musk-Deer, was in its way
even more bizarre-looking than the †chalicotheres; with an antelope-like
head, it had four horns, one pair over the eyes, curving inward, and
a shorter pair, with outward curvature, on the muzzle. Another genus
(_†Hypertragulus_) was very much smaller and very slender.
The camels were beginning to diversify and give rise to several phyla.
One of the genera (_†Protomeryx_), which did not much exceed a sheep
in size, probably represented the main stock, which led to the camels
and llamas of to-day. A second (_†Stenomylus_) was a still smaller
animal, with remarkably long and slender legs, and might be called a
“gazelle-camel,” while a third (_†Oxydactylus_, _see_ Fig. 209, p. 392),
which was larger and apparently the beginning of the †giraffe-camels,
was noteworthy for its long neck. All of these lower Miocene camels
had deer-like hoofs, the characteristic pad or cushion which gives
such an exceptional appearance to the feet of modern llamas and camels
not being fully developed till a later period. A very important new
element in the North American fauna was the appearance of the first deer
(_†Blastomeryx_), which came in the latter part of the Arikaree stage
and were the forerunners of a renewed immigration from the Old World,
which had been broken off during the upper Oligocene. This, however, is
a disputed point; Professor Osborn and Dr. Matthew believe that these
animals were truly indigenous and derived from a long line of American
ancestry. The same genus continued through the middle Miocene, as we have
already seen, and therefore no further description of it is called for.
The limits of the _South American_ Miocene are very doubtful. The Paraná
formation, here regarded as lower Pliocene, may prove to be more properly
referable to the upper Miocene. No other upper Miocene is known.
[Illustration: FIG. 131.—The †gazelle-camel (_†Stenomylus hitchcocki_)
of the lower Miocene. Restored from skeletons in the Carnegie Museum,
Pittsburgh.]
To the earlier, probably middle, Miocene may be referred the wonderful
Santa Cruz fauna of Patagonia. It is extremely difficult to convey to
the reader any adequate conception of this great assemblage of mammals,
because most of them belonged to orders which have altogether vanished
from the earth and are only remotely like the forms with which we are
familiar in the northern hemisphere. To one who knows only these northern
animals, it seems like entering another world when he begins the study of
the Santa Cruz fossils. If any North American mammals had then entered
South America, which is not probable, they had not extended their range
as far as Patagonia. Marvellously rich and varied as the Santa Cruz fauna
was, it did not contain everything that we should expect to find in it;
several recent families of undoubtedly indigenous South American origin
have left no ancestors in the early Miocene formations. For this, there
are several obvious reasons. In part, these gaps in the history are
merely due to the accidents of collecting and some of them will almost
certainly be filled by future exploration. Other absentees will probably
never be found, because the Santa Cruz beds are known only in the very
far south, and the Miocene climate of the region, though much milder and
more genial than the present one, must have been unsuitable for many
tropical animals. Again, the Patagonia of that time appears to have been
a country of open plains, with few trees, and hence arboreal forms were
rare.
[Illustration: FIG. 132.—Diagram to illustrate the comparative sizes of
the Santa Cruz mammals, a modern pointer dog, within the rectangle, to
give the scale. 1. _†Cladosictis lustratus_, predaceous marsupial. 2.
_†Protypotherium australe_, †typothere. 3. _†Eocardia excavata_, rodent.
4. _†Stegotherium tesselatum_, armadillo. 5. _†Propalæohoplophorus
australis_, †glyptodont. 6. _†Hapalops longiceps_, †ground-sloth. 7.
_†Thoatherium minusculum_, †litoptern. 8. _†Astrapotherium magnum_,
†astrapothere. 9. _†Prothylacynus patagonicus_, predaceous marsupial.
10. _†Theosodon garrettorum_, †litoptern. 11. _†Nesodon imbricatus_,
†toxodont.]
While great numbers of large, flightless birds, some of them of enormous
size, were entombed in the volcanic ash and dust which were spread
over such wide areas and to such great depths, the extreme scarcity of
reptiles is surprising; a few remains of lizards have been found, but no
snakes, crocodiles, or tortoises, and we have no information as to the
plant-life of the region at that time. The mammals were almost all of
small or moderate size; only one or two species were really large.
One very striking and characteristic feature of the Santa Cruz fauna is
the great abundance of marsupials which it contained and which resembled
more or less those of modern Australia. There were no true Carnivora and
their places were taken by a variety of carnivorous marsupials, some
of which (_e.g._ _†Prothylacynus_) were as large as wolves and were
closely similar to the so-called Tasmanian Wolf (_Thylacynus_). Another
genus (_†Borhyæna_) had a short, bullet head, not unlike a small Puma
in appearance and, besides, there were many smaller beasts of prey, in
size like badgers and minks. Opossums were common and there were many
very small herbivorous marsupials, which resembled, though perhaps but
superficially, the Australian phalangers. At the present day South
America contains no Insectivora, but in the Santa Cruz there was one
family (†Necrolestidæ) of this order which bore considerable resemblance
to the “golden moles” of South Africa. An extraordinary variety of
rodents inhabited Patagonia in Santa Cruz times, all of them belonging
to the Hystricomorpha, or porcupine suborder, and all referable to
existing South American families. There were none of the northern forms
of rodents, neither rats, mice, squirrels, marmots, hares, nor rabbits,
but a very numerous assembly of tree-porcupines, cavies, chinchillas,
coypus and the like. The genera, though closely allied to existing ones,
are all extinct, and the animals were very generally smaller than their
modern descendants. A few small monkeys of unmistakably Neotropical type
have been found, but like other arboreal and forest-living animals, they
are very rare among the fossils.
The Edentata were more abundant and diversified than at any other time
in South American history of which the record is preserved. Two of the
modern subdivisions of this order have not been certainly identified
in the Santa Cruz collections, the arboreal sloths and the anteaters,
and though they may be found there at any time, it will only be as
stragglers from the warmer forested regions to the north, where these
forms had doubtless long been present. Unfortunately, however, nothing
is directly known concerning the life of those regions in Miocene
times. On the other hand, three groups of edentates, two of them now
extinct, were very copiously represented in the Santa Cruz formation,
the armadillos, †glyptodonts and †ground-sloths. Of the many armadillos,
some quite large, others very small, only a few can be regarded as
directly ancestral to those now in existence; the truly ancestral
forms were probably then living in the forests of Brazil and northern
Argentina, in the same areas as the ancestral tree-sloths and anteaters.
In comparison with the giants of the Pliocene and Pleistocene, the Santa
Cruz †glyptodonts were all small, the carapace rarely exceeding two
feet in length, and, what it is particularly interesting to note, they
departed much less widely from the armadillo type than did their gigantic
successors. The †ground-sloths were present in actually bewildering
variety and they also were very small as compared with the huge animals
of the Pleistocene, none of them exceeding the Black Bear in height or
length, though proportionally much more massive, and many were no bigger
than foxes. They had small heads, long bodies, heavy tails and short,
thick legs; their teeth show that they were plant-feeders, but their feet
were armed with long, sharp and formidable claws. Among this great host
of Santa Cruz †ground-sloths may readily be noted the probable ancestors
of the gigantic creatures which were such characteristic elements of the
Pliocene and Pleistocene faunas.
There was an extraordinarily rich and varied assemblage of hoofed
animals, all utterly different from those of the northern hemisphere and
belonging to groups which have never been found outside of South and
Central America. Of these groups there were five, which by different
writers are variously regarded as orders or suborders, a matter of very
secondary importance. Individually, the commonest of the hoofed mammals
were the †Toxodonta, which ranged in size from a sheep to a tapir,
heavily built and clumsy creatures, with absurdly small, three-toed feet;
in some of the species there was a small median horn on the forehead. As
with the †glyptodonts and †ground-sloths, the contrast in size between
the Santa Cruz ancestors and the Pleistocene descendants was very
striking. A very numerous and varied group was that of the †Typotheria,
all small animals, some no larger than rabbits, others the size of
small foxes. It requires a decided effort to think of these †typotheres
as being really hoofed animals at all, as their whole appearance must
have been much more like that of rodents, yet their structure clearly
demonstrates their near relationship to the †toxodonts. Still a third
group of the same series, the †Entelonychia, is of great interest, for,
as in the †chalicotheres of the northern hemisphere, the hoofs had been
transformed into claws and their five-toed feet had a truly grotesque
appearance, not diminished by the long and powerful limbs and relatively
small head.
This is the third example of that paradoxical creature, a “hoofed animal”
with claws instead of hoofs, and in each of the three instances, there
is every reason to believe, the transformation proceeded independently.
Among the perissodactyls the †chalicotheres (p. 238) underwent this
change; in North America the †Agriochœridæ, a family of artiodactyls,
had a very similar history, while in South America the †Entelonychia
arose from the same stock as the †toxodonts, with which they were nearly
allied. They were among the largest animals of Santa Cruz times and
ranged in size from an ox to a rhinoceros.
There was a fourth group, the †Astrapotheria, concerning which our
knowledge is tantalizingly incomplete, some species of which were the
largest of known Santa Cruz mammals, while others were much smaller.
They had short, domed heads, with a considerable proboscis, and were
armed with formidable tusks, which were the enlarged canine teeth, the
only known instance of large canine tusks among the indigenous South
American hoofed animals. The limbs were long and not very massive, the
feet short, five-toed and somewhat elephantine in appearance. These
bizarre animals would seem to have held a rather isolated position among
the South American ungulates, and though they may be traced back to the
most ancient mammal-bearing beds of that continent, their relationships
are still obscure; much more complete material must be obtained before
this problem can be definitely solved. Both the †Astrapotheria and the
†Entelonychia died out shortly after the end of the Santa Cruz.
From many points of view the most interesting members of the Santa
Cruz fauna were the †Litopterna, an order which also went back to the
earliest South American Tertiary. In the Miocene and Pliocene the order
was represented by two very distinct families, the †Macrauchenidæ and
†Proterotheriidæ, which were superficially very unlike. In the Santa Cruz
beds is found a genus (_†Theosodon_) which was apparently the direct
ancestor of the Pampean _†Macrauchenia_. The Miocene genus was a much
smaller animal and had hardly more than an incipient proboscis, but
otherwise was very like its Pampean successor; it was somewhat larger and
heavier than a Llama and probably bore some resemblance to that animal
in appearance. The long, narrow head, with its prehensile upper lip,
must have had an almost reptilian likeness from the numerous uniform and
sharp-pointed teeth with which the front of the jaws was supplied; the
neck was elongate, the body short and rather slender and the legs long,
ending in three nearly equal toes.
The †proterotheres, on the other hand, were almost the only Santa Cruz
ungulates which had nothing _outré_ or grotesque about them to the eye of
one habituated to the faunas of the northern hemisphere. They were small,
graceful animals, very like the Miocene horses of the north in their
proportions, though having much shorter necks and shorter, heavier heads.
In some genera of this family (_e.g._ _†Diadiaphorus_, _†Proterotherium_)
the feet were three-toed and most surprisingly horse-like in shape, but
one genus (_†Thoatherium_) was absolutely single-toed, more completely
monodactyl than any horse. The horse-likenesses ran all through the
skeleton and are so numerous and so striking that several writers have
not hesitated to incorporate the †Litopterna with the Perissodactyla,
but this I believe to be an error. If the †proterotheres were not
perissodactyls, as I am convinced they were not, they afford one of the
most remarkable examples of convergent evolution among mammals yet made
known.
3. _Oligocene_
_North America._—The John Day formation of eastern Oregon represents
the upper Oligocene and has yielded a very extensive series of mammals,
though with some obvious gaps that remain to be filled by future work.
The land-connection with the Old World which had existed in the lower
Oligocene and was restored in the lower, or at latest in the middle,
Miocene, was interrupted in John Day times, and so the mammals assumed a
purely indigenous character.
No opossums or other marsupials have been found, and nothing is known
of the Insectivora. Of the Carnivora, there were but three families,
and one of these, the mustelines, was represented but scantily by a
few small species. Cats of the †sabre-tooth subfamily were common and
one species was quite large, almost equalling the Jaguar in length;
but most of the species were small, much smaller than the Pleistocene
members of the group. True cats are not definitely known to have been
present, but there were two genera (_†Nimravus_ and _†Archælurus_)
which have been called the “false †sabre-tooths,” which may prove to be
referable to that series. The dogs, on the other hand, were remarkably
numerous and diversified, more so than ever before or since; none of
them was very large, the largest but little exceeding the Timber Wolf
in size, and some were extremely small; but the number of distinct
genera and species and the differences among them are quite remarkable.
Both long and short-faced forms and early stages of the “†bear-dogs,”
and “†hyena-dogs,” and ancestral forms of the wolves and dholes may
be distinguished, a truly wonderful assemblage. The rodents also were
numerous and varied, including ancient and extinct genera of the
beavers, squirrels, mice, pocket-gophers and hares and the earliest
distinguishable ancestors of the sewellels (Aplodontiidæ).
The remainder of the known John Day fauna was composed of artiodactyls
and perissodactyls. The latter had suffered serious losses as compared
with the preceding or White River stage. Up to and through White River
times the perissodactyls had held their own in actual diversity, though
the rise of the artiodactyls had put an end to the dominant position
which they had maintained in the Eocene. With the John Day the actual
decline may be said to have begun. The rhinoceroses were represented
chiefly by the †diceratheres, with a transverse pair of horns, some
species of which were much larger than those of the lower Miocene.
Hornless rhinoceroses have not yet been certainly found, though there is
every reason to believe that they then existed, as they unquestionably
did both before and after. Tapirs occurred but rarely and the horses were
individually abundant, though in no great diversity; they were smaller
and lighter than the horses of the lower Miocene. Enough has been found
to demonstrate the presence of the clawed †chalicotheres, but not to show
how they differed from their immediate successors.
In the number of individuals, species, genera and families, the
artiodactyls of the John Day much exceeded the perissodactyls. The
peccaries were numerous, but smaller and more primitive than those of
the succeeding age, as were also the †giant pigs, or †entelodonts,
but the latter were very large. The peculiarly North American family
of the †oreodonts was very numerously represented, and one genus
(_†Promerycochœrus_), comprising animals not unlike the Wild Boar in size
and shape, was the probable beginning of the series of proboscis-bearing
†oreodonts, which led to such grotesque forms in the middle and upper
Miocene. A family closely allied to the †oreodonts, and by many
writers included in the latter, is the very remarkable group of the
†Agriochœridæ, which was distinguished by the long, stout and cat-like
tail and by the possession of claws instead of hoofs. The family is not
known to have existed later than the John Day and no trace of it has been
found in the succeeding formations. The camels seem to be all comprised
in a single genus (_†Protomeryx_) which was the same as that found in the
lower Miocene. A very small and dainty little creature (_†Hypertragulus_)
belonged to another family, the relationships of which are not clear.
To the middle and lower Oligocene is referred the great White River
formation of South Dakota, Nebraska, Wyoming, etc., which is divisible
into three clearly marked substages. The White River contains the
best-known fauna of all of the North American Tertiaries, for collecting
in these beds has been carried on for more than sixty years, and a
greater number of complete and nearly complete skeletons has been secured
than from any of the other formations. It is plainly evident that a
land-connection existed with the Old World, which was interrupted in the
John Day, as is shown by the intermigration of characteristic forms; but
some barrier, presumably climatic, prevented any complete interchange of
mammals, and very many genera and even families remained confined to one
continent or the other.
The aspect of the White River fauna changes in accordance with the
direction from which it is approached. If one comes to the study of it
from the Eocene, it displays a very modern aspect, given by the almost
complete disappearance of the archaic groups of mammals and by the great
multiplication of genera and species belonging to the progressive orders.
These genera, it is true, are all extinct, but many of them stood in an
ancestral relationship to modern forms. On the other hand, if approached
from the Miocene side, the White River mammals seem to be very ancient
and primitive and very different from anything that now lives. We speak
of horses and rhinoceroses, dogs and cats, in this fauna, but those
terms can be employed only in a very wide and elastic sense to designate
animals more or less distantly allied to those of the present day.
[Illustration: FIG. 133.—1. _†Archæotherium._ 2. Ancestral camel
(_†Poëbrotherium_). 3. _†Merycoidodon._ 4. _†Agriochœrus._ 5. Ancestral
horse (_†Mesohippus_). 6. _†Hoplophoneus._ 7. _†Bothriodon._ 8.
_†Hyænodon._ 9. †Cursorial rhinoceros (_†Hyracodon_). 10. _†Protoceras._
11. Hornless rhinoceros (_†Cænopus_).]
Several species of opossums, some of them very small, were the only
marsupials in North America then, as they are now. There was quite a
variety of Insectivora; some were survivals of a family that was abundant
in the Eocene, others, like the hedgehogs, moles and shrews, were
probably immigrants. Here we find the last of a group (order or suborder)
of ancient and primitive flesh-eaters, the †Creodonta, that had played a
great rôle in the Eocene and Paleocene of North America and Europe. In
White River times but a single family (†Hyænodontidæ), with two genera,
remained of the Eocene host. One of these genera (_†Hemipsalodon_), a
very large beast of prey, which was almost identical with the Old World
genus _†Pterodon_, was confined to the lower substage of the White River
beds in the Northwest Territory of Canada; the other, _†Hyænodon_, which
was also an Old World form, was represented abundantly in the United
States by many species. In size, these species ranged from a small fox
to a large wolf, but they all had disproportionately large heads, and
small, weak feet, with blunt claws, so that they must have been very
curious-looking creatures and were probably carrion-feeders rather than
active catchers of prey. The White River members of the family were
migrants from the eastern hemisphere, for, though small and primitive
representatives of it occurred in the North American Eocene, as well as
in the corresponding formations of Europe, the family appears to have
died out in America and to have been renewed by the Oligocene migration.
[Illustration: FIG. 134.—White River †titanothere (_†Titanotherium
robustum_) reduced to the same scale as Fig. 133.]
Coincident with this decline of the †creodonts and, no doubt, causally
connected with it, was the rise of the true Carnivora, which for the
first time were numerous and were divisible into three distinct families.
Small and primitive representatives of the wolves (_†Daphœnus_) and
possibly also of the foxes (_†Cynodictis_) were quite common, and there
were a few species of the musteline family, evidently immigrants and the
most ancient yet found in America. There were several species of the
†sabre-tooth cats (_†Dinictis_ and _†Hoplophoneus_) all of which, except
in the uppermost substage, were quite small, few of them exceeding the
Canada Lynx in size. A much larger animal (_†Eusmilus_, also European)
appeared in the latter part of the stage. None of the true cats, or
feline subfamily, has been obtained. Nothing is yet known of the time
and place of origin of the †sabre-tooth series, for they appeared at
approximately the same date in Europe and America, and in neither
continent have any possible ancestors been found in preceding formations.
The problem is like that of the Proboscidea (_see_ p. 234), but Egypt
has given no help in the case of the †sabre-tooths, and, by a process
of elimination, we reach the conclusion that these strange creatures
probably arose somewhere in Asia and sent out migrants eastward and
westward.
The Rodentia were fairly abundant and present a strange mixture
of ancient and comparatively modern types. One very common genus
(_†Ischyromys_), which was the last remnant of a family almost limited
to the North American Eocene, was associated with the earliest American
mice, arboreal and ground squirrels, beavers and rabbits; some, if not
all, of these were immigrants.
The hoofed mammals were present in fairly bewildering variety, but were
restricted to the two orders of the Perissodactyla and Artiodactyla. The
Perissodactyla, while they no longer had the relatively dominant position
which they held in the middle Eocene (_see_ p. 270), had suffered no
actual loss; and no less than seven families of them, or six by another
scheme of classification, had members in the North America of White
River times, a very notable difference from the present order of things,
when there are but three families in the entire world, none of which
enters North America. The Eocene family of the †titanotheres became
extinct at the end of the lower substage of the White River, but in that
substage there was a marvellous abundance of these huge beasts, some of
which were of almost elephantine stature and bulk. The pair of great
bony, horn-like protuberances on the nose varied much in size and form
in the different species, short to very long, triangular, cylindrical,
flattened and shovel-shaped, and gave these ungainly creatures somewhat
the appearance of strange and very large rhinoceroses. The †titanotheres
were a typically North American family, but sent migrants to the Old
World, at least two species reaching southeastern Europe. Rhinoceroses
too were extremely numerous and diversified throughout the stage and are
very plainly divisible into three strongly contrasted series, which are
sometimes regarded as three subdivisions of the same family and sometimes
put into two separate families. One of these series, the †hyracodonts
(_†Hyracodon_), was composed of small, long-necked and long-legged,
slender and lightly built, cursorial animals, but with short, heavy
heads, which gave them a somewhat clumsy look; having neither horns nor
tusks, they were entirely defenceless and depended for their safety
upon speed alone. The second series, or †amynodonts (_†Metamynodon_),
formed the very antithesis of the first,—large, heavy, short-necked, and
short-legged and probably amphibious in manner of life, they were armed
with formidable tusks; and their skulls were so curiously modified as
to bear a distinct resemblance to the skull of a huge carnivore. The
†amynodonts migrated to the Old World and occur in the Oligocene of
France, but the †hyracodonts would seem never to have left North America.
The third series, that of the true rhinoceroses, comprised several genera
at different levels in the White River beds (_†Trigonias_, _†Cænopus_,
etc.); they were of uncertain origin and it has not yet been determined
whether they were immigrants or of native stock. Many species have been
found, varying much in size, up to that of a modern tapir, and not unlike
one in proportions, for they were of lighter build and had relatively
longer legs than any existing rhinoceros. The species of the lower
and middle substages were all hornless, but in the uppermost substage
we find skulls with a pair of nasal horns in an incipient stage of
development. This was the beginning of the †paired-horned rhinoceroses
(_†Diceratherium_) which so flourished in the John Day and the lower
Miocene.
[Illustration: FIG. 135.—†Hornless rhinoceros (_†Cænopus tridactylus_)
of the White River stage. Restored from a skeleton in the American
Museum.]
Of the horses there was no great variety and all the species so far
discovered are included in a single genus (_†Mesohippus_), though there
was a decided increment in the size of the successive species from the
earlier to the later portion of the stage. Looked at superficially, it
seems absurd to call these little creatures “horses” at all and the term
can be justified only as implying that they were ancestral members of the
family. The largest of the White River species hardly exceeded a sheep in
size and all of them had comparatively short necks, long and slender legs
and three-toed feet. The low-crowned grinding teeth show that they were
browsers, not grazers. The abundant Eocene family of the †Lophiodontidæ
made its last appearance in the White River, where it was scantily
represented by slender, long-legged animals (_†Colodon_), with feet
singularly like those of the contemporary horses, except that there were
four toes in the front foot. Tapirs (_†Protapirus_) were very much less
common than rhinoceroses or horses and were hardly half as large as the
existing species of the family and of relatively far more slender form;
the development of the proboscis had already begun. Lastly, the presence
of the clawed †chalicotheres has been reported from the lower Oligocene
of Canada, but the material is too fragmentary for generic reference.
Though the number of artiodactyl families yet identified among the White
River fossils is no larger than that of the perissodactyl families,
the artiodactyls greatly preponderated in individual abundance. The
peccaries, which were fairly common, resembled those of the John Day,
but were considerably smaller. Of the camels, there were two series,
one of which (_†Eotylopus_), lately described by Dr. Matthew, is of yet
unknown significance, while the other (_†Poëbrotherium_) was apparently
the ancestor common to all the subsequent phyla of camels and llamas.
This extremely interesting genus had species which ranged in size from a
gazelle to a sheep, had two toes in each foot, a moderately elongate neck
and teeth which were beginning to assume the high-crowned character.
From this it may be inferred that those animals were, partly at least,
of grazing habit, which was rare among White River ungulates, most
of which fed upon leaves and soft and succulent plants. An extinct
family, the †Hypertragulidæ, were a greatly diversified group of dainty
little creatures, one of which (_†Hypisodus_) was no larger than a
rabbit and had high-crowned teeth. The other genera (_†Leptomeryx_,
_†Hypertragulus_) must have resembled in form and proportions the tiny
little chevrotains or “mouse-deer” of the East Indian islands. Late
in the age arose a larger form of this family, nearly equalling the
Musk-Deer in size, the extraordinary genus _†Protoceras_, which was,
especially the males, a grotesque object. The males had a pair of upper
canine tusks and two pairs of prominent long protuberances on the skull.
This, or some similar form, must have been the ancestor of the still more
bizarre _†Syndyoceras_ of the lower Miocene.
The †oreodonts were by far the commonest of White River mammals, and
evidently they roamed the woods and plains in great herds. There
were several species, larger and smaller, of the abundant genus
(_†Merycoidodon_) but the largest did not surpass a modern peccary in
size and was somewhat like that animal in appearance, but had a shorter
head and much longer tail. In the upper substage appeared a very peculiar
genus of this family (_†Leptauchenia_), animals with short, deep, almost
monkey-like heads, and presumably aquatic in habits. The _†agriochœrids_
were very much less common; they may be described roughly as †oreodonts
with very long, cat-like tails and clawed feet.
[Illustration: FIG. 136.—_†Merycoidodon culbertsoni_, the most abundant
of White River †oreodonts. Restored from a skeleton in the American
Museum of Natural History.]
All of the foregoing artiodactyl families were exclusively North
American in Oligocene distribution; even the camels did not reach
Asia till the Pliocene, and the other families never invaded the Old
World at all. There were, however, two additional families, which also
occurred in the eastern hemisphere, whence one of them, and possibly
the other, was derived. The unquestionably Old World family, that of
the †anthracotheres, was represented in the White River by two genera
(_†Bothriodon_ and _†Anthracotherium_), which were short-legged,
long-snouted, swine-like animals, which have no near relations in the
modern world. The other family, the †giant pigs, which we have already
met with in the lower Miocene and upper Oligocene, is of doubtful origin,
and nothing has yet been found in the preceding formations of either
North America or Europe which can be regarded as ancestral to them. The
White River genus (_†Archæotherium_) was very like the John Day and
Arikaree genera, but most of the species were much smaller and some were
not so large as a domestic pig. In the uppermost beds, however, are found
huge species, which rivalled those of the subsequent formations. That
these strange animals were rooters and diggers and therefore pig-like in
habits is indicated by the manner in which the teeth are worn.
[Illustration: FIG. 137.—†Giant pig (_†Archæotherium ingens_) from
the lower White River stage. Restored from a skeleton in the museum of
Princeton University.]
_South America._—The older continental Tertiary formations of South
America cannot be correlated with those of North America or Europe,
because they have nothing in common. Difficult as it is to give a
correct and adequate conception of the Tertiary mammalian life of the
northern hemisphere to one who has not made a study of it, it is far more
difficult in the case of South America. The stock of adjectives, such
as “peculiar,” “bizarre,” “grotesque” and the like, already overworked
in dealing with northern forms, is quite hopelessly inadequate where
everything is strange. In addition to this, we are seriously handicapped
in treating of the Oligocene and Eocene of South America by very
incomplete knowledge. Many fossils have been collected and named, but
the great majority of these are known only from teeth; a few skulls and
limb-bones have been described, but no skeletons, and therefore much is
very uncertain regarding these faunas.
The Deseado formation (Pyrotherium Beds) has been variously referred by
different writers from the upper Cretaceous to the lower Miocene, but
its most probable correlation is with the Oligocene. Though most of the
mammalian groups are the same as those of the Santa Cruz, the proportions
of the various orders in the two faunas are very different, but, to some
extent, the difference is probably illusory and due to the conditions
of fossilization, for, as a rule, the small mammals are much less
frequent and well preserved in the older beds. As in the Santa Cruz, the
marsupials were the only predaceous mammals, and some of them attained
gigantic size; but no such variety of these beasts of prey has been found
in these beds as occurred in the middle Miocene. In addition, there
were numerous small herbivorous marsupials. One of the most striking
differences from the Santa Cruz fauna was in the very much smaller number
of Edentata, which, instead of being extremely common, are quite rare
among the fossils. No doubt there was a real and substantial difference
in this respect, but it was probably not so great as it seems, and
the same three suborders are found in both formations. One of the few
†ground-sloths that have been obtained was very large (_†Octodontherium
crassidens_), a much larger animal than any species of the suborder that
is known from the Santa Cruz. The †glyptodonts were also rare, and only
two genera and species have been described from very scanty remains.
Armadillos, on the other hand, were much more common, and no less than
eleven genera have been named, three of which occurred also in the Santa
Cruz. Among these was the remarkable genus _†Peltephilus_, in which
the anterior two pairs of plates of the head shield were modified into
horn-like spines.
Equally striking was the remarkable diminution of the Rodentia, as
compared with those of the Santa Cruz, though, of course, this is an
inaccurate mode of stating the truth, occasioned by the fact that we are
following the history in reverse order. It would be preferable to say
that the rodents underwent a remarkable expansion in the Santa Cruz.
These rodents of the Deseado stage are the most ancient yet discovered
in South America and represent only two families, both belonging to
the Hystricomorpha, or porcupine group. If, as Dr. Schlosser and
other European palæontologists maintain, the Hystricomorpha were all
derived from a family of the European Eocene, this would necessitate a
land-connection between South America and the Old World independent of
North America, for the latter continent had no hystricomorph rodents
until the connection between the two Americas was established.
The great bulk of the Deseado fauna is made up, so far as individual
abundance is concerned, of hoofed animals belonging to the typically
South American groups. The †Toxodonta were represented partly by
genera which were the direct ancestors of the common Santa Cruz
genera (_†Pronesodon_, _†Proadinotherium_), and, more numerously, by
a very peculiar family, the †Notohippidæ, which had highly complex,
cement-covered grinding teeth. Still a third family of this suborder,
the †Leontiniidæ, was highly characteristic of the Deseado fauna and
is not known from the Santa Cruz. These were large animals, with a
small horn on the tip of the nose and low-crowned, comparatively simple
grinding teeth. Even more abundant were the †Typotheria, small forms
which were ancestral to the Santa Cruz genera, larger ones which died out
without leaving successors and one quite large animal (_†Eutrachytherus_)
which seems to have been the ancestor of the Pliocene and Pleistocene
_†Typotherium_. This series is not known to have been represented in
the Santa Cruz and may have withdrawn from Patagonia at the end of the
Deseado stage.
[Illustration: FIG. 138.—Horned †toxodont (_†Leontinia gaudryi_), Deseado
stage. Restored from a skull in the Ameghino collection.]
The †Entelonychia, those strange toxodont-like animals with claws instead
of hoofs, were much more numerous and varied than they were afterward in
the Santa Cruz, when they were on the verge of extinction, and included
both very small and very large species. The †Pyrotheria, a suborder which
is not met with in the Santa Cruz or later formations, likewise included
some very large forms. The typical genus, _†Pyrotherium_, included large,
relatively short-legged and very massive animals; the upper incisors
formed two pairs of short, downwardly directed tusks, and in the lower
jaw was a single pair of horizontally directed tusks; the grinding teeth
were low-crowned and had each two simple, transverse crests. These
grinding teeth and the lower tusks so resemble those of the ancestral
Proboscidea in the Oligocene of Egypt, that the †pyrotheres have actually
been regarded as the beginnings of the †mastodons and elephants, but this
is undoubtedly an error. The †Astrapotheria, another group which became
extinct at or soon after the end of the Santa Cruz, were relatively
abundant in the Deseado and counted some very large species. Finally,
the †Litopterna were represented by the same two families as continued
through the Pliocene and one of them far into the Pleistocene. The
horse-like †proterotheres were present, but not enough of them has been
obtained to show whether or not they were in a notably less advanced
stage of development than those of the Santa Cruz. The †macrauchenids
were quite similar to those of the latter formation, though considerably
smaller. In addition, there were a few genera, survivals from earlier
times, which were not referable to either of these families.
The large number of genera, especially among the †toxodonts and
†typotheres, which had high-crowned, cement-covered teeth, may be taken
as an indication that grazing habits had already begun to be prevalent.
Of this wonderful assemblage of hoofed animals, divisible into six
separate groups, whether of ordinal or subordinal rank, not a trace
remains to-day. Not only are all the species, genera and families
extinct, but the suborders and orders also. Further, this was a very
strictly autochthonous fauna, so far as the hoofed animals were
concerned, and no member of any of the six groups has ever been found
outside of the Neotropical region.
4. _Eocene_
_North America._—In the western interior of North America the Oligocene
followed so gradually upon the Eocene, that there is great difficulty
in demarcating them and much difference of opinion and practice obtains
as to where the boundary line should be drawn. Not to depart too widely
from the scheme used by Professor Osborn, the Uinta stage is here
treated as uppermost Eocene, though this is a debatable procedure. For
several reasons, the extraordinarily interesting and significant Uinta
fauna is far less completely known than that of the preceding Bridger
and succeeding White River stages. For one thing, it has been much less
thoroughly explored, and it may be confidently expected that future
exploration will greatly enlarge our knowledge.
The smaller mammals of the Uinta are particularly ill-known. No
Insectivora have yet been found, though this gap will assuredly be
filled; rodents are scanty in the collections and include only two
families, one the †ischyromyids, which were still common in the
White River, the other of doubtful position, but not improbably to
be considered as the beginning of the pocket-gophers (Geomyidæ). The
archaic flesh-eaters, or †Creodonta, were represented by two families,
one comprising smaller animals with somewhat cat-like, shearing
teeth (†Oxyænidæ), the other, very large beasts with crushing teeth
(†Mesonychidæ), neither of which continued into the White River. As
compared with the middle and lower Eocene, the †creodonts had greatly
diminished and, to replace them, the true Carnivora were beginning to
come in. As yet, however, only small and very primitive dog-like forms
are known and no trace of †sabre-tooths or mustelines has been found.
Indeed, it is very doubtful whether members of these families ever will
be found in the Uinta, for their presence in the succeeding White River
was probably due to immigration.
The Perissodactyla were the preponderant type of hoofed animals, and
ancestral forms of most of the White River genera have already been
identified. The †titanotheres (_†Diplacodon_, _†Protitanotherium_)
were much smaller and lighter than those of the lower White River and
had much shorter horns. The †hyracodonts, the lightly built, cursorial
rhinoceroses, were represented by a genus (_†Triplopus_) which was
smaller and more slender than the White River form (_†Hyracodon_) and its
teeth were of distinctly more primitive character. The heavy, massive
and presumably aquatic †amynodonts (_†Amynodon_) were likewise smaller
and less specialized than their descendants of the Oligocene. No member
of the true rhinoceros series has yet been identified in the Uinta,
but there is some reason to think that they were nevertheless present.
Tapirs are distinctly indicated by certain fossils, but they are still
too incompletely known to make possible any statement as to their degree
of development. The horses (_†Epihippus_), like the other families
mentioned, were much smaller and distinctly more primitive than their
successors in the Oligocene.
The Artiodactyla were, for the first time in the history of North
America, as numerous and as varied as the perissodactyls and, with the
exception of the peccaries and †anthracotheres, representatives of
all the White River families are known. The finding of the peccaries
is merely a question of further exploration, but the †anthracotheres
were migrants from the Old World, and there is no likelihood that they
will be discovered in the Uinta at any future time. Fairly large,
pig-like animals, probably referable to the †giant-pigs or †entelodonts,
occurred, but nothing has yet been found which can be considered as
the direct ancestor of the White River genus. As was true of the
perissodactyls, the Uinta artiodactyls were nearly all much smaller and
more primitive than their Oligocene descendants and the differences are
most interesting from the evolutionary point of view. The ancestral
camel (_†Protylopus_) was a little creature no bigger than a fox-terrier,
though the †hypertragulids (_†Leptotragulus_) were as large as
_†Leptomeryx_ and _†Hypertragulus_ of the White River. The most ancient
known members of the †oreodonts (_†Protoreodon_) and the †agriochœrids
(_†Protagriochœrus_) are found in the Uinta.
The middle Eocene fauna, Bridger stage, though it passed upward very
gradually into that of the Uinta, was yet, on the whole, very different
from the latter. It was exclusively indigenous and so radically distinct
from the mammals of corresponding date in Europe as to preclude the
possibility of a land-bridge with that continent. In the lower Eocene,
as will be shown in a subsequent page, the communication between the two
continents was broadly open and the faunas of the two continents were
much more closely similar than they have ever been since. It is really
remarkable to see with what comparative rapidity the two regions, when
severed, developed different mammals under the operation of divergent
evolution. Had the separation continued throughout the Tertiary and
Quaternary periods, North America would now have been as peculiar
zoölogically as South America is, a result which has been prevented by
the repeated renewal of the connection.
The characteristic features of the Bridger mammalian fauna were chiefly
due to the great expansion and diversification of certain families,
which began their career at an earlier stage, and to the disappearance
of many archaic groups which had marked the more ancient faunas. Other
archaic groups, however, survived and even flourished in the Bridger,
and of these it is particularly difficult to convey a correct notion
to the reader, because they were so utterly unlike anything that now
lives. One of these orders, the †Tæniodontia, which had so many points
of resemblance to the †ground-sloths that several writers have not
hesitated to include them in the Edentata, survived only into the older
Bridger, but the equally problematical †Tillodontia then reached their
culmination, though they were not very numerous. Though not at all
related to that group, the †tillodonts looked like huge rodents, with
their chisel-like incisor teeth. There was a remarkable assemblage of
Insectivora, more numerous and varied than in any subsequent formation,
no less than six families being known. One of these somewhat doubtfully
represented the moles and two others modern Asiatic groups. The very
unexpected discovery of an armadillo in the Bridger has been reported,
but the propriety of referring this animal to the armadillos, or even
to the edentates, has not yet been proved, and it would therefore be
premature to discuss its significance. The only marsupials were opossums.
So far as our information extends, there were no true Carnivora in the
Bridger, all the beasts of prey of the time belonging to the archaic
†Creodonta, which then reached their maximum development in numbers
and diversity. One family (†Oxyænidæ) included large and powerful
flesh-eaters, with cat-like dentition and short, rounded, lion-like
heads, long bodies and tails and short, heavy limbs, giving them the
proportions of otters. Another (the †Hyænodontidæ) comprised small,
long-headed, fox-like and weasel-like animals, which doubtless preyed
upon small mammals and birds. A third family (†Mesonychidæ) was made
up of moderate-sized, long-jawed creatures, which must have resembled,
rather remotely, short-legged and long-tailed wolves and hyenas. Their
habits and mode of life are somewhat problematical, for their grinding
teeth were blunt, not adapted to the shearing of flesh, and their claws
were broad, almost hoof-like. Such creatures could hardly have subsisted
by the pursuit of living prey and were probably carrion-feeders and more
or less omnivorous. The †Miacidæ, a family which connected the †creodonts
and true carnivores and might almost equally well be placed in either
group, were externally much like the small †hyænodonts, but were more
efficiently equipped for the capture and devouring of prey.
[Illustration: FIG. 139.—A mesonychid †creodont (_†Dromocyon velox_)
of the Bridger stage. Restored from a skeleton in the Museum of Yale
University.]
Of the archaic and extinct orders of hoofed animals, the only one which
persisted from earlier times into the Bridger and greatly flourished
there was the †Amblypoda, one family of which (†Uintatheriidæ) was
preëminently characteristic of middle Eocene life, becoming very rare
and then dying out in the upper Eocene. The †uintatheres of the Bridger
underwent considerable modification in size and appearance within the
limits of the stage, the larger and stranger species appearing toward
the end of the time. Most of these great creatures may fairly be called
gigantic, for they equalled the largest modern rhinoceroses and smaller
elephants in size. The body, limbs and feet were so elephantine in
character that they were once believed to be ancestral Proboscidea, but
the teeth and the fantastic skull were so radically different that this
belief was long ago abandoned. The upper canine teeth were converted, in
the males, into formidable spear-like or scimitar-like tusks, protected
by great flange-shaped expansions of the lower jaw; bony knobs on the
end of the nose probably supported a pair of dermal horns like those of a
rhinoceros and, in addition, a pair of high, cylindrical, horn-like, bony
protuberances arose above the eyes and another, more massive pair, near
the back of the head. It would be difficult to imagine more extraordinary
creatures than the †uintatheres, which were the largest land-mammals of
their time. The family was entirely confined to North America, no trace
of them having been found in any other continent.
While the backward and archaic orders, most of which have left no
descendants in the modern world, had thus a stately representation in
Bridger times, they were outnumbered in genera, species and individuals
by the progressive orders, which are still in more or less flourishing
existence. The Primates, whether lemurs or monkeys, were numerous,
and this, so far as is definitely known, was their last appearance in
extra-tropical North America. They may at any time be found in the
Uinta, but there is small probability that they will ever turn up in
the White River or later formations. The many rodents all belonged to
the †ischyromyids, an extinct family which, there is much reason to
believe, was ancestral to many families of the squirrel-like suborder of
Sciuromorpha. Most of them were species of a single genus (_†Paramys_)
and varied in size from a mouse to a beaver, or even larger.
The Perissodactyla may be said, in one sense, to have reached their
culmination in the Bridger; not that many of them, such as the horses
and rhinoceroses, did not advance far beyond their state of development
in the Eocene, but at no subsequent time did the order as a whole
possess such dominating importance. There were five or six families
of perissodactyls in the Bridger, and their remains are much the most
abundant fossils found there. Individually, the commonest perissodactyls
of the time were the †titanotheres, of which there were several genera
and many species, differing chiefly in size and proportions, though the
largest hardly exceeded a modern tapir in stature and was not dissimilar
in appearance. These Bridger †titanotheres were considerably smaller
than those of the Uinta and therefore very much more so than the White
River forms; it was not till the latter stage that the family lived up
to its name of “titanic beasts.” By far the commonest of the genera in
the middle and lower Bridger was _†Palæosyops_, which was hornless, while
in the upper part of the beds are found genera (_e.g._ _†Manteoceras_
and _†Dolichorhinus_) in which the horns were just beginning to appear.
Another extinct family, the †Lophiodontidæ, which was very abundant in
the European Eocene, formed a very subordinate element in this fauna and
included a number of small tapiroid genera (_e.g._ _†Helaletes_).
[Illustration: FIG. 140.—Some characteristic mammals of the Bridger
Eocene reduced to a uniform scale, with a pointer dog, in frame,
for comparison. 1. Primitive rhinoceros (_†Hyrachyus eximius_). 2.
†Tritemnodon agilis. 3. _†Patriofelis ferox_, and 4, _†Dromocyon
velox_, †creodonts. 5. Primitive rodent (_†Paramys delicatior_). 6.
_†Uintatherium alticeps._ 7. †Titanothere (_†Mesatirhinus superior_).]
The horses (_†Orohippus_) were very small and primitive creatures, no
bigger than a fox, with four toes in the front foot and three in the
hind. So completely different in appearance and proportions were these
little animals from any of the modern horses, that it requires an effort
of the imagination to think of them as belonging to the same family, and
it is only by employing the family to designate a _genetic series_ that
such a classification can be justified. The †hyracodonts, or cursorial
rhinoceroses, were very abundantly represented by a number of small and
medium-sized animals (_†Hyrachyus_) which had less specialized teeth,
shorter neck and limbs than their upper Eocene and Oligocene successors,
and four toes in the front foot; one genus (_†Colonoceras_) had a
pair of nasal horns, but would seem to have died out without leaving
descendants. In the upper part of the beds is found the Uinta genus
_†Triplopus_, with three-toed fore foot; and in the same division occurs
another Uinta genus, _†Amynodon_, the most ancient known species of the
supposedly aquatic rhinoceroses. True rhinoceroses, that is animals which
were directly ancestral to the modern members of the family, have not
been identified and may not have been present in North America; that is
still an open question. Tapirs, all of them quite small, were relatively
common, but are still very incompletely known. The earliest known members
of the clawed †chalicotheres were of Bridger date.
It is worth remarking that, except a single genus in the upper and later
portion of the stage (_†Triplopus_), all of the Bridger perissodactyls
had four toes in the front foot and three in the hind, while in the White
River beds above the lowest substage the number three in both fore and
hind feet was almost equally universal.
One of the most radical and striking differences between the Uinta and
Bridger faunas was the rarity of Artiodactyla in the latter, which is
in almost equally strong contrast with their abundance in the middle
Eocene of Europe. Most significant of these rare Bridger artiodactyls
were the little creatures (_†Homacodon_), hardly so large as a domestic
cat, which may fairly be regarded as a very early stage, if not the
actual beginning, of the great camel family, which was destined to
play so conspicuous a part in the life of America, North and South.
Small pig-like animals (_†Helohyus_) which were no doubt ancestral
to the peccaries, were fairly common and there were, in addition,
relatively large animals (_†Achænodon_) allied, but not ancestral, to the
†giant-pigs of the Oligocene; some of these were considerably larger than
a full-grown Wild Boar (_Sus scrofa_).
Among all the many hoofed mammals of the Uinta and Bridger there was not
a single one that had the high-crowned, persistently growing teeth of
the grazers; all of them must have had browsing habits and have fed upon
such soft vegetable tissue as did not rapidly abrade the teeth. The same
statement applies, _à fortiori_, to the stages antecedent to the Bridger
and therefore to the entire Eocene and Paleocene. From these facts it may
be inferred that the grasses had not yet taken possession of wide areas.
Concerning the Bridger fauna, Professor Osborn, who has done so much to
elucidate it, says: “On the whole, it is a very imposing, diversified and
well-balanced fauna, with an equal distribution of arboreal, cursorial,
aquatic, fossorial, carnivorous and herbivorous types.”
The lower Eocene is divisible into two stages, in descending order, the
Wind River and Wasatch, both extensively exposed in central Wyoming. As
would be expected from its stratigraphical position, the Wind River
fauna was largely transitional between that of the Bridger above and that
of the Wasatch below. Unfortunately, the fossils are far less numerous
than those of the Bridger and not so well preserved, and therefore give
us a less adequate conception of the life of that time. The archaic,
non-progressive orders were strongly represented, but already the
progressive groups were in a numerical majority of species; most of
these archaic orders may be most advantageously described in connection
with the Wasatch. Opossums were almost certainly present, though the
available specimens are too fragmentary for assured determination.
The †tillodonts, †tæniodonts and insectivores differed little from
the Wasatch representatives of these orders, except that the Bridger
†tæniodont, _†Stylinodon_, which had rootless, persistently growing
teeth, was associated with the Wasatch genus _†Calamodon_. On the other
hand, the primitive flesh-eaters, or †creodonts, which were referable to
Wasatch families, were less numerous and varied and formed a mixture of
Bridger and Wasatch genera. The †Oxyænidæ, the family with cat-like teeth
and head, had both the smaller Wasatch genus _†Oxyæna_ and the very large
Bridge _†Patriofelis_. Of the blunt-toothed †Mesonychidæ, one very large
animal (_†Pachyæna_) survived from the Wasatch. The small forms of the
family †Hyænodontidæ were common, and there were numerous species of the
progressive family †Miacidæ.
Among the hoofed animals there were two of the antique orders which
became extinct before the end of the Eocene, indeed, one of these groups,
the †Condylarthra, made its last appearance in the Wind River. This
extremely primitive group, which, in a sense, connected the hoofed with
the clawed mammals, will be described under the more ancient faunas.
The other order, the †Amblypoda, was represented by two very different
families, one of which, the †uintatheres, was so flourishing in the
Bridger, where it formed the most characteristic and by far the most
striking element of the fauna. The Wind River genus (_†Bathyopsis_)
was a very much smaller animal than any of the Bridger forms and its
horn-like protuberances were in an incipient state, while in various
other respects it was decidedly more primitive than its successors. The
second family was represented by the genus _†Coryphodon_, which did
not survive into the Bridger, but was especially characteristic of the
Wasatch fauna, with which it will be described.
Turning now to the progressive orders, we note that the rodents, lemurs
and monkeys were very similar to those of the Bridger and belonged to
the same families, but were decidedly less numerous. This difference,
however, may be rather apparent than real and due to the much more
favourable conditions for the preservation of small mammals in the middle
Eocene. Among the Perissodactyla, the horses were intermediate in size
and structure between those of the Bridger and those of the Wasatch, but
were decidedly nearer to the latter. The †lophiodonts, so far as known,
were represented by a single genus (_†Heptodon_) which also occurred in
the Wasatch. The modest beginnings of the †titanotheres, the family which
became so very conspicuous in the middle and upper Eocene and lowest
Oligocene, may be noted in the Wind River fauna, in which there were
two genera. One of these (_†Eotitanops_), the very probable ancestor
of all the subsequent genera, was quite small, about two-thirds the
size of a modern tapir, while the other (_†Lambdotherium_) was a much
smaller, lighter and more slender animal and apparently belonged to an
abortive, short-lived phylum. Then, too, the first of the †hyracodonts,
or cursorial rhinoceroses, made their appearance here in the genus
_†Hyrachyus_, which was afterward so common in the Bridger.
No Artiodactyla have yet been found in the Wind River, though there can
be little doubt that they then inhabited North America, as they did both
before and afterward.
The Wind River fauna was of so much less peculiar and isolated character
than that of the Bridger as to suggest a connection with the eastern
hemisphere, a suggestion which is strengthened by the unheralded
appearance of the †titanotheres and †hyracodonts, of which no forerunners
have been found in the Wasatch.
The lowest and most ancient of the Eocene faunas is that of the Wasatch
formation, which is extensively developed in central and southern
Wyoming, Utah and New Mexico. The fauna of this stage is plainly
divisible into two groups: (1) those types which were the descendants of
American Paleocene mammals and were therefore indigenous, and (2) the
immigrants from other continents. The indigenous mammals, which almost
all belonged to orders now extinct, few of which survived later than the
Eocene, must have given a very bizarre appearance to the assemblage,
especially as they were more numerous, varied and, for the most part,
larger and more conspicuous than the newcomers. Marsupials have not yet
been found, but the occurrence of opossums in the Bridger and probably
in the Wind River gives some reason to believe that they were in North
America during Wasatch times also. The †Tæniodontia, which bore a certain
resemblance to South American edentates, had one pair of incisor teeth
above and below enlarged and chisel-shaped, somewhat like those of
rodents. The †Tillodontia were much smaller than those of the Bridger,
and their incisors were only beginning to take on the chisel-like form.
Insectivora were quite abundant, and three, or perhaps four, families
were represented in the Wasatch; some of these resembled the modern
aquatic insectivores of the west African rivers and others were more like
European hedgehogs.
The flesh-eaters all belonged to the †Creodonta, and, though rather less
diversified than those of the Bridger, were yet relatively abundant.
In size, they ranged from little creatures not larger than a weasel up
to truly enormous beasts, and differed, no doubt, largely in habits
and manner of life. For the most part, the families were the same as
those of the Bridger †creodonts, but the genera all were different. The
†oxyænids (_†Oxyæna_) were much smaller and lighter than the large and
massive representatives found in the middle Eocene, and their teeth were
not so cat-like. Another group of predaceous animals (_†Palæonictis_)
which also inhabited Europe, but did not survive the lower Eocene
in either continent, had short, broad and very cat-like heads. The
†mesonychids were far larger than those of the Bridger, a departure from
the ordinary rule, and the several species of the common Wasatch genus
(_†Pachyæna_) had grotesquely large heads. A family (†Arctocyonidæ),
of very extensive geographical range and great antiquity, had its last
representatives here in a very curious animal (_†Anacodon_) which had the
flat-crowned, tuberculated grinding teeth of the bears and the enlarged,
scimitar-like upper canines of the †sabre-tooth cats. Such a combination
seems utterly incongruous and no one would have ventured to predict
it. The progressive family of †creodonts (†Miacidæ) was already quite
numerously represented, but only by small forms, which must have preyed
upon small mammals, birds and lizards.
Two archaic orders of hoofed mammals were fairly numerous. One, the
†Condylarthra, comprised quite small, five-toed animals, with long
tails and short feet and extremely primitive in structure. A genus
(_†Phenacodus_) of this order was long regarded as being ancestral to
most of the higher orders of ungulates, but this belief has proved to
be untenable. More numerous were the †Amblypoda, one genus of which
(_†Coryphodon_), though persisting into the Wind River, was especially
characteristic of the Wasatch. The †coryphodonts were the largest of
lower Eocene mammals, and some of the species equalled a tapir or
small rhinoceros in length and height, but had heavier limbs; as the
skeleton conclusively shows, these must have been heavy, clumsy and
exceptionally ugly brutes, with formidable tusks, large head, but
relatively more slender body, short and massive limbs and elephantine
feet. In appearance, these strange beasts were not altogether unlike
the Hippopotamus and were perhaps more or less amphibious in habits.
The other family of †Amblypoda, the †uintatheres, have not yet been
registered from the Wasatch, but they will undoubtedly be found there, as
they were unquestionably present at that time.
[Illustration: FIG. 141.—_†Phenacodus primævus_, the best known Wasatch
representative of the †Condylarthra. Restored from a skeleton in the
American Museum of Natural History.]
[Illustration: FIG. 142.—The commonest of Wasatch ungulates, the
†amblypod, _†Coryphodon testis_. Restored from a skeleton in the American
Museum of Natural History.]
All of the preceding groups were of the archaic, non-progressive type
and have long been extinct. With the sole exception of one †creodont
family (†Miacidæ) and perhaps some of the insectivores, they have no
descendants or representatives in the modern world. All of them appear to
have been indigenous and derived from North American ancestors, though
it is possible that a few were immigrants. We now turn to the orders
which were more significant of the future, because they had within them
the potency of a far higher development. These progressive groups were
all immigrants, coming to North America from some region which cannot
yet be positively identified, but most probably was Asia. From the same
region and at a corresponding period of time Europe received many of the
same forms, and so many genera were at that time common to the latter
continent and North America that a broad and easy way of intermigration
must have been open.
One of these immigrant orders, the Rodentia, the most ancient known
members of which were these species from the North American Wasatch,
was represented by the same family (†Ischyromyidæ) and some of the same
genera (_†Paramys_, _†Sciuravus_) as throve also in the Bridger stage.
There were two orders of hoofed mammals, which were newcomers to the
western world, Perissodactyla and Artiodactyla. Of the former was a genus
(_†Eohippus_) of the most ancient American horses. These most interesting
little animals, no larger than small foxes and domestic cats, would
hardly be called horses, were it not for the long series of gradual and
successive modifications which led from _†Eohippus_ up to the modern
horses. The graceful little creatures had a short neck, curved back, and
relatively short, slender limbs, with four functional toes in the front
foot and three in the hind; and, though they differed from existing
horses in almost every detail of teeth and skeleton, there was something
unmistakably equine about them. From the abundance of their remains it
may be inferred that herds of them swarmed in the forests and glades of
Wasatch times. The second perissodactyl family, the †Lophiodontidæ, which
comprised considerably larger animals, never attained to importance in
America, but flourished and became greatly diversified in Europe. What
are believed to be the most ancient tapirs yet discovered (_†Systemodon_)
were individually very common in the Wasatch. This tapir was no larger
than a Coyote, had no proboscis and was so little like a tapir in outward
appearance that an observer might well be pardoned for overlooking the
relationship; even the skeleton is of so indifferent a character that the
reference of this genus to the tapirs cannot be positively made.
Of equal significance for the future was the arrival of the Artiodactyla,
of which there were members of three families in the Wasatch, though
individually they were much less common than the horses. These were
geologically the oldest known artiodactyls, Europe having yielded none
of this date, and are still too imperfectly known to justify any very
positive statements about them. One genus, however (_†Trigonolestes_),
tiny little creatures, like rabbits in size, would seem to represent the
beginnings of the great ruminant tribe, now so very important a factor in
the life of the world. A second genus (_†Eohyus_), considerably larger,
is very doubtfully referable to the pigs; while a third (_†Parahyus_),
still larger, was the first in the short-faced series of the
†entelodonts, which persisted in ever increasing size through the whole
Eocene, but could hardly have been ancestral to the true †entelodonts,
or †giant-pigs, of the Oligocene, the place and time of whose origin are
unknown.
Another immigrant order of great interest, since we ourselves belong
to it, the Primates, made its first appearance in North America in the
Wasatch, but was not destined to long life or great importance in this
continent, where it did not survive the Eocene. Several different kinds
of small, lemur-like and monkey-like creatures dwelt in the tree-tops
of the Wasatch forests. One genus (_†Anaptomorphus_) had a remarkable
likeness to the modern Tarsier (_Tarsius spectrum_) of the Malay
peninsula and islands.
_South America._—The Eocene of South America, referred by some writers to
the upper Cretaceous, is very incompletely and unsatisfactorily known.
The Casa Mayor formation (or Notostylops Beds), which has yielded a
great variety of mammals, for the most part very fragmentary, probably
contains not one but several successive faunas which have not yet been
fully discriminated, and that of the next succeeding Astraponotus Beds is
still but a scanty list. This list, however, includes the most ancient
†glyptodonts yet discovered and the most ancient †astrapotheres in the
narrow sense of the term. The Astraponotus Beds may be either Eocene or
Oligocene in date.
Taking the Casa Mayor faunas as a whole, they were a very numerous and
diversified assemblage of small mammals, without a single large one
among them. There were no monkeys or rodents; otherwise, the orders
were in almost all cases the same as those which made up the Santa Cruz
fauna. The marsupials were represented by the opossums and by several
of the carnivorous kinds, the only beasts of prey that South America
had until the migrations from the north brought in the true Carnivora,
late in the Miocene or very early in the Pliocene. There were also
numerous small marsupials of peculiar type, of which the last living
survivor is _Cænolestes_, of Ecuador. Throughout the stage, armadillos
were present in considerable variety, but are known only from the bony
plates of the carapace, and therefore little can be determined as to
their relationships to the modern families. Only a single and very
problematical genus of the †ground-sloths, which afterwards throve so
mightily in the Miocene and Pliocene, has been obtained and that in the
later portion of the stage.
The orders of hoofed mammals were represented by many small animals,
most of which are known only from the teeth, which show these Casa Mayor
genera to have been far more primitive and less specialized than their
descendants in the Deseado and Santa Cruz stages. All of them had the
low-crowned grinding teeth of the browsers, and no grazers were then in
existence, so far as is known. No †toxodonts, in the more restricted
sense of that term, have been found, but the two allied suborders of
the †Typotheria and †Entelonychia were numerously represented. Of the
former there were two families and of the latter three, which is more
than in the Deseado or Santa Cruz formations. One of the families of the
†Entelonychia (†Notostylopidæ) consisted of very small, rodent-like
animals, with a pair of chisel-shaped incisors in upper and lower jaw,
and a second family (†Homalodontotheriidæ) contained genera which would
seem to have been directly ancestral to those of the Santa Cruz, but
were very much smaller than their successors. The very large and massive
†Pyrotheria of the Deseado stage were represented by small animals,
in which the grinding teeth had two pairs of conical tubercles, not
yet united into transverse crests. Two families of the †astrapotheres,
in the broad sense, were far smaller than their Oligocene and Miocene
descendants. To the †Litopterna are referred a number of genera, in which
the grinding teeth were tuberculated and had very imperfectly developed
crests, so as strongly to suggest the teeth of the †Condylarthra.
However, until something is ascertained regarding the skeleton,
especially the feet, of these animals, their relationships will remain
more or less doubtful.
It will be observed that these Casa Mayor faunas not only were made up
exclusively of small animals, but also that they already were typically
and characteristically South American and bore the stamp which remained
essentially the same until the successive waves of migration from the
north so greatly modified the composition of the Neotropical fauna. The
absence of rodents and monkeys and the comparative unimportance of the
Edentata gave a somewhat different character to these ancient faunas from
those of the Santa Cruz and later formations.
5. _Paleocene_
_North America._—A very important discovery is one lately made by
American Museum parties of a formation intermediate between the Wasatch
and Torrejon. The interesting fauna of these beds has not yet been
described, but it may be remarked that it contained none of the immigrant
orders.
The vegetation of the Paleocene was already very modern in character,
and nearly all of the common forest-trees were represented by species
which differed but slightly from those of the present. The grasses were
already in existence, but, there is good reason to believe, they had not
attained to much importance and did not cover the plains and open spaces
as they did in the Miocene and still continue to do. As the grasses
afford the principal food-supply of so many grazing animals, the matter
of their abundance and extension is a very significant one in the history
of mammalian development, and, as we have already learned, eventually led
to widespread and profound modifications of structure, especially of the
teeth. While there is thus nothing very strange about the plant-world of
Paleocene times, the higher animal life was almost totally different from
that of modern times and made up a most curious and bizarre assemblage,
from which nearly all the familiar Recent types were absent. The reptiles
had been greatly impoverished by the world-wide and, as yet, unexplained
destruction which overtook them at the end of the Mesozoic era, but it is
possible that in both North and South America a few of the huge Dinosaurs
survived the decimation of the class. Very characteristic of the
Paleocene in North America and Europe were large, lizard-like reptiles,
allied to the New Zealand Tuatara, while crocodiles and tortoises
abounded; snakes were present, but do not appear to have been very common.
It is the mammals which were the strangest element of Paleocene life,
and our imaginary observer would find no creature that he had ever seen
before. The difference from modern mammalian life was not merely one of
species, genera or even families, but of orders, for only one, or at most
two, of the orders now living were then to be found in North America, and
both of these (marsupials and insectivores) were primitive and archaic
groups, which seem like belated survivals in the modern world. There
were no rodents, or true carnivores, no lemurs, monkeys, artiodactyls,
perissodactyls or proboscideans.
In the _Torrejon_, or upper Paleocene, there were many herbivorous
marsupials, with very complex grinding teeth and chisel-like incisors,
but no carnivorous or insectivorous members of the order have been found.
Insectivora were present. Of the †creodonts, or primitive flesh-eaters,
there were no less than five families; the bear-like †Arctocyonidæ, which
died out in the Wasatch, were quite numerous, and the problematical
†Mesonychidæ were much smaller and more primitive mammals than those of
the Eocene. Passing over two families which did not survive the Torrejon,
we may note the first of the †Miacidæ, the progressive family which led
eventually to the true Carnivora. The hoofed animals all belonged to
the archaic †Condylarthra and †Amblypoda; of the former there were many
genera and species referable to three families, one of which contained
the forerunners of the Wasatch _†Phenacodus_. The genus _†Pantolambda_ of
the Amblypoda may well have been ancestral to both the †coryphodonts and
the †uintatheres of the Eocene.
[Illustration: FIG. 143.—The Torrejon forerunner (_†Pantolambda
bathmodon_) of _†Coryphodon_. Restored from a skeleton in the American
Museum of Natural History.]
The _Puerco_ fauna was much like that of the Torrejon, but even less
advanced and diversified. The herbivorous marsupials were more abundant,
and some of them (_†Polymastodon_) larger than those of the Torrejon;
Insectivora may have been present, but this is doubtful. The †creodonts,
so far as they have been discovered, were less numerous, varied and
specialized than those of the Torrejon and included but one of the
families which passed over into the Eocene. The †Condylarthra were much
less common and the †Amblypoda but doubtfully represented, but the
edentate-like †Tæniodontia were conspicuous.
[Illustration: FIG. 144.—Head of an †allotherian marsupial
(_†Polymastodon taöensis_) from the Puerco stage. Restored from a skull
in the American Museum of Natural History.]
Not only were the Paleocene faunas radically different from the mammals
of our time, but they could not have been ancestral to the latter, being
hardly more than an advanced and diversified Mesozoic assemblage. It is
true that some of its elements, such as the †Condylarthra, †Amblypoda
and †Creodonta, developed greatly and played an important part in the
life of the Eocene, but of these only a few †creodonts continued into the
Oligocene and all became extinct without leaving any descendants behind
them. Another curious fact concerning the Paleocene mammalian faunas is
that they were made up entirely of small and very small animals; not a
single mammal as large as a sheep has yet been found in these beds, and
the same is true of Europe.
That a land-connection with the Old World existed during the Paleocene
epoch, is indicated by the similarity of the faunas of North America and
Europe.
CHAPTER VIII
HISTORY OF THE PERISSODACTYLA
In attempting to trace the evolutionary history of the various mammalian
groups, it is necessary to bear in mind the inevitable limitations of
work of this kind. Speaking of plants, Dr. D. H. Scott says: “Our ideas
of the course of descent must of necessity be diagrammatic; the process,
as it actually went on, during ages of inconceivable duration, was
doubtless infinitely too complex for the mind to grasp, even were the
whole evidence lying open before us. We see an illustration, on a small
scale, of the complexity of the problem, in the case of domesticated
forms, evolved under the influence of man. Though we know that our
cultivated plants, for instance, have been developed from wild species
within the human period, and often within quite recent years, yet nothing
is more difficult than to trace, in any given instance, the true history
of a field-crop or garden plant, or even, in many cases, to fix its
origin with certainty.”[5] With some mammalian groups the task, though
difficult enough, is not so hopeless, because of more complete records,
yet in dealing with mammals a very troublesome complication is introduced
by the existence within the families, and even within the genera, of
two or more parallel phyla, or genetic series. Without complete and
perfect material it is impossible to make sure that we are not confusing
the different phyla with one another and placing in one series species
and genera that properly belong in a different one. Thus, Osborn
distinguishes no less than seven such phyla among the true rhinoceroses
of the Old and New Worlds, which long followed parallel, but quite
independent, courses of development, and five phyla among the American
horses. While these phyla add so much to the difficulty of working out
the genealogical series, it is possible to simplify the problem and treat
it in a broad and comprehensive manner that will sufficiently establish
the essential steps of change.
[Illustration: FIG. 145.—Left manus of Tapir (_Tapirus terrestris_).
_S._, scaphoid. _L._, lunar. _Py._, pyramidal. _Pis._, pisiform. _Td._,
trapezoid. _M._, magnum. _Un._, unciform. The metacarpals are erroneously
numbered. _Mc. I._, second metacarpal. _Mc. II._, third do. _Mc. III._,
fourth do. _Mc. IV._, fifth do. _Ph. 1_, first phalanx. _Ph. 2_, second
do. _Ung._, ungual phalanx.]
In external appearance and general proportions the different families
of existing perissodactyls have very little in common; that tapirs and
rhinoceroses should be related is not surprising, but the horses would
seem to be as far removed from both of the former as possible. Why, then,
should they be included in the same order? A study of the skeleton,
however, reveals the community of structure which obtains between the
three families, a community which removes them widely from all other
hoofed mammals. In all existing perissodactyls, though not in most of
the Eocene genera, all the premolars, except the first, have the size
and pattern of the molars. The foramina of the skull, or perforations by
which blood-vessels and nerves enter and leave the cranium, are arranged
in a way characteristic of the order and different from that seen in
other hoofed mammals. The femur always has the third trochanter. The
number of digits in each foot is usually odd, 1, 3 or 5, but four-toed
forms occur, as the tapirs, which have four toes in the front foot, three
in the hind; the important character is that the median plane of the foot
bisects the third digit, which is symmetrical. The third and fourth, each
asymmetrical, together form a symmetrical pair. Especially characteristic
is the form of the astragalus and calcaneum (ankle and heel bones); the
astragalus has but a single, deeply grooved and pulley-like surface, that
for the tibia, the lower end is nearly flat and rests almost entirely
upon the navicular, covering but little of the cuboid (see Figs. 146,
148). The calcaneum does not articulate with the fibula and its lower end
is broad and covers most of the cuboid.
While the foregoing list includes the most important of the structural
features which are common to all perissodactyls and differentiate them
from other hoofed animals, there are many others which it is needless to
enumerate.
[Illustration: FIG. 146.—Left pes of Tapir. _Cal._, calcaneum. _As._,
astragalus. _N._, navicular. _Cn. 1_, _Cn. 2_, _Cn. 3_, first, second
and third cuneiforms. _Mr. II_, _III_, _IV_, second, third and fourth
metatarsals.]
The subjoined table gives the families and principal genera of the
American Perissodactyla; extinct groups are marked †.
Suborder CHELODACTYLA. Normal Perissodactyls
I. EQUIDÆ. Horses.
_†Eohippus_, low. Eoc. _†Orohippus_, mid. Eoc.
_†Epihippus_, up. Eoc. _†Mesohippus_, low. Oligo.
_†Miohippus_, up. Oligo. _†Anchitherium_, up.
Oligo. _†Parahippus_, low. Mioc. to low. Plioc.
_†Desmatippus_, mid. Mioc. _†Hypohippus_, mid. Mioc.
to low. Plioc. _†Merychippus_, mid. Mioc. to low.
Plioc. _†Protohippus_, up. Mioc. _†Pliohippus_, up.
Mioc. and low. Plioc. _†Neohipparion_, up. Mioc.
and low. Plioc. _†Hipparion_, Plioc. _†Hippidion_,
Pleist., S. Amer. _†Hyperhippidium_, Pleist., S. Am.
_Equus_, Pleist., N. and S. Amer.
II. †TITANOTHERIIDÆ. †Titanotheres.
_†Lambdotherium_, low. Eoc. _†Eotitanops_, low. Eoc.
_†Palæosyops_, mid. Eoc. _†Telmatherium_, mid. Eoc.
_†Dolichorhinus_, up. Eoc. _†Diplacodon_, up. Eoc.
_†Titanotherium_, low. Oligo.
III. TAPIRIDÆ. Tapirs.
_†Systemodon_, low. Eoc. _†Isectolophus_, mid. and
up. Eoc. _†Protapirus_, Oligo. _†Tapiravus_, mid.
Mioc. _Tapirus_, Pleist., N. Amer., Pleist. and
Recent, S. Amer.
IV. †LOPHIODONTIDÆ. †Lophiodonts.
_†Heptodon_, low. Eoc. _†Helaletes_, mid. Eoc.
_†Colodon_, low. Oligo.
V. RHINOCEROTIDÆ. True Rhinoceroses.
_†Trigonias_, low. Oligo. _†Cænopus_, Oligo. and low.
Mioc. _†Diceratherium_, up. Oligo. and low. Mioc.
_†Aphelops_, mid. Mioc. to low. Plioc. _†Teleoceras_,
mid. Mioc. to low. Plioc.
VI. †HYRACODONTIDÆ. †Hyracodonts and †Amynodonts, cursorial and
aquatic Rhinoceroses.
_†Hyrachyus_, low. and mid. Eoc. _†Triplopus_, mid.
and up. Eoc. _†Colonoceras_, mid. Eoc. _†Hyracodon_,
low. Oligo. _†Amynodon_, up. Eoc. _†Metamynodon_,
low. Oligo.
Suborder †ANCYLOPODA. †Clawed Perissodactyls
VII. †CHALICOTHERIIDÆ. Chalicotheres.
_†Moropus_, up. Oligo. and low. Mioc.
_?†Schizotherium_, low. Oligo. _†Eomoropus_, mid. Eoc.
The earliest perissodactyls of which we have any knowledge are found in
the older part of the lower Eocene (Wasatch stage) of Europe and North
America, into which they must have migrated from some other region
yet unknown, for no probable ancestors of the group are found in the
Paleocene of either continent.
I. SUBORDER CHELODACTYLA. NORMAL PERISSODACTYLA.
1. _Equidæ. Horses_
In order to make intelligible the evolutionary changes which have led up
to the modern horses, it will be necessary to say something concerning
the dental and skeletal features which characterize these animals. Using
the term _horses_ in a broad sense to include all the existing members
of the family Equidæ, true horses, asses, zebras and quaggas, we find a
greater uniformity in the skeleton and teeth than would be expected from
the external appearance. The differences in appearance are, however,
largely due to colouring, growth of mane and tail and the size of the
ears, which leave no record in the skeleton.
[Illustration: FIG. 147.—Asiatic Wild Horse (_Equus przewalskii_).—By
permission of the N.Y. Zoölog. Soc.]
The teeth (Figs. 45, p. 95; 154, p. 306) are extremely high-crowned,
or hypsodont, and do not form roots till an advanced age; the incisors
have a deep, enamel-lined pit, the “mark” in the centre of the grinding
surface; the first premolar in each jaw is very small and of no
functional importance; the other premolars have the same pattern as the
molars, which is excessively complex in the arrangement of the enamel
ridges and the areas of dentine and cement.
The skull (Fig. 154, p. 306) is long, especially the facial portion, the
eye-socket (orbit) being shifted behind the teeth, which otherwise, on
account of their great height, would press upon the eye itself; the orbit
is completely encircled in bone. The lower jaw is deep vertically and
the ascending ramus (see p. 66) very high, on account of the hypsodont
character of the teeth, which thus necessitates a remodelling of the
skull in several respects. The neck is long, each of its seven vertebræ
being elongate; except in the atlas and axis, the anterior face of
each centrum is strongly convex and the posterior of all except the
atlas is deeply concave; the odontoid process of the axis (see p. 71)
is spout-shaped, concave on the upper and convex on the lower side,
lodging and protecting the spinal cord. The spines of the anterior
dorsal vertebræ are very high, making a low hump at the withers between
the shoulder-blades; the trunk-vertebræ are so arranged as to make the
back almost straight and horizontal. The limbs and especially the feet
are very long. The two bones of the fore-arm, the ulna and radius, are
coössified into a single piece (Fig. 30, p. 81), but the limits of each
are still plainly to be seen, especially in a young animal; and it is
evident that the ulna is greatly reduced in size and has lost its middle
portion, while all the weight is borne by the radius. Similarly, in the
hind leg the enlarged tibia, or shin-bone, alone supports the weight; and
only the two ends of the fibula are preserved (Fig. 38, p. 87), and these
are indistinguishably fused with the tibia in the adult animal, but may
be made out in the colt. The thigh-bone has a very characteristic shape,
which is difficult to describe without an undue use of technical terms,
but the unusual prominence of the great trochanter (Fig. 35, p. 85) and
of the rotular groove is an important factor in producing this appearance.
[Illustration: FIG. 148.—Left pes of Horse. _Cal._, calcaneum. _As._,
astragalus. _N._, navicular. _Cn. 3_, third cuneiform. _Mt. III_,
functional (third) metatarsal. _Mt. II_ and _Mt. IV_, splints.]
[Illustration: FIG. 149.—Left manus of Horse, front side; to the right,
rear view of the metacarpus. _S._, scaphoid. _L._, lunar. _Py._,
pyramidal. _Pis._, pisiform. _Td._, trapezoid. _M._, magnum. _U._,
unciform. _Mc. II_, _Mc. IV_, rudimentary second and fourth metacarpals,
or splints.]
The very long and slender feet are so raised from the ground that the
animal walks upon the very tips of the toes, the wrist being what
horsemen call the “knee” and the heel is the “hock,” and the gait is
thoroughly unguligrade. Each foot has but a single functional toe, the
third or middle one of the primitive five-toed foot; and, as this toe
has to carry the whole weight supported by its leg, it is necessarily
much larger than in animals which distribute the weight among several
digits. The horses are therefore said to be _monodactyl_, or single-toed,
but the term is not strictly accurate, for on each side of the functional
digit is a rudimentary or vestigial one, the 2d and 4th of the original
five. These rudimentary digits, which are not visible externally, have
no phalanges and are merely “splint-bones,” metapodials (see p. 90)
which have very slender shafts and end below in blunt points. The single
functional metapodial has encircling its lower articular end a prominent
ridge or keel, which fits into a corresponding groove on the upper end
of the first phalanx and serves to prevent lateral dislocation. In most
mammals this keel is merely a projection from the lower articular surface
and is confined to the posterior side, so as not to be visible from the
front. The terminal or ungual phalanx is much enlarged to carry the great
weight which it supports and is enclosed in the characteristic hoof,
unlike that of any other mammal.
In brief, the whole structure of the horses is pre-eminently adapted to
swift running; they are admirable “cursorial machines,” as they have been
called, and every part of the skeleton has been modified and specialized
to that end; the narrow, rigid hoofs fit them for walking on firm ground
and they speedily are made helpless in quicksand or bog. Did we know
nothing of their mode of life, we might confidently infer from their
teeth that the horses were grazers, feeding principally upon grass. A
long-legged, grazing animal must needs have a neck of sufficient length
to enable the mouth to reach the ground easily, unless a long proboscis
is developed; and so we shall find in the history of the horses that the
elongation of the head and neck kept pace with the lengthening of the
legs and feet.
Though it can hardly be doubted that the horses passed through most
of their development in North America, yet the immediate ancestry of
all the existing species must be sought in the Old World, none of
the many Pleistocene species of the western hemisphere having left
any descendants. In North America all of the known Pleistocene forms
belonged to the genus _Equus_, but the True Horse, _E. caballus_, was
not among them. The more abundant and important of these species have
been sufficiently described in Chapter VII (p. 199); it need only be
recalled that there were ten or more distinct forms, ranging in size from
the great _E. †giganteus_ of Texas to the minute _E. †tau_ of Mexico,
while the plains and forests were the feeding grounds of moderate-sized
species, about 14 hands high.
In the latest Pliocene, and no doubt earlier, species of the modern genus
_Equus_ had already come into existence; and in association with these,
at least in Florida, were the last survivors of the three-toed horses
which were so characteristic of the early Pliocene and the Miocene.
However, little is known about those earliest recorded American species
of _Equus_, for the material so far obtained is very fragmentary. In the
absence of any richly fossiliferous beds of the upper Pliocene generally,
there is a painfully felt hiatus in the genealogy of the horses; and it
is impossible to say, from present knowledge, whether all of the many
species of horses which inhabited North America in the Pleistocene were
autochthonous, derived from a purely American ancestry, or how large a
proportion of them were migrants from the Old World, coming in when so
many of the Pleistocene immigrants of other groups arrived. It is even
possible, though not in the least likely, that all of the native American
stocks became extinct in the upper Pliocene and that the Pleistocene
species were all immigrants from the eastern hemisphere, or the slightly
modified descendants of such immigrants; but, on the other hand, it is
altogether probable that some of these numerous species were intruders.
Unfortunately we are in no position yet to distinguish the native from
the foreign stocks.
In the middle Pliocene, which also has preserved but a meagre and scanty
record of its mammalian life, we again meet with horses in relative
abundance, but of a far more primitive type. They are still incompletely
known, but it is clear that they belonged to three parallel series,
or phyla, of three-toed grazing horses, with teeth which, though
high-crowned, had not attained to the extreme degree of hypsodontism seen
in the species of _Equus_ and had a somewhat less complex pattern of the
grinding surface, though distinctly foreshadowing the modern degree of
complication. One of the genera (_†Pliohippus_) was not improbably the
ancestor of a very peculiar horse (_†Hippidion_) of the South American
Pleistocene. These middle Pliocene genera were much smaller animals than
the Pleistocene horses, aside from the pygmy species of the latter, of
light and more deer-like proportions, and with three functional toes or
digits. The median digit (3d of the original five) was much the largest
and carried most of the weight, on hard ground practically all of it;
the lateral digits (2d and 4th) which in existing horses are represented
by the rudimentary metapodials, or “splints,” though much more slender
than the median digit, yet had the complete number of parts and each
carried a small hoof. Mere “dew-claws” as these lateral toes were,
they may have been of service in helping to support the weight in mud
or snow. In all parts of the skeleton there are little details which
show that these species of the middle Pliocene were not so advanced
and differentiated as are their modern successors, but it would be
unprofitable to enumerate these details, which are of interest only to
the anatomist.
In the lower Pliocene the horses were very much more numerous and varied
than in the middle portion of the epoch. The same three genera of grazing
animals, represented by less advanced and modernized species, are found;
and, in addition, there was an interesting survival (_†Merychippus_) from
the middle Miocene of an intermediate type, together with several species
of browsing horses (_†Parahippus_ and _†Hypohippus_). In these browsing
forms the teeth were all low-crowned and early formed their roots, and
the crowns were either without cement or with merely a thin film of it
in the depressions of the grinding surface. The pattern of the grinding
surface is so very much simpler than in the high-crowned, prismatic teeth
of the grazers that it requires close analysis to detect the fundamental
identity of plan. Such teeth imply that their possessors must have fed
habitually upon a softer and less abrasive diet than grass, probably the
leaves and soft shoots of trees and bushes and other succulent vegetable
substances, very much in the fashion of existing deer, and must therefore
have been chiefly inhabitants of the woods and groves and thickets along
streams, as the grazing species were of the plains and open spaces.
“This assemblage of the progressive and conservative types of horses was
certainly one of the most distinctive features of Lower Pliocene time in
North America” (Osborn).
[Illustration: FIG. 150.—Three-toed, grazing horse (_†Neohipparion
whitneyi_) of the upper Miocene. Restored from skeletons in the American
Museum of Natural History.]
In the upper Miocene very much the same conditions prevailed and, for
the most part, the same genera of horses, with different and somewhat
less advanced species, were found as in the lower Pliocene, so that no
particular account of them is needed. In the middle Miocene, however,
there was a change, the typically grazing horses being very rare or
absent and those with intermediate forms of teeth taking their place.
Evidently, it was about this time that the horses with more plastic
organization and capable of readjustment to radically different
conditions began to take to the grazing habit, while other phyla, less
capable of advance, retained the ancient, low-crowned type of grinding
teeth and, after persisting, as we have seen, into the lower Pliocene,
became extinct before the middle of that epoch. It is of great interest
to observe that in the genus (_†Merychippus_) intermediate between the
browsing and grazing types, the milk-teeth retained the older and more
primitive character of low crowns without covering of cement, while the
permanent grinders had much higher, cement-covered and complex crowns.
In the lower Miocene, the variety of horses was much diminished and all
had the low-crowned, cement-free, browsing type of teeth. Reversing the
statement, we see that in the middle and still more in the upper Miocene
the primitive and more or less distinctly homogeneous phylum branched out
into several series, like a tree, some of the branches continuing and
further subdividing through the Pliocene and Pleistocene, while others,
less progressive and less adaptable, underwent but little change and had
died out before the middle Pliocene.
[Illustration: FIG. 151.—Skeleton of _†Neohipparion whitneyi_, American
Museum.]
The Oligocene horses deserve more particular attention, for they were
almost the half-way stage of development in the long backward ascent to
the earliest known members of the family in the lower Eocene. We may pass
over the John Day horses (_†Miohippus_), which were somewhat larger than
those of the White River, but otherwise very like them, merely noting the
presence of a slightly different genus (_†Anchitherium_) which was the
probable ancestor of _†Hypohippus_ and the other non-progressive types of
the Miocene and Pliocene. The genus (_†Mesohippus_) which characterizes
the White River, or lower Oligocene, was a group of species of different
sizes, becoming smaller as we go back in time, the commonest one being
considerably smaller than a sheep and differing more or less in all its
parts from the horses of the upper Miocene and all subsequent formations.
The teeth were very low-crowned and fitted only for the mastication of
soft vegetable tissue; but it is of particular interest to observe the
beginnings of the “mark” in the upper incisors in the form of a low
enamel-ridge arising behind the cutting edge of the tooth; the lower
incisors still had the simple chisel-like crowns of the more ancient
genera; all the premolars, except the first, had already acquired the
molar-pattern.
[Illustration: FIG. 152.—The small, browsing, three-toed, short-necked
horse (_†Mesohippus bairdi_) of the middle White River. Restored from a
skeleton in the American Museum.]
The skull resembled that of a very small modern horse, but with many
differences of detail, the most obvious of which is the shallowness of
the jaws, for depth was not needed to carry the very low-crowned teeth,
and, for the same reason, the ascending ramus of the lower jaw was short.
The face was relatively short and the eye-socket, which was incompletely
surrounded by bone, was directly above the hindmost teeth; the cranium
was proportionately large and capacious and the brain, as is shown by the
cast, was richly convoluted. The neck was relatively far shorter than in
the Miocene genera, the ball-and-socket joints between its successive
vertebræ were less elaborated and the odontoid process of the axis was in
the first stage of assuming the spout-like form, being semicylindrical,
with convex lower and flat upper surface. The trunk was proportionately
long and the back sloped forward, owing to the greater length of the
hind legs. The limbs and feet were elongate and very slender, but the
fore-arm bones are only partially coössified, and the ulna, though
greatly attenuated, was still complete. The same is true of the bones
of the lower leg; the shaft of the fibula was hardly more than a thread
of bone, but its full length was preserved. In the fore foot there were
three functional digits, the median one enlarged and supporting most
of the weight, but its hoof was much thinner and flatter than in the
corresponding digit in the Miocene and subsequent genera; the lateral
digits touched the ground and were not entirely functionless and, in
addition, there was a small splint, the rudiment of the fifth digit. The
hind foot was three-toed, without splint.
The little Uinta horse (_†Epihippus_) is still very incompletely known,
but gives us one point at least of greater primitiveness than the
White River genus in that only the last two premolars had taken on the
molar-pattern, the forward two being smaller and simpler. The known
species of the Uinta genus was very much smaller than any of the White
River forms and even smaller than some of those of the preceding Bridger
formation; but it should be remembered that the Uinta has been but
partially explored and much remains to be learned regarding its fauna.
The Bridger horses are fortunately much better known. There are
several species of the genus _†Orohippus_, which form a connected and
progressive series; and, though much smaller than the smallest and oldest
of the White River forms, they were somewhat larger than the known
representative of the Uinta, _†Epihippus_, but distinctly more primitive
in all other respects. The incisors were simple cutting teeth, with no
trace of even an incipient “mark,” and only one premolar in each jaw,
the hindmost one, had taken on the molar-pattern. The orbit was farther
forward in the skull and less enclosed behind than in _†Mesohippus_, the
cranium narrower and less capacious; the neck was even shorter and the
odontoid process of the axis still retained the primitive peg-like form.
The limbs and feet were conspicuously shorter in proportion than those of
the White River genus; the ulna and fibula were stouter and less reduced
and entirely separate from the radius and tibia respectively. The front
foot had four functional toes; the fifth digit, which in _†Mesohippus_
had been reduced to a splint, was completely developed in the Bridger
horses, but the hind foot was three-toed.
[Illustration: FIG. 153.—The “Dawn Horse” (_†Eohippus_) of the lower
Eocene. Restored from a skeleton in the American Museum.]
Passing over, for lack of space, the transitional forms of the Wind
River, we come finally to the most ancient known horses, the Wasatch
species comprised in the genus _†Eohippus_, the “Dawn Horse,” as its name
signifies; these were little creatures ranging in size from a cat to a
small fox. Despite an unmistakably equine look in the skeletons of these
diminutive animals, it is only the long intermediate series of species
and genera, together forming a closely linked chain, which we have traced
back from the Pleistocene to the lower Eocene, that leads us to regard
_†Eohippus_ as the ancestral type of the horses. Were only the two ends
of the chain known, he would be a daring speculator who should venture
to connect them. In these little Wasatch horses we have the evidence of
a still more ancient form with five fully developed toes in each foot,
since the front foot had four functional digits and indication of a
splint, and splints, as the whole history of the long series teaches,
always are found to be functional digits in the ancestor; the hind foot
had three toes and perhaps two splints. This preceding form is hardly to
be looked for in America or Europe; it will be found, if ever, in the
region whence the great migration came. In all other respects, as well,
_†Eohippus_ was what we should expect the forerunner of the Wind River
and Bridger horses to be. The premolars were all smaller and simpler than
the molars and the latter in the upper jaw are particularly interesting,
for they had no crests and ridges of enamel, but four principal conical
cusps, arranged in two transverse pairs, and between the cusps of each
pair was a tiny cuspule no bigger than the head of a pin. These cuspules
were the first step in the formation of the transverse crests, which
were destined to assume such importance in the subsequent members of the
series. The neck was very short, the body long, with curved or arched
back, the limbs and feet short, and the hind limb much longer than
the fore, making the relative proportions of the various parts of the
skeleton very different from what they afterwards became.
Reviewing this marvellous history of steady and long-continued change,
beginning with the most ancient genus, _†Eohippus_, the following
modifications may be noted:
(1) There was a nearly constant, if somewhat fluctuating, increase in
size, leading by slow gradations from the diminutive horses of the lower
Eocene to the great animals of the Pleistocene.
(2) The molar teeth, originally made up of conical cusps, changed to
a highly complex pattern of crests and ridges, and the premolars, one
by one, assumed the size and pattern of the molars; the low-crowned,
rooted and cement-free teeth, fitted only for browsing, became very
high-crowned, prismatic and cement-covered, admirably adapted to grazing.
Beginning in the upper incisors of the White River _†Mesohippus_, the
“mark” became established as an enamel-lined pit, growing in depth as the
teeth increased their length.
(3) The face grew relatively longer, the eye-socket being shifted behind
the teeth and becoming completely encircled in bone, and the jaws were
greatly increased in depth to accommodate the very long teeth.
(4) The short neck was greatly elongated and the individual vertebræ
modified so as to give flexibility with no loss of strength. The
primitive peg-like odontoid process of the axis became first
semicylindrical and then spout-shaped.
(5) The arched back was straightened and the neural spines, especially of
the anterior dorsals, elongated.
(6) The limbs grew relatively much longer; the bones of the fore-arm and
lower leg were fused together, the one on the inner side (radius and
tibia) enlarging to carry the entire weight and the external one (ulna
and fibula) becoming more or less atrophied.
[Illustration: FIG. 154.—Series of horse skulls in ascending geological
order. _A._, _†Eohippus_, lower Eocene (after Cope). _B._, _†Mesohippus_,
lower and middle Oligocene. _C._, _†Protohippus_, upper Miocene (after
Cope). _D._, _Equus_.]
[Illustration: FIG. 155.—Right manus and left pes of _Equus_.]
[Illustration: FIG. 156.—Right manus and left pes of _†Protohippus_.]
(7) The feet were much elongated and the median (3d) digit of each
gradually enlarged until it carried the whole weight, at the same time
modifying the shape of the hoof so as to fit it to be the sole support
of the body. The other toes gradually dwindled and became functionless,
though often retained as splints. The first digit (pollex and hallux)
was first lost, then the fifth, then the second and fourth were reduced
to dew-claws and finally to splints. Thus the pentadactyl horses of the
lower Eocene were transformed into the monodactyl species of the Pliocene
and Pleistocene.
In South America the story of the horses was a brief one, for they were
among the immigrants from the north and did not reach the southern
continent till the Pliocene, probably late in that epoch, for none
of the three-toed genera have been found in South America. So far as
known, these southern equines were small and medium sized animals, with
large heads, relatively short feet and somewhat ass-like proportions.
There were two well-defined groups of these animals: (1) species of
the genus _Equus_, which thus, at one time or another, inhabited every
one of the continents, Australia excepted; (2) three genera peculiar
to South America and developed there from northern ancestors, probably
_†Pliohippus_. Two of these genera (_†Hippidion_ and _†Onohippidium_)
displayed curious modifications of the nasal bones, which were
extremely slender and attached to the skull only at their hinder
ends, instead of being, as is normally the case, supported for nearly
their whole length by lateral articulation with other bones. What can
have been the significance and function of these excessively slender,
splint-like nasals, it is difficult to conjecture. The third genus
(_†Hyperhippidium_) was a small mountain-horse, with extremely short
feet, which were well adapted to climbing.
[Illustration: FIG. 157.—Right manus and left pes of _†Mesohippus_.]
[Illustration: FIG. 158.—Right manus and pes of _†Eohippus_.]
This is the merest outline sketch of a most wonderful series of gradual
and progressive modifications, a sketch that might readily be expanded
into a volume, were all the details filled in. While each set of organs,
teeth, skull, neck, body, limbs and feet, might appear to advance
independently of the others, in reality there was no such independence,
for at every stage of the progression all the parts must have been so
coördinated into a harmonious whole, that the animal could thrive and
hold its own in the stress of competition. Could we but discover all the
facts of environment, on the one hand, and organization, on the other,
we should doubtless learn that the little _†Eohippus_ was as exquisitely
fitted to its place in the Wasatch world, as are the horses, asses and
zebras of the present day to theirs. It was the response to changing
needs, whether of food, climate, disease or competition, that was the
main factor of development.
[Illustration: FIG. 159.—Skeleton of a Pampean horse (_†Hippidion
neogæum_). National Museum, Buenos Aires. For restoration, see Fig. 119,
p. 214. Note the splint-like nasal bones attached only at the hinder end.]
2. _†Titanotheriidæ. †Titanotheres_
This family, all of whose members vanished from the earth ages ago, was
a comparatively short-lived group and nearly the whole of its recorded
history was enacted in North America; only a few belated stragglers
reached the eastern hemisphere, though the family may, nevertheless, have
originated there.
[Illustration: FIG. 160.—White River †titanothere (_†Titanotherium
robustum_) males fighting. Restored from a skeleton in the American
Museum of Natural History.]
In the lowest of the three substages of the White River Oligocene the
most conspicuous and abundant fossils are the †titanotheres, the latest
members of which were huge animals of almost elephantine proportions.
They belonged to four parallel, or rather slightly divergent, phyla,
differing in the development of the horns, in the shape of the head and
in the relative length and massiveness of the limbs. The teeth were all
low-crowned, or brachyodont, the canines much too small to have been of
any service as weapons and the incisors had curious little, button-shaped
crowns, which can have had little or no functional importance, since
they show hardly any wear, even in old animals. With such front teeth,
a prehensile lip and long tongue would seem to have been necessary for
gathering and taking in food.
[Illustration: FIG. 161.—Second upper molar, left side, of
_†Titanotherium_. _A._, masticating surface; _B._, outer side of crown.]
The †titanotheres were one of two perissodactyl families in which the
premolars never became so large and complex as the molars. The upper
molars had a longitudinal outer wall, composed of two deeply concave
cusps, and two internal conical cusps, but no transverse ridges; the
lower molars were composed of two crescents, one behind the other, a
pattern which was very widely distributed among the early and primitive
artiodactyls and perissodactyls.
The so-called “horns” were not strictly such, but a pair of bony
protuberances from the front of the skull and, from their shape, could
hardly have been sheathed in horn. The long, immensely broad and massive
head resembled that of some fantastic rhinoceros, as did also the body
and limbs. The brain was quite absurdly small, the cavity for it, lost in
the huge skull, would hardly contain the fist of an ordinary man; these
great beasts must have been incredibly dull and stupid, surpassing even
the modern rhinoceroses in this respect. As is generally true in mammals
which have horns, antlers, or similar weapons borne upon the skull, or
very large tusks, the bones of the brain-case were made enormously thick
and yet lightened by an intricate system of communicating cavities or
“sinuses,” separated by many bony braces and supports connecting the
inner and outer denser layers, which form the surfaces of the bones.
In this way the skull is made strong enough without any proportionate
increase of weight to endure the severe shock of impact, when the horns
or tusks are made use of. The principle is the same as the engineer
employs in designing a steel truss-bridge. The upper profile of the head
was deeply concave, just as it is in those rhinoceroses which are armed
with nasal horns.
[Illustration: FIG. 162.—Skull of _†Titanotherium elatum_. American
Museum.]
The neck was of moderate length and the body, as indicated by the long,
arched ribs and the greatly expanded hip-bones, was extremely bulky and
massive. The spines of the anterior dorsal vertebræ were excessively
long, forming a great hump at the withers. The limbs and feet were
columnar, like those of an elephant; the feet were supported on a great
pad, while the hoofs were mere excrescences on the periphery of the
foot. The bones of the fore-arm were entirely separate and the ulna was
very stout; in the lower leg also the bones were not coössified, but
the fibula was but moderately heavy. This is a sharp contrast to the
arrangement found in the horses and in those hoofed animals generally
which are swift runners and have slender, elongate limbs and feet,
such as deer, antelopes, camels, etc. Heavy, slow-moving animals, like
elephants, tapirs, rhinoceroses, etc., almost always have separate
fore-arm and leg-bones and generally a heavy ulna. The number of digits
was four in the front foot and three in the hind. The genera differed
in the proportions of limbs and feet, one having them longer and less
ponderous than another, and, no doubt, the former was of swifter gait.
At a certain level in the White River beds the †titanotheres abruptly
cease, disappearing with what seems like startling suddenness. In all
probability, however, the extinction was more gradual and its apparent
abruptness was due, partly at least, to the break in the deposition
of the beds, which is very obvious. Such a break, or “unconformity,”
as geologists call it, almost always implies an unrecorded lapse of
time, which may have been very long. However it came about, gradually
or suddenly, the extinction of these great animals is difficult to
explain; no Carnivora of the time could have been formidable enemies
and they had no rivals in their own walk of life. Their stupidity may
have been a factor, but it seems more likely that the onset of some
new infectious disease, perhaps imported by incoming migrants from the
eastern hemisphere, gave the _coup de grace_. In the lower substage,
beneath the unconformity, where the remains of †titanotheres are so
abundant, successive changes may be observed. The species with great
“horns,” rounded, flattened or triangular, are confined to the upper
levels; in the middle section other species, somewhat smaller and with
shorter “horns,” are found, while in the bottom levels the animals are
much smaller and have still smaller “horns.”
The Uinta †titanotheres were much more numerous and varied than those of
the White River; in the upper part of these beds are found two genera
(_†Diplacodon_ and _†Protitanotherium_) which already had quite prominent
bony protuberances on the nose; their canines were large enough to be of
value as weapons and the incisors were well developed and functional.
Evidently, there was a change here in the manner of feeding, the front
teeth were used for cropping and browsing, a function which in the White
River members of the family must have been largely taken over by the
lips and tongue, while the growth of the horn-like protuberances on the
skull rendered the canines superfluous as weapons. This latter change is
one which recurs frequently in different phyla of the hoofed animals,
in which the earlier and more primitive members had canine tusks, and
the later, more advanced representatives developed horns, the tusks
diminishing as the horns increased. While this rule is a general one, it
is not entirely without exceptions.
In the lower Uinta and in the Bridger the †titanotheres were extremely
abundant and numerically they are the commonest of all fossils in those
beds; no less than five series or phyla may be distinguished, three of
them being added in the upper Bridger. The differences between the phyla,
however, principally concern the forms of the teeth and the shape of the
skull; in some the head was short and broad, in others long and narrow,
and in others again of medium proportions; some had broad and extremely
low-crowned grinding teeth, which in others were higher and more erect.
But these are matters of minor detail, useful as they are in pointing
the way to a proper arrangement of the various species; in essentials,
the forms all agreed and constituted several series of closely allied
genera. Comparing these Bridger animals with the great †titanotheres
of the lower White River, the first and most obvious difference that
strikes the observer is the very much smaller size of the more ancient
types. With some variation in this respect, hardly any of the Bridger
species exceeded a modern tapir in stature and very much resembled one in
proportions. The canine teeth were tusks as large as those of a bear and
must have been very effective weapons; the molar-pattern was identical
with that found in the great Oligocene beasts, but the premolars were
simpler and relatively smaller.
[Illustration: FIG. 163.—†Titanothere (_†Mesatirhinus superior_) with
long, narrow head; Bridger stage. Restored from a skeleton in the
American Museum.]
[Illustration: FIG. 164.—Second upper molar, right side of a Bridger
†titanothere (_†Palæosyops_).]
The skull had a straight upper profile, though in several of the phyla
small bony protuberances were developed over the eyes, and must clearly
be regarded as incipient stages of the “horns” which were subsequently
to become so long and prominent. Instead of being broad on top as it
was in the White River genera, the cranium carried a high ridge of
bone, the sagittal crest, which served for the attachment on each side
of the great temporal muscle, one of the most important of the muscles
of mastication. The trunk was less massive and the limbs were lighter
than in the Oligocene genera, but the number of digits was the same,
four in the front foot and three in the hind, and the hoofs were much
better developed, serving actually to carry the weight and not being
mere excrescences upon the periphery of a pad. Aside from the proboscis,
which lends such a characteristic appearance to the existing tapirs, the
†titanotheres of the Bridger must have looked much like tapirs, and in
early days, when the mutual relationships had not been satisfactorily
determined, they were frequently described as “tapiroid.” The term is
unobjectionable in so far as it is understood that a merely superficial
likeness is implied, not any real relationship other than that which
unites all the perissodactyl families.
As noted above, the phyla of the †titanotheres were much more numerous
in the later than in the earlier part of the Bridger stage, when they
were reduced to two. In the still older Wind River stage these two united
into one. The Wind River animals (_†Eotitanops_) were similar, but much
smaller, and occurred in incomparably less variety and abundance. Indeed,
one of the most striking differences between the Wind River and the
Bridger faunas consists in the great increase and diversification of the
†titanotheres in the latter. There was, it is true, a second phylum of
the family in the Wind River, represented by the genus _†Lambdotherium_,
but this was a short-lived series, which left no descendants in the
Bridger or subsequent formations. These were the smallest known members
of the family and were light, slender-limbed animals, a very notable
difference from the others.
With the Wind River the history of the †titanotheres breaks off short,
and from present information, can be carried no farther back. Possibly,
there was a Wasatch ancestor, which only awaits discovery, but it seems
more likely that these earliest known genera were belated immigrants
from the same as yet unknown region, whence came the modernized and
progressive elements of the Wasatch fauna. Except for its obscure
beginning, the family was pre-eminently characteristic of North America,
and only two representatives of it have been found outside of that
continent, one in Hungary and one in Bulgaria. No doubt others will yet
be found in Asia.
Both in its resemblances and its differences, as compared with the
far longer and more complex story of the horses, the history of the
†titanotheres has instructive bearings upon evolutionary theory.
(1) Starting with two phyla, one of which speedily died out, the other
ramified into four or five, which continued until the disastrous end,
pursuing a nearly parallel course of development.
(2) There was a great increase in size and especially in bulk and
massiveness from species no bigger than a sheep in the Wind River stage
to those which rivalled small elephants in the lower White River.
(3) The teeth underwent comparatively little change; the incisors
dwindled and lost functional importance and the canines were reduced,
horn-like growths taking their place as weapons; the premolars grew
larger and more complicated, but never attained the full size and
complexity of the molars, as they did in other perissodactyl families.
(4) Horn-like, bony protuberances appeared first as small humps and knobs
over the eyes and steadily enlarged, at the same time shifting their
position forward, until they finally attained great size and were on the
nose.
(5) The skull was modified so as to support these weapons and endure the
shock of impact when they were put to use, (_a_) by making the upper
profile strongly concave from before backward; (_b_) by greatly widening
the top of the cranium, where in the older and more primitive genera the
high and thin sagittal crest was placed; (_c_) by immensely increasing
the thickness of the cranial bones and at the same time hollowing them by
means of an intricate system of cavities; in this way sufficient strength
was secured without undue increase in weight.
[Illustration: FIG. 165.—Series of heads of †titanotheres in
ascending geological order. _A._, _†Palæosyops_, lower Bridger. _B._,
_†Manteoceras_, upper Bridger. _C._, _†Diplacodon_, Uinta. _D._,
_†Titanotherium_, extreme development of horns, White River. From models
in the American Museum and Princeton University.]
(6) The growth of the brain did not keep pace with the increase in the
size and weight of the body and head, and this deficiency may have been a
factor in determining the early extinction of the family.
(7) To support the huge head, stout ligaments and powerful muscles were
needed in the neck and trunk and these in turn required large bony,
surfaces for their attachment. To meet this need, the spines of the
anterior trunk-vertebræ were very much lengthened, so as to form a hump
at the shoulders, and this elongation of the spines went on in proportion
to the growing weight of the head.
(8) The limb-bones increased in thickness until they became extremely
massive, to carry the immense weight of the body, and they eventually
lost the marrow-cavities, which were filled up with spongy bone, a
great gain in strength. As is generally, though not universally, true
of the large and heavy mammals, there was no coössification between the
limb-bones and no great increase in their proportionate length. The
thigh-bone, or femur, lost the cylindrical shape of the shaft, becoming
flattened and very broad, and acquiring something of the appearance of
the same bone in the elephants.
[Illustration: FIG. 166.—Right manus of †titanotheres. _A._,
_†Titanotherium_, White River (after Marsh). _B._, _†Palæosyops_,
Bridger, Princeton University Museum.]
(9) There was no loss or coössification of elements in wrist (carpus)
or ankle (tarsus) and _no reduction of digits_ within the limits of the
family. In the latest, largest and most specialized genera, as well as
in the earliest, smallest and most primitive, there were four toes in
the front foot and three in the hind. We have the most cogent reasons
for assuming that all mammals were derived from ancestors which had
five toes in each foot, neither more nor less. If this be true, then
the most ancient known †titanotheres, which were small and light, had
already suffered the loss of the first digit in the fore foot and of the
first and fifth digits in the hind foot, but there reduction ceased.
With the growing body-weight, long, narrow and slender feet would have
been a detriment, whereas in swift-running animals, like horses and
deer, long and very slender feet are a great advantage. The contrast is
both striking and instructive, showing the importance of a short, broad,
polydactyl and pillar-like foot to very large and heavy mammals, all of
which have feet of this character.
(10) The hoofs, as shown by the terminal bones (ungual phalanges) which
formed their bony cores, were reduced in size until they became mere
nail-like excrescences around the border of the massive foot.
3, 4. _Tapiridæ and †Lophiodontidæ. Tapirs and †Lophiodonts_
The history of the tapir family is not at all satisfactorily known,
partly because tapirs are comparatively rare as fossils in all of the
Tertiary formations, and still more for the reason that the specimens so
far collected are so fragmentary, not a single half-complete skeleton
among them. Had these animals actually been as rare in North America
as the fossils would seem to indicate, they could not possibly have
maintained themselves for so long a time, throughout nearly the whole of
the Tertiary and Quaternary periods. For some reason, probably because
they have always been forest-haunting animals, their habits must have
kept them in places remote from the areas where the accumulation of
sediments was in progress, and thus only occasional stragglers were
buried and preserved.
The rarity and incompleteness of the material render it impossible to
give any such full account of the tapirs as is practicable for the horses
and †titanotheres, but the circumstance is less unfortunate in the case
of the tapirs than in that of many other families. This is because these
creatures have been so conservative and unprogressive, that they have
undergone comparatively little change since their earliest recorded
appearance. They have been aptly termed “living fossils” and seem like
belated survivors from some older world, out of place in the modern order
of things. Attention has already been directed (p. 137) to the remarkable
geographical distribution of the tapirs at the present time; Central and
South America, southeastern Asia and the adjoining islands.
[Illustration: FIG. 167.—American Tapir (_Tapirus terrestris_). By
permission of W. S. Berridge, London.]
The tapirs are all of moderate size, going back to very small forms
at the beginning of their history and never at any period developing
into large animals. The only striking and unusual feature about any of
the existing members of the family is the long proboscis, a flexible,
dependent snout, and, were they all extinct and nothing known of them but
the skull, this proboscis could have been confidently predicated of them
from the great shortening of the nasal bones. Small tusks, not showing
when the mouth is closed, are formed in an exceptional way by the
enlarged external upper incisor and the lower canine, the upper canine
being much reduced and without function. The grinding teeth have very
low crowns, premolars (except the first) and molars are all alike and of
a very simple pattern, which has been independently repeated in several
different orders of herbivorous mammals; in both upper and lower teeth,
there are two elevated, straight, transverse crests.
[Illustration: FIG. 168.—Skull of American Tapir, right side.]
Except for the modification of the skull which is conditioned by the
development of the proboscis, the skeleton might belong to any one
of several Eocene or Oligocene families, and it is this generalized,
indifferent character which has led to the dubbing of many early
perissodactyls as “tapiroids.” The limbs are short and moderately heavy,
the bones of the fore-arm and lower leg all separate and the number of
toes is four in the front foot and three in the hind. The toes end in
well-formed separate hoofs, but behind them is a pad, which carries most
of the weight. The body is covered with smooth, short hair, which in the
American species is of a uniform dark brown, but in the Asiatic species
the head, neck and limbs are black and the body is white. In both,
however, the young have longitudinal, light-coloured stripes and spots on
a dark ground (_see_ Fig. 6, p. 47) indicating what the colour-pattern of
the ancestral forms must have been. As might be inferred with certainty
from the low-crowned teeth, the tapirs are browsing, not grazing,
animals, feeding upon leaves and shoots and other soft vegetable tissues.
They are shy and solitary in habit and live usually in thick forests and
near water, which they frequently enter, both for bathing and as a place
of refuge when pursued. Under modern conditions, the only perissodactyls
of the western hemisphere are the tapirs of the Neotropical region, North
America proper, which for ages was the principal home of the order, not
having a single representative now.
In the Pleistocene, tapirs were apparently more abundant than in any
of the Tertiary epochs, but this was probably due to the fact that the
Pleistocene of the forested regions is far more fully recorded than is
any Tertiary stage. One species, which was hardly distinguishable from
the Recent Central American form, was common in the forested region east
of the Mississippi and in California, and a second species (_Tapirus
†haysii_) was larger and heavier than the other. Except in Texas, none
have been found in the Great Plains area, nor are they likely to be, for
that region, then as now, appears to have been devoid of forests. No
doubt, these Pleistocene species had substantially the same habits as the
existing ones, but they were adapted to a colder climate and a different
vegetation, for, except the Pinchaque Tapir (_T. roulini_) of the high
Andes, all the modern species are tropical in distribution.
Concerning the Pliocene and Miocene tapirs, but meagre information has
been obtained. Enough material has been gathered by the collectors to
demonstrate the continuous presence of the family in North America
throughout those epochs, but the broken and fragmentary specimens are
insufficient to show what the structural changes were. It should be
remembered, however, that it is only in the region of the Great Plains
and the Great Basin of Nevada that any considerable quantity of Miocene
and Pliocene mammals have been found, and in those regions tapirs
probably never were common. If the Peace Creek formation of Florida is
properly classified as latest Pliocene, then at that time the American
tapirs were essentially what they are to-day, for the Florida species is
hardly separable from the modern _T. terrestris_.
Not till we reach the lower Oligocene, or White River beds, do we get
material which permits the making of definite statements regarding the
course of developmental changes. The White River genus, _†Protapirus_,
which is also found in the middle Oligocene of Europe, was a much smaller
animal than any of the known Pleistocene or Recent species, barely more
than half the size, in fact. The teeth show that the small tusks were
canines, both above and below, and that the curious substitution of
the external upper incisor for the canine had not yet taken place. The
grinding teeth were identical in pattern with those of the existing
genus, but not all the premolars had yet acquired the form and size of
the molars. In the skull the nasal bones had begun to shorten, but the
change had not yet made much progress, and the proboscis must have been
in merely an incipient stage of development. What little is known of the
skeleton other than the skull was like that of the modern genus, but the
bones were much smaller and proportionately lighter.
[Illustration: FIG. 169.—Skull of White River tapir (_†Protapirus
validus_), left side. Princeton University Museum. N.B. This figure is
much less reduced than Fig. 168.]
The Eocene tapirs are still very imperfectly known; all that can be said
of them is that they become successively smaller as they are traced
backward in time, and that in them the premolar teeth were all smaller
and simpler than the molars. The Wasatch genus (_†Systemodon_) is the
most ancient member of the series yet discovered. Dating from the Eocene
immigration, the tapirs are to be regarded as a North American family,
for there is here a complete continuity from the lower Eocene to the
Pleistocene, while in Europe they first appeared, probably by migration
from North America, in the middle Oligocene.
[Illustration: FIG. 170.—Head of the White River tapir (_†Protapirus
validus_). Restored from a skull in the museum of Princeton University.]
In South America the history of the tapirs is even shorter and less
eventful than that of the horses; the latter, as we have seen, reached
the southern continent in the Pliocene and there gave rise to a number of
peculiar and characteristic genera, but the tapirs have been found only
in the Pleistocene of Argentina and Brazil and only the modern genus is
represented.
Wofully broken and incomplete as the developmental history of the tapirs
still is, the fragments are nevertheless sufficient to show a mode of
evolution differing in certain important respects from that followed by
the horses or †titanotheres. Certain features are common to all three
groups, such as the increase in size and in proportionate stoutness
from stage to stage and the gradual enlargement and complication of
the premolar teeth. On the other hand, the tapirs have been very
conservative, and they underwent far less radical changes than did either
of the other families. Aside from the proboscis and the modifications of
the skull which the development of that organ necessitated, these animals
remain to-day very nearly what they were in Oligocene times. This, then,
is an example of development practically restricted to a few organs,
while all the other parts of the structure changed but little.
[Illustration: FIG. 171.—Upper teeth, left side, of tapirs, showing
comparative sizes. _A_, _†Protapirus validus_, White River Oligocene.
_B_, _Tapirus terrestris_, modern. _i3_, external incisor. _c_, canine.
_m1_, first molar.]
The extinct †lophiodonts, like the tapirs, of which they would seem to
have been near relatives, are known only from incomplete material, and
comparatively little has been learned regarding their history. While they
were abundant and varied in Europe, during the Eocene epoch, they never
were a striking or prominent element among the mammals of North America,
where they persisted one stage later, and they did not reach South
America. In North America they are found from the Wasatch to the White
River.
The White River genus (_†Colodon_), which is fairly well known, might
almost be described as combining the characters of horses and tapirs;
but such an expression is not to be interpreted as meaning that this
genus is in any sense a connecting link or transition between the two
families, but merely that in certain important respects its course of
development ran parallel with that followed by the horses. The teeth
were very tapir-like, especially those of the lower jaw, which, indeed,
are hardly distinguishable from those of a tapir, and the premolars had
the molar-pattern. The limbs were very light and slender and the feet
long and narrow; the fore foot retained a small fifth digit; the feet,
especially the hinder one, had a resemblance to those of the contemporary
horses (_†Mesohippus_), though the median digit was not so much enlarged,
nor the lateral ones so far reduced. It is highly probable that, had this
family persisted till the Pleistocene, instead of dying out in the lower
Oligocene, it would have eventually terminated in monodactyl forms.
The †lophiodonts of the Eocene are represented by very fragmentary
material; so far as that material goes, it does not show much change from
the White River genus, except that the premolar teeth were smaller and
simpler, the limbs and feet retaining the same characteristics of length
and slenderness. The Wasatch genus (_†Heptodon_) had a similar lightness
of limb and narrowness of feet, these characters thus appearing at the
very beginning of the family history, so far as their North American
career is concerned.
5. _Rhinocerotidæ. True Rhinoceroses_
The history of the great group of rhinoceroses and rhinoceros-like
animals is a very long and complicated one, inferior in its completeness
only to that of the horses. The complexity of the story arises from the
large number of phyla into which the families are divisible, and, despite
the great wealth of material and the admirable preservation of much of
it, it is extremely difficult to find a clew through the mazes of this
labyrinthine genealogy. From the standpoint of the existing geographical
distribution of animals, few mammals could seem more foreign and exotic
to North American life than do the rhinoceroses, and yet for a very long
time that continent was one of the chief areas of their development, so
far, at least, as that development can be followed. It is even probable,
though not clearly demonstrable, that the family originated here and
subsequently spread to the Old World, but not to South America, where no
member of it has ever been found. The later history of the rhinoceroses
ran its course in the Old World entirely, and the highest specializations
within the family are to be found there; in North America these animals
are not known to have persisted beyond the lower Pliocene, and if they
did survive, it was only as a few stragglers in out of the way places.
The modern rhinoceroses are restricted to Africa, southern Asia and some
of the larger Malay islands, Borneo, Sumatra and Java, and within these
wide geographical limits are to be found the terminal representatives of
at least three separate and quite distinct phyla, the African, Indian and
Sumatran genera respectively (_Opsiceros_, _Rhinoceros_, _Dicerorhinus_).
It will be advisable to begin the study of this peculiarly interesting
family with a brief examination of its modern members, even though none
of these are found in the western hemisphere.
[Illustration: FIG. 172.—Skull of the Javan Rhinoceros (_R. sondaicus_).
Note the single upper incisor, and the rough surface on the nasal bones
for the attachment of the single horn.]
All the existing rhinoceroses are large and massive animals, ranging
from four feet to six feet six inches in height at the shoulder, and all
have solid dermal horns, except in most females of the Javan species[6]
(_R. sondaicus_). The Indian and Javan species have a single horn on
the nose, while those of Africa and Sumatra have, in addition to the
nasal horn, a second one on the forehead. The horns, thus, do not form
a transverse pair, but are placed in the median line of the head, one
behind the other; it should also be noted that these horns are solid,
dermal structures, made up of agglutinated fibres or hairs and not having
a bony core formed by outgrowths of the skull, as do the horns of most
ruminants, such as oxen, sheep and antelopes, which are therefore called
“hollow-horned” (Cavicornia). The skull, however, betrays the presence of
horns by the extremely rough areas which serve for their attachment and
thus the presence or absence of these weapons may be readily determined
in the case of an extinct species of which only the skeleton remains.
The skin is very thick and coarse, typically “pachydermatous,” and is
quite naked in most of the species; but in the Sumatran form there is a
sparse coat of hair, which is quite thick in the young animal. In the
Indian _Rhinoceros unicornis_ the enormously thick skin has conspicuous
and regularly arranged folds, which make the creature look as though
encased in armour; the ears and tail are tufted with hair. In the African
and Sumatran genera the folds are obscurely marked and not definitely
arranged, giving the body a smoother appearance. All the existing
species, except one, are browsers and feed upon leaves and twigs, and
they frequent forests and marshes where their food is abundant. Not that
these and other browsing animals do not occasionally eat grass, but it is
not their principal diet. The exception noted is the largest of all the
living species, the Broad-Lipped Rhinoceros (erroneously called “White”)
of Africa, _Opsiceros simus_, which is strictly a grazing animal and
therefore frequents more open country than the other African species, _O.
bicornis_.
There are considerable differences in proportions and general appearance
among the various species, but they all have short necks, very long
and massive bodies, short and heavy limbs and short, columnar feet,
which look much like those of elephants, but have only three toes
each. In all but two of the living species the upper lip is prehensile
and characteristically pointed and can be used to pick up very small
objects, like the “finger” on an elephant’s trunk; in the Sumatran
species (_Dicerorhinus sumatrensis_) the lip, though pointed, is
horny and inflexible, while in the African _O. simus_ it is broad and
straight-edged.
The teeth of the modern rhinoceroses are extremely characteristic and
may always be recognized at a glance. In the African genus (_Opsiceros_)
there are no front teeth, all the incisors and canines being lost;
the other genera have on each side a single large and trenchant upper
incisor, in shape like a broad, obliquely edged chisel, which shears
against a still larger elongate and tusk-like lower incisor, that is
procumbent and points directly forward. The Indian Rhinoceros (_R.
unicornis_) is said to use its tusks as weapons in very much the same
fashion as the Wild Boar. Between the large lower tusks there is a pair
of very small incisors, which can have little or no functional value;
the third lower incisor has been suppressed, as have also the canines
of both jaws. The dental formula then is: _i_ 1/2 or 0/0, _c_ 0/0, _p_
4/4, _m_ 3/3, × 2 = 28 or 34 (see p. 93). The premolars, except the
first, though somewhat smaller than the molars, have essentially the same
pattern. The upper molars have moderately high crowns, yet they are
purely brachyodont, except in the grazing, broad-lipped African species
(_O. simus_), in which they may fairly be called hypsodont. The external
wall of the tooth is broad and nearly smooth, not divided into cusps,
as it is in the horses and tapirs, and the two transverse crests, which
in the tapirs are directly transverse, are very oblique. In all the
existing species additional complications are given by the short spurs,
which project inward from the outer wall or from the transverse crests.
The lower molars are formed each of two crescents, one behind the other,
but their arms or horns are angulate, not curved as they are in other
perissodactyls which have crescentic lower teeth.
The upper surface of the skull is very concave in the antero-posterior
direction and very broad over the cranium, where there is no sagittal
crest. The nasal bones are immensely thick and strongly arched, with the
convexity upward; both this arching of the nasals and the fore-and-aft
concavity of the skull are devices for giving a strong and solid
attachment to the great nasal horn, for the attachment of which these
bones have an extremely rough surface, and in the two-horned species,
a second roughened area on the forehead marks the place of attachment
of the frontal horn. The bones of the cranium are very thick, but
lightened by the many chambers which traverse them. The articulation of
the lower jaw with the skull is in some respects unique among mammals;
the postglenoid process is a long spike, which fits inside of a bony
lump (the _postcotyloid process_) behind the condyle of the lower jaw,
and the posterior margin of the latter is greatly thickened. The neck
is short and stout, the trunk very long, broad and deep, the long and
strongly arched ribs and the widely expanded hip-bones providing space
for the great mass of viscera. The bones of the limbs are short and very
massive; the humerus has a very prominent deltoid ridge and the femur an
unusually large third trochanter; the bones of the fore-arm and lower
leg are separate, as in the massive ungulates generally. The foot-bones
are likewise extremely short and heavy, and the number of digits is
three in each foot. Each of the five or more existing species has its
skeletal peculiarities, every portion of the bony structure showing
characteristic features; but these are only minor modifications of the
general plan and may be neglected in any comprehensive account of the
living representatives of the family.
[Illustration: FIG. 173.—Left manus of Indian Rhinoceros (_R.
unicornis_).]
In order to find any American members of this family, it is necessary
to go back to the lower Pliocene, where a great abundance of them is
encountered, representing, according to Osborn’s view, four or five
phyla; and just as in the case of the horses of the same formation, they
were an assemblage curiously made up of progressive and old-fashioned,
conservative genera,—some were persistent native stocks, others the
descendants of immigrants from the Old World, which reached America
in the middle Miocene. There was great variety of form, size and
proportions among these animals, North America at that time having a
larger number of genera and species than Africa and Asia combined have
now. Some were quite small, some large, though none equalled the larger
modern species. Some of the genera had relatively long legs, but in one
genus, _†Teleoceras_ (Fig. 125, p. 230), an Old World type, they were
most grotesquely short, the belly almost touching the ground, as in a
hippopotamus. Most of these rhinoceroses were hornless, but _†Teleoceras_
had a small horn on the very tip of the nose. In consequence of the
lack of horns, the nasal bones were thin and weak, in marked contrast
to the massive, convex nasals of the modern species, and, for the same
reason, the upper profile of the skull was nearly straight. Except
for minor details, the dentition was in very nearly the modern stage
of development; there was a single trenchant upper incisor on each
side, a procumbent lower tusk and between the tusks a pair of small
incisors; the other incisors and the canines were already lost. One genus
(_†Peraceras_) had lost all the upper front teeth. The grinding teeth had
the same character as in the existing species, but were somewhat simpler,
owing to less development of the accessory spurs. In the more progressive
types the teeth were rather high-crowned, though in none were they
actually hypsodont; while the persistent ancient genera had teeth with
much lower crowns.
Aside from the differences in the skull, which are obviously to be
correlated with the absence or very small size of the horn, the skeleton
in these Pliocene genera differed but little from the type common to the
existing rhinoceroses, and in all the species the feet were three-toed.
In short, the dentition and skeleton, except the skull, had already
attained to substantially the modern conditions. While the Old World at
that time had both horned and hornless rhinoceroses in abundance, none
of the genera with large and fully developed horns ever migrated to
the western hemisphere. This is the more remarkable in that the great
†Woolly Rhinoceros (_Opsiceros †antiquitatis_) of the Pleistocene, which
had two very large horns, inhabited Siberia with the †Mammoth (_Elephas
†primigenius_). The latter extended its range through Alaska and the
northern United States, but the rhinoceros, for some unknown reason, did
not accompany it in its eastward wanderings.
The rhinoceroses of the upper Miocene did not differ sufficiently from
those of the lower Pliocene to call for particular attention. Needless to
say, there were differences between the species of the two epochs, but
in such a sketch as this only the broader and more obvious changes can
be taken into account. Even in the middle Miocene the only feature which
calls for notice was the first appearance in North America of the Old
World genus _†Teleoceras_, which became so abundant in the upper Miocene
and lower Pliocene. The middle Miocene species (_†T. medicornutus_) would
seem to have been descended from _†T. aurelianensis_ of the lower Miocene
of France; the two species agreed not only in having a small horn on the
tip of the nose, but also in the presence of a still smaller one on the
forehead.
In the lower Miocene but two phyla of rhinoceroses have been found,
both of which were the comparatively little changed descendants of
Oligocene ancestors; and there was thus a notable difference from the
rhinoceroses of the middle Miocene and subsequent stages, which were
decidedly more modern in character. One of these phyla was constituted
by those rhinoceroses (_†Diceratherium_, Fig. 129, p. 239) which had a
transversely placed pair of horns on the nose, not one behind the other,
as in all of the subsequent two-horned species, of which North America
had but the one middle Miocene form (_†T. medicornutus_) mentioned
above. The lower Miocene species of _†Diceratherium_ was a very small
animal, and smaller than any member of the family from later formations.
The †diceratheres originated in North America, and the stages of their
development may be clearly made out; they also migrated to the eastern
hemisphere and have been found in France, though it is possible that
the genus was not truly monophyletic and arose independently in both
hemispheres.
The second phylum is that of the hornless forms (_†Cænopus_) which were
so abundantly represented in the Oligocene and persisted with little
change into the Pliocene.
In the upper Oligocene, or John Day, the †diceratheres are the only
rhinoceroses certainly yet obtained, and of these there were several
species, large and small. The hornless forms may have been present in
Oregon, but this has not been clearly demonstrated. That they continued
to exist somewhere during that stage is hardly open to question, for they
reappeared in the lower Miocene.
From the White River, or lower Oligocene, many well-preserved
rhinoceroses, including complete skeletons, have been gathered in the
various collections and display very interesting differences in the three
substages of the White River beds. In the uppermost substage is found the
apparent beginning of the †dicerathere phylum, though it may be traced
back to the middle substage; the nasal bones had become much thickened
so as to serve as a support for the horns, and these are indicated by a
small, but very rough, area on the outer side of each nasal. Comparing
this White River species with those of the upper Oligocene and lower
Miocene, two differences may be observed: in the later species the
horn-supports were well defined bony knobs or prominences, and these
knobs were close to the anterior ends of the nasals; while in the White
River animal the places for the attachment of the horns were mere
roughened areas, and these were well behind the tips of the nasals.
This is not an infrequent sort of change, that horns should shift their
position forward or that the portion of the nasals in front of the horns
should be shortened. Parallel changes occurred among the †titanotheres.
In the middle White River all the rhinoceroses were hornless, but the
same two phyla may be distinguished; the actual starting point of the
†diceratheres had no indication of the nasal horns, but may be identified
as such by their close resemblance in other respects to the species of
the upper substage in which the incipient horns appeared. Much commoner
were the members of the typical hornless line (see Fig. 135, p. 256),
which, though true and unmistakable rhinoceroses, were yet far removed in
many details of structure from the progressive genera of the middle and
upper Miocene. There are several species in this phylum, which constitute
a series of diminishing size almost in proportion to their increasing
antiquity. The dentition was already thoroughly and characteristically
rhinoceros-like, but a more primitive feature was the presence of a
second upper incisor, a small tooth placed behind the trenchant one,
making the incisor formula 2/2; the third incisor and the canines of
both jaws were already lost. The assumption of the molar-pattern by
the premolars varied much in degree of completeness in the different
species; the upper molars, while having all the essentials of the
rhinocerotic plan of structure, had a much less complex appearance than
in the Recent genera, because of the absence of the accessory spurs; and
all the grinding teeth were very low-crowned, in strong contrast to the
high-crowned (yet not properly hypsodont) teeth of the middle Miocene and
subsequent genera.
[Illustration: FIG. 174.—Skull of †hornless rhinoceros (_†Cænopus
tridactylus_); middle White River stage. (After Osborn.)]
[Illustration: FIG. 175.—Second upper molar, left side, of _†Cænopus_,
showing the masticating surface.]
As already mentioned, there was much variation in size among the species,
but none was as large as those of the Miocene and Pliocene genera, not
to mention the enormous animals of the Pleistocene and Recent epochs in
the Old World. The commoner species of the middle White River substage
(_†Cænopus occidentalis_) was an animal nearly equalling in size the
American Tapir (_T. terrestris_) and quite like that species in its
proportions, the limbs being relatively longer and less heavy and the
feet narrower than in the rhinoceroses of the subsequent geological
epochs. The skull, being hornless, had thin, pointed and nearly flat
nasal bones, an almost straight and horizontal upper profile, and a short
and low, but distinct, sagittal crest; the cranial bones were quite
thin, there being no extensive development of sinuses within them. The
articulation of the lower jaw with the skull was only beginning to take
on the characteristic peculiarities seen in the later genera, and the
hinder margin of the lower jaw was not much thickened. Thus, many of the
features which distinguish the skull in all Recent and Pleistocene and
most Pliocene, and upper and middle Miocene rhinoceroses were entirely
lacking in _†Cænopus_, yet no anatomist could doubt that the White River
animal was a genuine rhinoceros.
The neck was short, but not very heavy, the trunk elongate, but not
massive, the ribs not being inordinately long nor strongly arched, and
the hip-bones so little expanded that they were tapiroid rather than
rhinocerotic in appearance. The limb-bones were relatively much more
slender than in any existing species, and, although every one of them was
characteristically that of a rhinoceros, yet the comparative lightness of
body and slenderness of limb gave to these bones a certain resemblance
to those of tapirs. The feet, which were moderately elongate and rather
narrow, were three-toed, as in all subsequent North American species and
in all existing members of the family.
The most ancient and primitive representative of the true rhinoceroses
so far discovered occurs in the lowest division of the White River beds
and is of particular interest as throwing light upon the origin of
the family. The genus (_†Trigonias_) differed from that (_†Cænopus_)
which was so abundant in the middle White River substage in several
highly significant particulars, though on a merely casual inspection
one might easily be misled into thinking that the two animals were
nearly identical, for _†Trigonias_ was an undoubted rhinoceros.
Such an identification, however, would be a great mistake, for the
differences, though not striking, are very important. In the upper jaw
the first or anterior incisor had already assumed the characteristic
trenchant, chisel-like shape, but two other incisors were present also,
thus bringing the number up to the original three, common to all early
perissodactyls; even more interesting is the presence of a small upper
canine. The lower jaw likewise had three incisors on each side, the
first and third small, the second enlarged and tusk-like, but the canine
had already been suppressed, and thus the dental formula was: _i_ 3/3,
_c_ 1/0, _p_ 4/4, _m_ 3/3, × 2 = 42, or 14 more than the formula of the
existing African species. The premolars were smaller and less complex
than the molars.
[Illustration: FIG. 176.—Skull of _†Trigonias osborni_, lower White
River. (After Hatcher.)]
[Illustration: FIG. 177.—Anterior end of right upper jaw of _†Trigonias
osborni_ (after Lucas). _c._, canine. _i 3_, external incisor. _i 2_,
middle incisor. _i 1_, first incisor.]
From this ancient genus may readily be inferred the steps by which the
peculiar characters of the anterior teeth in the true rhinoceroses were
attained. The first stage was undoubtedly an animal in which, as in all
other Eocene perissodactyls, there were three well-developed incisors on
each side of both jaws, 12 in all, and moderately prominent canine tusks;
all these teeth were erect. The second stage was the enlargement of the
first upper and second lower incisors, the latter becoming less erect and
beginning to assume the recumbent position; at the same time the other
incisors and the canines were reduced in size and were so little used
that they lost their functional importance. The third stage, in which
the first and second lower incisors were horizontal and pointed directly
forward, and the first upper and second lower teeth were still further
enlarged, the non-functional teeth reduced in size and the lower canine
suppressed, was realized in the genus _†Trigonias_. There were thus but
two hypothetical stages between this lower White Region genus and the
tapir-like forms of the middle Eocene, so far, at least, as the anterior
teeth are concerned.
[Illustration: FIG. 178.—Anterior end of left upper jaw of _†Cænopus_,
_A_, adult; _B_, immature animal (after Osborn). _I 1_, first incisor; _I
2_, second incisor; _C_, canine.]
The skeleton of _†Trigonias_ was, on the whole, very much like that of
the succeeding genus, _†Cænopus_, of the middle substage of the White
River, but with the important exception that the front foot had four
digits instead of three. The pollex, or first of the original five,
almost always the first to disappear, had been suppressed, the third
or median digit was already the largest of the series, both in length
and breadth; the second and fourth, somewhat shorter together made a
symmetrical pair, while the fifth, though much the most slender of all,
was still functional and had retained all of its parts. In the hind foot
the digits had been reduced to three. This arrangement, four toes in
the manus and three in the pes, is the same as is found in the existing
tapirs and in the Eocene perissodactyls generally, with only two or three
known exceptions. In the Oligocene, on the other hand, all the genera
except the †titanotheres, tapirs, †lophiodonts and †amynodonts were
tridactyl both before and behind.
[Illustration: FIG. 179.—Left manus of _†Trigonias osborni_. (After
Hatcher.)]
With _†Trigonias_ the definitely known history of the true rhinoceroses
breaks off abruptly, and it is possible that that genus was an immigrant,
though it is perhaps more likely that its ancestors existed in the
upper and middle Eocene (Uinta and Bridger stages) of North America.
Some fragmentary specimens from the Uinta beds, too imperfect for
any definitive identification, are an encouragement to hope that the
forerunner and direct ancestor of _†Trigonias_ may yet be discovered in
that formation. It is also quite possible that one of the larger species
of the genus _†Hyrachyus_, so abundant in the Bridger and going back to
the Wind River, may take its place in the same series.
6. _†Hyracodontidæ. †Cursorial and †Aquatic Rhinoceroses_
The luxuriant diversification of the rhinoceros-stem was not exhausted by
the many phyla of what we have called the true rhinoceroses. Two other
series, very distinctly marked and rather distantly connected with the
first, are yet to be considered. These two series, the †hyracodonts (in
the narrow sense) and the †amynodonts, ran courses which, in certain
respects, were singularly alike; both were of North American origin and
one, the †hyracodonts, was entirely confined to that continent, while
the other sent out late migrants, which entered Europe, no doubt through
Asia, and both ended their careers before the close of the White River
time. Their history was thus a brief one when compared with that of the
true rhinoceroses, three phyla of which persist to the present day,
though their geographical range is greatly restricted in comparison with
what it was in the Miocene and Pliocene, when they ranged over every
continent except Australia and South America.
Just how to classify these three series of rhinoceroses and
rhinoceros-like animals, so as most accurately to express their mutual
relationships, is a question that has received several answers. One
method suggested is to include them all in a single family and to make
a subfamily for each of the three well-distinguished series; this is
the arrangement which personally I should prefer. A second plan is to
accord family rank to each of the three groups; while the most elaborate
scheme, that of Professor Osborn, is as follows: for the rhinoceroses,
in the broader sense, he makes two families, the Rhinocerotidæ and the
†Hyracodontidæ, and divides the former into four subfamilies, which
include all of the true rhinoceroses, living and extinct, of the Old and
New Worlds, and the latter into two subfamilies, the †Hyracodontinæ and
†Amynodontinæ. It is not a matter of very great moment as to which of
these three schemes is followed, and I shall therefore adopt the one
proposed by Professor Osborn, in order to avoid, so far as possible, the
confusing effect of different methods of classification.
[Illustration: FIG. 180.—†Cursorial rhinoceros (_†Hyracodon
nebrascensis_), White River stage. Restored from a skeleton in the Museum
of Princeton University.]
As before mentioned, the subfamily of the †hyracodonts (†Hyracodontinæ)
became extinct in White River times, during most of which it was
represented by the single genus _†Hyracodon_, whence are derived the
names for the family and subfamily. The series was purely North American,
and no member of it has ever been found in any other continent. The
species of _†Hyracodon_ were altogether different in appearance and
proportions from the true rhinoceroses, being lightly built, slender,
cursorial creatures, suggestive rather of horses than of rhinoceroses,
to which they bore much the same relation as the slender-limbed,
narrow-footed †lophiodonts did to the tapirs (_see_ p. 326); in size,
they were somewhat taller and considerably heavier than a sheep.
The low-crowned grinding teeth had the unmistakable rhinoceros-pattern,
and between them and the teeth of the contemporary _†Cænopus_ the
difference was merely one of size, except for one small, but not
insignificant feature. The last upper molar had not perfectly acquired
the triangular form characteristic of all the true rhinoceroses, caused
by the complete fusion of the outer wall with the posterior crest, but
the wall projected a little behind the crest, as in perissodactyls
generally. Premolars (except the first) and molars were alike in
structure and of nearly the same size. While the grinding teeth were
thus hardly to be distinguished from those of the true rhinoceroses, the
anterior teeth, incisors and canines, were totally different; they were
very small and had simple, pointed and slightly recurved crowns, and
were all very much alike in size and form. Thus, there were in the front
of the mouth eight small, hook-like teeth, above and below, which were
obviously quite useless as weapons; and as the skull had no horn, the
animal was defenceless, and must have depended entirely upon speed for
its safety from the attacks of the larger and more powerful beasts of
prey.
The skull was short, deep and thick, and the head must have been heavy
and clumsy, quite out of keeping with the body and limbs. The neck was
surprisingly long, longer indeed proportionately than in the contemporary
genus of horses (_†Mesohippus_), but the neck-vertebræ were relatively
stout and strong, as was required for the muscles to move and control
the heavy head. The body was rather elongate, but not deep or massive,
and the limbs were proportionately much longer than in any of the
known rhinoceroses. The limb-bones, one and all, despite their length
and slenderness, bore an unquestionable likeness to those of the true
rhinoceroses. In this elongation of the limbs the fore-arm and thigh were
the parts most affected, and the slenderness, though in notable contrast
to the proportions both of the true rhinoceroses and the †amynodonts, was
yet much less marked than in the middle Eocene representatives of the
†hyracodonts themselves. The feet were long and narrow, approximating,
though not actually attaining the proportions of the feet in the White
River horses (_†Mesohippus_). There were three digits in each foot, and
the median toe (third of the original five) was so much enlarged and the
lateral toes (second and fourth) so reduced, though still functional,
as strongly to suggest a monodactyl foot as the outcome of this course
of development, had not the early extinction of the subfamily put an
end to it. It is interesting to reflect that, had the †lophiodonts and
†hyracodonts continued their existence to the present time and had
persisted in advancing along their particular lines of specialization, we
should, in all probability, have had monodactyl tapirs and rhinoceroses,
as well as horses.
[Illustration: FIG. 181.—Left manus of †cursorial rhinoceroses. _A_,
_†Triplopus cubitalis_ (after Cope), upper Bridger. _B_, _†Hyracodon
nebrascensis_, White River.]
As in the case of so many other mammalian series, the †hyracodonts of the
but partially explored Uinta formation are still very imperfectly known.
Almost all that can be positively stated about them is that they were
smaller than their White River successors and that the assumption of the
molar-pattern by the premolars was incomplete. In the upper Bridger beds
also not very much is known regarding the then representatives of the
series, (_†Triplopus_). So much is clear, however, that they were still
smaller and lighter animals, that the limbs were very light, and that the
number of digits in the fore foot had already been reduced to three, the
only known Bridger perissodactyl of which this is true, all the others
having four digits in the manus and three in the pes.
[Illustration: FIG. 182.—Primitive †cursorial rhinoceros (_†Hyrachyus
eximius_), lower Bridger. Restored from a skeleton in the American Museum
of Natural History.]
In the middle and lower Bridger, and even in the Wind River, occurs a
genus (_†Hyrachyus_) which contained a large number of species, ranging
in size from a full-grown modern tapir to creatures no larger than foxes.
It is among these smaller species that the most ancient member of the
†hyracodont line is to be sought, though it is not yet practicable to
select any particular one. _†Hyrachyus_, indeed, may very possibly have
contained among its many species the ancestors of all three lines of the
rhinoceroses and rhinoceros-like animals, and thus formed the starting
point from which they developed in diverging series. It is always a very
significant fact when two or more groups approach one another the more
closely, the farther back in time they are traced, because that can only
be interpreted to mean that ultimately they converged into a common
term, even though that common ancestor should elude discovery.
_†Hyrachyus_ may be described as a generalized, relatively
undifferentiated perissodactyl, from which almost any other family of the
order, except the horses and the †titanotheres, might have been derived.
The incisors, present in undiminished number, were well developed and
functional, but not large, and the canines were moderately enlarged,
forming small tusks. The premolars were all smaller and less complex than
the molars, which had a strong resemblance to those of the tapirs; in
the lower jaw they were identical with the latter, but in the upper jaw
there was more than a suggestion of likeness to the rhinoceroses. The
skull was long, narrow and low, hornless, and with thin, slender nasals
and straight, horizontal upper contour. The neck was short, the body very
long and the limbs of medium length and weight; though relatively stouter
than in _†Triplopus_ of the upper Bridger and Uinta beds, they cannot be
called heavy. The feet were not especially elongate and rather slender;
the manus had four toes and the pes three.
[Illustration: FIG. 183.—Skull of _†Hyrachyus_. (After Osborn.)]
A brief and short-lived branch of this stock existed in the Bridger
stage, but was not, so far as is known, represented in any of the
subsequent stages, and was made up of a single genus (_†Colonoceras_)
which had a small pair of dermal horns upon the nasal bones. In other
respects, it was like _†Hyrachyus_. It is surprising to find that the
horned series should have so speedily died out, while the defenceless
forms not only persisted, but actually became more defenceless through
the reduction of the canine tusks. _À priori_, one would have expected
the opposite result, but the key to the enigma is probably to be found in
the more perfect adaptation of the surviving kinds to swift running.
The second subdivision (†Amynodontinæ) of this family contains a series
of animals which developed in a very divergent fashion and went to
quite the opposite extreme from the cursorial †hyracodonts, resembling
the latter (aside from the fundamental characteristics common to all
rhinoceroses, in the broadest sense of that term) only in the pattern
of the molar teeth and in the absence of horns. The terminal member of
the †amynodont series was a White River genus (_†Metamynodon_) of which
the remains have been found almost exclusively in the consolidated and
cemented sands filling the old river-channels of the middle substage
of the White River beds. This fact, together with certain structural
features of the skull and skeleton, leads at once to the suggestion
that these animals were chiefly aquatic in their habits and somewhat
like hippopotamuses in mode of life. _†Metamynodon_ was quite a large
animal, the heaviest and most massive creature of its time, after the
disappearance of the giant †titanotheres, but was low and short-legged.
The true rhinoceroses, save those which, like the existing African
species, have lost all the front teeth, all agree in the peculiar
differentiation of the incisors, which was fully described in the
preceding section of this chapter. The †hyracodonts had a second scheme,
the incisors and canines being all similar in shape, small, pointed and
recurved, while still a third mode of development was exemplified by the
†amynodonts, in which the canines became large and formidable tusks, a
very notable difference from all other rhinoceroses whatever.
[Illustration: FIG. 184.—Supposedly †aquatic rhinoceros (_†Metamynodon
planifrons_) of the White River. Restored from a skeleton in the American
Museum.]
In _†Metamynodon_ the incisors were not enlarged, but were unreduced and
functional; the upper canine was a short, heavy tusk, obliquely truncated
by the abrasion of the lower tusk, which was very large. Another striking
difference from all the other groups of rhinoceroses was the reduction
of the premolar teeth, which, instead of equalling the molars in size,
were much smaller and were diminished to three in the upper, two in the
lower jaw. The molars were of the characteristic rhinoceros-pattern,
but were very narrow, especially the inferior ones, in which the enamel
did not surround the whole crown, as it normally does, but was lacking
along vertical bands, where the dentine formed the surface. The skull
was extremely peculiar and, with its very long and high sagittal crest
and immensely expanded and heavy zygomatic arches, had a surprising
likeness to the skull of some great beast of prey. The face was very much
shortened and the skull depressed, so that the head was remarkably low,
broad and flat, proportions which did not recur in any other group of
rhinoceroses. The neck was short, the body very long and very massive, as
is shown by the long and strongly arched ribs. The limbs were short and
stout and the feet quite primitive in character, the front foot retaining
four fully developed and functional digits. No other perissodactyls of
the middle White River beds, except the †lophiodonts and tapirs, had more
than three digits in the manus, and thus _†Metamyodon_ was a belated
exception to the general rule.
The Uinta member of this series was _†Amynodon_, a similar but smaller
and lighter animal. The canine tusks were of more moderate size and none
of the premolars had been lost, but were considerably smaller than the
molars, and the last two had assumed the molar-pattern. The face was
not conspicuously shortened and the zygomatic arches of the skull were
not so heavy or so widely expanded as in the White River genus, and the
skeleton was less massive.
The genus _†Amynodon_ is also represented in the upper Bridger beds, but
by a species different from that of the Uinta stage. This more ancient
species was a smaller animal than its upper Eocene successor and had less
enlarged canine tusks, but it already possessed the typical rhinoceros
molar teeth, the only Bridger mammal of which this is true. Beyond this
species the line, as at present understood, cannot be traced, though
probably some species of _†Hyrachyus_, or an allied genus, will prove to
be the ancestor sought; but the connecting link has not yet been brought
to light.
The history of the rhinoceroses and rhinoceros-like animals, of which
a very much simplified sketch has just been given, is a highly complex
one, much more so than that of the horses, †titanotheres, or tapirs,
and is less fully recorded, the earlier chapters of the story being
still missing. However, in the progress of discovery these chapters will
almost certainly be recovered, and it is already possible to draw close
inferences as to what they will reveal. The complexity of the history is
chiefly due to the fact that, as compared with the other perissodactyl
groups, the rhinoceros stem ramified more widely and gave rise to
more divergent and diversified forms. At one extreme, we find huge,
massive, slow-moving types; and, at the other, light, slender, cursorial
creatures, almost horse-like in appearance, with intermediate forms of
moderate size. Some were long and others short legged, mostly adapted to
terrestrial life, but some with aquatic habits. The three very different
sorts of modification which the anterior teeth (incisors and canines)
underwent in the three principal series may be taken as an illustration
of this divergent development, and to these may be added a fourth, the
complete suppression of all the incisors and canines above and below, as
is exemplified by the modern African species.
Of the three rhinoceros groups, whatever rank be assigned them, family
or subfamily, much the most prolific in divergent forms was that of the
true rhinoceroses (Rhinocerotidæ) of which seven or more phyla have been
distinguished, three of them surviving to the present time. Only in this
series were horns frequently present, the brief experiment, as it might
be called, of the Bridger genus _†Colonoceras_, being the only known
instances of horns among the †hyracodonts, and the †amynodonts were all
hornless. In making the comparison as to degree of ramification among
the three series, it should be borne in mind that the true rhinoceroses
were the only long-lived group, the other two dying out before or at
the end of the White River stage. Within the series or family of the
true rhinoceroses, there was no great divergence of type, and all the
members were much alike, heavy and slow animals, but with very great
variety in the details of structure. Take, for instance, the matter of
horns; we find both hornless and horned genera, the former preceding the
latter in time, but, so far as North America is concerned, continuing in
association with them till the end. Among the horned genera, the horn may
be single, double in a transverse pair (_†Diceratherium_) or arranged
one behind the other in the median line of the head (_Dicerorhinus_,
_Opsiceros_, etc.). The single horn may be on the nose or the forehead;
if on the nose, it may be on the upper side of the nasal bones
(_Rhinoceros_) or on the extreme tip and pointing obliquely forward
(_†Teleoceras_). The single frontal horn was much less common, but in the
extraordinary _†Elasmotherium_, of the European and Siberian Pleistocene,
the horn was of gigantic size and the surface for its attachment an
enormous, dome-like boss on the forehead.
All three of the series had their most ancient known representatives
in North America, and it seems probable, though by no means certain,
that they all originated here by divergence from a common stock, which
was represented more or less closely by the genus _†Hyrachyus_ of the
Bridger and Wind River stages of the Eocene. However that may be, true
rhinoceroses flourished exceedingly in the Old World from the upper
Oligocene to the Pleistocene, the events of the latter epoch restricting
them to their present range. The significance of the American genera for
the ancestry of the modern types can be found only in the most ancient
forms, _†Trigonias_ and _†Cænopus_; the subsequent development which
led up to the existing species of Asia and Africa went on entirely
in the eastern hemisphere. The †hyracodont subfamily had no known
representatives outside of North America, but the †amynodonts sent out
emigrants, which appeared for a brief time in the Oligocene of Europe.
In the varied history of the rhinoceroses, the principles of evolutionary
change which may be deduced from the recorded development of the horses,
tapirs and †titanotheres are found to be applicable.
(1) There was the same gradual increase in size from the earlier to the
later geological stages. Not that all the phyla kept equal pace in this
respect, and even within the same phylum it was the rule rather than the
exception to find larger and smaller contemporary species.
(2) In all of the early forms, up to the middle Miocene, the teeth were
low-crowned; after that time there was a decided increase in the height
of the teeth, though only in _†Elasmotherium_ was the fully hypsodont,
cement-covered crown attained. In the existing African Broad-Lipped
Rhinoceros (_Opsiceros simus_), which is a grazing animal, the high,
cement-covered teeth may also fairly be called hypsodont.
(3) In all of the lines, as in the other perissodactyl families, the
premolars gradually took on the pattern of the molars; only in the
†amynodonts were the premolars notably reduced in number and size.
(4) The three different modes of development of the anterior teeth,
exemplified by the true rhinoceroses, the †hyracodonts and †amynodonts
respectively, need not be recapitulated here. It is sufficient to call
attention to the fact that the three kinds of modification diverged
from a common starting-point such as may be seen in the middle Eocene
perissodactyls generally, and that in each series the transformation was
gradual.
(5) The modification of the skull followed several different courses, as
designated by the major and minor subdivisions of families, subfamilies
and phyla. The development of horns, whether single or double, in
transverse or longitudinal pairs, was the most important single influence
in transforming the skull, as determined by the mechanical adjustment
necessary to make these weapons effective, but even in the hornless
forms changes went on, and in all the phyla the skull departed more and
more widely from the primitive Eocene type in each succeeding geological
stage. The most aberrant form of skull was that of the hornless and
presumably aquatic _†Metamynodon_, in which the greatly shortened face,
high sagittal crest and extremely wide zygomatic arches were altogether
exceptional.
(6) When the history of any horned phylum is at all complete, the
development of the horns may be followed step by step from the marks
which they left upon the skull. As a rule, the story was one of gradual
enlargement, but, in one case at least, an incipient horn apparently
failed to enlarge and was eventually lost.
(7) In the light, slender and cursorial †hyracodonts the mode of
development resembled that of the horses, as appears in the elongation
of the neck, limbs and feet, in the enlargement of the median toe and
concomitant reduction of the lateral digits. Also, as in the horses, the
elongation of the limbs began to be noteworthy while the body-weight
was small and was consequently accompanied by great slenderness; as the
body-weight increased, the limbs became stouter, to yield the necessary
support.
(8) In the phyla composed of massive animals the principle of change
agreed with that exemplified by the †titanotheres, increasing
body-weight being the determining factor in both cases. When this
increase began to be decided, the reduction of digits ceased at the point
which had already been reached in any particular series, three in both
manus and pes in the true rhinoceroses, four in the manus and three in
the pes in the †amynodonts. Very heavy animals require broad, columnar
feet to support them, and hence the similarity of appearance in such
widely separated groups as elephants, rhinoceroses and hippopotamuses,
not to mention several extinct orders and families. Among the larger and
heavier rhinoceroses, as in those of the present time, there was great
variety in the proportionate lengths of the limbs, body and feet.
In brief, the great complexity of the history of the rhinoceroses is due
to the many divergent and parallel phyla into which these animals may
be grouped. Broadly speaking, they may be subdivided into the slender,
cursorial types and the heavy, slow-moving types, the former developing
in a manner similar to that shown by the horses, while the latter were
modified after the fashion of the †titanotheres. Obviously the load to be
supported by the legs and feet was a very important factor in determining
the character of evolutionary change.
II. †ANCYLOPODA. †CLAWED PERISSODACTYLS
The very extraordinary and aberrant animals which are referable to this
suborder have been understood only since the year 1888, for, as was shown
in an earlier chapter (p. 41) their scattered parts had been assigned to
two different mammalian orders, the skull to the perissodactyls and the
feet to the pangolins, or scaly anteaters (Pholidota) of the Old World,
since it occurred to no one that the same animal could have such a skull
and teeth in combination with such feet.
The history of the Ancylopoda is still very incomplete, only four genera,
of the lower Pliocene, middle and lower Miocene, and the middle Eocene
respectively, being at all adequately known, but even in this imperfect
form the story is worth telling. The suborder was probably of American
origin and its most ancient known member existed in the middle Eocene.
Both in Europe and North America the group persisted into the lower
Pliocene and it is believed, though not clearly demonstrated, that in
eastern Asia it continued even into the Pleistocene. All the genera of
the suborder may be included in a single family.
7. _†Chalicotheriidæ. †Chalicotheres_
The specimens which so far have been found in the American middle and
upper Miocene and lower Pliocene are very fragmentary, consisting of
little more than teeth, and give no information other than to demonstrate
the presence of the family in North America during that period of time.
On the other hand, the European genera of the middle Miocene and lower
Pliocene are well known and may or may not have been closely similar
to their American contemporaries, though they were undoubtedly larger.
In these most peculiar and grotesque animals (_†Macrotherium_ and
_†Chalicotherium_) the head was relatively small, the teeth were very
low-crowned and adapted only to a diet of soft vegetable substances
and the mode of feeding must have been that of browsing upon leaves
and shoots of trees and bushes; the premolars had not acquired the
molar-pattern, which was very exceptional for perissodactyls of so
late a time, such a difference between the two classes of teeth being
characteristic of the Eocene members of the order; the incisors and
canines were reduced, but the formula is not definitely known.
The neck was of moderate length, the body very long, and the limbs were
also elongate, especially the anterior pair, in consequence of which the
back sloped downward from the shoulders to the rump; the two fore-arm
bones were fused together, and these, with the thigh-bones, were the
longest segments of the limbs. The special peculiarity of these animals
was in the character of the feet, which had three toes, each armed with
a huge claw, instead of terminating in a hoof, as it does in all normal
perissodactyls. The external digit, which, in the absence of the fifth,
was the fourth, was the largest of the series and apparently bore the
most of the weight, a notable departure from the normal perissodactyl
symmetry, in which the third or median toe is the largest. The hind feet
were considerably smaller than the fore, but had similar claws.
Many suggestions have been offered as to the manner in which these great
claws were employed. The teeth demonstrate that these animals could not
have had predaceous habits, but must have been inoffensive plant-feeders.
As no such herbivorous creatures are living now, it is impossible to
reach a definitive solution of the problem, which is further complicated
by the fact that in two other orders of hoofed mammals, Artiodactyla and
†Toxodontia, a more or less similar transformation of hoofs into claws
took place, and among the edentates the large, herbivorous †ground-sloths
(†Gravigrada) had enormous claws. It is inadmissible to suppose that
these great †chalicotheres could have been burrowers, or tree-climbers,
or that they pursued and slaughtered prey of any kind, for, aside from
the character of the teeth, such heavy and slow-moving beasts would
have been utterly inefficient at work of that sort. No doubt, the claws
were used, to some extent, as weapons of defence, as the existing South
American Ant-Bear (_Myrmecophaga jubata_) uses his formidable claws;
probably also some, if not all, of these clawed ungulates would employ
the fore feet in digging for roots and tubers, as is done by the bears
generally. Many years ago, the late Sir Richard Owen suggested with
reference to the †ground-sloths that the principal use of the fore feet,
other than that of locomotion, was to draw down within reach of the long
tongue and prehensile upper lip the branches upon which they browsed.
This explanation may perhaps be applicable to all of these aberrant and
exceptional groups of hoofed animals.
[Illustration: FIG. 185.—Left manus of lower Miocene †chalicothere
(_†Moropus_). (After Peterson.)]
In the lower Miocene (Arikaree stage) of North America well-nigh complete
skeletons of a large †chalicothere (_†Moropus_, Fig. 130, p. 240) have
been obtained, an animal which considerably exceeded a large horse in
bulk and stature. In structure this genus had departed less widely from
the normal perissodactyl type than the genera of the European Miocene
and Pliocene above described and was in many respects more primitive.
It could not, however, have been directly ancestral to the European
forms, though indicating in a general sort of way what the ancestral type
must have been. _†Moropus_ had a relatively small, slender and pointed
head, a long neck, much longer than in the European genera, and long
fore legs; the shorter hind legs gave the back a steep inclination from
the shoulders to the rump. The proportions of the head, neck and limbs
suggest those of a giraffe, in less exaggerated form, but the likeness
is more marked in the skeleton than in the restoration and is at best
a distant one. The feet were armed with the great claws characteristic
of the suborder, but the fore foot, in addition to the three functional
toes, had a long splint, representing the rudimentary fifth digit; of the
first, or pollex, no trace remained. The perissodactyl plan of symmetry
had not yet been lost, the third or median digit being the longest of
the series. In the hind foot, which had only three toes, the departure
from the perissodactyl arrangement had already begun, and the third and
fourth digits (_i.e._ of the original five) were of nearly equal size,
both in length and thickness, while the second was smaller.
The family is represented in the John Day, or upper Oligocene, by
specimens which are sufficiently characteristic to prove that they are
properly referable to this group. They have been assigned to the same
genus as that of the lower Miocene, but whether the identification is
justified remains to be determined.
In the lower White River beds of Canada is found a much smaller animal
of this family, but the material is too fragmentary for generic
identification. Something more is known of a genus (_†Schizotherium_)
from the European Oligocene, likewise much smaller than the Miocene and
Pliocene forms, which had four, or possibly even five, functional digits,
in the manus, but it has not been ascertained whether the transformation
of hoofs into claws had already taken place.
It is not yet practicable to determine the relationships of the European
and American †chalicotheres to one another, because of the imperfect
nature of most of the material.
The molar teeth of the †chalicotheres were suggestively like those of the
†titanotheres, and, were the teeth alone to be taken into account, no one
could hesitate to regard the two families as closely related.
The most ancient known member of the family is the genus _†Eomoropus_,
from the Bridger Eocene, which will be described by Professor Osborn
in a paper soon to appear. _†Eomoropus_ was much nearer to the normal
perissodactyls than were the genera from the Oligocene and Miocene above
described.
CHAPTER IX
HISTORY OF THE ARTIODACTYLA
The artiodactyls are and for a very long time have been a very much
larger and more variegated group than the perissodactyls, and the Old
World has been and still is their headquarters and area of special
development, where they are represented in far greater number and variety
than in the New; the perissodactyls, on the other hand, flourished
especially in North America, as was shown in the preceding chapter. At
the present time the artiodactyls are the dominant ungulate order, far
outnumbering all the others combined, and include an assemblage of varied
types, which, when superficially examined, appear to be an arbitrary
and unnatural group. What could seem more unlike than a dainty little
mouse-deer, no larger than a hare, a stag, a camel, a giraffe, a bison
and a hippopotamus? Yet, in spite of this wonderful diversity of size,
proportions, appearance and habits, there is a genuine unity of structure
throughout the order, which makes their association in a single group
altogether natural and proper, especially as these structural characters
are not found united in any other group.
It would be superfluous to enumerate all of the diagnostic characters
which, on the one hand, unite all the living and extinct artiodactyls
and, on the other, distinguish them from all other hoofed animals, and it
will suffice to mention a few of the more significant of these features.
As the name implies, the artiodactyls typically have an even number of
toes in each foot, four or two; though this rule may be departed from
and we find members of the order with five digits or three, just as the
tapirs and nearly all the Eocene genera of perissodactyls had four toes
in the manus. Much more important is the fact that the plane of symmetry,
which in the perissodactyls bisects the third digit and is therefore
said to be _mesaxonic_, passes between the third and fourth digit and
is _paraxonic_. The third and fourth digits always form an equal and
symmetrical pair and are the “irreducible minimum,” beyond which the
number of toes cannot be diminished. A single-toed artiodactyl would seem
to be an anatomical impossibility; at all events, such a monstrosity was
never known. Hence the term “cloven” or “divided” hoof, which seems to
take the solid hoof of the horse as the norm; but “cloven or divided,”
while expressing the appearance of the foot with sufficient accuracy, is
erroneous, if taken to mean the splitting of what was once continuous.
[Illustration: FIG. 186.—Left fore-arm bones of the Domestic Pig (_Sus
scrofa_). _R._, radius. _U._, ulna. _ol._, olecranon.]
[Illustration: FIG. 187.—Left manus of Pig. _S._, scaphoid. _L._, lunar.
_Py._, pyramidal. _Pis._, pisiform. _Td._, trapezoid. _M._, magnum. _U._,
unciform. _Mc. I_, second, _Mc. II_, third, _Mc. III_, fourth, _Mc. IV_,
fifth, metacarpals.]
[Illustration: FIG. 188.—Left pes of Pig. _Cal._, calcaneum. _As._,
astragalus. _N._, navicular. _Cb._, cuboid. _Cn. 2_, _Cn. 3_, second and
third cuneiforms. _Mt. II-V_, second to fifth metatarsals.]
[Illustration: FIG. 189.—Bunodont upper molar of peccary (_Tagassu_).]
[Illustration: FIG. 190.—Selenodont upper molar of deer (_Odocoileus_).]
Especially characteristic of the order is the structure of the ankle,
or “hock-joint” of the hind limb. The ankle-bone, or astragalus, has
a double pulley, the upper and lower ends being of quite similar
shape; its lower end is almost equally divided between the cuboid and
navicular, which are made concave to receive it. This type of astragalus
is altogether peculiar to the artiodactyls, all of which possess it;
it is unlike that of any other mammal whatever and may be recognized
at a glance. The calcaneum, or heel-bone, has a large convex facet, by
means of which it articulates with the fibula, or external leg-bone;
there is no such articulation in the perissodactyls. The lower end of
the calcaneum is narrow and fits into a step cut in the cuboid, which
is thus every whit as peculiar and characteristic as the calcaneum and
astragalus. The femur never has the third trochanter, which is always
present in the perissodactyls. Another respect in which the artiodactyls
differ from all perissodactyls except the horses is in the much more
complex mode of articulation between the vertebræ of the lumbar and
posterior dorsal regions, which the former display, and even the horses
have no such elaborate arrangement. Finally, another very marked
difference from the perissodactyls is in the teeth, for the premolars
and molars are never alike, and only in very rare instances does the
last premolar assume the molar-pattern. Of this pattern, there are two
principal kinds, one exemplified by the peccaries, in which the crown
supports a series, fundamentally two pairs, of conical cusps, and called
_bunodont_, and the other, to be seen in all the ruminating animals, in
which the crown is composed of two pairs of crescents and is therefore
said to be _selenodont_. The bunodont was the primitive type, whence the
other was derived, and many transitional forms are known.
The classification of the immense horde of living and extinct genera and
species which are referable to the artiodactyls is an extremely difficult
problem, which has found no thoroughly satisfactory solution and will
not until much more is learned concerning the history of the order and
conflicting opinions can be reconciled. The most important American
families and genera are given below, though the arrangement is but
tentative.
Suborder A. ARTIODACTYLA †PRIMITIVA. (Extinct genera of
doubtful affinities)
I. †TRIGONOLESTIDÆ.
_†Trigonolestes_, low. Eoc.
II. †LEPTOCHŒRIDÆ.
_†Leptochœrus_, low. Oligo. _†Stibarus_, low. Oligo.
III. †DICHOBUNIDÆ. _†Homacodon_, mid. Eoc. _†Bunomeryx_, up. Eoc.
IV. †ANTHRACOTHERIIDÆ.
_†Anthracotherium_, low. Oligo. _†Bothriodon_, do.
_†Arretotherium_, do.
V. ?†OREODONTIDÆ.
† _Protoreodon_, up. Eoc. _†Merycoidodon_, low.
Oligo. _†Eporeodon_, up. Oligo. _†Promerycochœrus_,
up. Oligo. to up. Mioc._†Merycochœrus_, Mioc.
and low. Plioc. _†Pronomotherium_, up. Mioc.
_†Mesoreodon_, low. Mioc. _†Merychyus_, low. Mioc.
to low. Plioc. _†Leptauchenia_, low. Oligo. to low.
Mioc. _†Cyclopidius_, mid. Mioc.
VI. †AGRIOCHŒRIDÆ.
_†Protagriochœrus_, up. Eoc. _†Agriochœrus_, Oligo.
Suborder B. SUINA. Swine-like Animals
VII. TAGASSUIDÆ, Peccaries.
_†Helohyus_, mid. Eoc. _†Perchœrus_, low. Oligo.
_†Thinohyus_, up. Oligo. _†Desmathyus_, low.
Mioc. _†Prosthennops_, up. Mioc. and low. Plioc.
_†Platygonus_, mid. Plioc. to Pleist. _Tagassu_,
Recent, Pleist. in S. A.
VIII. †ENTELODONTIDÆ. †Giant Pigs.
_†Parahyus_, low. Eoc. _†Achænodon_, mid. and up.
Eoc. _†Arcæotherium_, low. Oligo. _†Boöchœrus_, up.
Oligo. _†Dinohyus_, low Mioc.
Suborder C. TYLOPODA. Camels and Llamas
IX. CAMELIDÆ.
_†Protylopus_, up. Eoc. _†Eotylopus_, low. Oligo.
_†Poëbrotherium_, Oligo. _†Pseudolabis_, low.
Oligo. _†Protomeryx_, up. Oligo. and low. Mioc.
_†Oxydactylus_, low. Mioc. _†Miolabis_, mid. Mioc.
_†Protolabis_, mid. and up. Mioc. _†Alticamelus_,
mid. Mioc. to low. Plioc. _†Stenomylus_, low.
Mioc. _†Procamelus_, up. Mioc. and low. Plioc.
_†Pliauchenia_, up. Mioc. to mid. Plioc. _Camelus_,
Pleist. _Lama_, Plioc. to Recent, S. A.
X. †HYPERTRAGULIDÆ.
_†Leptotragulus_, up. Eoc. _†Leptoreodon_, up. Eoc.
_†Leptomeryx_, low. Oligo. _†Hypertragulus_, Oligo.
_†Hypisodus_, low. Oligo. _†Protoceras_, low. Oligo.
_†Syndyoceras_, low. Mioc.
Suborder D. PECORA. True Ruminants
XI. CERVIDÆ. Deer.
_†Blastomeryx_, low. Mioc. to low. Plioc. _Cervus_,
Pleist. and Rec. _Rangifer_, Pleist. and Rec. _Alce_,
Pleist. and Rec. _†Cervalces_, Pleist. _Odocoileus_,
Pleist. and Rec., N. and S. A. _Mazama_, Pleist. to
Rec., S. A.
XII. †MERYCODONTIDÆ. †Deer-Antelopes.
_†Merycodus_, mid. Mioc. to low. Plioc.
_†Capromeryx_, Pleist.
XIII. ANTILOCAPRIDÆ. Prong-Bucks.
_Antilocapra_, Pleist. and Rec. _?†Dromomeryx_, mid.
and up. Mioc.
XIV. BOVIDÆ. Antelopes, Sheep, Goats, Oxen, etc.
_†Neotragocerus_, _†Ilingoceros_, _†Sphenophalus_,
low. Plioc. _†Preptoceras_, _†Euceratherium_,
_†Symbos_, Pleist. _Ovibos_, Pleist. and Rec.
_Bison_, Pleist. and Rec.
This list of families and genera, portentous as it is, would be greatly
increased by the addition of the Old World forms, which outnumber those
of the western hemisphere.
SUBORDER SUINA. SWINE-LIKE ANIMALS
The history of the American types of pig-like forms is, in one sense,
very full and complete in that the successive genera may be traced
back to the Eocene, but, in another sense, the story is exasperatingly
imperfect, because so much of the material is fragmentary. Of most of the
genera, nothing is known but teeth and jaws, and these, though sufficient
for identification, tell but little of the structural changes which it
is desirable to know. It is merely a question of time, when more adequate
material will be obtained.
1. _Tagassuidæ. Peccaries_
The peccaries, or American swine, are now chiefly of Neotropical
distribution, extending into the Sonoran region only as far as Arkansas;
but this has been true only since the Pleistocene, for nearly the
entire history of the family has been enacted in North America. In many
points of structure the peccaries of the present day are more advanced
and specialized than the far more varied and diversified true swine of
the Old World, for it is a singular fact that such a long-lived and
persistent stock as the peccaries should have given rise to so few
variants and side-branches. Existing peccaries all belong to a single
genus (_Tagassu_) and are relatively small animals, of unmistakably
pig-like character and appearance, but far smaller than the Wild Boar
(_Sus scrofa_) of Europe, or the Wart Hog (_Phacochœrus æthiopicus_) of
Africa, to mention only two of the Old World swine.
[Illustration: FIG. 191.—Dentition of the Collared Peccary (_Tagassu
tajacu_) left side. _i 3_, external incisor. _C_, canine, _p 2_, second
premolar (the first is lost), _m 1_, first molar.]
One characteristic and thoroughgoing difference between the peccaries and
the swine is the shape of the canine tusks. In the former, the tusks,
though very effective weapons, are not very large and are straight and
have a vertical direction, while in all the true swine the upper tusk
is curved upward and outward, projecting strongly from the side of the
jaw, and the great, curved lower tusk wears against its anterior side.
The peccaries further have smaller and simpler molars, each with four
principal, conical cusps (quadrituberculate pattern) arranged in two
transverse pairs, with numerous very small cuspules around and between
them, obscuring the plan. In the true swine the teeth are much larger
and covered with innumerable wart-like cusps, large and small, seldom
arranged according to any definite plan.
In the following particulars the modern peccaries show advance over
the Old World swine: (1) the last lower premolar has taken on the
molar-pattern, a very exceptional feature among the artiodactyls; (2) the
ulna and radius are coössified; (3) there are but two functional digits
in each foot; the fore foot has, in addition, two complete, but very
reduced and slender, lateral digits and the hind foot only one, whereas
in all the pigs of the eastern hemisphere there are four functional toes
in each foot; (4) in the hind foot the two functional metatarsals, the
third and fourth, have coalesced to form a “cannon-bone,” a structure
which is not found in any other family of the suborder; (5) the stomach
is complex, approximating that of a ruminant.
In the North American Pleistocene the predominating kind of peccary was
a genus (_†Platygonus_) which was more advanced than the existing form
(_Tagassu_), and, to all seeming, better fitted to survive, though for
some inexplicable reason it failed to do so. It was a considerably larger
animal, with proportionately longer and heavier legs. Its molar teeth
are of special interest because they reproduced a type which has been so
often repeated and independently acquired in so many different groups
of mammals. In this molar the two conical cusps of each pair were fused
into a high, transverse ridge or crest. Precisely the same modification
took place among the true swine in the genus _†Listriodon_ of the French
middle Miocene. _†Platygonus_ first appeared in the middle Pliocene,
and its predecessor in the lower Pliocene and upper Miocene showed the
crests of the molars in process of formation. In the latter stage it was
accompanied by a true peccary with tuberculated teeth, which differed
from the modern species in the simplicity of the hindmost premolar, which
had not taken on the molar-pattern. If the feet and limbs of this upper
Miocene peccary were known, they would doubtless prove to be much more
primitive than those of _Tagassu_, but they still await discovery.
Little can be said of the peccaries of the middle and lower Miocene
other than to record the fact of their presence in those formations, but
those of the upper Oligocene (John Day) are, however, represented by
well-preserved skulls, which show that more than one phylum of the family
had arisen, though there was no great difference between them; they were
considerably smaller animals than those of the Pliocene and Pleistocene.
Still smaller was the White River genus (_†Perchœrus_) of which some
fragmentary skeletons have been obtained. Although an undoubted peccary,
this animal was not far from what the common progenitor of the peccaries
and the true swine might be expected to resemble. The molars were
quadrituberculate without the numerous accessory cuspules of the modern
genus; the bones of the fore-arm were separate and the feet had four
functional digits each, while there was no cannon-bone in the pes, the
metatarsals remaining free.
No peccaries have yet been found in the Uinta, but probably this is a
mere accident of collecting. It is, however, possible that the White
River genus was not of American derivation, but an immigrant from the
Old World. In the middle Eocene, or Bridger stage, this series is known
only from teeth and jaws and a very few scattered foot-bones, and these,
though probably referable to the family, cannot be definitively assigned
to it without more complete material. Several species, larger and
smaller, of the genus _†Helohyus_ occurred in the Bridger, where they
were not uncommon, considering the general rarity of artiodactyls in that
stage. Thus, the peccaries, though none of them were large, followed the
usual law of mammalian development, and, beginning with very small forms,
increased in size with each succeeding geological stage down to the
Pleistocene.
2. _†Entelodontidæ. †Giant Pigs_
The †giant pigs, a most remarkable group of swine-like forms and of as
yet unknown origin, appeared for the last time in North America in the
lower Miocene, where the genus of that date (_†Dinohyus_) was the largest
of known suilline animals, the hippopotamuses excepted. In nearly every
part of the skeleton these great beasts displayed an unusual and aberrant
kind of development. The incisors were long and pointed, and the canines
formed stout and heavy, though not very long, tusks, which in shape
were more like those of a bear than those of either peccaries or swine.
The premolars were very simple, of compressed conical and trenchant
shape, and occupied a very long space in the jaws, while the molars
were relatively small and quadrituberculate, the crowns covered with
very thick, coarsely wrinkled enamel. The skull was immensely elongate,
especially the facial region in front of the eyes, while the brain-case
was so absurdly small as to give the skull a reptilian aspect, when
viewed from above. Evidently, these great pigs were profoundly stupid,
in this respect rivalling the †titanotheres of the White River (p. 311).
Beneath each eye-socket was a long, descending, bony flap, or process,
and on the under side of the lower jaw were two pairs of prominent
knobs, the function of which, as of the flaps beneath the eyes, is quite
problematical. The eye-sockets themselves were completely encircled in
bone, a rare character in the suborder.
The neck was short, as in the pigs generally, the body not very elongate
and the tail of moderate length; at the shoulders, the spines of the
dorsal vertebræ were very long, making a decided hump, and in the lumbar
and posterior dorsal region the processes for articulation between the
vertebræ were extremely elaborate. For one of the pigs, the limbs were
very long and gave quite a stilted look to the animal. As in the modern
peccaries, the fore-arm bones were indistinguishably fused together
and the feet had only two toes each, the only members of the suborder
in which digital reduction had proceeded so far, though the existing
peccaries approximate this condition. There were, however, nodular
vestiges of two other digits, which prove the derivation of this form
from at least a four-toed type; no cannon-bone was formed. In view of the
size of the animal, the hoofs were surprisingly small, which suggests
that the weight was chiefly borne upon a pad. _†Dinohyus_ was a very
large animal, six feet or more in height at the shoulder.
[Illustration: FIG. 192.—Right manus of †entelodont (_†Archæotherium
ingens_) from lower White River beds. Princeton University Museum.]
[Illustration: FIG. 193.—Skull of White River †entelodont
(_†Archæotherium mortoni_). Princeton University Museum. For restoration,
see Fig. 137, p. 260.]
In the upper Oligocene were very large species of another, but
closely similar, genus (_†Boöchœrus_) though somewhat smaller than
those of _†Dinohyus_, and the species of the upper White River beds
(_†Archæotherium_) were little, if at all, smaller than those of the John
Day. A number of specimens in the museum of Princeton University throw
a welcome light upon the habits of these strange creatures. In one, the
external, or third, upper incisor tooth has a deep, triangular notch worn
in its postero-external face, and the lower canine has a well-defined
groove worn on the posterior side at the base of the crown; other
individuals show less distinct marks of similar kind. (See Fig. 194.) It
is out of the question to suppose that these grooves and notches could
have been produced by abrasion with other teeth, for no other teeth could
reach the worn areas, and it is altogether probable that they were made
in digging up roots. The root, held firmly in the ground at both ends
and looped over the teeth which pulled until it broke, and being covered
with abrasive grit, would wear just such marks as the teeth actually
display.[7] While the †entelodonts were thus rooters, they were doubtless
omnivorous, like other pigs, and did not disdain a meal of carrion when
they could get it. It is likely that the heavy canine tusks were also
used as weapons, both in defence against the attacks of carnivores and
in fighting between the males of the same species. It must have been in
some such encounter that the animal represented by a complete skeleton in
the Princeton Museum received its broken rib; that the fracture was made
during life is demonstrated by the large callus growths on the broken
ends, but the pieces did not knit.
[Illustration: FIG. 194.—Specimen showing characteristic grooves of wear
in the anterior teeth of †entelodont (_†Archæotherium_) from upper White
River beds. Princeton University Museum.]
In the middle and lower substages of the White River the genus
(_†Archæotherium_) was the same as in the upper substage of these
beds, but the species were all smaller and some of them very much so,
not exceeding an ordinary pig in size. Throughout the series, as we
now have it, from the lower Oligocene into the lower Miocene, there
is very little change except in size, all the essential features of
structure remaining the same; the genera are therefore distinguished by
modifications of very secondary importance, and it is a question whether
all the species should not be included in a single genus. The European
genus _†Entelodon_, which gives its name to the family, is so like the
American forms that by most writers the White River species are referred
to it. It is of interest to note that the †giant pigs have also been
found in the marine Miocene of New Jersey, one of the few records of the
Tertiary land mammals of the Atlantic seaboard.
At present, the †entelodonts proper cannot be traced back of the lower
White River beds, nor are they found in any more ancient formations in
Europe. It is, therefore, probable that they were immigrants in both of
these continents, presumably from Asia.
[Illustration: FIG. 195.—Skull of †short-faced pig (_†Achænodon
robustus_) from the Bridger Eocene. Princeton University Museum.]
The whole Eocene of North America had a series of pig-like animals,
called the †achænodonts or †short-faced pigs, which seem to have been
related to the †entelodonts. They ended their career in the Uinta just
before the appearance of the †entelodonts, and it would be natural to
suppose that the latter were descended from them. If, however, the
principle that an organ or structure once lost can never be regained,
is valid, then there can be no relation of ancestor and descendant
between the two groups, for of the †achænodonts, even their most ancient
representatives had lost the first premolar, giving the formula _p_ 3/3,
while in the †entelodonts it is constantly _p_ 4/4. The †achænodonts,
which are much less fully known than the †entelodonts, had teeth very
similar in form to those of the latter; and their most conspicuous
feature was the shortness of the face and jaws, as contrasted with the
extreme elongation of these parts in the †entelodonts, nor did they have
the bony flaps under the eyes or the knobs on the lower jaw which gave
such a fantastic appearance to the †entelodont skull. Little is known of
the skeleton except that there were four functional digits in the manus.
The Uinta and Bridger genus (_†Achænodon_) was larger than the Wasatch
form (_†Parahyus_), which was an immigrant, probably from the same region
as afterwards sent out the †entelodonts to America and Europe; this
would account for the similarity and probable relationship of the two
subfamilies.
SUBORDER ARTIODACTYLA †PRIMITIVA. †PRIMITIVE ARTIODACTYLS
No doubt, this suborder is an artificial assemblage of unrelated
families, a sort of waste-basket, into which are thrown the groups of
which no other disposition can be made in the present state of knowledge.
As information becomes more complete, the various families will be
redistributed among the groups with which they had a genuine relationship.
3. _†Anthracotheriidæ. †Anthracotheres_
This family was abundantly represented in Europe from the middle Eocene
through the Oligocene, in Asia persisting even into the Pliocene, and
were abundant in the Oligocene of Egypt. Migrants from the Old World
reached America in White River times, but speedily died out, as they
did not survive into the upper Oligocene. The most fully known of these
animals is an American species of a European genus _†Bothriodon_. Almost
complete skeletons of this genus have been obtained in the channel
sandstones of the upper White River substage. In size and proportions,
_†Bothriodon_ was not unlike a domestic pig, but had a very long head
with slender, pointed snout; it had also a short neck, long body, short
limbs and feet. The primitive character of this genus is made clear
by many features of its structure; the molar teeth were extremely
low-crowned and their cusps were so imperfectly crescentic in form as to
be called _buno-selenodont_, as indicating their transitional nature,
and the upper molars had five cusps instead of four, a very primitive
feature. Another very significant character was the five-toed manus; the
first digit, or pollex, was much smaller than the others.
[Illustration: FIG. 196.—_†Bothriodon brachyrhynchus_, upper White River
stage. Restored from a skeleton in the museum of Princeton University.]
The second genus of the family which had American representatives was
_†Anthracotherium_, which was much like _†Bothriodon_, but even more
archaic in character; the molars could hardly be called selenodont at
all.
4. _†Oreodontidæ. †Oreodonts_
This was one of the most characteristic of North American artiodactyl
families, and its members were exceedingly abundant throughout the upper
Eocene, the whole Oligocene and Miocene, ending their long career in
the lower Pliocene. In distribution the family was exclusively North
American, and no trace of it has been found in any other continent.
In the course of their long history the †oreodonts underwent many
transformations and branched out into several distinct phyla, yet
through all these changes they remained singularly conservative, for the
transformations, some of them sufficiently bizarre, affected chiefly the
teeth and skull, the remainder of the skeleton changing but little. The
†oreodonts were all small or of moderate size, none of them surpassing
the Wild Boar in stature, nor was there any decided increase in size from
stage to stage. One and all, they were strange beasts. Dr. Leidy, who
first described and named most of the genera, spoke of them as combining
the characters of camel, deer and pig, and called them “ruminating hogs,”
a conception expressed in the names which he gave to some of them, such
as _†Merychyus_ and _†Merycochœrus_, both of which mean ruminant swine.
The general proportions of most of the species were quite as in the
peccaries, though, for the most part, with much longer tails; they
had a short neck, elongate body, short limbs and feet. In one genus
(_†Mesoreodon_) of the lower Miocene a rudimentary collar-bone has been
found, and probably all of the more ancient genera possessed it, but
only by an unusually lucky chance would so small and loosely attached a
bone be preserved in place. As the collar-bone is superfluous in hoofed
animals, in which the limbs are used only for locomotion and move in
planes parallel with that of the backbone, it is almost universally
absent in them, and in only one other group of ungulates, the extinct
†Typotheria of South America, has its presence been demonstrated. In
all of the †oreodonts the bones of the fore-arm and lower leg remained
separate. The teeth were in continuous series, and there was a peculiar
feature in the dentition common to nearly every one of the genera.
On casual examination, one would say that the animals had four lower
incisors on each side and that the lower canine closed behind the upper
one, a most exceptional arrangement. More careful study shows that the
apparent fourth incisor was the canine, a transformation which has also
taken place in all of the ruminants except the camels, and the tooth
which had assumed the form and function of the lower canine was really
the first lower premolar; this latter change is not found among the
ruminants, but was repeated in a few other extinct families.
[Illustration: FIG. 197.—Head of _†Merycochœrus proprius_, lower Miocene
to lower Pliocene. Restored from a skull in the American Museum of
Natural History.]
Only two genera of †oreodonts (_†Merychyus_ and _†Merycochœrus_) survived
into the lower Pliocene. Both had the proportions common throughout the
family, but _†Merychyus_ was much more slender and lightly built, its
lateral digits were reduced in size and very thin and it had hypsodont
grinding teeth; while _†Merycochœrus_ was of larger size (about that of
a large domestic pig) and stouter build and had low-crowned teeth; its
head, however, had a very different appearance, given by the possession
of a short proboscis, the presence of which is indicated by the greatly
reduced nasal bones; the jaws and face were also much shortened. The
eye-sockets presented obliquely forward and upward, instead of laterally,
as is usual among mammals, and were placed high in the head. This
position of the eyes and of the entrance to the ear renders it probable
that _†Merycochœrus_ was largely aquatic in its habits. Both genera had
short, four-toed feet, as was general throughout the family and in no
genus did the reduction of digits proceed beyond the loss of the first of
the original five, the pollex and hallux.
The two genera above described, representatives of two distinct phyla
within the family, held over, as it were, from the upper Miocene without
essential change. The phylum of the hypsodont and slender _†Merychyus_
went back, with only minor modifications, into the upper substage
of the lower Miocene, but cannot as yet be traced to an Oligocene
ancestry; it is therefore still impossible to say just where and when
it branched off from the main stem of the family. Future discoveries in
the Oligocene will no doubt clear up this problem. The real terminal
and most highly specialized member of the _†Merycochœrus_ phylum and
the most extraordinary member of the entire family was confined to the
upper Miocene. The extreme peculiarity of this genus (_†Pronomotherium_)
was displayed only in the head, which was an exaggeration of the
_†Merycochœrus_ type, the face being excessively shortened and the nasals
so reduced as to show that the proboscis was much better developed than
in the parent genus. The shortening of the face and the great vertical
height of the skull and lower jaw gave a decided likeness to the skull
of a great ape, though the proboscis would mask any such resemblance in
the living head. _†Merycochœrus_ itself went back to the upper division
of the lower Miocene, but in the lower division it was replaced by an
ancestral genus, _†Promerycochœrus_, which had an elongate face and jaws
and no proboscis; but in other characteristic features, such as the
extreme thickness and roughness of the zygomatic arches, it was like
its descendant. _†Promerycochœrus_ contained the largest known species
of †oreodonts, some of them equalling a Wild Boar in stature, and its
remains are found so abundantly in the middle and lower Miocene and
upper Oligocene, that there must have been great herds of these animals
over the plains. Probably it was itself derived from some of the larger
species of _†Eporeodon_ of the upper White River beds, but there is a gap
in the history, due to the fact that the lower part of the John Day is
almost barren of fossils and the connecting link has not been recovered.
[Illustration: FIG. 198.—Head of _†Pronomotherium laticeps_, upper
Miocene. Restored from a skull in the Carnegie Museum, Pittsburgh.]
It is an interesting and significant fact that ancestral and
derivative genera may continue to live side by side in the same region.
_†Promerycochœrus_, it is believed, gave rise to _†Merycochœrus_, but
survived with it into the middle Miocene. _†Merycochœrus_, in its turn,
produced _†Pronomotherium_, and, so far from being replaced by the
latter, actually outlived it and persisted into the lower Pliocene.
[Illustration: FIG. 199.—_†Promerycochœrus carrikeri_, lower Miocene.
Restored from a skeleton in the Carnegie Museum, Pittsburgh.]
A third phylum of the †oreodonts, which appeared for the last time in
the middle Miocene (genus _†Cyclopidius_), was a series of small and
very small species, of which the skull was almost as peculiar as that of
_†Pronomotherium_, but in a different fashion. The face was very much
shortened and on each side a great vacuity reduced the nasal bones to
mere splints; the elevated position of the eye-sockets, which projected
above the forehead, and of the tubular entrance to the ear is an evidence
of an aquatic or amphibious mode of life, such as is illustrated by the
hippopotamuses, which can float almost completely submerged, with only
the ears, eyes and nostrils above the surface of the water. The tympanic
bullæ (see p. 66) or bony chambers into which the ear-tubes opened, were
of relatively enormous size and added much to the unusual appearance of
the skull. The incisors were very small and the grinding teeth narrow and
completely hypsodont, this and the _†Merychyus_ series being the only two
phyla of the family in which the hypsodont molar was fully acquired. The
remainder of the skeleton differed but little from the type common to the
whole family, except for a somewhat shorter tail.
[Illustration: FIG. 200.—Skull of _†Leptauchenia nitida_, upper White
River.]
The animals of this series were common in the middle and lower Miocene
and in the upper substage of the White River, but have not been found in
the intermediate John Day. This may have been a matter of geographical
distribution, these creatures not extending west of the main ranges of
the Rocky Mountains. In the upper White River the genus _†Leptauchenia_
is extremely common, but below that level they suddenly and completely
vanish and, as in the case of the _†Merychyus_ phylum, it is not yet
practicable to determine the point in time or space of their branching
off from the main stem of the family. Were the †oreodonts not entirely
confined to North America, we should, as a matter of course, explain the
seemingly sudden appearance of _†Leptauchenia_ as due to immigration,
and it is entirely possible that the series did actually originate in
some part of North America which has left no record of its Eocene or
Oligocene terrestrial life. On the other hand, no one can imagine that
everything that can be known of the mammals of the middle and lower White
River has already been learned, and at any time the sought-for ancestor
of _†Leptauchenia_ may be found in those beds.
[Illustration: FIG. 201.—_Leptauchenia nitida_, upper White River.
Restored from a skeleton in the Museum of Princeton University.]
The fourth phylum may be regarded as the main or central stem of the
family and was the one which underwent the least change, though it
probably gave rise to all the other phyla, which branched off from it
at various stages in its history. This series terminated in the middle
Miocene and comprised several genera, all very much alike, in the lower
stages of that epoch. One of these genera (_†Mesoreodon_) displayed a
very remarkable peculiarity of structure in the ossification of the
great cartilage of the larynx, which seems to point to the possession of
uncommon vocal powers. It is impossible to say whether this feature was
confined to the single genus, or was general in the family, for only in
rare instances would so extremely delicate a structure be preserved. In
the John Day the genus _†Eporeodon_, which was very abundant, was the
representative of this phylum, and the same, or a closely similar, genus
lived in the latter part of the White River stage.
In the middle and lower White River substages †oreodonts are the
commonest of fossils, so that the collector soon wearies of them (see
Fig. 136, p. 259); they must have lived in great herds in the forests
and along the streams. There were several species, varying principally
in size, the largest about as long as a wolf, but with shorter legs, and
the smallest not so much as half of that size. All belonged to a single
genus, for which the rigid law of priority compels us to use a most
cumbrous name (_†Merycoidodon_), the widely used term _†Oreodon_ being
a synonym. This genus was the central stock of the family, from which
most, if not all, the others were directly or indirectly derived, though,
as previously pointed out, we cannot in all cases trace the connection.
In these White River animals the grinding teeth were very low-crowned
and had considerable resemblance to those of a deer; the molars were
typically selenodont and made up of two pairs of crescentic cusps. The
skull differed little from that of the succeeding genera of this phylum;
the neck was short, body and tail long. An especially interesting fact is
that the fore foot had five digits, the first, or pollex, very small and
of no functional value, but complete in all its parts; the hind foot was
four-toed. In all of the subsequent genera of the family the number of
digits was uniformly four in both manus and pes.
[Illustration: FIG. 202.—Skull of _†Merycoidodon culbertsoni_, middle
White River. (After Leidy.)]
In the Uinta stage of the upper Eocene lived the most ancient and
primitive member of the family yet discovered, the genus _†Protoreodon_,
which is in every respect what the ancestor of the White River genus
should be. The functional transformation of the lower canine into a
fourth incisor and the replacement of the canine by the first lower
premolar had already taken place, but the molars were much more primitive
than those of the White River and succeeding genera; the crescents
were thicker and less complete, plainly indicating their derivation
from conical cusps, and a small fifth cusp was present between the
anterior pair of the upper molars, as in the †anthracotheres and other
European families of the Artiodactyla †Primitiva. Before the discovery
of _†Protoreodon_, the character of its molars was predicted by Dr.
Schlosser, of Munich. The skull resembled that of the White River genera,
except that the eye-socket was open behind, and there was no glandular
pit in front of the eye. The skeleton is but partially known, but it has
been ascertained that there were five toes in the manus and probably also
in the pes.
[Illustration: FIG. 203.—Skull of _†Protoreodon parvus_, Uinta Eocene.
Princeton University Museum. N.B. This skull is actually much smaller
than that shown in Fig. 202.]
Nothing has yet been discovered in formations older than the upper Eocene
which can be regarded as ancestral to the †oreodonts, and this is not
surprising in view of the extremely meagre and unsatisfactory nature
of our information regarding the artiodactyls of the Bridger. On the
whole, however, it seems rather more probable that the Uinta genus was an
immigrant (whence, we cannot say) than that the Bridger will ever yield
the desired ancestral forms. So long as the early Tertiary mammals of
northern and central Asia remain unknown, this and many similar problems
can find no definitive solution. The question of relationship with other
families is bound up with that of the origin of the †oreodonts; many
characters point to a connection with the †anthracotheres and, from the
standpoint of present knowledge, that appears to be the most probable
affinity; but, on the other hand, there are structural features which
suggest relationship with the primitive camels. Between these and other
alternatives, only the recovery of the middle and lower Eocene forms can
finally decide.
Reviewing the long history of the oreodont family from the evolutionary
point of view, we find a course of development which differs in several
respects from that exemplified by most of the families previously
considered:
(1) There was a general increase in size, though it was far from steady,
and almost every genus had larger and smaller species, and in some of
the phyla the species were far larger than in others. The members of the
_†Leptauchenia_ phylum were very small and no member of the family ever
attained to more than moderate size.
(2) The upper molars early lost the fifth cusp, and after that there
was little change in the dentition, except that in the _†Merychyus_ and
_†Leptauchenia_ phyla the grinding teeth became hypsodont.
(3) There was great variety in the modifications of the skull, each
phylum having its own peculiarities. The orbit, which was open behind
in the Uinta _†Protoreodon_, was closed in the White River and all
succeeding genera. In the _†Merycochœrus_ series, the skull first
enlarged, with little change in proportions, then elongated the facial
region, then shortened the face and so reduced the nasals as to indicate
the presence of a proboscis, culminating in the grotesque, ape-like skull
of _†Pronomotherium_. In the _†Leptauchenia_ phylum the skull became
depressed and flattened and the face was invaded by great openings, or
vacuities; the tympanic bullæ were enormously inflated and the orbits
and ear-openings raised, presumably in adaptation to an amphibious mode
of life. These were the extremes of change within the family; the other
phyla need not be considered.
(4) At an early stage the digits were reduced from five to four, first
in the pes and then in the manus, and there reduction ceased; though in
_†Merychyus_, especially in the upper Miocene species, the lateral digits
were very slender and, had this series survived, it would probably have
led to didactyl forms.
[Illustration: FIG. 204.—Left manus of †oreodonts. _A_, _†Merycoidodon
culbertsoni_, White River. _B_, _†Merycochœrus proprius_, upper Miocene.]
In other respects there was very little difference in the skeletons of
the various phyla and herein lies the peculiarity in the history of the
family, great variety in the form of the skull, and, relatively speaking,
hardly any change in the body, limbs or feet. In the horses, rhinoceroses
and †titanotheres the modifications of the successive genera affected
all parts of the structure, but in the †oreodonts, except for the loss
of one digit in manus and pes and variations in the length of the tail,
the skeletons of the latest genera did not differ in any important
respect from those of the earliest. Such a combination of mutability and
plasticity in the skull with extreme conservatism in the remainder of the
bony structure is an exception to the usual mode of development, though
something of the same sort has already been pointed out in the case of
the tapirs (p. 325) and will recur in that of the elephants (Chap. X).
5. _†Agriochœridæ. †Agriochœrids_
This family, one of the strangest and most aberrant of ungulate groups,
was very closely allied to the †oreodonts and by many authorities is
included in the same family. The history of the successive steps of
discovery, by which the structure of these extraordinary animals was
gradually made plain, is much the same as in the case of the even more
peculiar perissodactyl family of the †chalicotheres (p. 356). The various
parts, found scattered and at long intervals of time, had been referred
to no less than three different mammalian _orders_! for, until the
discovery of †chalicothere skeletons gave the clue, no one imagined that
such discordant parts could belong to the same animal.
[Illustration: FIG. 205.—Skull of _†Agriochœrus latifrons_, White River.
(After Wortman.)]
[Illustration: FIG. 206.—_†Agriochœrus antiquus_, White River. Restored
from a skeleton in the American Museum of Natural History.]
The †agriochœrids had a very much shorter career than the allied family
of the †oreodonts, extending only through the upper Eocene and the
Oligocene (Uinta to John Day, inclusive); and only two genera of the
family are yet known, _†Agriochœrus_ of the John Day and White River,
and _†Protagriochœrus_ of the Uinta. In the former the teeth were not in
a continuous, closely crowded series, but there were open spaces behind
the upper canine and first lower premolar; the same exceptional character
of the lower teeth which was found in the †oreodonts was repeated in
the present family, the canine assuming the form and functions of an
incisor and the first premolar those of the canine; the upper incisors
were extremely small and were shed in the adult, just as in the true
ruminants. The molars had the selenodont pattern, but the upper molars
were very different in shape from those of the †oreodonts, resembling
rather those of the †anthracothere _†Bothriodon_ (see p. 370). Another
difference from the †oreodont dentition was that the last lower premolar
had acquired the molar form and the last upper one nearly so, a very
unusual feature among the artiodactyls. The skull was almost exactly like
that of the White River †oreodonts, save in a few details; the face was
somewhat longer, the orbit was open behind and there was no glandular pit
on the face in front of the eye. The neck was short and the body long,
and the backbone in the region of the loins very stout, the vertebræ of
this region having much resemblance to those of the great cats, as though
_†Agriochœrus_ were an agile and powerful leaper. Another likeness to
the cats was in the very long and heavy tail, which was much longer
than in the †oreodonts, and its vertebræ were hardly distinguishable
from those of a Leopard. The limbs were relatively longer than those
of the †oreodonts and the separate bones had a suggestive likeness to
those of carnivores, and, more specifically, of cats. The feet, save in
one particular, were not only artiodactyl, but also characteristically
†oreodont in structure and, as in the earlier members of that family,
there were five toes in the manus and four in the pes. The exception
was that, instead of narrow and slender hoofs, the feet were armed with
sharp, though not very large claws, which were not comparable in relative
size to the great claws of the †chalicotheres.
Altogether, a strange jumble of incongruous characters was united in
this skeleton. Were only the skeleton known without the skull, one would
be tempted to call it that of a carnivorous artiodactyl, but the teeth
make such a suggestion absurd, since they could have been used only for
masticating a diet of soft vegetable substances. No flesh-eater has,
or ever had, teeth in the remotest degree like these, which were of
characteristically herbivorous type. How such a creature lived and what
were its habits, are questions to which no satisfactory answer has been
found.
[Illustration: FIG. 207.—Right manus of _†Agriochœrus latifrons_, White
River. (After Wortman.)]
_†Protagriochœrus_ of the upper Eocene is, unfortunately, known only from
very imperfect and fragmentary specimens, which, however, are sufficient
to determine some significant points. These remains show that, while
the two families of the †agriochœrids and the †oreodonts were already
distinct in the Uinta, they were decidedly nearer together than they
became in the Oligocene. In other words, it is clear that the two groups
were converging back to a common ancestry. This may be discovered in the
Bridger, but it seems more probable that these forms were immigrants.
Another fact concerning the Uinta genus, which is important, is that the
upper molars possessed the fifth or unpaired cusp which also occurred in
the contemporary †oreodonts, as well as in the †anthracotheres and other
Old World families.
SUBORDER TYLOPODA. CAMELS AND CAMEL-LIKE ANIMALS
Existing Tylopoda are all included in a single family, the Camelidæ,
and by several authorities no other family, even of extinct forms, is
admitted to the suborder. My own preference, however, is to refer the
problematical little †hypertragulids to this group, as will be shown
subsequently.
6. _Camelidæ. Camels and Llamas_
Under modern conditions, no mammals could seem more completely foreign
to North America than those of the camel family, which, now restricted
to two well-defined genera, inhabit central Asia and the colder parts
of South America. Yet, as a matter of fact, this family passed through
nearly the whole of its development in North America and did not emigrate
to the other continents before the late Miocene or early Pliocene, and it
is this North American origin of the family which explains its otherwise
inexplicable distribution at the present time. To all appearances, the
whole family had completely disappeared from this continent in the later
Pleistocene, but in the middle and earlier portions of that epoch both
true camels and large llama-like animals were very abundant on the Great
Plains and in California, while they seem to have avoided the forested
regions.
In order to appreciate the changes through which the camels and llamas
have passed, it will be necessary to consider briefly the skeletal and
dental structure which characterizes the modern genera. In the true
camels (_Camelus_) the first and second upper incisors have been lost,
but the third remains as a large, sharp-pointed tooth, as are also the
upper canine and first premolar; thus there are three pointed, spike-like
teeth in a row, with spaces between them, constituting with the lower
canine a very effective lacerating apparatus. Behind the first premolar
is a long gap, the second being suppressed; the third and fourth are
grinding teeth, but unusually small. The molars are selenodont and
high-crowned, though not extremely hypsodont. The lower incisors are
large and shovel-shaped, the canine large and erect and there are but two
lower premolars. The dental formula thus is: _i_ 1/3, _c_ 1/1, _p_ 3/2,
_m_ 3/3.
The skull is long, with the facial region much and abruptly narrowed,
which gives a triangular appearance to the head when seen from above; the
orbit is completely encircled with bone and the sagittal and occipital
crests are very prominent. The tympanic bullæ are large and filled with
spongy bone. The condyle of the lower jaw is hemispherical and not,
as it is in most ungulates, semicylindrical, and a curious, hook-like
angulation is on the posterior border of the bone. The neck is very long,
and the vertebræ have the exceptional peculiarity that the canal for the
vertebral artery runs through the side of the neural arch, instead of
perforating the transverse process, and thus is invisible externally; the
odontoid process of the axis is spout-like. The legs and feet are very
long; the humerus has a double bicipital groove and the fore-arm bones
are coössified, and the ulna is so reduced that the radius carries the
whole weight; in the lower hind leg the tibia supports the weight, and
of the fibula only the lower end remains as the malleolar bone. There
are but two digits in each foot, the third and fourth, the metapodials
of which have coalesced to form a cannon-bone, which differs from that
of the true ruminants, or Pecora, in the curious way in which the lower
ends, separated by a Λ-shaped notch, diverge from each other, and by
the fact that the keels of the lower articular surfaces are confined
to the posterior side, not visible from the front. The ungual phalanges
are small and nodular, and the hoofs, which carry no part of the weight,
are hardly more than nails. Under the other phalanges is a broad pad of
elastic tissue, upon which the weight rests, and the separation of the
toes is very partial. The peculiar external appearance of the camels
is largely due to structures which leave no trace in the skeleton, and
especially to the great humps, one or two according to the species, which
are accumulations of fat; the ears are short and rounded and the hair is
not woolly, but almost straight.
The teeth and skeleton of the llamas (_Lama_) are closely similar
to those of the camels, but the absence of humps, the long, pointed
ears, the woolly hair and the much smaller size and lighter build give
to the living animals a more marked difference of appearance from
the camels than one would expect from a comparison of the skeletons
alone. The dental formula is: _i_ 1/3, _c_ 1/1, _p_ 2/2, _m_ 3/3. The
remaining upper incisor, the third, is recurved, as is also the canine,
but the spike-shaped first premolar of the camels is absent and the
other premolars are much smaller than in the latter. In the skull the
brain-case is larger, and the sagittal and occipital crests are much less
prominent. The skeleton differs hardly at all from that of the camels,
except for its smaller size and more slender proportions. The toes are
more distinctly separated, each having its own pad. Thus, among the
existing representatives of the family are two very well-defined phyla,
each characteristic of a different continent.
[Illustration: FIG. 208.—Guanaco (_Lama huanacus_).—By permission of the
New York Zoölogical Society.]
The Blanco stage of the middle Pliocene, which has preserved but a meagre
representation of the life of its time, has yielded a number of very
large, llama-like species, not, however, ancestral to the modern species,
for they had but one premolar in each jaw. From the lower Pliocene
we have fuller information. In the Snake Creek stage the separation
of the two modern phyla was complete, and there was a third one, now
extinct, that of the browsing or “†giraffe-camels” (_†Alticamelus_)
(see Fig. 127, p. 236), a term which must not be taken as implying any
relationship with the giraffes, but merely a resemblance to them in
proportions. These browsing camels were very large animals, but with
relatively small heads and low-crowned teeth not suited for grazing; the
neck was extremely long, made so by the great elongation of five of the
vertebræ (second to sixth, inclusive), and the legs were also very long,
fitting their possessors to browse upon trees. Much of the description
of the appearance and habits of the Giraffe given by Flower and Lydekker
would no doubt be applicable to these extinct camels. “To produce the
extremely elongated neck the seven cervical vertebræ are proportionately
long, which gives a somewhat stiff and awkward motion to the neck....
The Giraffe feeds almost exclusively on the foliage of trees ... for
browsing on which its prehensile tongue and large free lips are specially
adapted.”[8]
In teeth and skeleton the phyla of the true camels and of the llamas in
the lower Pliocene did not differ very strongly from the living forms;
the upper incisors were already reduced to one, but the premolars were
not so small; the ulna and radius had coalesced and of the fibula only
the lower end remained; the cannon-bones were completely formed, and
that the pads of the feet had already been developed is shown by the
phalanges, especially the irregular, nodular unguals.
The most ancient known camels of the Old World are found in the Pliocene
of India, and the first llamas recorded in South America are also
Pliocene. Since both camels and llamas existed together in North America,
it may reasonably be asked why only one phylum migrated to Asia and only
the other to South America. Why did not each continent receive migrants
of both kinds? Without knowing more than we are ever likely to learn
about the details of these migrations, it will not be possible to answer
these questions, though plausible solutions of the problem suggest
themselves. It is to be noted, in the first place, that a migration
from the central portion of North America to Asia was by way of the far
north and thus involved very different climatic conditions from those
which must have been encountered in passing through the tropics to South
America. It is perfectly possible that animals which lived together
in temperate North America should have had very different powers of
adaptation to heat and cold respectively, and the northern route may have
been impassable to one and the southern route to the other. To this it
might perhaps be objected that the llamas are cold-country animals, but
this is true only of the existing species, for fossil forms are found
abundantly in the Pleistocene of Ecuador, Brazil and Argentina. Another
possibility is that both phyla did actually migrate to both continents
and that only the camels succeeded in permanently establishing themselves
in Asia and only the llamas in South America, though for this solution
the fossils afford no evidence.
The camels of the upper Miocene did not differ sufficiently from those
of the lower Pliocene to call for special notice other than to remark
that the two phyla of the true camels and the llamas were hardly
distinguishable and one genus (_†Procamelus_) may have been ancestral
to both. In the middle Miocene the browsing camels (_†Alticamelus_)
reached the acme of their importance and made no great progress
subsequently. The generalized stock, from which the upper Miocene and
lower Pliocene _†Procamelus_ descended, was represented by _†Protolabis_
and _†Miolabis_, smaller animals, which had a full set of upper incisors
and premolars and the grinding teeth were not so high-crowned. In most
of the species the metapodials had not fused to form cannon-bones and
probably there were no pads on the feet, though _†Alticamelus_, the
†Giraffe-Camel, had already developed both cannon-bones and pads.
[Illustration: FIG. 209.—Lower Miocene †giraffe-camel (_†Oxydactylus
longipes_). Restored from a skeleton in the Carnegie Museum, Pittsburgh.]
In the lower Miocene the †giraffe-camels were represented by the genus
_†Oxydactylus_, which was a considerably smaller animal than its
successor _†Alticamelus_, of the middle Miocene and later formations,
and had shorter neck and legs. The teeth, though brachyodont, were not
very low-crowned. There was no cannon-bone, the two metapodials of each
foot remaining separate. An especially noteworthy feature in this genus
is to be observed in the character of the hoofs, which, as the ungual
phalanges demonstrate, were narrow and pointed, like those of antelope
and deer, and carried most of the weight. The member of the grazing
series (_†Protomeryx_) was smaller in every way than its contemporary
(_†Oxydactylus_) of the browsing line and had shorter neck and legs,
though these were already long. The teeth were present in undiminished
number, and the grinders, while not properly to be called hypsodont,
showed a decided tendency to assume that character. The feet were in the
same stage of development as in _†Oxydactylus_, that is to say, with two
free digits and pointed, deer-like hoofs. We have thus the remarkable
and most significant fact that, while the grazing and browsing camels
of the lower Miocene were already distinctly separated, neither had yet
attained to the type of foot-structure which _both_ of them afterwards
independently acquired. This is a very instructive example of parallel
evolution in closely related series.
[Illustration: FIG. 210.—Skeleton of _†Oxydactylus longipes_. Lower
Miocene. (After Peterson.) For restoration, see Fig. 209.]
Of still another phylum of the camel family, the lower Miocene contains
the only representatives yet discovered, the little “†gazelle-camels,”
as they may be called. The single known genus (_†Stenomylus_, Fig. 131,
p. 242) of this series was quite a small animal, much smaller than its
contemporaries of the grazing or browsing series. _†Stenomylus_ was an
extremely slender, cursorial creature and had a very exceptional feature
in its dentition in the apparent presence of ten lower incisors, five
on each side, the canine and first premolar having assumed the form and
functions of the incisors; the molars were low-crowned. The head was
rather small and rounded, the neck long and light, the limbs and feet
elongate and excessively slender. The feet had two digits each, which
were separate, not forming a cannon-bone, and the hoofs were narrow,
pointed and deer-like. These delicate and graceful little animals had
but a brief career, which seems to have reached its close in the lower
Miocene. Perhaps their complete defencelessness made it impossible for
them to maintain themselves against their enemies, despite their evident
capacity for swift running.
The camels of the upper Oligocene (John Day) are still incompletely
known, but appear all to have belonged to the series of grazers which led
up to the modern genera. Future discovery may bring to light in the John
Day earlier members of the †giraffe-camel series, of which a possible
member is found in the uppermost substage of the White River, or perhaps
both phyla united in the upper Oligocene, a question which remains to be
determined. At all events, in the middle substage of the White River,
or lower Oligocene, there is no evidence of more than a single phylum,
from which the others were almost certainly derived, branching off from
the main stem at different levels. First was given off the branch of
the †giraffe-camels, then (or perhaps even earlier) that of the little
†gazelle-camels, and, finally, the main stem bifurcated into the two
phyla of the llamas and the true camels. The point of origin of the
†gazelle-camels is still uncertain.
[Illustration: FIG. 211.—The White River camel (_†Poëbrotherium
labiatum_). Restored from a skeleton in the museum of Princeton
University.]
The typical White River genus (_†Poëbrotherium_) included a series of
species which increased in size from the earlier to the later portions
of the stage, but showed no such structural changes as to call for
special notice. The larger of these species was somewhat taller than
a sheep, but of much lighter proportions, with small, pointed head,
long neck and body and long, very slender limbs and feet. The teeth
were present in undiminished number, 44 in all; the lower incisors
were small, simple, nearly erect and chisel-shaped, very different
from the large, procumbent and shovel-like teeth of the modern genera,
and the trenchant canines were much smaller than in the latter. The
first premolar had an isolated position, the second and third were
trenchant and much extended antero-posteriorly, quite as in many other
groups of primitive artiodactyls. The molars, which were typically
selenodont, were low-crowned in the upper jaw, but in the lower showed
an incipient tendency to hypsodontism. The skull, by its shape and the
characteristic narrowing of the face, immediately suggests the modern
type, but differed in many details of structure, the most obvious of
which were the incompletely closed orbits, the shallow and slender
jaws, and the very large, hook-like process from the angle of the
lower jaw, which, in greatly reduced form, is present in both of the
Recent genera. The neck was relatively long, though by no means so long
proportionately as it subsequently became, and the vertebræ had already
acquired the peculiarity found in all the succeeding camels, of the
exceptional position of the canal for the vertebral artery, save in the
sixth vertebra, where it pierced the transverse process, as in mammals
generally; the odontoid process of the axis was neither spout-like nor
peg-like, but of intermediate form, convex below and flat above. The
body was long and light, and the ribs were much more slender than in
the Recent genera. The fore and hind limbs, which were of nearly equal
length, were very slender; the humerus had a single bicipital groove;
the fore-arm bones were fully coössified and in the lower leg only the
two ends of the fibula remained. The feet were already in the stage of
development which persisted through the lower Miocene in all of the
phyla, with two separate digits and nodular remnants of two others, and
deer-like hoofs.
It would be of interest to compare this little White River camel with
its contemporary genus of horses, _†Mesohippus_, and to observe in
how many respects they have followed a parallel course, and how nearly
_†Poëbrotherium_ occupied the same position with reference to the modern
camels and llamas as _†Mesohippus_ did to the Recent horses; but such
a comparison would involve too many technicalities to be profitably
undertaken here. Suffice it to say that in many details there was a
genuine parallelism in the progress of these two widely separated
families from a polydactyl ancestry towards an extreme of digital
reduction, ending in the horses in the single-toed and in the camels in
the two-toed foot. The members of the two series kept nearly equal pace
in their slow progress, with the camels a little in advance, since they
were the first to attain the modern state of development in the height
of the teeth and the structure of the feet, though eventually the horses
surpassed them in both respects.
In the upper Eocene (Uinta stage) there were at least two kinds of
camels, the time-relations of which to each other are not known, that
is, whether they were contemporary or successive. The best-known genus,
_†Protylopus_, may perhaps not be in the direct line of camel descent,
but it so nearly represents the proper ancestral stage that, for all
practical purposes, it will serve nearly as well. It was a much smaller
animal than the smallest of the White River species, and was hardly
larger than a “jack-rabbit.” The teeth of each jaw were in continuous
series and the canines were but slightly longer than the incisors; the
premolars had less antero-posterior extension than in _†Poëbrotherium_,
and all the molars above and below were very low-crowned. The skull was
almost a miniature copy of that of _†Poëbrotherium_, but more primitive
in a number of details, the most important of which was that the tympanic
bullæ were much smaller and hollow, not filled with spongy bone. The
neck, concerning which it would be very desirable to have information, is
almost the only part of the skeleton that is not known. The fore limb was
considerably shorter than the hind, making the back slope downward from
the rump to the shoulders; in the fore-arm the two bones were entirely
separate and in the lower leg the fibula, though very slender, was still
complete. In the manus there were four functional digits, the laterals
not very much smaller than the median pair; but in the pes the lateral
metatarsals were reduced to mere bony threads, to which small phalanges,
in full complement, were attached, making tiny dew-claws.
With _†Protylopus_ ends the genealogy of the camels so far as it can be
definitively traced, but in the middle of the Bridger stage is found a
genus, _†Homacodon_ (family †Dichobunidæ), which is a probable member of
the series. However, until the connecting link can be found in the upper
Bridger, this conclusion cannot be demonstrated and _†Homacodon_ itself
is incompletely known. It was a very small animal, even less in size than
_†Protylopus_, and had not yet acquired the selenodont molars. These
teeth were quadritubercular, _i.e._ with four principal cusps arranged,
in the upper molars, in a square, and with a minute cuspule between each
transverse pair, while the lower molars were narrower and had only the
four principal cusps. The cusps were not conical, as they are in the
pigs, but angular and pyramidal, the first step toward the assumption
of the selenodont form. The skull was not specifically cameline in
appearance, but rather indifferent, as though almost any kind of an
artiodactyl might have been derived from it. The feet were decidedly more
primitive than those of the Uinta genus, having four functional digits
each, perhaps five in the manus. While it cannot be positively stated
that _†Homacodon_ was the actual ancestor of _†Protylopus_, it nearly
represents what we should expect that ancestor to be.
[Illustration: FIG. 212.—Diagram to illustrate the development of
the skull and molar teeth in the camel tribe, in ascending geological
order. _A_, _†Protylopus petersoni_, Uinta Eocene. _B_, _†Poëbrotherium
wilsoni_, White River. (After Wortman.) _C_, _†Procamelus gracilis_,
upper Miocene. (After Cope.) _D_, _Lama huanacus_, the modern Guanaco.]
In the lower Eocene (Wasatch stage) lived a tiny creature,
_†Trigonolestes_ (family †Trigonolestidæ), smaller even than _†Homacodon_
of the Bridger, and one of the most ancient and primitive of known
artiodactyls, but, unfortunately, still represented only by very
imperfect specimens, so that much which it would be highly desirable to
learn must await the finding of better material. The upper molars were
triangular and tritubercular, _i.e._ with three principal cusps arranged
in a triangle, and are hardly to be distinguished from those of other
early mammalian orders. From the teeth alone the artiodactyl nature of
the animal would not have been suspected, and, in fact, they were, when
first discovered, referred to primitive monkeys. The feet probably had
five toes each, but this is not certain, and the femur had the third
trochanter, the only known artiodactyl of which this is true. As this
little Wasatch genus is so imperfectly known, it would be premature to
claim it as the starting point of the camel family, and yet it may very
well have been so. Better material of this genus and the links of the
chain which belong in the upper Bridger and the Wind River respectively
must be recovered before this earliest portion of the family history can
be written in more than tentative fashion.
[Illustration: FIG. 213.—Right manus of camels. _A_, _†Protylopus_,
Uinta. _B_, _†Poëbrotherium_, White River. _C_, _†Procamelus_, upper
Miocene. (After Cope.) _D_, Recent Guanaco.]
[Illustration: FIG. 214.—Right pes of camels. _A_, _†Protylopus_. _B_,
_†Poëbrotherium_. _C_, _†Procamelus_. (After Cope.) _D_, Guanaco.]
The mode of evolution displayed by the camels does not differ in any
significant respect from that seen in the horses. There was the same
increase in bodily stature and in the relative lengths of the limbs
and feet, the same kind of diminution in the number of digits from
the original five, the same reduction of the ulna and its coalescence
with the radius and the loss of the fibula save for its two ends. There
was also a similar development of the high-crowned, or hypsodont,
grinding teeth, from the low-crowned, or brachyodont, type. In still
another respect there was a similarity in the mode of development of
the two families, namely, in the way in which the several phyla of
each originated. For the earlier portion of their history there was in
each but a single distinguishable series, though it is very possible
that fuller knowledge and more complete material would enable us to
distinguish more than one. This monophyletic condition continued through
the Eocene and most of the Oligocene, but in the upper portion of the
latter and, more markedly in the lower Miocene, the two families branched
out, each in its own fashion.
Of course, there were differences in the development of the camels
and horses, some conditioned by the fundamental distinction between
artiodactyl and perissodactyl, such as the didactyl foot as the
possible minimum and the formation of cannon-bones in the camels. Other
differences are characteristic of the latter family, such as the great
elongation of the neck and the peculiar structure of its vertebræ, the
formation of pads on the feet and concomitant reduction of the hoofs. In
a general way, the two families kept quite an even pace in their advance
from the more primitive to the more specialized condition and, though
the camels were the first to acquire certain modifications, the horses
ultimately surpassed them.
Even more close was the parallelism in evolution between the camels and
the true ruminants (suborder Pecora), and this case is of particular
importance as clearly demonstrating the development, in two independent
but related lines, of similar structures not derived from a common
ancestry. This comparison must naturally await the description of the
Pecora.
7. _†Hypertragulidæ. †Hypertragulids_
This was a very peculiar family, of exclusively North American
distribution and of doubtful systematic position, the known history of
which extended from the upper Eocene into the lowest Miocene and then
abruptly terminated. None of its members attained to considerable size,
the largest hardly surpassing a sheep, and some were extremely small.
In view of its comparatively brief career, this family was surprisingly
ramified, and no less than four phyla may be distinguished within its
limits.
[Illustration: FIG. 215.—_†Syndyoceras cooki_, lower Miocene. Restored
from a skeleton in the museum of the University of Nebraska.]
One of the phyla which persisted into the lower Miocene was there
represented by a most fantastic creature (_†Syndyoceras_) with four
horn-like outgrowths from the skull, one pair arising from the anterior
part of the face and curving outward away from each other, and the hinder
pair, which were placed over the eyes, curved toward each other at the
tips and were shaped much like a cow’s horns in miniature. The shape of
these bony protuberances makes it unlikely that they were sheathed in
horn and probably they were merely covered with skin like the horns of
the giraffes. This description applies only to the skull of the male;
that of the female is not yet known, but there is good reason to believe
that in that sex the horns were much smaller or wanting, as in nearly
all existing deer. The skull was long, narrow and low; the orbits were
small, completely enclosed in bone and unusually prominent; the nasal
bones were exceedingly short, as though indicating the existence of a
proboscis, but this can hardly have been the case, for the nasal opening
was divided into anterior and posterior portions by the bony bridge which
united the bases of the forward pair of horns. In no other known mammal
does such a division of the nasal opening occur. The upper incisors had
all disappeared, but there was a small upper canine tusk and another
formed by the first lower premolar, while the real lower canine had gone
over to the incisor series. This exceptional arrangement is a point of
resemblance to the †oreodonts (see p. 372). The grinding teeth were
brachyodont. The fore limb is not known, but the hind limb has been
completely recovered; it was stout and not very long in proportion to
the length of the head. The fibula was completely reduced, only the ends
remaining, and the pes was didactyl, the two metatarsals uniting in a
cannon-bone; the hoofs were like those of deer and antelopes.
[Illustration: FIG. 216.—_†Protoceras celer_, upper White River; males
on the right and left, female in the middle. Restored from skeletons in
the American Museum and Princeton University.]
No representative of this series has yet been found in the upper
Oligocene; and it is not yet possible to say whether their absence
from the John Day beds, as in several other cases already referred to,
was due to an actual geographical difference in contemporary faunas,
or whether it is merely one of the accidents of preservation and
collecting. In the upper White River, however, was another most curious
animal (_†Protoceras_), a forerunner, if not a direct ancestor, of
_†Syndyoceras_. The exact relationship between the two forms can hardly
be determined, until the genera, one or more, which once connected them
shall have been recovered, though it is obvious that they belonged to the
same series. _†Protoceras_ was a smaller animal and, if anything, an even
more bizarre-looking object, for the anterior protuberances were broad,
prominent and everted plates of bone, not even suggesting horns in their
form, and the posterior pair were short and club-shaped; in the female
neither pair was more than indicated. The dentition was very similar
to that of _†Syndyoceras_, except that the upper tusk was considerably
larger and scimitar-shaped; the female had no tusks. In the fore-arm
the two bones were just beginning to coalesce, but in the lower leg the
fibula was completely reduced. The manus had four complete and functional
digits, the laterals not very much shorter and thinner than the median
pair; but the pes was already didactyl, though the metatarsals were
separate, not fused into a cannon-bone; two long and pointed splints
were the vestigial remnants of the second and fifth digits.
[Illustration: FIG. 217.—_†Protoceras celer_, skull of male. (After
Osborn and Wortman.)]
It is not yet possible to trace this phylum below the level of the
uppermost White River beds, yet that will very probably be accomplished
by future exploration.
The second phylum of the family was represented in the lowest Miocene
by _†Hypertragulus_, a genus of much smaller animals than those of the
preceding series, which went back to White River times without essential
change, and was abundant in the John Day stage. Despite this fact, the
structure of the genus is still incompletely known and much remains
to be learned, but enough has already been ascertained to justify
the association of this phylum with the _†Protoceras-†Syndyoceras_
series in one family as reasonable. The number of upper incisors in
_†Hypertragulus_ has not been ascertained, but the canines were enlarged
and tusk-like, the lower one not having gone over to the incisors, as it
had in the preceding group. The skull had much resemblance to that of the
contemporary camels, the sudden narrowing of the facial region giving it
a very llama-like appearance; the orbit was open and on the face in front
of it was a conspicuous vacuity. The ulna and radius were coössified and
there were four digits in the manus, two in the pes, but no cannon-bone
was formed.
[Illustration: FIG. 218.—Skull of _†Leptomeryx evansi_, White River.
(After Matthew.)]
The third phylum, that of _†Leptomeryx_, had about the same range in
time as the preceding one, though it has not yet been found in the
John Day, and the genus is assuredly known only from the White River
beds, in which it is not uncommon. _†Leptomeryx_ comprised a number of
species, all very small animals, and none larger than a jack-rabbit. (See
Fig. 277, p. 563.) In size, proportions and appearance, these dainty
little creatures must have been very like the existing chevrotains or
“mouse-deer” of Asia and the Malay islands, and by many writers they have
been classed in the same suborder, the Tragulina. The upper incisors had
been suppressed and the upper canine reduced to very small size, while
the lower canine had become functionally one of the incisors. The skull
had a very long and slender facial region, but had a less llama-like
appearance than in _†Hypertragulus_. The neck was short and the fore
limbs much shorter than the hind, so that the back sloped downward from
the rump to the shoulders, as in the chevrotains. There was a remarkable,
indeed quite unparalleled, difference between the fore and hind limbs and
feet, the hinder extremity being not only much longer, but also much more
specialized, while the anterior one retained in very large degree its
primitive characteristics. Thus, in the fore-arm the ulna was complete
and separate from the radius, but in the lower leg the fibula was reduced
to its minimum. In the manus there were four entire and functional
digits, in the pes only two, which were joined in a cannon-bone.
Finally, there was a fourth phylum, that of _†Hypisodus_, which was
confined to the White River stage and is still incompletely known. This
was a tiny creature, much smaller than any of the preceding ones, and
is the only known White River ungulate with fully hypsodont grinding
teeth. Another very exceptional peculiarity of its dentition was that in
the lower jaw it had ten incisor-like teeth; not only the canine, but
the first premolar as well, had assumed the character of the incisors.
This same peculiarity is found in the lower Miocene †gazelle-camel,
_†Stenomylus_ (see p. 394), but in no other mammal.
A considerable assemblage of genera belonging to this family occurs in
the upper Eocene, but the material yet obtained is too fragmentary to
permit the assignment of these forms to the different phyla, though it is
very probable that among them are to be found ancestors of all the White
River and subsequent genera.
While there is little difference of opinion as to the propriety of
including in the family †Hypertragulidæ the four phyla described in the
foregoing pages, the systematic position and the relationships of that
family as a whole are matters of debate and likely long to remain so. Dr.
Matthew refers the entire group to the suborder Tragulina and regards
_†Leptomeryx_ as being closely related to the direct ancestry of the
American deer, a view which is accepted by Professor Osborn, but in which
I am unable to concur. My own belief is that the family was an early
offshoot from the cameline stock and therefore referable to the Tylopoda,
in which suborder they are here included. It would be out of place to
enter upon a discussion of this perplexing problem, which can hardly
receive a definitive solution until the artiodactyls of the Uinta stage
are thoroughly understood. As in so many other series, the key of the
mystery lies hidden in the Uinta fauna, which is still so inadequately
known.
SUBORDER PECORA. TRUE RUMINANTS
This is the most advanced, specialized and diversified group of the
artiodactyls, though the ruminating habit is shared by both Tylopoda and
Tragulina. In this multitude of forms, giraffes, deer, antelopes, sheep,
goats, oxen, buffaloes, bisons, etc., it is difficult to find a clue to
a natural arrangement or classification. As a whole, the suborder is a
well-defined group, and many structural characters, not all of which
is it needful to enumerate here, are common to all of its members. The
upper incisors are invariably absent, and, save in a few of the deer,
the upper canine also, while the lower canine has become incisiform; the
premolars are always three in number in each jaw and the molar-pattern is
selenodont throughout. The odontoid process of the axis is spout-shaped.
Except in a few deer, the Pecora all have bony outgrowths of the skull
in the form of antlers or horns, at least in the males, many females
being hornless. The ulna is coössified with the radius and the fibula
is lost, except the lower end, which is a separate malleolar bone. There
is always, in both fore and hind feet, a cannon-bone, the lower ends of
which are parallel, not divergent, as they are in the Tylopoda, and each
articular surface is encircled all around by a prominent median keel, as
in the horses, which in the other suborders, as in mammals generally, is
confined to the posterior side and not visible from the front. (_Cf._
Figs. 220 and 214, p. 401.) In no existing member of the Pecora are there
complete lateral digits, and in several modern genera they have been
completely suppressed; but in most there is, behind the functional pair
of digits, a pair of “dew-claws,” the bones of which are more or less
completely reduced, often to mere nodules. The stomach, which in the
Tylopoda and Tragulina is three-chambered, is in the Pecora divided into
four distinct parts.
[Illustration: FIG. 219.—Left manus of Patagonian Deer (_Hippocamelus
bisulcus_). _S._, scaphoid. _L._, lunar. _Py._, pyramidal. _Td._,
_M._, coössified trapezoid and magnum. _Un._, unciform. _Mc. II_ and
_V_, rudimentary second and fifth metacarpals. _Mc. III_ and _IV_,
cannon-bone. _Ph. 1_, _2_, first and second phalanges. _Ung._, ungual
phalanx.]
[Illustration: FIG. 220.—Left pes of Patagonian Deer. _Cal._, calcaneum.
_As._, astragalus. _N._, _Cb._, coössified navicular and cuboid. _Mt.
III_, _IV_, cannon-bone. Other letters as in Fig. 219.]
As already intimated, the subdivision of the Pecora into smaller groups
is far from easy. “The great difficulty which all zoölogists have felt
in subdividing them into natural minor groups arises from the fact that
the changes in different organs (feet, skull, frontal appendages, teeth,
cutaneous glands, etc.) have proceeded with such apparent irregularity
and absence of correlation that the different modifications of these
parts are most variously combined in different members of the group.”[9]
Two main sections of the suborder are, however, sufficiently well
defined, (1) the Cervicornia and (2) the Cavicornia.
SECTION CERVICORNIA. DEER AND GIRAFFES
This section includes two families, the giraffes and the deer. Inasmuch
as the former have not now and never did have any representatives in the
western hemisphere, for the purposes of this book the section becomes
identical with the deer family.
8. _Cervidæ. Deer_
In most of the deer now existing the male has antlers. The antler is
a bony outgrowth from the frontal bone of the skull and is annually
shed and replaced, increasing, as a rule, in size and in the number of
branches with each renewal. During the period of growth the antler is
richly supplied with blood-vessels and covered with skin and is then said
to be “in the velvet,” which dries and peels off when growth is complete;
after the rutting season a layer of bone at the base of the antler is
resorbed, loosening the antler, which is then shed. There is, however,
a permanent, cylindrical process, of greater or less length, from each
frontal, the “_pedicle_,” from which the antler is annually reproduced,
and around the base of the antler and shed with it is a roughened ring,
the “_burr_.” Among the different genera of deer there is great variety
in the form and size of the antler, from a single spike to the immense
and complicated appendages of the Wapiti (_Cervus canadensis_). As a
rule, the “_beam_” or main stem of the antler and its branches or
“_tines_” are cylindrical and tapering; but in some cases, as in the
Moose (_Alce_) and the Fallow Deer (_Dama_), the antler is very broad
and flat and is then said to be “_palmated_.” Except in the Reindeer and
Caribou (_Rangifer_) the female is without antlers.
In the skeleton there is little difference between the deer and the
Cavicornia, but there are some differences in the teeth. In the males
of those deer which have no antlers, such as the Musk-Deer (_Moschus
moschiferus_) and the Chinese Water-Deer (_Hydropotes inermis_), as well
as in certain forms with very small antlers, like the muntjacs of Asia
(_Cervulus_ and _Elaphodus_), the upper canine is a long, thin, recurved
and sabre-like tusk, a very effective weapon. Speaking of the Indian
Muntjac or “Barking Deer” (_Cervulus muntjac_), Flower and Lydekker
say, “When attacked by dogs the males use their sharp canine teeth with
great vigour, inflicting upon their opponents deep and even dangerous
wounds.” In other forms of deer the upper canines are small or absent.
The grinding teeth are brachyodont, but in the existing genera they have
higher crowns than in the Tertiary progenitors of the family, and in the
Axis and Hog Deer of India (_Axis axis_ and _A. porcinus_) the molars are
quite hypsodont.
As was shown in Chapter V, the existing deer of North America are of
two kinds: (1) the northern, which are plainly of Old World origin and
so closely similar to Old World species that many naturalists deny
the necessity of making distinct species for the American forms. The
best known of these are the Wapiti (_Cervus canadensis_), the Caribou
(_Rangifer caribou_) and the Moose (_Alce americanus_). (2) The southern
deer, of which the common Virginia Deer (_Odocoileus virginianus_)
is a familiar example, though overlapping in their range that of the
northern genera, are peculiar to the Americas, and, though not exactly
autochthonous, they must have had a long American ancestry. In the
Pleistocene we find the same genera and mostly the same species, their
distribution over the continent shifting in accordance with the many
climatic changes of that epoch. There was, however, at least one
Pleistocene genus (_†Cervalces_) different from any now living and
different from any known in the eastern hemisphere. The most complete
specimen of this animal is a skeleton in the museum of Princeton
University, found beneath a bog in northern New Jersey, though other
bones, collected in Kentucky and elsewhere, are very probably referable
to it. _†Cervalces_ was very nearly related to the Moose, the neck, body,
limbs and feet being almost identical in the two genera, but the skull
and antlers were notably different; the nasal bones were not nearly so
much shortened as in the Moose, indicating that the proboscis-like snout
was not so large or inflated as in the latter. The antlers were quite
unique; though in general like those of the Moose, they were much less
palmated and they had, in addition, a great trumpet-like plate of bone on
the lower side of each antler (see Fig. 117, p. 209), such as occurs in
no other known member of the family. Although _†Cervalces_ has not been
found in the Old World, it was almost certainly an immigrant from eastern
Asia.
The Moose, Caribou and Wapiti were unquestionably immigrants and came in
not earlier than the Pleistocene. Nothing is known in the Pliocene or
more ancient Tertiary epochs of North America which could be twisted into
forms ancestral to these typically Old World genera. With the southern
deer (_Odocoileus_, etc.) the matter stands differently, for these have
a probable American ancestry extending back to the lower Miocene and
possibly much farther. On the other hand, it is not altogether certain
that these may not have been Pliocene immigrants, for their genealogy
is still in an extremely fragmentary and unsatisfactory condition. The
North American genus, _Odocoileus_, extended back to the Pliocene with
very little change. The annoying, unrecorded gap of the upper Pliocene
and the meagre representation of the middle Pliocene mammals given by the
Blanco leave us without information regarding the deer of that time. In
the lower Pliocene and through the whole Miocene we meet with frequent
remains of a genus (_†Blastomeryx_) which was quite probably the ancestor
of the American types of deer. It was considerably smaller than any of
the existing North American species and had no antlers, but possessed
the sabre-like, upper canine tusks, which characterize the muntjacs
and hornless deer of Asia. The limb-bones had already attained nearly
their present state of development, as regards the reduction of ulna and
radius, formation of cannon-bones, etc. _†Blastomeryx_ probably entered
North America in the lower Miocene, but, as was mentioned previously (p.
409), Dr. Matthew and Professor Osborn regard the genus as autochthonous
and descended from the †Hypertragulidæ.
[Illustration: FIG. 221.—Lower Miocene †hornless deer (_†Blastomeryx
advena_). Restored from a skeleton in the American Museum of Natural
History.]
In the middle Miocene _†Blastomeryx_ gave rise to an aberrant genus
(_†Merycodus_) which has been made the type of a distinct family
(†Merycodontidæ, see table, p. 362), but this is perhaps unnecessary.
_†Merycodus_ had deer-like antlers, but completely hypsodont teeth such
as no known member of the Cervidæ possesses. The middle Miocene species
(_†M. osborni_) was a little creature, not more than eighteen or twenty
inches high at the shoulder, and had a branched antler of three tines,
which was considerably longer than the skull, while in the species of the
upper Miocene (_†M. furcatus_) the antler was shorter and simply forked.
From the number of specimens of these animals found in which the burr is
incomplete or absent, it may be inferred that the antler was not always
deciduous. The legs were long and very slender, and apparently there was
no trace of the lateral digits, even in the fore foot. These peculiar
hypsodont deer persisted even in the older Pleistocene.
[Illustration: FIG. 222.—Miocene †deer-antelopes (_†Merycodus osborni_,
middle Miocene, and _†M. furcatus_, upper Miocene). Restored from
specimens in the American Museum.]
Deer are the only members of the Pecora which inhabit South America,
where there are several genera of them, all much more nearly allied to
North American than to Old World forms. No record of the presence of the
family in the southern continent has been found in beds older than the
Pleistocene, but in view of the degree of specialization which they have
there undergone, it is probable that the immigration took place in the
Pliocene.
SECTION CAVICORNIA. HOLLOW-HORNED RUMINANTS
In the animals of this second and far larger section of the Pecora there
are bony outgrowths of the skull, from the frontal bones, outgrowths
which are permanent and non-deciduous; these are the _horn-cores_, which
are tapering and unbranched. The horn-core is, in turn, covered with a
sheath of horn, likewise unbranched and permanent, but growing from year
to year until the maximum size is attained, a process which is familiarly
illustrated in the growth of a calf. Among Recent Cavicornia there is
but one exception to the rule that the horny sheath is non-deciduous and
unbranched and that one is the Prong Buck (_Antilocapra americana_). In
the Cavicornia it is the very general rule that both sexes are horned,
though the females commonly have smaller horns and in several genera
of antelopes the does are hornless. There is almost as great variety
in the shape and size of the horn as of the antler; we find small,
medium-sized and enormously large horns, which may be straight, simply
curved, complexly curved, spiral, lyrate or twisted. The antelopes have
many types of horns, as have the sheep and goats, the oxen, buffaloes and
bisons; but only a few of them are exemplified in the western hemisphere,
which now, as in the preceding geological periods, is singularly poor in
representatives of the Pecora.
9, 10. _Antilopidæ and Antilocapridæ. Antelopes_
Two very different kinds of antelopes are found in North America at
the present time; one of them, the erroneously named Rocky Mountain
Goat (_Oreamnos montanus_), is evidently a late immigrant from the Old
World, and fossil remains of it have been found in the Pleistocene
cave-deposits of California. This animal is a member of the true
antelope family (Antilopidæ) and belongs to the chamois group of
mountain-antelopes; it has no near relatives among other American
mammals, living or extinct.
The Prong Buck, or Prong-horned Antelope (_Antilocapra americana_),
occupies a very isolated position, so much so that a distinct family,
the Antilocapridæ, has been created for its reception. It differs from
all other Cavicornia in having a branched horn, though the bony core
is simple, and in annually shedding and renewing the horny sheath; the
horn is directly over the eye; there are no dew-claws and all traces
of the bones of the lateral digits have completely disappeared. The
grinding teeth are thoroughly hypsodont. The genus occurred in the older
Pleistocene, where it was associated with the last of the †deer-antelope,
or _†Merycodus_ series (_†Capromeryx_), and which, so far as it is known,
would seem to connect the two families, though this is doubtful. A middle
Miocene genus (_†Dromomeryx_ Fig. 128, p. 237) would be a more probable
ancestor of the Prong Buck, if it were not for the long, unfilled gap
of the upper Miocene and the whole Pliocene. _†Dromomeryx_ had erect
horn-cores placed directly above the eyes as in the modern genus, but
low-crowned grinding teeth; it was the most ancient American cavicorn
yet known. It remains to be determined by future exploration, whether
this middle Miocene genus was actually the ancestor of _Antilocapra_, or
merely an anticipation of it.
In the lower Pliocene have been found the remains, very incomplete, of
several antelopes, which appear to have been immigrants from the Old
World, but are too imperfectly known for any definitive reference. One
resembles the flat-horned, or goat-horned, antelopes of the European
Miocene and Pliocene. Others had spirally twisted horns like those of the
Recent strepsicerine, or twisted-horn antelopes of Africa and Asia, but
may, nevertheless, be referable to the Antilocapridæ.
Antelopes even penetrated to South America, and three genera of them
have been reported from the Pleistocene of the Brazilian caverns and
the Argentine pampas, but they were less successful in establishing a
foothold than were the deer, and form no part of the modern Neotropical
fauna.
11. _Bovidæ. Sheep, Bisons, Oxen, etc._
A series of genera, of disputed systematic position, is represented
to-day by the so-called Musk-Ox (_Ovibos moschatus_), which is now
exclusively North American, but in the Pleistocene ranged over northern
Asia and Europe as far west as Great Britain. The Musk-Ox, which is at
present found only in the extreme north, is a heavy, short-legged animal,
three and a half to four feet high, and six feet or more in length; the
body is covered with a dense coat of woolly hair overlaid by a thatch of
long, straight hair, which gives the animal a very shaggy appearance.
The horns are broad at the base, especially so in old males, in which
they meet in the middle line and cover much of the head as with a horny
casque; they curve downward and then upward and forward, with the tips
directed toward the front; in the females and young males the horns are
very much smaller.
This series cannot be traced back of the Pleistocene, in which epoch
it was not only far more widely distributed, but also very much more
diversified, no less than three extinct genera, in addition to the
existing one, having been found in the North American Pleistocene. One
of these (_†Symbos_), which extended from Alaska to Arkansas, had horns
which were smaller and shorter than in the modern genus, and, even
when fully developed, did not meet in the middle line of the head. The
other two genera, from California (_†Euceratherium_ and _†Preptoceras_
Fig. 116, p. 203), are of great interest as showing affinities to the
Musk-Ox and also to sheep and to certain antelopes, such as the Takin
(_Budorcas_) of northern India and Tibet. They serve to connect the
musk-oxen with other Cavicornia, but the origin of all these animals is
to be sought in Asia.
In Recent North America there are four or five species of sheep (_Ovis_)
which are confined to the mountainous and broken areas of the western
part of the continent and extend from Alaska to Mexico. The “Bighorn”
or Rocky Mountain Sheep (_Ovis canadensis_) is characterized by great,
spirally coiled horns in the rams, in the ewes the horns are very much
smaller and nearly straight; the other species differ but slightly from
this type. The species _O. canadensis_ has been found in the Pleistocene,
but nothing further is known of its history. Evidently, the sheep were
late immigrants.
“The geographical distribution of wild sheep is interesting. The immense
mountain ranges of Central Asia, the Pamir and Thian Shan of Turkestan,
may be looked on as the centre of their habitat.” “Sheep are essentially
inhabitants of the high mountainous parts of the world, for dwelling
among which their wonderful powers of climbing and leaping give them
special advantages. No species frequent by choice either level deserts,
open plains, dense forests or swamps. By far the greater number of
species are inhabitants of the continent of Asia, one extending into
North America [should read, four or five] one into Southern Europe and
one into North Africa.... No remains that can be with certainty referred
to the genus [_Ovis_] have been met with in the hitherto explored
true Tertiary beds, which have yielded such abundant modifications of
Antelopes and Deer.”[10]
The only other division of the family which is represented in North
America is that of the bisons, of which the fast vanishing remnant of
a single species[11] (_Bison bison_) is all that is left of what was
once an extensive and varied assemblage. The bisons differ from the
true oxen in the form and structure of the skull, in the shoulder-hump,
which is produced by the very long spines of the dorsal vertebræ and in
consequence of which the back slopes downward from the shoulders to the
croup. They differ further in the character of the hair, which is short
and woolly on the body and hind quarters, very long and shaggy on the
head and neck. In the Pleistocene of North America there were at least
seven recognizable species of bisons, which ranged over the continent
from Alaska to Florida, though it is not probable that they were all
contemporary. One of the earliest and by far the largest of these was the
gigantic _B. †latifrons_, a specimen of which in the American Museum of
Natural History measures six feet across the horns in a straight line;
this was a Mississippi Valley species and extended from Ohio to the Gulf
of Mexico and westward to Kansas and Texas. Another gigantic species (_B.
†crassicornis_) lived in Alaska in association with a second and smaller
species (_B. †occidentalis_) which ranged as far south as Kansas. _B.
†occidentalis_, though smaller than the preceding species, was larger
than the existing one and was remarkable for the great size of the hump.
The bisons were migrants from the Old World and are the only members of
the great ox-tribe that ever reached America. At present the Old World
has but a single species of _Bison_ (_B. bonasus_), which has been saved
from extermination only by the most rigid protection.
Neither sheep nor bison extended their range to South America; both are
and have been essentially northern groups and seem to have been unable to
cross the tropics.
* * * * *
From the foregoing account, confused as it unavoidably is, one thing at
least stands out clearly, that North America played a very insignificant
rôle in the evolution of the Pecora, and has only two peculiar groups,
the Prong Buck and the American types of deer, and of these, the probable
American ancestry does not extend back of the lower Miocene and perhaps
not so far. Even in the Old World the story, so far as it has been
deciphered, is by no means clear and consistent, which is no doubt due to
the fact that the regions from which Tertiary mammals have been obtained
are so small in comparison with those that have yielded nothing. Certain
broad outlines of the history may, nevertheless, be discerned.
The suborder Pecora at an early date became divided into the two great
branches of the Cervicornia and Cavicornia, the former giving off the
giraffe series, which in the Miocene and Pliocene ramified and extended
through Asia and southern Europe, though now confined to Africa. In
the lower Miocene of Europe the muntjac-like deer and the antelopes,
the first of the Cavicornia, were already well distinguished. From
the primitive antelopes arose not only the wonderful assemblage of
modern antelopes, but also the goats and sheep and the great and varied
ox-tribe. From the middle Oligocene forms it may obviously be inferred
that both Cervicornia and Cavicornia united in a single trunk, or, traced
in the other direction, diverged from a common stock, to which also the
suborder of the Tragulina goes back.
On the other hand, it is equally obvious that the camels and llamas have
been separated from the Pecora at least since the middle Eocene, and,
consequently, the many points of agreement between the two suborders,
other than those shared with all artiodactyls, are not due to inheritance
from a common ancestry, but have been independently acquired in the
two series. It will be instructive to note some of the more important
of these independent similarities: (1) the selenodont and more or
less hypsodont character of the grinding teeth; (2) the spout-shaped
odontoid process of the axis; (3) the great reduction of the ulna and
its coössification with the radius; (4) the loss of the fibula, except
for its lower end, which persists as a separate malleolar bone; (5)
the formation of cannon-bones by the fusion of the third and fourth
metapodials; (6) the development of a complex, many-chambered stomach.
Other points of likeness might be cited, but those already given will
suffice to show how very important this parallel mode of evolution often
proves itself to be.
CHAPTER X
HISTORY OF THE PROBOSCIDEA
Utterly foreign as the elephant-tribe appears to be to present-day North
America, it was a very conspicuous element in the fauna of that continent
from the middle Miocene to the end of the Pleistocene, and in the latter
epoch it spread over South America also. Like so many others of the
mammals which have, from time to time, flourished in the Americas, the
elephants and their allies, the †mastodons, were immigrants from the
Old World, and, until comparatively lately, the region of their origin
was a complete mystery. They appeared suddenly and unheralded and at
approximately the same time in Europe and North America and nothing is
known from preceding geological formations of either continent which
could with any plausibility be regarded as ancestral to them. The mystery
was dispelled by the discoveries of Dr. C. W. Andrews in Egypt, which
demonstrated that these strange and huge beasts had originated in Africa
and had migrated thence through Asia to Europe, on the one side, and to
North America on the other.
The proboscideans occupy a very isolated position among the hoofed
mammals, and in structure they display a curious mingling of high
specialization with an extreme conservatism of primitive characters,
the specialization being exemplified in the teeth and head and the
conservatism in the body and limbs, very much as in the †oreodont family
of artiodactyls (p. 382). The most conspicuous of the external features
in the order is the long trunk, or proboscis, which gives its name to the
group, and is a great prolongation of the nose, with the nostrils at the
end and a finger-like tip, which can be used to pick up minute objects.
[Illustration: FIG. 223.—Molar of the African Elephant (_Loxodonta
africanus_) showing the oblique mode of wear. Heavy black lines indicate
enamel, enclosing areas of dentine, cement covering the whole tooth.]
In the true elephants the dental formula is: _i_ 1/0, _c_ 0/0, _p_ 0/0,
_m_ 3/3, × 2 = 14, though this formula is misleading, to the extent that
the milk premolars, three in number in each jaw, take the place and
perform the functions of the premolars, thus adding 12 to the effective
number of teeth. The single upper incisor on each side grows into an
immense tusk, which has enamel only on the tip, where it is speedily worn
away; the lower jaw is without incisors and there are no canines above
or below. The grinding teeth are very large and have a highly complex
structure and a most exceptional method of eruption on coming into use.
They are thoroughly hypsodont and each is composed of a large number of
high, broad and thin plates of dentine covered with enamel and the spaces
between the enamel ridges are filled with cement (see Fig. 47, p. 97);
indeed, the whole tooth is so thickly covered with cement that, when
unworn, it looks like a mere lump, with no ridges showing on the surface.
The teeth increase in size and in the number of component ridges from
before backward, and in the Indian species (_Elephas maximus_) the number
of ridges in the six grinding teeth, including the milk premolars, is:
4, 8, 12, 12, 16, 24. In the African Elephant (_Loxodonta africanus_)
the teeth are not so high and have fewer and thicker plates, the formula
being: 3, 6, 7, 7, 8, 10. The teeth do not succeed one another vertically
in the normal mammalian fashion, but come in successively from behind
and the series moves forward, so that the foremost tooth is pushed out,
when it is so worn down as to be of no further service. As these teeth
are very large and the jaws are relatively short, only one tooth on
each side, above and below, is in use at the same time, though part of
a second may also be involved. The movement of the successive teeth is
not directly forward, but oblique, an upper tooth coming forward and
downward and a lower tooth forward and upward. In consequence of this
arrangement the teeth are abraded obliquely, the anterior part first
coming into use, and, by the time a tooth is fully in place, the front
portion is worn down to less than half the height of the hinder part. All
of these peculiarities in the dental system imply a very high degree of
specialization and a notable difference from other mammals.
The skull is equally specialized, as is indeed required by the character
of the teeth and the development of the long and heavy proboscis. The
premaxillæ are converted into sheaths for the great tusks; the nasals
are extremely abbreviated and the anterior nasal opening is shifted to
the top, directly above the posterior opening, so that the nasal canal
passes vertically downward through the skull. All of the bones forming
the cranium are enormously thickened and at the same time lightened by
the formation of an extensive system of communicating sinuses, and thus
the brain-chamber is, as it were, hidden away in the middle of the huge
mass of the skull. This explains the difficulty of killing an elephant
by shooting it in the head; the shot must be so directed as to reach the
brain, which requires knowledge and skill.
The neck is short, the body long and extremely massive, the tail of
moderate length. The shoulder-blade is very large and has a prominent
metacromion given off from the spine; the hip-bones are immensely
expanded in correlation with the breadth of the thorax and abdomen. The
limbs are long, massive and columnar, their upper segments, especially
the thigh, are very long, so that the knee-joint is brought below the
body and free from it to the position of the hock-joint in the Horse;
hence, the hind leg appears to bend in the opposite direction from the
bend in the legs of ordinary quadrupeds, in which the true knee-joint is
concealed. The fore-arm bones are separate and, for most of its length,
the ulna is far heavier than the radius, a wide departure from the
proportions usual in hoofed animals. The femur has no pit in its head for
the round ligament and no third trochanter; the shaft is broad and much
flattened, having quite lost the normal cylindrical shape. The bones of
the lower leg are also separate, but the fibula, though stout, is very
much more slender than the ulna. The long bones have no marrow-cavities,
but are filled with spongy bone. The feet are extremely short and broad
and of columnar shape, the weight resting upon a pad of elastic tissue
and the small, nail-like hoofs are mere excrescences upon the periphery.
There are five digits in manus and pes, but not all of them have hoofs;
in the Indian and West African species the number of hoofs is five in
the fore foot and four in the hind, in the East African four and three
respectively. In the adult the skin is quite hairless, though the young
calf has a considerable quantity of hair.
[Illustration: FIG. 224.—Right manus of the Indian Elephant (_E.
maximus_).]
At present, the Proboscidea are restricted to the warmer parts of Asia
and Africa, where five species, four of them African, are recognized.
This is a very great reduction in the number of species and in the area
inhabited during the Pleistocene epoch, when they ranged through every
continent, except Australia, and were adapted to every climate from
the tropics to the shores of the Arctic Sea. Four distinct species of
proboscideans existed in Pleistocene North America, three elephants and a
†mastodon, though not all in the same areas, nor probably all at the same
time, their ranges both in time and space overlapping to a greater or
less degree, but not exactly coinciding in either respect.
[Illustration: FIG. 225.—Vertical section through the manus of the Indian
Elephant. _U_, lower end of ulna. _L_, lunar. _M_, magnum. _III_, third
metacarpal. _1_, _2_, _3_, phalanges. _E_, pad of elastic tissue. (After
M. Weber.)]
The first species was an immigrant, the northern †Mammoth (_Elephas
†primigenius_), which extended over the greater part of the northern
hemisphere, both in the Old World and in the New. This is the species
of which complete carcasses with hide and hair have been found in the
frozen gravels of northern Siberia, its structure and appearance being
thus almost as well known as those of any modern elephant. That the
†Mammoth was perfectly adapted to life in a climate of severe cold is
shown not only by the contents of the stomach, which are comminuted
fragments of present-day Siberian vegetation, but also by the dense coat
of woolly hair, covered by long, coarse outer hair, which afforded full
protection against the cold. The tusks, with considerable variation of
form, had a tendency to spiral curvature, curving first downward and
outward, then upward and inward; the grinding teeth were characterized
by their relative breadth and the numerous thin enamel-ridges which
traversed them. The number of these ridges was very variable in different
individuals, but may be expressed for the six successive teeth as
follows: 3-4, 6-9, 9-12, 9-15, 14-16, 18-27. The skeleton was more
like that of the Indian Elephant than of the other species, though
with a number of small differences in the skull. In size, the †Mammoth
was comparatively small, standing about nine feet six inches at the
shoulders. In North America its range was from Alaska southeastward
across the continent to New England.
The second species, the †Columbian Elephant (_E. †columbi_ Fig. 114, p.
198), was eighteen inches or more taller than the †Mammoth and rivalled
the largest existing elephants in stature; its huge tusks curved first
downward and then upward and inward, their tips crossing when full-grown.
The grinding teeth had fewer and thicker enamel plates than those of
the †Mammoth. The range of the †Columbian Elephant overlapped the
southern border of that of the †Mammoth, but was, on the whole, much
more southern; it crossed the continent from ocean to ocean and covered
nearly the whole of the United States, extending down to the southern end
of the Mexican plateau. The two species were very closely related and in
some cases are so intergraded that it is difficult to distinguish them;
the †Mammoth was an undoubted immigrant and the †Columbian Elephant was
probably a local North American variant of it, adapted to a somewhat
warmer climate. Nothing is known of the skin or hair in the latter
animal, but, from the fact that it was not a tropical species and was
exposed to very cold winters, it may be inferred that it had a hairy
covering of some sort.
The third species of elephant (_E. †imperator_) was older geologically
than the others, as it was more characteristic of the lower Pleistocene
and uppermost Pliocene; its range coincided with the western half of
the region covered by _E. †columbi_, extending far into Mexico, but not
occurring east of the Mississippi River. It was an enormous creature, the
largest of known elephants, with an estimated height of thirteen and a
half feet at the shoulder (Osborn). The grinding teeth had thicker and
more crumpled enamel plates than in either of the other species.
[Illustration: FIG. 226.—The American †Mastodon (_†Mastodon
americanus_), Pleistocene. Restored from a skeleton in the museum of
Princeton University.]
The fourth of the Pleistocene proboscideans of North America was a member
of a different and much more ancient genus, _†Mastodon_, which in the
Old World became extinct before the end of the Pliocene. The American
†Mastodon (_†M. americanus_) was thus a belated survival of an ancient
type, seemingly out of place even in the strange Pleistocene world,
which had so many bizarre creatures. The distinguishing characteristic
of the genus was in the simple, low-crowned and comparatively small
grinding teeth, which had three or four prominent transverse ridges,
covered with heavy enamel, and, usually, with no cement on the crowns.
As these teeth were so much smaller than those of the elephants, as many
as three on each side of each jaw might be in simultaneous use. In this
species there was no vertical succession of teeth, but in some of the
Tertiary †mastodons such succession has been observed. The long tusks
were directed nearly straight forward and were almost parallel, with but
slight curvature, the convexity downward. In the males there was a short
single tusk or, less commonly, a pair of such tusks, in the lower jaw,
which were probably not visible externally; these were the vanishing
remnants of an earlier stage of development, when the †mastodons had a
fully developed pair of lower tusks, nearly as large as the superior pair.
[Illustration: FIG. 227.—Last lower molar of the American †Mastodon.]
The skull, while essentially proboscidean, was yet much lower and flatter
and less dome-like than in the elephants; the thickening of the cranial
bones was less extreme. The remainder of the skeleton differed so little
from that of the elephants as to require no description. In size, this
species about equalled the †Mammoth, the larger individuals measuring
nine feet six inches at the shoulder. Remains have been found which prove
that the American †Mastodon had a covering of long, coarse hair, and that
it fed upon the leaves, shoots and small branches of trees, especially of
conifers. There is much reason to believe that the species outlived the
elephants in this continent and persisted until after the establishment
here of the American Indian, and it may well have been human agency which
finally extinguished the dwindling race. The range of the species nearly
coincided with that of the †Columbian Elephant, but did not extend so
far into Mexico, and in the central part of the continent reached much
farther north, even into Alaska.
In the Pliocene of Texas, Nebraska and Idaho lived the American
representatives of a genus (_†Stegodon_) which was a connecting link
between the elephants and the †mastodons, and which was especially
characteristic of the Pliocene of India. The tusks, which were confined
to the upper jaw, had lost their enamel and the last molar, above and
below, had five or six enamel ridges, but the crowns, which in the
Asiatic species were buried in cement, had but a small amount of this
material. Several species of _†Mastodon_ occur in the same beds, but only
isolated teeth have been found.
The †mastodons, in a broad sense of the term, have been divided into
several genera and subgenera in accordance with different schemes; the
simplest perhaps is to group into a second genus those species which had
fully developed lower tusks. This four-tusked genus has received several
names, of which _†Tetrabelodon_ is most commonly used in this country,
but the term _†Gomphotherium_ is much older and, according to the law
of priority, must therefore be employed. The lower Pliocene species of
_†Gomphotherium_ had a pair of large lower tusks, of cylindrical shape,
and both upper and lower tusks had longitudinal bands of enamel, and
in order to support the weight of these great tusks the symphyseal, or
chin, region of the lower jaw was greatly elongated; the molars had four
cross-crests.
[Illustration: FIG. 228.—Head of upper Miocene †mastodon (_†Gomphotherium
productum_) showing the chisel-like lower tusks. Restored from a skull in
the American Museum of Natural History.]
In the upper Miocene is found another and more primitive stage of
proboscidean development. In these species the grinding teeth were
three-ridged; the upper tusks were quite short and curved downward,
diverging somewhat from each other, and they had enamel bands. The
lower tusks were still shorter and of depressed, flattened and somewhat
chisel-like form and so worn as to show that they were regularly
employed in cropping and browsing. The skull was low and broad and the
symphysis of the lower jaw was greatly prolonged to carry the tusks.
A very important fact concerning these early †mastodons is that they
had the normal method of tooth-succession, permanent premolars forming
beneath (in the lower jaw, above in the upper) the milk-teeth and pushing
them out at maturity.
Of the middle Miocene proboscideans not much is known beyond the mere
fact of their presence in North America at that time and indeed little of
the skeleton, other than the skull, has yet been found in the American
Miocene; but well-nigh complete skeletons have been obtained from the
middle Miocene of Europe, and these bring out the surprising fact that
the body and limbs of these species did not differ in any noteworthy
manner from those of the existing elephants; the modern skeletal
structure of these animals had been attained at a time when the dentition
and skull were still in a far less advanced stage of development. In
size, however, there was a decided difference, the species of the
American Miocene rarely attaining a height of six feet.
Proboscidea have been reported from the lower Miocene of the Great
Plains, but the material is insufficient for a definitive judgment.
There is no doubt as to their presence in Europe at that time, but in
neither continent can the history be traced any farther and we must
turn to Africa for a backward continuation of the story. In the lower
Oligocene of the Fayûm, southwest of Cairo in Egypt, occurs the highly
interesting genus _†Palæomastodon_, which was much more primitive than
any of the genera described above, though it was an unmistakable member
of the order and even of the family Elephantidæ. The dentition was
already much reduced, giving the formula: _i_ 1/1, _c_ 0/0, _p_ 3/2,
_m_ 3/3. The upper tusks were short, compressed, directed downward, and
slightly divergent, and had a broad band of enamel on the outer side;
the lower tusks were still shorter and procumbent, pointing straight
forward, and were covered with enamel, which was very thick on the
lower side and thin or wanting on the upper. All of the grinding teeth
were in place and function at the same time, which was not true of any
of the genera previously considered, and each of the premolars had its
predecessor in the milk-series, which it succeeded and displaced in the
normal vertical manner. The premolars were smaller and simpler than the
molars, which were made up of three pairs of conical tubercles arranged
to form a three-crested crown. The skull, as compared with that of the
elephants, was long and narrow, the premaxillaries extending into a long
snout; the nasals were shortened, though not so much as in the succeeding
genera, and there was probably rather a long and flexible snout than a
true proboscis. The skull had a long and well-defined sagittal crest,
which none of the later genera had, and the development of sinuses in the
cranial bones, though considerable, was much less than in the elephants.
The occiput was relatively high and the thickened parietals did not tower
above it to any such degree as they do in the elephants. The symphysis
of the lower jaw was greatly prolonged, extending out beyond the ends of
the upper tusks, and this implies that the lower lip had a corresponding
prolongation.
The skeleton is still incompletely known, though it may be said that the
neck was probably longer than in the subsequent genera of the family.
The limb-bones were already proboscidean in character, differing only
in details from those of the more typical members of the order, but the
animal was more lightly built and had less massive limbs. The presence
of the third trochanter on the femur, which is lacking in all of the
succeeding forms, is an interesting approximation to other and still more
primitive groups of ungulates. The several species of _†Palæomastodon_
represent a considerable range in size, from animals which were not much
larger than a tapir to those which equalled a half-grown Indian Elephant.
It is possible to take another and very long step back from
_†Palæomastodon_, so long, indeed, as to make it apparent that one or
more links in the chain are still missing. The genus _†Mœritherium_ is
found together with _†Palæomastodon_ in the lower Oligocene, but also
occurs separately in the upper Eocene. It seems likely that it is a
persistent middle Eocene type and that the known species of it were
somewhat aside from the main line of descent, but that it very closely
represents, nevertheless, a very early stage in the elephant genealogy.
These known species were quite small animals, about the size of a tapir,
and therefore not much less than the smaller members of _†Palæomastodon_.
The dental formula of _†Mœritherium_ was: _i_ 3/2, _c_ 1/0, _p_ 3/3, _m_
3/3, × 2 = 36. The first or median upper incisor was a relatively small
and simple tooth, but the second was quite a large, downwardly directed
tusk, which was much smaller and less curved than in _†Palæomastodon_,
and was not capable of indefinite growth. The third incisor and the
canine were small, spike-like teeth of no functional importance, but
their presence is significant as approximating the primitive, unreduced
dentition of the ungulates. The lower incisors were nearly procumbent,
with a slight upward inclination; the first one was long and the second a
thick, enamel-covered tusk, with a chisel-like edge, which was produced
by wear. The premolars were smaller and simpler than the molars, which
were quadritubercular, the four conical cusps arranged so as to form
two transverse crests, giving a pattern like that of the early pigs and
peccaries and of precisely the kind that might have been predicted from
the teeth of _†Palæomastodon_.
The skull had an utterly different appearance from that of
_†Palæomastodon_, the difference being much greater than between the
latter and the Miocene _†Gomphotherium_. It was long and narrow, and,
except for the very prominent zygomatic arches, of nearly uniform,
tubular shape, the brain-case being of small capacity, though, as
compared with other Eocene mammals, the brain was proportionately large.
“It is possible that the early tendency toward a considerable cerebral
development shown in these primitive Proboscidea is one of the causes
why the group has survived and flourished through so long a period”
(Andrews). The cranium was very long and the facial region extremely
short, the premaxillaries not being prolonged into a snout, as they
were in _†Palæomastodon_; the occipital bones formed nearly the entire
posterior surface of the cranium and even encroached slightly upon the
roof. There was a long, but not very prominent, sagittal crest, and some
of the cranial bones were much thickened; in one species the hinder part
of the cranial walls was distinctly inflated, a beginning of the enormous
thickening which has culminated in the true elephants. The nasal bones
were already much shortened, though they were twice as long as those of
_†Palæomastodon_, and the animal would appear to have had an incipient
proboscis.
The neck was of moderate length and the body very long, with at least
twenty pairs of ribs, and there was probably a long tail. The hip-bone
differed remarkably in its extreme narrowness from that of the later
Proboscidea and the limb-bones were much more slender, though not
dissimilar in shape.
At a very early period the order became divided into two main branches,
one of which includes all the forms so far considered, and the other the
very strange _†Dinotherium_. The †dinotheres entered Europe together
with the †mastodons in the lower Miocene and continued into the Pliocene
without much change and then died out, leaving no descendants. They
never invaded North America, probably because they were of more or
less aquatic habit, like the hippopotamuses, and therefore less likely
to find suitable conditions in the narrow and unstable land-bridges
which connected the Old World with the New, than were animals of purely
terrestrial habitat. The †dinotheres were of huge size, equalling the
larger elephants in this respect and closely resembling them in the
skeleton of the body and limbs. As usual in this order, the generic
peculiarities were to be found in the teeth and skull. There were no
superior tusks, all the upper incisors and canines being lost, but there
was a pair of large lower tusks, which were directed downward, with a
strong backward curvature. The dental formula then was: _i_ 0/1, _c_ 0/0,
_p_ 2/2, _m_ 3/3, × 2 = 22. The grinding teeth were relatively quite
small and had, except the first molar, two transverse crests, giving a
pattern singularly like that seen in the tapirs. The skull was remarkably
long, low and flat, and no doubt these animals had a proboscis of some
sort. That the †dinotheres were derived from the same ancestral stock as
the †mastodons and elephants is perfectly obvious and is not questioned
by any one, but it is not yet possible to trace the connection.
The proboscideans were late immigrants into South America, being known
there only in the Pleistocene and late Pliocene times, and only the
†mastodons entered the southern continent, where they gave rise to
several peculiar local species in Argentina, Bolivia, Chili and Brazil;
one of these (_†Mastodon andium_) had a deposit of cement on the
crowns of the grinding teeth. Why the elephants, which extended to the
northern border of the Neotropical region, should have failed to reach
South America and maintain themselves there, is but one of many similar
questions to which no assured answer can be given.
The evolution of the Proboscidea was, in a certain sense, very similar
to that of the †oreodont family (p. 381) among the Artiodactyla, in
that the developmental changes affected chiefly the dentition and the
skull, the skeleton of the body and limbs having very early acquired a
character which was afterward but little modified. Were the skull and
teeth of the lower Miocene _†Gomphotherium_ not known, we should hardly
hesitate to refer the skeleton to the genus _Elephas_, and even in the
Oligocene _†Palæomastodon_ all the bones of the skeleton, other than the
skull, were characteristically and unmistakably proboscidean. On the
other hand, the transformations of the teeth and skull were very profound
and far-reaching, very much more so than those which took place in the
†oreodonts.
[Illustration: FIG. 229.—Evolution of the Proboscidea: on the right, a
series of skulls; on the left, last lower molar. (After Lull, modified by
Sinclair.) N.B. _†Tetrabelodon_ should read _†Gomphotherium_.]
In the dentition we may consider separately the development of the
tusks and of the grinding teeth. The first step in the known series, as
exemplified by _†Mœritherium_, was the enlargement of the second incisor
in each jaw to form a tusk which, though actually quite long, was very
small when judged by the proboscidean standard. The upper tusk was
directed vertically downward and the lower one was procumbent, pointing
almost directly forward; the third incisor and the canine were small and
in the lower jaw already lost. In the next known stage, _†Palæomastodon_,
all of the anterior teeth, except the tusks, had been suppressed; the
upper tusks were longer and more curved and of an oval cross-section;
they extended less directly downward and more forward, while the enamel
was restricted to the outer side of the tusk; the lower tusks were more
fully procumbent than in the preceding genus. The third stage, that
of the lower Miocene _†Gomphotherium_, showed the upper tusks greatly
elongated and directed more forward than downward, while the lower
tusks were but little larger than before. From the middle Miocene two
phyla may be distinguished by the tusks alone; in one, which was not
destined to long life, the lower pair increased greatly both in length
and in diameter, while in the other series they rapidly diminished and
eventually disappeared. Even in the Pleistocene, however, the American
†Mastodon had remnants of these tusks in the males. In the later
†mastodons, the †stegodonts, and true elephants, the upper tusks, which
alone remained, lost the enamel bands and attained enormous proportions,
differing in the various genera and species in the extent and direction
of curvature. An aberrant mode of tusk development was to be seen in the
†dinotheres, in which the upper pair was suppressed and the lower pair
enlarged and so curved that the points were directed backward.
The grinding teeth underwent much more radical and striking changes.
At first (_†Mœritherium_) they were small, very low-crowned and of
simple pig-like or quadritubercular pattern, making two interrupted
cross-crests; all were in use simultaneously and the succession of
milk-teeth and premolars was by vertical replacement, as in normal
mammals generally. In _†Palæomastodon_ there were three pairs of
tubercles on the molars and in _†Gomphotherium_ these coalesced into
ridges, but in all the †mastodons there was more or less distinctness
of the conical tubercles. In one or more phyla the three-ridged plan
persisted for a long time, one such phylum terminating in the Pleistocene
_†Mastodon americanus_. In the other series the number of ridges
increased, first to four, then to five, six and more (_†Stegodon_); the
crowns of the teeth became much larger and higher, and the ridges, as
their number increased, became much thinner, and the valleys between
them were filled with cement, and finally, in the true elephants, with
their fully hypsodont, many-crested teeth, were thickly covered all over
with cement. The vertical succession of milk-teeth and premolars was
retained in _†Gomphotherium_, at least in some species, but the large
molars, which could not find room to be exposed while the premolars were
in place, came in successively from behind. This horizontal mode of
succession is the only one to be seen in the true elephants, in which but
one tooth, or parts of two, on each side of each jaw are in simultaneous
use and the premolars have entirely disappeared, but the milk-teeth are
retained.
The changes in the skull, which amounted to a reconstruction, were
very largely conditioned by the great increase in the length and
consequent weight of the tusks, in the size of the grinding teeth and the
development of the proboscis. In the earliest known type (_†Mœritherium_)
the skull had little about it that would, at first sight, suggest
proboscidean affinities; it was long and narrow, with sagittal crest
and occiput of normal type, very long cranial and very short facial
region. The nasal opening was directed forward and the nasal canal was
relatively long and horizontal in direction, but the nasal bones were
already much shortened, indicating that the proboscis was probably in
an incipient stage. The symphysis of the lower jaw was procumbent and
somewhat elongated, but to only a comparatively slight degree.
While the skull of _†Mœritherium_ was not obviously proboscidean, that
of its successor, _†Palæomastodon_, was unmistakably so, yet retained
several primitive features, which were lost in all of the subsequent
genera, such as the sagittal crest, the relatively low cranium and
moderate thickening of the cranial bones, the forward direction of the
nasal opening, etc.; the symphysis of the lower jaw was very greatly
prolonged.
As the tusks enlarged and the proboscis grew longer, the weight of
the head and its appendages necessitated a largely increased area
of attachment for the neck-muscles, and this was attained by a very
great thickening of the cranial roof, the occiput not increasing
proportionately; at the same time, the thickened bones were honeycombed
with sinuses, so as to reduce their weight without sacrifice of strength.
In those species of the Miocene _†Gomphotherium_ which had large and
heavy tusks, this thickening was not very much less than in the true
elephants. The enlargement of the tusks had other consequences, as, for
example, in lengthening and broadening the premaxillaries and, in the
elephants, in their downward bending, so as to shorten still further the
facial region of the skull. With the development of the proboscis, the
nasal bones were reduced to a minimum and the anterior nasal opening was
no longer directed forward, but obliquely upward, while the nasal passage
lost its horizontal direction and became almost vertical. The lower
jaw continued to elongate the symphysis, reaching a maximum in certain
species of _†Gomphotherium_; but the reverse process of shortening this
anterior region of the jaw began with the reduction of the lower tusks,
and, when these had disappeared, nothing remained of the immensely
elongated symphysis, except the short spout of the elephant’s jaw. As
the grinding teeth increased in height, there was a concomitant increase
in the vertical depth of the jaws for their lodgment.
It was an obvious advantage in the mechanical problem of supporting the
enormous weight of head, tusks and trunk to shorten the neck and thus
bring the weight nearer to the point of support at the withers, the
lengthening proboscis rendering it unnecessary for the mouth to reach the
ground in feeding or drinking. The other parts of the skeleton underwent
comparatively little change, the degree of modification being greatest
between _†Mœritherium_ and _†Palæomastodon_. Throughout the series the
bones of the fore-arm and lower leg remained separate, and the feet very
short and five-toed. In size also the great stature and massiveness were
attained early. After the great migration of the Proboscidea to the
northern continents, we find considerable differences of size between the
various phyla, though all were very large, and even as early as the lower
Miocene of France, there were species which rivalled the modern elephants
in bulk. It was this rapid attainment of great size and weight which
appears to have been the determining factor in the conservatism of the
skeleton. After the skeleton had become fully adjusted to the mechanical
necessities imposed by immense weight, and that adjustment, as we have
seen, was effected at a comparatively early period in the history of the
order, then no further modification of importance would seem to have
been called for. No doubt the habits and mode of life of these massive,
sedate and slow-moving animals underwent but little change from the
lower Oligocene onward. There is reason to think that _†Mœritherium_ was
semi-aquatic and a haunter of marshes and streams, but, if so, the change
to a life on dry ground was complete in the lower Oligocene, for the
structure of _†Palæomastodon_ gives no reason for supposing that it was
anything but a dweller on solid land and a denizen of forests.
Although this book does not undertake to deal with the obscure problems
connected with the marine mammals, it may be noted in passing that one
of these problems has been brought near to solution, if not actually
solved, by the discoveries in Egypt and that is the question concerning
the origin of the Sirenia. The order includes the existing Manatee or
Sea-Cow (_Manatus_) of the coast of Florida, northeastern South America
and western Africa, and the Dugong (_Halicore_) of the Indian Ocean.
These are mammals which are adapted to a strictly marine habitat and are
incapable of existence on land, having lost the hind limbs and converted
the fore limbs into swimming paddles. Unlike the whales, porpoises
and other Cetacea, the Sirenia are herbivorous and feed upon seaweed
and eel-grass and the aquatic plants of large rivers. The Egyptian
discoveries tend very strongly to the conclusion that the Sirenia and
Proboscidea were both derived from a common stock and that the genus
_†Mœritherium_ was not very far removed from the probable ancestor from
which both of the orders descended.
CHAPTER XI
HISTORY OF THE †AMBLYPODA AND †CONDYLARTHRA
These are two orders of hoofed animals which long ago vanished from the
earth and no member of either is known to have survived later than the
Eocene epoch; both were of great antiquity, dating back to the Paleocene,
perhaps even to the Cretaceous. The last of the †Amblypoda are found in
the lowest Uinta or highest Bridger, but they were relatively abundant in
all the more ancient beds. The following table gives the more important
American forms:
Order †AMBLYPODA. †Short-Footed Ungulates
Suborder †TALIGRADA
I. †PERIPTYCHIDÆ.
_†Periptychus_, Paleoc.
II. †PANTOLAMBDIDÆ.
† _Pantolambda_, Paleoc.
Suborder †PANTODONTA
III. †CORYPHODONTIDÆ.
_†Coryphodon_, Wasatch and Wind River.
Suborder †DINOCERATA
IV. †UINTATHERIIDÆ.
_Bathyopsis_, Wind River. _†Elachoceras_, Bridger.
_†Uintatherium_, do. _†Eobasileus_, do.
As is shown in the table, the suborder †Taligrada is entirely Paleocene
in distribution, the †Pantodonta are lower Eocene and the †Dinocerata
chiefly middle Eocene, though persisting into the upper. The †Dinocerata
were the most striking and characteristic of Bridger mammals, and two
or three phyla of them may be distinguished, though for our purposes
this is hardly necessary, for these strange and bizarre creatures
were all very much alike. From the commonest and best-known genus
(_†Uintatherium_) they are called †uintatheres. They were large and
ponderous animals, the veritable giants of their time, far exceeding any
of their contemporaries. In appearance they were among the most fantastic
of the many curious beasts which the fossils have revealed.
The skull carried three pairs of bony protuberances, or horn-like
outgrowths; one pair on the nasal bones suggest by their shape and
character that they formed the support of dermal horns like those of
the paired-horn rhinoceroses (_†Diceratherium_) of the Oligocene and
lower Miocene. (See p. 239.) The second pair, which were moderately
high and thick prominences, almost cylindrical in shape and tapering
but slightly to their bluntly rounded ends, were chiefly outgrowths of
the maxillaries, or upper jaw-bones. From their shape, it is likely
that these were not sheathed in horn, but were merely covered with
skin, as were also the third pair, which arose from the parietals.
These were massive, club-shaped prominences, eight or ten inches high
and broadening to the free ends, a shape which makes it impossible to
suppose that these were true horn-cores covered with horny sheaths. A
high crest of bone, representing the occipital crest, enclosed the top
and back of the cranium, connecting the posterior pair of “horns” and
dying away in front of them. The top of the cranium had thus a deep,
basin-like character, such as is to be found in no mammal outside of this
suborder and was one of the most peculiar features of this extraordinary
skull. The brain-cavity was absurdly small, the growth of the brain not
having kept pace with that of the body; the cavity is hidden away in
the postero-inferior portion of the skull, the immense thickness of the
cranial walls being somewhat lightened by the formation of sinuses, but
these were much less extensive and pervasive than in other very large,
horned or tusk-bearing mammals, such as elephants, rhinoceroses, etc.
Probably, as in the case of the †titanotheres and †entelodonts, this
deficiency of brain-development was at least one of the factors which led
to the early extinction of the group. The premaxillaries were slender
and rod-like bones, which did not meet in the middle line and carried no
teeth. The long and massive nasal bones and the position of the nasal
opening show that these animals cannot have had a proboscis of any kind.
The lower jaw was remarkable for the great bony flange which, in the
males, descended on each side from the lower border, near the anterior
end, and served to protect the great canine tusks from fracture.
[Illustration: FIG. 230.—Skull of †uintathere (_†Uintatherium alticeps_),
lower jaw supplied from another species. Princeton University Museum. For
restoration, see Fig. 231, p. 447.]
The female skull differed in two respects from that of the male: (1) the
horn-like protuberances were much more slender and less prominent; (2) as
the upper canine did not form a tusk, the lower jaw had no flanges. The
skull of the artiodactyl _†Protoceras_ (p. 406) was remarkably similar to
that of the †uintatheres.
The dental formula was: _i_ 0/3, _c_ 1/1, _p_ 3/3, _m_ 3/3, × 2 = 34.
The upper incisors were completely lost and the lower ones had the very
unusual peculiarity of being bilobate, or having the crown separated
into two well-defined cusps. The upper canines in the males were very
large, relatively thin, recurved and sabre-like tusks, with acute points
and sharp edges, which must have been terrible weapons, though it is
difficult to see how they were used; probably the mouth was widely
opened, so as to clear the points of the tusks, and the animal then
struck with them, as a snake does with its fangs. The lower canine was
very small and was included in the incisor series, the shape and function
of which it had assumed. Thus, the †uintatheres, with their toothless
premaxillaries and, to all appearances, eight lower incisors, formed a
curious parallel to the true ruminants (Pecora), and, as in the latter,
they must have had a firm elastic pad on the premaxillaries, against
which the lower incisors could effectively bite, when cropping the soft
plants which formed the diet of these great beasts. The grinding teeth
were low-crowned and surprisingly small in comparison with the size of
the skull. The premolars and molars were nearly alike and had two or more
transverse crests.
Aside from the altogether exceptional character of the skull, the
skeleton was quite strikingly elephantine in appearance, so much so, in
fact, that these animals have repeatedly been referred to the Proboscidea
and some writers are still of the opinion that the two orders were
related. There is, however, no sufficient ground for this view; the
undeniable likenesses are much more probably to be ascribed to the
operation of convergent development.
[Illustration: FIG. 231.—One of the elephantine †amblypods
(_†Uintatherium alticeps_) of the Bridger stage. Male in foreground,
female behind. Restored from specimens in the museums of Yale and
Princeton universities.]
The neck was of moderate length, sufficiently long to enable the
animal to reach the ground with the lips, a necessity in the absence
of a proboscis. The body was very long and, as is shown by the length
and curvature of the ribs and the great breadth of the hip-bones,
extremely bulky. The limbs were very massive, and the long bones had
lost the marrow-cavities, being filled with spongy bone, as in the
elephants, †titanotheres and most other very heavy mammals. The bones
of the fore-arm and lower leg were separate. The hip- and thigh-bones
and shin-bones were remarkably elephantine in character and, if found
isolated, might readily be referred to some unknown proboscidean, but the
bones of the fore limb were quite different from those of the elephants.
The feet likewise had a very proboscidean appearance, notwithstanding
important and significant deviations in structure; they had the same
shortness and massiveness and a similar reduction in the size of the
hoofs, and the presence of all five digits added to the resemblance.
Undoubtedly, the feet had the same columnar shape and arrangement of
elastic pads. The living animal must have had an appearance quite similar
to that of a rather small elephant, not exceeding six or seven feet in
height at the shoulders and therefore not surpassing the largest modern
rhinoceroses, the broad-lipped species of Africa (_Opsiceros simus_).
Of course, the head must be excepted from the comparison, as that was
totally unlike the head of any existing creature; with its long and
narrow shape, its fantastic protuberances and its lack of a proboscis,
it had no suggestion of likeness to any proboscidean. Whether the great
body was naked, or clothed with hair, it is of course impossible to
determine with confidence, but, all things considered, it seems unlikely
that the hair should have been completely lost in any terrestrial mammal
at so early a period. As we have seen in the preceding chapters, hairy
elephants and rhinoceroses continued into and through the Pleistocene,
not only in the cold regions of the north, as is shown by the hair of
the American †Mastodon. In the tropics conditions were different, and in
that uniformly warm climate the loss of hair by the very large mammals
probably took place long before the Pleistocene. At all events, it is
a significant fact that no hairless land mammals are now known in any
region which has severe winters. It is true that the middle Eocene
climate over most of North America was warm-temperate or subtropical,
and the †uintatheres may, in consequence, have been hairless, but there
is no evidence of this.
[Illustration: FIG. 232.—Skull of _†Elachoceras parvum_ (lower jaw
restored). Princeton University Museum.]
Within the limits of the †uintathere family, considerable modification
and change may be traced, which, as in the case of the Proboscidea,
principally affected the skull and the general stature. It is hardly
worth while to deal separately with the two or more phyla which may
be distinguished, for the differences between them are relatively
unimportant. In the uppermost part of the Bridger stage almost the latest
representatives of the family are found and the genus (_†Eobasileus_)
was of the largest size. These animals had remarkably long and narrow
heads and very large, shovel-shaped nasal protuberances; in the males the
upper canine tusks were very long and curved back nearly in a semicircle.
In the middle portion of the stage the species of _†Uintatherium_ were
somewhat smaller and had shorter, wider and higher heads, the tusks,
though well developed, were not quite so long, nor so strongly recurved;
in some species they were nearly straight, with “hastate” or spear-head
point. In the same horizon is found a third genus (_†Elachoceras_)
which was probably a survival persisting from the lower Bridger, in
which none of these animals and little of anything else has yet been
found. _†Elachoceras_ was hardly half as large as the common species of
_†Uintatherium_ and its skull might be described as a preliminary sketch
for that of the latter; the nasal horns were extremely small, or, more
probably, entirely absent; the median pair were mere low knobs, hardly
an inch in height, and the posterior pair were simply thickenings of the
crest which enclosed the top of the cranium on three sides, scarcely
rising above it. This crest itself was much less prominent than in
_†Uintatherium_ and the basin-like top of the skull, in consequence, very
much shallower. The upper incisors and the first premolar had already
been lost and the upper canine enlarged into a sabre-like tusk, which,
however, was relatively smaller than in the succeeding genera. The
grinding teeth were quite the same as in the latter. Unfortunately, the
skull of _†Elachoceras_ is the only part of the animal which is known,
but, so far as that is concerned, it is precisely what we should expect
the forerunner of _†Uintatherium_ to be; an ancestor made to order could
hardly be more diagrammatic. It might, of course, be objected that no
such relation as that of ancestor and descendant could obtain between
these two genera, because they were contemporaries, but the case is like
that of the ancestral elephants described in the preceding chapter.
_†Mœritherium_ and _†Palæomastodon_ are found together in the Egyptian
Oligocene, the former surviving for a considerable time after it had
given rise to the latter, and in the upper Eocene only _†Mœritherium_
occurs. Many similar instances might be given, just as grandfathers often
live long with their grandchildren.
In the Wind River stage, or upper division of the lower Eocene, lived the
still incompletely known _†Bathyopsis_, of which, however, sufficient
material has been obtained to show that it was much less specialized than
any of the Bridger genera. This genus comprised animals much smaller
than its successor, _†Elachoceras_ of the middle Eocene, being smaller
than a tapir; it stood in much the same relation to _†Elachoceras_ as
the latter did to _†Uintatherium_. In the American Museum of Natural
History is a highly interesting skull of _†Bathyopsis_, which will
shortly be described by Professor Osborn. The premaxillaries have not
been preserved, and it is therefore impossible to say whether the upper
incisors had already been suppressed or not, and though the upper canine
has not been found, there can be no reasonable doubt that it was a tusk.
The lower canine had not yet gone over to the incisor series, but was
a thin though large tusk. There was one more lower premolar, four in
all, than _†Uintatherium_ possessed, and all the premolars were somewhat
smaller and simpler than the molars. The small skull had a broad and
somewhat concave cranial roof, with slightly raised enclosing crest,
and the horn-like protuberances of the posterior and median pairs were
present in an incipient stage. Whether those of the nasal pair were also
indicated is not known, but probably they were not. The lower jaw was of
very peculiar shape; the flange of the inferior border was not so well
defined as in _†Uintatherium_, but had no hinder margin and rose very
gradually backward.
The series of genera in descending order, _†Eobasileus_, _†Uintatherium_,
_†Elachoceras_ and _†Bathyopsis_, immediately impresses the observer
as being a natural phylogenetic series of successive ancestors and
descendants. Unfortunately, only the skull is known in the two last
named, but there is no ground for supposing that the discovery of the
skeletons would require any alteration in the series as we now have
it. No member of this series has yet been found in the Wasatch, but
there can be no doubt that it was represented in that stage, for a
recent expedition from the American Museum has collected teeth of a
_†Bathyopsis_-like form in still older beds.
SUBORDER †PANTODONTA
During the older part of the lower Eocene the †uintatheres must have
been a rare and unimportant element of the fauna, at least in those
parts of the continent whose history we know. Their place was taken by
another suborder, the †Pantodonta, which was not ancestral to them, but
collaterally related and descended from a common ancestry. The largest
and most dominating of Wasatch mammals was the genus _†Coryphodon_, which
also occurred in the lower Eocene of Europe, and the species of which
ranged in stature from a tapir to an ox, though of much heavier form than
the latter. The latest surviving species lived in the Wind River stage as
a contemporary of _†Bathyopsis_, but then the suborder gave way to the
†uintatheres.
In _†Coryphodon_ (see Fig. 142, p. 279) the number of teeth was
unreduced, a fact which is recorded in the name of the suborder, the
dental formula typical of all the primitive ungulates being applicable
to the genus. This formula was: _i_ 3/3, _c_ 1/1, _p_ 4/4, _m_ 3/3, ×
2 = 44. The upper incisors were rather small, but functional, and the
canines of both jaws were formidable tusks, though not rivalling in
size the great sabres of the †uintatheres; the premolars had a simpler
structure than the molars, which resembled those of the †uintatheres in
a general way, but not closely. The skull differed greatly from that of
the †uintatheres in having no horn-like protuberances, and was relatively
large and heavy, the cranium having a broad, flat roof and no sagittal
crest, and the lower jaw had no descending flange from the inferior
border; in every way this skull was more normal and less bizarre-looking.
The neck was proportionately longer than in the †uintatheres, the body
long and the tail of medium length; the trunk-vertebræ had surprisingly
small and weak spines, perhaps an indication of aquatic habits. The limbs
were quite short and very heavy, and the bones, in comparison with those
of the †uintatheres, were less proboscidean and more perissodactyl in
character. For example, the femur retained the third trochanter and the
long bones had marrow-cavities. The feet, on the contrary, were very like
those of the †uintatheres, being extremely short and five-toed and with
reduced, nodular hoof-bones; even in the details of the wrist and ankle
joints there was no important difference between the two groups.
SUBORDER †TALIGRADA
None of the ungulate series considered in the foregoing chapters can be
traced back to a time earlier than the Wasatch, and many of them not
so far, but in the case of the †Amblypoda the line may be carried down
through the Paleocene. In the upper stage of that epoch (Torrejon) the
order was represented by _†Pantolambda_ (Fig. 143, p. 285), a member of
the third suborder, †Taligrada. The best-known species of the genus was
an animal with head and body somewhat smaller than those of a sheep and
much shorter legs. The teeth were present in unreduced number, 44 in
all; the canines were tusk-like, but very much smaller proportionately
than those of _†Coryphodon_; the premolars were smaller and simpler than
the molars, which closely represent the common starting point, whence
the curious tooth-patterns found in the subsequent genera of the various
families were derived. The skull was long and narrow and had a prominent
sagittal crest; the neck was of ordinary length, about equal to that
of the head; the body was long and the tail very long, much as in the
great cats. The hip-bones were narrow and slender and not bent outward,
having no such breadth as in _†Coryphodon_. The limbs were short and
relatively heavy, and the various bones were of such primitive character
that, if found isolated and not in association with teeth or foot-bones,
one would hardly venture to consider them as belonging to any hoofed
animal; the humerus had a very prominent deltoid crest and an epicondylar
foramen, and the femur had the third trochanter. The five-toed feet
were very short, and the digits were arranged in a spreading manner and
were relatively much more slender than in _†Coryphodon_. Each digit
terminated in a flat, pointed, well-developed hoof; evidently there was
no elastic pad to bear the weight, such as recurs in nearly all very
heavy ungulates. The gait of the animal was probably semi-plantigrade,
the hoofs being the principal points of support.
While _†Pantolambda_ was an undoubted ungulate and a member of the
†Amblypoda, there were many structural features in its skeleton which
point to a relationship with the primitive flesh-eaters. In the lower
stage of the Paleocene, the Puerco, the genus _†Periptychus_ would seem
to be the most ancient known member of the order, but it is still very
imperfectly understood.
* * * * *
In the mode of evolution of the †Amblypoda, so far as that is recorded by
the fossils, there is much to recall the development of the Proboscidea,
though the story began and ended at far earlier dates and may be traced
back to a much more primitive stage.
(1) There was a rapid increase of stature, especially of bulk, in the
†coryphodonts, but decidedly more gradual in the †uintatheres, which
eventually attained a far larger size.
(2) The upper incisors were suppressed and the canines grew into
formidable tusks, at first straight, then the superior one, enlarging
still farther, acquired a curved, scimitar-like shape, while the inferior
one dwindled and became functionally one of the incisors.
(3) The grinding teeth remained low-crowned throughout, but acquired a
more complex pattern, and the premolars became almost like the molars.
(4) The skull underwent a most remarkable transformation. Beginning with
a form that might have belonged to almost any of the ancient mammals,
hoofed or clawed, having very prominent sagittal and occipital crests,
long cranium and short face, it became in _†Coryphodon_ flat-roofed,
with moderately elongated face, while in the †uintatheres the top of
the cranium gradually took on a deeply concave basin-shape and, with
equal gradualness, three pairs of horn-like protuberances; the lower jaw
developed a great bony flange for the protection of the upper tusks.
These peculiarities grew more and more exaggerated and were most
striking in the terminal genus of the series, _†Eobasileus_.
(4) Unfortunately, nothing is yet known of the skeleton of _†Bathyopsis_
and _†Elachoceras_, so that it is not practicable to follow out all the
stages of skeletal modification, though the general course of development
is sufficiently plain. The neck did not change greatly, except to become
very strong and heavy and to grow shorter proportionately as the skull
was lengthened. The body remained long throughout the series, but gained
greatly in bulk, as the stature of the animal increased.
(5) The limb-bones lost their primitive character, such as the
epicondylar foramen of the humerus and the third trochanter of the
femur, and then, with the great increase of the weight to be supported,
the marrow-cavities were filled with spongy bone and the hip-bones
increased enormously in width; the femur lost its cylindrical shape
and was flattened antero-posteriorly, which gave it a very elephantine
appearance. None of the limb-bones was suppressed or greatly reduced in
size, nor was there any coössification between them.
(6) The feet early gained their definitive character; at no time was
there any loss of digits, but the originally divided toes were, in the
genera of the Wasatch and subsequent stages, united into the columnar
foot, and the hoofs were reduced from their primitively pointed shape to
nodular form.
As in the Proboscidea, therefore, there was comparatively little change
in the skeleton after the massive and bulky proportions had been
acquired, but great and continual modification of the skull. At the time
when the †Amblypoda finally disappeared, no ungulate had acquired the
hypsodont dentition. Had the group survived till the middle Miocene, a
time when the spread of grassy plains so profoundly affected the feeding
habits of many herbivorous mammals, the high-crowned teeth might have
been developed in them also, and this, in turn, would have produced other
changes in the skull, making closer the parallel with the Proboscidea.
In conclusion, a few words may be said concerning the geographical
distribution of the †Amblypoda. In the Paleocene the only known
representatives of the order were those of North America, but the
†coryphodonts of the lower Eocene migrated to the Old World; indeed, the
genus _†Coryphodon_ was first described and named from English specimens,
but there were no such abundance and variety of these animals in Europe
as there were in the western United States. The †uintatheres were
strictly North American in distribution and no member of the suborder
has ever been found outside of this continent. Animals referred to the
†Amblypoda by some authorities have been obtained in the Oligocene
and Miocene of South America, but the assignment has been made upon
insufficient evidence. (See p. 508.)
ORDER †CONDYLARTHRA
The †Condylarthra were a group of exceedingly primitive ungulates, which
served to connect the hoofed and clawed mammals in quite an intimate
manner. So few indeed were the distinctively ungulate characters which
they had acquired, that it is still premature to make any positive
statements regarding their geographical distribution, because unusually
well-preserved specimens are required to make sure of their presence
in any particular region. Concerning North America there is no room
for question, and there is hardly any doubt that they existed in the
Paleocene of Europe. The South American remains which have been referred
to this order may very well prove eventually to belong to it properly,
but until both feet and skulls have been obtained in unequivocal
association, the reference can be only tentative. In North America they
ranged through the Paleocene and lower Eocene, but are not known from any
subsequent formation, and even in the Wind River only a few stragglers
survived.
The principal American families and genera are as follows:
I. †MENISCOTHERIIDÆ.
_†Meniscotherium_, Wasatch and Wind River.
II. †PHENACODONTIDÆ.
_†Protogonodon_, Puerco. _†Euprotogonia_, Torrejon.
_†Phenacodus_, Wasatch and Wind River.
1. _†Phenacodontidæ_
The typical Wasatch genus _†Phenacodus_, which is very fully known from
nearly complete skeletons, included species which varied in size from a
fox to a small sheep; the same genus occurred in the Wind River, but not
later. _†Phenacodus_ had the unreduced dental formula: _i_ 3/3, _c_ 1/1,
_p_ 4/4, _m_ 3/3, × 2 = 44.
[Illustration: FIG. 233.—Skeleton of the Wasatch †condylarth,
_†Phenacodus primævus_. American Museum. For restoration, see Fig. 141,
p. 278.]
The incisors were small and simple, the canines tusk-like, but of no
very great size, the premolars smaller and simpler than the molars. The
latter were of the quadrituberculate pattern, of four simple, conical
cusps arranged in two pairs, a pattern which is common to the earlier
and less specialized members of many ungulate groups. The skull was
long, narrow and low, with long and well-defined sagittal crest. As
in primitive skulls generally, the cranial region was long and the
face short, the eyes being very far forward; this does not imply large
brain-capacity, indeed, the brain was very small, but merely that the
portion of the skull behind the eyes was relatively long. The jaws were
short and shallow, in accordance with the small and low-crowned teeth
which they carried. The neck was of medium length, but the body was
elongate and the tail was very long and stout. The hip-bones were narrow
and slender, as in primitive ungulates generally. The limbs were short
and stout and retained many very primitive characteristics. The humerus
had a prominent deltoid crest and an epicondylar foramen; the fore-arm
bones were separate and the ulna quite unreduced, being almost as stout
as the radius. The femur had the third trochanter and the leg-bones were
distinct, though the fibula was slender. The feet, which were short, had
five digits each, but the third toe was enlarged, while the first and
fifth were shortened, as though preparing to disappear and thus give rise
to a three-toed perissodactyl foot. The ankle-bone (astragalus) had a
rounded, convex lower end, fitting into the navicular, so that it might
readily be taken for that of a clawed mammal.
2. _†Meniscotheriidæ_
A second family of Condylarthra was represented in the lower Eocene by
the genus _†Meniscotherium_ and was in some respects considerably more
advanced than the †phenacodonts. These were small animals, in which the
molars had acquired a crescentic pattern, recalling that seen in the
early horses and in the †titanotheres and †chalicotheres, and other
perissodactyl families. In the upper molars the two external cusps
had been so extended as to form a continuous outer wall, each of the
cusps having a concave external face and the two uniting in a prominent
median ridge. The lower molars had two crescents, one behind the other,
as in several families of both perissodactyls and artiodactyls. The
body and tail were long, the limbs relatively longer and lighter than
those of _†Phenacodus_ and the five-toed feet were so like those of
the modern conies, or klipdasses, of Africa and Asia Minor, that by
some investigators the family has been referred to the same order, the
Hyracoidea, but the suggestion is not a probable one. It is much more
likely that these problematical little †meniscotheres were merely a
short-lived branch of the †Condylarthra.
[Illustration: FIG. 234.—Lower Eocene †condylarth, _†Meniscotherium
terræ-rubræ_. Restored from a skeleton in the American Museum.]
The †condylarths were quite abundantly represented in the Paleocene,
where the genus _†Euprotogonia_ was the forerunner of the Wasatch
_†Phenacodus_, but had an even more primitive type of dentition. The
upper molars were not quadritubercular, but tritubercular, the three
cusps arranged in a triangle, the two outer ones forming the base and
the single inner one the apex. This type of upper molar was, or is
still, common to the primitive and unspecialized members of a great
many mammalian orders, marsupials, insectivores, rodents, carnivores,
lemurs, artiodactyls, etc., and there is strong reason to believe that
the tritubercular molar was the common starting point for almost all
types of mammalian dentition. However that may be, _†Euprotogonia_ is of
great interest as materially helping to close the gap between the clawed
and the hoofed mammals, belonging, as it did, to the latter and yet
retaining in dentition, limbs and feet so many characteristics of the
former.
†Condylarthra were probably present in the lowest Paleocene (Puerco
stage), but the material so far obtained is so fragmentary that there can
be no certainty on this point.
It is not at all probable that any of the North American †Condylarthra
should be regarded as ancestral to any of the more advanced ungulate
groups; on the contrary, they would appear to have come to an end in
the Wind River, leaving no descendants behind them. It is further true,
as was mentioned above, that the presence of †Condylarthra in other
continents, while very probable, cannot be positively asserted, because
the evidence is incomplete. Yet it would be a great mistake to assume,
for this reason, that these most primitive of ungulates were devoid of
evolutionary importance and interest. As is so often the case, where,
in the absence of the direct ancestry, the collateral relations afford
very valuable information as to the course of descent and modification,
the †Condylarthra throw useful light upon the origin of the ungulate
groups. It is extremely probable that the †condylarths, or some very
similar series of primitive hoofed mammals, had a very wide and perhaps
cosmopolitan range at the end of the Cretaceous and beginning of the
Tertiary period, and that, in the still unidentified region, where the
artiodactyls and perissodactyls arose, it was from a condylarthrous
ancestry. Possibly, all the other ungulate orders may yet be traced
back to the same stock, but it is rather more likely that the ungulates
include several series of quite independent origin. At all events, it is
quite certain that the clawed mammals long antedated the hoofed types and
that the latter arose, either once or at several separate times, from the
former. The †Condylarthra show how one, at least, of these transitions
was effected, and thus, in principle, how all were accomplished.
CHAPTER XII
HISTORY OF THE †TOXODONTIA (OR †NOTOUNGULATA)
It is a regrettable circumstance that, while the successive Tertiary
faunas are very fully represented in South America, approximately
complete skeletons have, as yet, been obtained from only a few of the
various stages; from the others the known material is very fragmentary
and largely made up of teeth and jaws. No doubt, the history of
fossil-collecting in North America will, in due course of time, be
repeated in the southern continent and more and more complete and
satisfactory specimens be obtained. At present, however, it is not
possible to trace the modifications of structure in any given series
with such detail as in those which were developed within the limits of
Arctogæa. No such story as that of the horses, the rhinoceroses or the
camels, can yet be told of the South American groups, whatever future
exploration may teach us. Nevertheless, much has already been learned
concerning the strange creatures that once inhabited the Neotropical
region and long ago vanished completely, leaving no trace in the modern
world.
As was mentioned in Chapter VI, on the present geographical distribution
of mammals, South America is to-day the richest and, after Australia,
the most peculiar zoölogically of all the regions. All of the modern
hoofed animals found in that continent at present, the tapirs, peccaries,
llamas and deer, are immigrants derived at a comparatively late date from
the north, but throughout the Tertiary and the Pleistocene there were
several indigenous types of ungulates, and of these the largest and most
varied assemblage was that included in the order †Toxodontia. The most
important and best known of the families and genera are listed in the
table:
Suborder †TOXODONTA. †Toxodonts Proper
I. †TOXODONTIDÆ.
_†Toxodon_, up. Plio. and Pamp. _†Xotodon_, do.
_†Trigodon_, Monte Hermoso. _†Nesodon_, Santa
Cruz. _†Adinotherium_, do. _†Pronesodon_, Deseado.
_†Proadinotherium_, do.
II. †NOTOHIPPIDÆ.
_†Notohippus_, Patagonian. _†Rhynchippus_, Deseado.
_†Morphippus_, do.
III. †LEONTINIIDÆ.
_†Leontinia_, Deseado. _†Colpodon_, Patagonian.
Suborder †TYPOTHERIA. †Typotheres
I. †TYPOTHERIIDÆ.
_†Typotherium_, Plioc. and Pleist. _†Eutrachytherus_,
Deseado.
II. †INTERATHERIIDÆ.
_†Interatherium_, Santa Cruz. _†Protypotherium_, do.
III. †HEGETOTHERIIDÆ.
_†Hegetotherium_, Santa Cruz. _†Pachyrukhos_, Santa
Cruz to Pampean.
IV. †NOTOPITHECIDÆ.
_†Notopithecus_, Casa Mayor. _†Adpithecus_, do.
V. †ARCHÆOPITHECIDÆ.
_†Henricosbornia_, Casa Mayor.
VI. †ARCHÆOHYRACIDÆ.
_†Archæohyrax_, Deseado.
Suborder †ENTELONYCHIA. †Homalodotheres
I. †NOTOSTYLOPIDÆ.
_†Notostylops_, Casa Mayor.
II. †ISOTEMNIDÆ.
_†Isotemnus_, Casa Mayor. _†Pleurocœlodon_, Deseado.
III. †HOMALODONTOTHERIIDÆ.
_†Homalodontotherium_, Santa Cruz. _†Asmodeus_,
Deseado. _†Proasmodeus_, Astraponotus Beds.
_†Thomashuxleya_, Casa Mayor.
Suborder †PYROTHERIA. †Pyrotheres
†PYROTHERIIDÆ.
_†Pyrotherium_, Deseado. _†Propyrotherium_,
Astraponotus Beds. _†Carolozittellia_, Casa Mayor.
_†Paulogervaisia_, do.
SUBORDER †TOXODONTA. †TOXODONTS PROPER
Among the remarkable animals which Charles Darwin found in the Pampean
deposits of Argentina and took with him to England, was a skull of
one which Sir Richard Owen named _†Toxodon_, or “Bow-Tooth,” from the
strongly curved grinding teeth, those of the opposite sides almost
meeting in the median line above the hard palate. For many years
_†Toxodon_, of which hardly anything was known, save the skull and teeth,
was a zoölogical puzzle and no one was able to reach any satisfactory
conclusion as to its systematic position and relationships, as all the
attempts made to force it into one of the known ungulate groups were
obvious failures. The discovery of complete skeletons, two of which
are mounted in the La Plata Museum, showed the necessity of making a
new group for its reception, as Owen had originally proposed. Through
the exploration of Argentina and its Patagonian provinces, the history
of the suborder was followed far back into the Tertiary period and its
indigenous character demonstrated. This and all the other subdivisions
of the †Toxodontia were exclusively Neotropical in distribution, and
none have been found farther north than Nicaragua and there only in the
Pleistocene.
The suborder was represented in the Pampean beds by several genera,
which differed in size and in the complexity of the grinding teeth,
but only of _†Toxodon_ is the skeleton at all fully known. The Pampean
species of this genus were massive, elephantine creatures, rivalling
the largest rhinoceroses in bulk, but not equalling them in height. The
teeth were all thoroughly hypsodont and apparently continued to grow
throughout life without forming roots; the dental formula was: _i_ 2/3,
_c_ 0/0, _p_ 3/3, _m_ 3/3, × 2 = 34. The first upper incisor was broad
and chisel-shaped, the second more tusk-like, but in some species these
proportions were reversed; the lower incisors were procumbent, pointing
straight forward, and of these the third was the largest. The canines
were lost and there was a long, toothless gap behind the incisors. The
premolars were smaller and simpler than the molars, and the anterior ones
were very small and were frequently shed at an early stage, making the
number of these teeth variable in different specimens. The upper molars
also were of quite simple pattern; the broad and smooth external wall
showed no distinct signs of a division into cusps, and from it arose
two oblique transverse ridges; the deep cleft or valley which separated
these ridges was divided and made Y-shaped on the grinding surface by a
prominent spur from the outer wall between the two principal crests. The
lower molars were composed of two crescents, one behind the other, of
which the posterior one was very much longer, and both were very narrow
transversely.
The skull had shortened nasal bones, an indication that some sort of a
proboscis or prehensile upper lip was present. There was no trace of a
horn, and the general aspect of the skull was not unlike that of one
of the hornless rhinoceroses, except for its great vertical depth; the
sagittal crest was very short and had almost disappeared. The auditory
apparatus was very extraordinary, though it can hardly be described
without an undue employment of anatomical terms; suffice it to say that
in addition to the usual outer ear-chamber, formed by the inflated
tympanic bone, there was a second chamber in the rear wall of the skull,
communicating with the first by a canal. This arrangement would seem to
imply an unusual keenness in the sense of hearing. The external entrance
to the ear was placed very high up on the side of the head, as in the
pigs and in many aquatic mammals, suggesting that _†Toxodon_ was more or
less amphibious. The anterior, or symphyseal, region of the lower jaw was
very broad, flattened and shovel-like, hardly projecting at all below the
plane of the lower incisors.
The neck was short and stout, the body long and extremely bulky, having
an immense, almost hippopotamus-like girth; the spines of the anterior
dorsal vertebræ were very long, making a high hump at the shoulders. The
limbs were short and very heavy, the bones very massive and with large
projections for muscular attachments. The fore leg was much shorter than
the hind, depressing the neck and head in very curious fashion. The
shoulder-blade was rather narrow, the spine without acromion or distinct
metacromion; the hip-bones were greatly expanded and turned outward,
quite in elephant-like fashion, a character which almost invariably
accompanies great increase in bodily mass. The thigh-bone was also very
elephantine in appearance, a likeness due to its shape and proportions,
to the loss of the third trochanter and the flattening of the shaft, so
that the width much exceeded the antero-posterior thickness. All of these
characters are, as a rule, associated with greatly augmented weight and
have been independently acquired in several series of large and massive
animals, elephants, †uintatheres, †titanotheres, and to this list should
be added the †toxodonts. In the fore-arm the bones were separate and the
ulna was quite unreduced and very stout, but in the lower leg, which
was very short in comparison with the thigh, the tibia and fibula were
coössified at the upper end, but not at the lower, a most exceptional
arrangement. The feet were surprisingly small and had but three digits,
the reduction from the original five having proceeded to that extent
before the process was arrested by augmenting weight. The heel-bone
(calcaneum) was so articulated with the other bones of the tarsus as to
project almost straight backward, nearly at a right angle to the position
normal in a digitigrade foot, a feature which is not known to occur in
any other mammal. The hoof-bones were so small and nodular that the foot
must have been of the columnar type, the weight resting upon the usual
elastic pad.
The restoration (Fig. 121, p. 217) shows _†Toxodon_ as a very heavy,
slow-moving, water-loving animal; the aquatic habits are, of course,
conjectural, but the general proportions are accurately given by the
skeleton.
From the Pleistocene, _†Toxodon_ may be followed back without notable
change to the Pliocene, but there it was in association with the last
of a curious phylum, the genus _†Trigodon_ (Fig. 138, p. 263), as yet
known only from the skull. In these animals a very prominent bony knob
or boss on the forehead clearly demonstrates the former presence of a
large, rhinoceros-like, frontal horn. But very few of the indigenous
South American ungulates possessed horns, or horn-like protuberances of
the skull, and all of these so far discovered belonged to the suborder
†Toxodonta. _†Trigodon_ was, from present knowledge, the only horned
creature of its time and region, for the deer and antelopes which had
probably arrived in South America had not advanced so far south as
Argentina. Another very peculiar feature of this genus was that the lower
incisors were present in uneven number, two on each side and one in the
middle. Nothing has been found of the skeleton, but it was doubtless that
of a smaller and somewhat lighter _†Toxodon_.
[Illustration: FIG. 235.—Skull of _†Toxodon_, Pampean formation, the
upper molars much broken. La Plata Museum.]
The material from the lower Pliocene adds nothing to our knowledge
of this suborder, but in the Santa Cruz time of Patagonia, which was
Miocene, it was very abundantly represented and preponderatingly by
the genus _†Nesodon_, which was the first discovered member of the
marvellous Santa Cruz fauna, named nearly 70 years ago by Sir Richard
Owen. It also chanced that Owen’s specimen was the imperfect lower jaw
of a young animal with the milk-teeth, which were mistaken for the
permanent dentition, and when the latter was found long afterwards, it
was naturally supposed to belong to a different animal and received a
different generic name. Nor was this all; the changes which took place
in the appearance and relative size of the permanent teeth within the
life-time of the individual were so remarkable, that the successive
stages of development were by several investigators supposed to be
distinct genera and species and named accordingly. In this way nearly 30
different names have, at one time or another, been assigned to the common
species, _†N. imbricatus_; and it was not until the late Dr. Ameghino had
brought together a complete series of skulls and jaws illustrating these
changes, and showing the gradual transition from one to the other, that
the confusion could be cleared up.
[Illustration: FIG. 236.—Skull of Santa Cruz †toxodont, _†Nesodon_; same
scale of reduction as Fig. 235.]
There was a long hiatus in time between _†Toxodon_ and _†Nesodon_ and
so great was the structural difference between them, that there is much
doubt whether the latter was directly ancestral to the former; in any
event, _†Nesodon_ so nearly represents what the desired ancestor must
have been, as to serve for all practical purposes of the study.
All the species of this Santa Cruz genus were much smaller animals than
the species of _†Toxodon_, _†N. imbricatus_ being no longer than a tapir,
with considerably shorter legs, and of much slighter and more slender
build than _†Toxodon_, though every tooth and every bone proclaims its
relationship to the latter.
In _†Nesodon_ the dental formula was unreduced; _i_ 3/3, _c_ 1/1, _p_
4/4, _m_ 3/3, × 2 = 44, though several of the teeth were much reduced
in size, so as to have lost their functional importance, and frequently
individuals are found in which one or more of these insignificant teeth
are lacking. The first upper incisor was a broad, chisel-shaped tooth,
which continued to grow for a period, then formed its root, and growth
ceased; the second incisor was a pointed, triangular tusk, which grew
throughout life, becoming longer with advancing age; while the third,
which was lost in _†Toxodon_, was small and unimportant. In the lower
jaw the first and second incisors were chisel-like and had a limited
growth; being rather narrow, they both bit against the broad first upper
incisor; the third incisor was a persistently growing tusk, not so large
as the upper one, against the posterior face of which it impinged and was
obliquely truncated by wear, so that its length was limited, while the
upper tusk continued to elongate and was made narrower and sharper by
wear. All the lower incisors were far less procumbent than in _†Toxodon_,
and were directed obliquely upward and forward. The remarkable changes
of appearance which, as mentioned above, took place within the life-time
of the individual, were largely due to the differential growth of the
incisors. The milk-incisors were all nearly alike and formed no tusks;
when the permanent incisors were first protruded, the first upper and
the first and second lower were large and the tusks were not visible,
and, when the latter did appear, they were for some time smaller than the
other incisors. These, however, formed roots and ceased to grow, actually
becoming smaller with advancing age, for the crowns narrowed to the roots
and, the more they were worn down, the smaller they became. The tusks,
on the other hand, grew throughout life and became larger as the other
incisors were reduced by wear, and thus the whole appearance of the
anterior part of the jaw was totally changed.
This mode of forming the tusks by the enlargement of the second upper and
third lower incisor is an unusual one, though it was repeated in another
South American ungulate order, the †Litopterna, and nearly so in the
Proboscidea, in which both upper and lower tusks were the second of the
three original incisors.
In both jaws, the canines of _†Nesodon_ were insignificant and sometimes
absent. The premolars, which were smaller and simpler than the molars,
had quite high crowns, but early ceased to grow and formed long roots.
The molars were truly hypsodont and formed no roots till late in life;
they were constructed on the same plan as those of _†Toxodon_, but were
decidedly more complex, the upper ones having several spurs and crests
given off inward from the external wall, in addition to the two principal
transverse crests, and they had a certain superficial likeness to the
teeth of a rhinoceros. As in _†Toxodon_, these upper molars were curved
inward, so as almost to meet those of the opposite side above the palate.
The lower molars had the same bicrescentic plan as in _†Toxodon_, but
were more complicated, and in the concavity of the hinder crescent was a
vertical pillar, which was well-nigh universal among the indigenous South
American ungulates.
If _†Nesodon_ was really the ancestor of _†Toxodon_, then the development
of the grinding teeth must have been a process of completing the
hypsodontism, until the teeth grew persistently, never forming roots,
and, at the same time, of simplifying the pattern. This is contrary to
the usual course of evolution, in which the pattern grew more complex in
the successive stages; but such steadily increasing complexity was not
invariable, and several instances of undoubted simplification are known
among mammals, though not yet in other ungulates. Only the recovery of
the intermediate genera will enable us to determine whether _†Nesodon_
was the actual ancestor of _†Toxodon_, or whether it was merely one of a
short-lived branch from the main stem, in which the teeth had acquired an
unusual degree of complexity.
[Illustration: FIG. 237.—_†Nesodon imbricatus_, Santa Cruz stage.
Restored by C. Knight from a skeleton in the museum of Princeton
University.]
A few years ago Dr. Ameghino announced the very surprising discovery
that, instead of having merely the normal arrangement of two dentitions,
the milk and the permanent, _†Nesodon_ developed three successive
dentitions, one preceding the milk-series, and therefore called
_pre-lacteal_. In certain other mammals traces of a pre-lacteal series
had already been found, in the shape of tooth-germs, which never attain
full development or even cut the gum; and quite recently Dr. Ameghino has
shown that in the tapir at least one functional pre-lacteal premolar is
formed. The significance of this fully developed pre-lacteal dentition in
_†Nesodon_ is not yet clear, though it seems reasonable to suppose that
it was the almost uniquely late retention of a primitive character.
The skull was closely similar to that of _†Toxodon_, on a smaller
scale, but there were several minor differences, which were, in part,
conditioned by the larger and much more completely hypsodont teeth of
the Pampean genus, as well as by its generally increased size and bulk.
In _†Nesodon_ the sagittal and occipital crests were much more prominent
and the former was much longer, while the thickening of the cranial bones
was in only an incipient stage. The nasal bones were considerably longer.
The jaws were lower and shallower, in correlation with the less perfectly
hypsodont teeth, and in the lower jaw the chin was much more erect and
rounded. The entire head of this curious Santa Cruz animal had something
remarkably rodent-like in its appearance, though it is quite inadmissible
to suppose that the likeness was due to relationship.
The skeleton was far smaller and lighter and otherwise differently
proportioned from that of _†Toxodon_, but there was, nevertheless, a
close agreement between the two genera. The neck was of moderate length
and thickness, the body long and heavy, but with no such relative bulk
as in the Pampean genus. The hump at the shoulders, as indicated by the
spines of the anterior dorsal vertebræ, though already well defined,
was less prominent. The shoulder-blade (scapula) was relatively broader
than in _†Toxodon_, its spine had a distinct acromion and two very long
and conspicuous processes given off backward from the spine, only one of
which, and that a mere vestige, is indicated in _†Toxodon_. The hip-bones
were almost parallel with the backbone and were not nearly so broad or so
everted as in the latter, a difference which is amply accounted for by
the great discrepancy in girth.
[Illustration: FIG. 238.—Left pes of _†Toxodon_. La Plata Museum.
_Cal._, calcaneum. _As._, astragalus. _N._, navicular. _Cn. 1_ and _2_,
coössified internal and middle cuneiforms. _Cn. 3_, external cuneiform.
_Cb._, cuboid.]
The limbs were of nearly equal length and there was no such shortening
of the fore-arm or elongation of the thigh as in _†Toxodon_, and so the
descent of the backbone forward, which gave such grotesqueness to the
skeleton of the latter, was far less pronounced. The limb-bones were
rather slender, in size and proportions not unlike those of a tapir,
but in structure very like the very much larger and more massive ones
of _†Toxodon_. The bones of the fore-arm were separate, but those of
the lower leg were coössified in the same exceptional manner as in
the Pampean genus, that is, the upper ends, but not the lower, were
fused together. The thigh-bone was not flattened, but had the normal
cylindrical shaft and a conspicuous third trochanter. The feet, in which
the digits were already reduced to three, were extremely small in
comparison with the size of the animal; in structure, they were almost
identical with those of _†Toxodon_, but were far narrower and more
slender. The heel-bone (calcaneum) articulated with the other bones of
the tarsus in a normal manner. The digits were well separated and the
hoof-bones quite strongly developed, indicating that the hoofs were
functional, supporting most of the weight. In short, the difference in
the external appearance of the feet between the two genera was much the
same as between the tapirs and rhinoceroses.
The species of _†Nesodon_, of which many have been named on very
questionable grounds, differed but little in size and were of such
variable and fluctuating character that a proper discrimination of them
is exceedingly difficult. One of these species (_†N. cornutus_) gives
indications of having possessed a small dermal horn on the forehead and
was thus a possible ancestor of _†Trigodon_.
[Illustration: FIG. 239.—Left pes of _†Nesodon_, Princeton University
Museum. Letters as in Fig. 238 and scale of reduction the same.]
A second phylum of the suborder was represented in the Santa Cruz stage
by the genus _†Adinotherium_, the species of which, not equalling a
sheep in size, were very much smaller animals than those of _†Nesodon_,
but closely like them in other respects. The dentition, including the
pre-lacteal series, and the skull were almost identical in the two
genera, with the exception that a large proportion of the individuals
of _†Adinotherium_ had the small frontal horn, while others had no
trace of it. While it is quite possible that the presence or absence
of the horn, which was always inconspicuous, may have been a matter of
specific distinction, a more probable explanation is that it was a sexual
character, the males horned and the females hornless. Much the same thing
is to be observed in the modern Javan Rhinoceros (_R. sondaicus_) in
which the females have a very small horn, or none at all, and the males a
large one.
In the skeleton also there were few differences, other than those of
size, between _†Adinotherium_ and _†Nesodon_; the former was not only
smaller, but also lighter and more slender proportionately, and there
was no hump at the shoulders, the spines of the dorsal and lumbar
vertebræ all reaching the same level, so that the back must have been
nearly straight in the living animal. From the more general and constant
presence of the frontal horn, _†Adinotherium_ was more probably the
ancestor of the horned _†Trigodon_ than was _†Nesodon_, but until the
intermediate forms shall have been recovered, no definite decision can be
made.
[Illustration: FIG. 240.—_†Adinotherium ovinum_, small, horned †toxodont
of the Santa Cruz. Restored from a skeleton in the museum of Princeton
University.—Note the minute horn on the forehead.]
The same or very nearly the same genera of the family †Toxodontidæ lived
in the Patagonian and Deseado stages, but there the record breaks off
and can, for the present at least, be followed no farther. It remains to
be determined whether the series originated in regions farther to the
north, or whether the ancestral types will be found in Patagonia.
The other two families are still very incompletely known, but
sufficiently to justify their inclusion in the present suborder. In the
†Leontiniidæ, which are known only from the Deseado stage (_†Leontinia_),
we have a curious variant of the †toxodont type. The tusks were decidedly
smaller than in the Santa Cruz members of the preceding family, the
grinding teeth with lower crowns and simpler structure. The skull was
much like that of _†Nesodon_, but the anterior nasal opening was of quite
a different shape, being carried much farther back on the sides, so that
the nasal bones had a far longer portion which was freely projecting and
unsupported; these bones were shorter and much thicker than in the Santa
Cruz genera and, to all appearances, supported a small, median horn on
their anterior ends. The feet, so far as they have been recovered, did
not differ in any significant manner from those of the preceding family.
[Illustration: FIG. 241.—Skull of _Adinotherium_, top-view, showing the
rugosity on the forehead for the small frontal horn.—Princeton University
Museum.]
Another imperfectly known family, that of the †Notohippidæ, occurred in
the Patagonian stage, but was most abundant in the Deseado, where several
genera of it have been found. These animals had mostly hypsodont teeth,
forming roots in old age, and the teeth were in closed series, but there
was no tusk-like enlargement of the incisors. In the later genera, those
of the Patagonian stage (_†Notohippus_, _†Argyrohippus_), the crowns of
the grinding teeth had a thick covering of cement, and those of the lower
jaw had some resemblance, though not at all a close one, to the teeth of
horses. The skull also had a certain suggestion of likeness to the horses
and Dr. Ameghino was persuaded that these animals were ancestors of the
horses. The family went back to the Astraponotus stage, but can be traced
no farther.
SUBORDER †TYPOTHERIA. †TYPOTHERES
This suborder was composed of much smaller animals than the †Toxodonta
and contained no large forms; some, indeed, were exceedingly small, no
larger than rabbits. It was much the most diversified of the suborders,
as is made evident by the table of families and genera. Two of these
families, the †Typotheriidæ and the †Hegetotheriidæ, continued into the
older Pleistocene. Of the former there was the genus first named and
described, _†Typotherium_, which has given its name to the family and
suborder, and the species of which were much the largest of the entire
group, almost equalling a large pig in size. At the first glance this
genus might easily be mistaken for a large rodent, and indeed it has
actually been referred to that order, but the resemblance was a purely
superficial one and involved no relationship.
In _†Typotherium_ the teeth were considerably reduced in number, the
formula being: _i_ 1/2, _c_ 0/0, _p_ 2/1, _m_ 3/3, × 2 = 24. The first
incisor in each jaw was a broad, scalpriform, persistently growing tooth,
which much resembled the corresponding tooth in the rodents, but was not,
as it is in the latter, worn to a sharp chisel-edge by attrition, but was
abruptly truncated. There was a second similar, but much smaller, tooth
in the lower jaw; the other incisors and all the canines had been lost
and the premolars reduced to two in the upper and one in the lower jaw.
The molars were large, persistently growing and thoroughly hypsodont; in
pattern they were very similar to those of _†Toxodon_. The skull without
the lower jaw was low and the cranial portion broad and flattened,
but retaining a long sagittal crest. The eye-sockets were nearly, but
not quite, closed behind by the very long and slender postorbital
processes of the frontal bones. In front of the eyes the face was
suddenly constricted into a long, narrow rostrum, and it is this shape
of the skull which, together with the persistently growing, scalpriform
incisors, gave such a rodent-like appearance to the head. The auditory
region had the same remarkable structure as in the †Toxodonta. The lower
jaw had a short horizontal portion and very high vertical portion, which
gave the head great vertical depth.
The skeleton, so far as it is known, was decidedly more primitive than
that of the contemporary _†Toxodon_, as is shown by the presence of
collar-bones (clavicles) and by the larger number of digits, five in the
front foot and four in the hind. The hoof-bones, or ungual phalanges,
were narrow, pointed and nail-like, though in the hind foot they were
broader and more hoof-like.
Little can be done as yet in tracing back the history of this family, the
Santa Cruz beds having yielded no member of it. In the Deseado stage, the
genus _†Eutrachytherus_ differed surprisingly little from _†Typotherium_,
in view of the long hiatus in time between them. The Deseado genus
already had thoroughly hypsodont and rootless teeth, and the molar
pattern was quite the same as in _†Typotherium_, but the teeth were much
more numerous, the formula being: _i_ 3/2, _c_ 1/1, _p_ 4/4, _m_ 3/3,
× 2 = 42. Nothing is known of the skeleton. The family arose probably
from one of the Eocene families (†Archæopithecidæ or †Acœlodidæ) with
low-crowned teeth, but the connection cannot be made out. Presumably, the
development of this family ran its chief course in some part of South
America far to the north of the fossil-beds of Patagonia.
The second family which was represented in Pampean times was that of the
†Hegetotheriidæ, and the sole genus of it which survived so late was
_†Pachyrukhos_, a little creature no larger than a rabbit. The genus
went back without any noteworthy change to the Santa Cruz stage of the
Miocene, from which complete skeletons have been obtained. The dental
formula was nearly as in _†Typotherium_: _i_ 1/2, _c_ 0/0, _p_ 3/3, _m_
3/3, × 2 = 30, and the enlarged, rootless and scalpriform incisors were
similar. The grinding teeth were thoroughly hypsodont and had a thin
coating of cement; the molar-pattern was fundamentally like that of
_†Nesodon_, in simpler form, but can be seen only in freshly erupted and
unworn teeth.
The skull was very rodent-like in appearance, its flat top and narrow,
tapering facial region, and the gnawing incisors adding much to the
resemblance. The very large eye-sockets and the enormously developed
auditory region suggest nocturnal habits, and, no doubt, the timid,
defenceless little creatures hid themselves by day, perhaps in burrows.
The enlargement of the accessory auditory chambers, which all of the
†Toxodontia possessed, reached its maximum in _†Pachyrukhos_, and the
chambers formed great, inflated protuberances at the postero-external
angles of the skull. The neck was short, the body long and the tail very
short, much like that of a rabbit. Collar-bones were present, as they
probably were in all of the other members of the suborder †Typotheria,
though this has not been definitely ascertained in all cases. The limbs
were relatively long, especially the hind legs, and very slender; the
bones of the fore-arm were separate, but those of the lower leg were
coössified at both ends. The feet, which had four digits each, were of
unequal size, the posterior pair being much longer than the anterior,
and the hoofs were long, slender and pointed, almost claw-like. The
entire skeleton suggests a leaping gait and its proportions and general
appearance were remarkably like those of a rabbit-skeleton. In the
restoration (Fig. 300, p. 639) Mr. Knight has followed these indications
and drawn an animal which might readily be mistaken for a curious,
short-eared rabbit; and there is every justification for doing this,
though the character of the fur and the form of the ears are, of course,
merely conjectural. Perhaps the ears are too small.
Associated with _†Pachyrukhos_ in the Santa Cruz stage was another
genus of the family, _†Hegetotherium_, which, though it cannot possibly
have been ancestral to the former, yet serves to indicate, in general
terms, what the ancestor must have been. This is another example of
the long-continued survival of the more primitive together with the
more advanced and specialized form. _†Hegetotherium_ persisted into the
Pliocene, but is not known from the Pleistocene. In this genus one upper
and two lower incisors were already enlarged, rootless and scalpriform,
but none of the teeth had been lost; it is interesting to note, however,
that the teeth which were lacking in _†Pachyrukhos_ were all very small
and ready to disappear. The Santa Cruz species of _†Hegetotherium_ were
considerably larger and more robust animals than those of _†Pachyrukhos_.
Both of these genera were preceded by very similar, almost identical
forms in the Patagonian, Deseado and Astraponotus stages, but the family
cannot be definitely traced farther back than the lower Oligocene, but it
very probably arose from some one of the groups, with low-crowned teeth,
of the Casa Mayor stage.
[Illustration: FIG. 242.—Santa Cruz †typothere (_†Protypotherium
australe_) and armadillo (_†Stegotherium tesselatum_). Restored by C.
Knight from skeletons in the museum of Princeton University.]
The family †Interatheriidæ was, in most respects, more conservative and
underwent less change than either of the preceding groups. A persistently
primitive type was the genus _†Protypotherium_, which appeared for the
last time in the Pliocene of Monte Hermoso, but was much more abundant
and better preserved in the Santa Cruz. The animal was small and had the
full complement of teeth, which were arranged in each jaw in a continuous
series, and were fully hypsodont and rootless, except incisors and
canine, which were rooted. None of the incisors was specially enlarged,
but there was a gradual transition of increasing size and complexity from
the incisors to the molars. A remarkable feature of this genus was the
deeply cleft form of the lower incisors, giving them a fork-like shape,
somewhat as in the modern Hyracoidea. The ulna and radius in the fore-arm
and the tibia and fibula in the lower leg were separate, but the digits
were already reduced to four in each foot. This was one of the few Santa
Cruz ungulates which possessed a long and heavy tail. The limbs were
relatively long and the feet were armed with such slender hoofs that they
looked almost like claws. The restoration shows the animal to have had,
like nearly all of the †Typotheria, a very rodent-like appearance, a
likeness which may, perhaps, be unduly increased by the form given to the
ears.
In the allied genus, _†Interatherium_, from which the family is named,
the head was short, broad and deep, almost bullet-like; the first incisor
was enlarged and chisel-shaped, and the other incisors and the canines
were much reduced in size. It is an interesting fact, observed as yet
only in this genus, but probably true also of all the smaller members
of the suborder which had hypsodont teeth, that the milk-premolars were
rooted and comparatively low-crowned, while their permanent successors
were completely hypsodont and rootless. The limbs were considerably
shorter than those of _†Protypotherium_ and the tail long and thick,
except for which, the general appearance of the skeleton suggests that
of the modern “conies” or “klipdases” (Hyracoidea) of Africa and Syria,
a suggestion which Mr. Knight has followed in the drawing (Fig. 297, p.
636).
This family was represented in the Deseado stage by a genus
(_†Plagiarthrus_) in which the teeth developed roots in old age, but is
not known from more ancient formations. Their probable ancestors of the
Eocene were very small animals, with brachyodont teeth, the premolars
smaller and of simpler pattern than the molars. The upper molars had
a continuous external wall, with indication of separate cusps, and
two transverse crests, as in the †Toxodonta, and the lower molars
were composed of two incomplete crescents. The teeth were present in
undiminished number and the anterior incisors were but little enlarged.
Nothing is known of the skeleton.
SUBORDER †ENTELONYCHIA. †HOMALODOTHERES
This third suborder of the †Toxodontia was in some respects the most
peculiar of all; no representatives of it have been found in formations
later than the Santa Cruz, and the group attained its culmination in the
still older Deseado stage, in which there were very large members of it.
These most extraordinary beasts are still incompletely known, and little
can be done as yet in the way of following out the steps of change which
led up to their exceptional characters, though the suborder itself may be
traced back to the Eocene by means of jaws and teeth alone.
The Santa Cruz genus labours under the portentous name of
_†Homalodontotherium_, which may be shortened to the vernacular form of
†homalodothere. In this genus the dentition was unreduced in number,
and the teeth, though having rather high crowns, were all rooted and
placed in continuous series, with a gradual transition in shape from
the incisors to the molars. The canines were tusks of very moderate
size, which projected but little above and below the plane of the other
teeth; the premolars, except the last, which was nearly molariform, were
smaller and simpler than the molars, which had a pattern fundamentally
the same as in the †Toxodonta. Those of the upper jaw were, however,
less complicated by spurs and accessory crests, and they had a somewhat
stronger resemblance to the rhinoceros pattern, though the resemblance is
demonstrably superficial and not indicative of relationship.
The skull was very like that of the Santa Cruz †toxodonts, _†Nesodon_,
etc., and had the same unusual structure of the auditory region as was
found throughout the order, but differed in many details, which it is
not worth while to enumerate, though it may be said that the nasal bones
were so much shortened that some kind of a proboscis or prehensile upper
lip was probably present. The head was quite small in proportion to the
size of the animal as a whole. Such of the vertebræ as are known were
quite similar to those of _†Nesodon_, but the limbs were far longer and
quite stout, though not massive. The humerus was remarkable for the great
development of the ridges for the attachment of the deltoid and supinator
muscles and for the prominence of the epicondyles, all of which gave to
the bone the appearance of the humerus of a huge burrower, yet it is
impossible to believe that so large an animal could have had burrowing
habits. The fore-arm bones were separate and very long, the ulna almost
as heavy as the radius; the latter is not known from a complete specimen,
but there would appear to have been some power of rotation, a power which
is conditioned by the shape of the upper end of the radius, and its mode
of articulation with the humerus in the elbow-joint. The thigh-bone was
long and heavy and its shaft was much flattened, having lost the normal
cylindrical shape, but retained a small third trochanter. The bones of
the lower leg were separate and relatively short, and the fibula was
uncommonly heavy.
So far, there was nothing very unusual, save in the shape of the humerus,
about the skeletal structure of the †Entelonychia, the remarkable
characters having been confined to the feet. Were it not for these, the
group might be included in the suborder †Toxodonta without difficulty.
The feet, which were five-toed, differed notably in size, the manus being
more than twice as long as the pes. In the former the metacarpals were
very long and, though actually stout, were slender in proportion to their
length; there was also a very unusual feature in an ungulate foot, that
the heaviest of the digits was the fifth, or external one. The mode of
articulation of the metacarpals with the first row of phalanges was very
exceptional, indicating an extraordinary mobility of the toes, and the
hoofs had been transformed into large, bluntly pointed claws, somewhat
like those of the †chalicotheres, those aberrant perissodactyls (see p.
354), but not so large or so sharp. In the pes, the ankle-bone had hardly
any groove for the tibia, and its lower end was hemispherical, as in
the †Condylarthra and the clawed mammals generally. The toes were quite
grotesquely short in comparison with those of the fore foot, and, as in
the latter, the fifth was the heaviest of the series. The hind foot was
apparently plantigrade, the heel-bone and the entire sole being applied
to the ground in walking, while the fore foot was probably digitigrade,
the wrist being raised and the metacarpals vertical. The weight was
carried upon the metacarpals and one or more pads under the phalanges,
as in the digitigrade carnivores, such as dogs and cats. In describing
the †chalicotheres, it was pointed out that it was uncertain whether each
foot had a single large pad, or whether there was a separate one under
the phalanges of each digit, and a larger one, the “ball of the foot,”
under the metacarpals collectively. The same doubt applies to the manus
of the †homalodotheres.
This is the third instance to be cited of the acquisition of claws by
a hoofed mammal and, as in the other two cases, the †chalicotheres and
†agriochœrids (p. 383), we are confronted by the seemingly incompatible
association of teeth which could have masticated only soft vegetable
tissues with feet like those of a beast of prey. As in the other
two groups, the problem as to the habits and mode of life of the
†homalodotheres is an unsolved one, chiefly because no mammal now living
is at all like these extraordinary creatures and one can therefore form
but vague conjectures as to the use of such feet to herbivorous animals.
Possibly they subsisted largely upon roots and tubers and used the great
claws for digging up food, the principal employment that bears now make
of their claws. This remarkable transformation of hoofs into claws
took place in three unrelated groups of hoofed animals and must have
occurred independently among the Artiodactyla, the Perissodactyla and the
†Toxodontia. By no possibility, so far as we are able to comprehend the
course of evolutionary change, could this common characteristic have been
due to inheritance from a common ancestry.
The †homalodotheres were among the largest of Santa Cruz mammals, but
they were then already approaching extinction, while in the Deseado stage
they were more numerous and varied and some of them very much larger.
This is an exception to the more common rule, according to which the
successive members of a phylum increased in stature until the maximum was
reached and this, in many cases, was followed by extinction. The rule is,
however, by no means without exceptions and several have already been
referred to. The largest of American proboscideans was the †Imperial
Elephant (_Elephas †imperator_) of the upper Pliocene and Pleistocene
and in many other phyla the Pleistocene species were much larger than
the Recent. So with the †homalodotheres; they reached their culmination
in size and importance in the Deseado stage, fewer and smaller forms
surviving into the Santa Cruz, after which the entire suborder vanished.
The family may be traced back to the Eocene, where it is represented
chiefly by a genus (_†Thomashuxleya_) which had larger canine tusks
and much more brachyodont teeth, but there is no way of determining
when the transformation of the hoofs took place. The other two families
(†Notostylopidæ, †Isotemnidæ) flourished chiefly or exclusively in the
Eocene and were small animals still very imperfectly understood.
SUBORDER †PYROTHERIA. †PYROTHERES
This suborder was a remarkable group, still incompletely known, of
elephant-like animals, which reached their culmination and died out in
the Oligocene, their last appearance being in the Deseado stage. The
genus _†Pyrotherium_ from the Deseado (also called the Pyrotherium Beds)
was the latest, largest and best known of the suborder. The dentition
was much reduced as is shown by the formula: _i_ 2/1, _c_ 0/0, _p_ 3/2,
_m_ 3/3, × 2 = 28. The upper incisors were two downwardly directed
tusks, the first quite small, the second considerably larger; the single
lower incisor of each side was a stout, but not very long, horizontally
directed tusk, with the enamel confined to a longitudinal band; the other
incisors and the canines had disappeared. The premolars, except the
foremost one, had the molar-pattern, which very rarely occurred among
the indigenous South American ungulates. The grinding teeth were similar
above and below and each had two elevated, transverse crests, which,
when quite unworn, carried a row of bead-like tubercles. These teeth
are decidedly reminiscent of the dentition of the aberrant proboscidean
_†Dinotherium_, from the Miocene and Pliocene of Europe (p. 435), and
this resemblance, together with the form of the tusks, has led to the
reference of this group to the Proboscidea, but the assignment is
undoubtedly erroneous, as is shown by the character of the skull and
skeleton.
[Illustration: FIG. 243.—Head of _†Pyrotherium_, showing the two pairs of
upper tusks. Restored from a skull in the museum of Amherst College.]
The skull, hitherto unknown, was obtained by the Amherst College
Expedition to Patagonia and its description by Professor F. B. Loomis is
anxiously awaited. In advance of that, he has published a brief account,
with a figure. This skull was long and narrow, with very short facial
region and nasal bones so shortened that the nasal canal passed almost
vertically down through the head, as in the elephants, and there must
have been a considerable proboscis. Despite this great modification, the
skull was plainly of the †toxodont and not of the proboscidean type.
The legs were extremely massive and the fore legs were considerably
shorter than the hind, with such a difference in length that the head
must have been carried low, as in the Pampean _†Toxodon_. The upper arm
and thigh were much longer than the fore-arm and lower leg respectively.
The humerus was immensely broadened, especially the lower end, and
the processes for muscular attachment were extremely prominent. The
femur was long, with broad and flattened shaft, and had no trace of
the third trochanter, quite strongly resembling the thigh-bone of an
elephant, which, as we have repeatedly seen, is the type more or less
closely approximated by all of the very heavy ungulates. In the standing
position, the femur was in nearly the same vertical line as the tibia
and the whole leg must have been almost perfectly straight, with the
knee-joint free from the body. The short and massive fore-arm bones were
coössified, at least in some individuals, as were the equally heavy bones
of the lower leg, the fibula being exceptionally stout. Little is known
of the feet, but that little renders probable the inference that they
were short, columnar and five-toed.
The Eocene representatives of the Pyrotheria are known only from very
fragmentary material. _†Propyrotherium_, of the Astraponotus Beds, was
smaller than the Deseado genus and still smaller was _†Carolozittellia_
of the Casa Mayor, which was not so large as a tapir. In the latter the
molars were of the same type as in the succeeding forms and small tusks
had already begun to develop. The older Eocene genus _†Paulogervaisia_
was probably a member of this suborder; if so, it shows that the molars
with transverse crests were derived from quadritubercular teeth, just as
happened in the Proboscidea and several other ungulate groups.
CHAPTER XIII
HISTORY OF THE †LITOPTERNA AND †ASTRAPOTHERIA
Besides the four well-defined groups which make up the †Toxodontia (or
†Notoungulata) there are two other extinct orders of indigenous South
American ungulates, which remain to be considered. These did not have
the exceptional development of the auditory region of the skull which
characterized the †Toxodontia. The best known and most important genera
of the †Litopterna are listed in the following table:
†LITOPTERNA. †Litopterns
I. †MACRAUCHENIDÆ.
_†Macrauchenia_, Plioc. and Pleist.
_†Scalibrinitherium_, Paraná. _†Theosodon_, Santa
Cruz. _†Cramauchenia_, Patagonian. _†Protheosodon_,
Deseado.
II. PROTEROTHERIIDÆ.
_†Epitherium_, Monte Hermoso. _†Diadiaphorus_,
Santa Cruz and Paraná. _†Proterotherium_, do.
_†Thoatherium_, Santa Cruz. _†Deuterotherium_,
Deseado. † _Prothoatherium_, do.
III. DIDOLODIDÆ.
_†Didolodus_, Casa Mayor. _†Lambdaconus_, do.
_†Notoprogonia_, do. _†Proectocion_, do., etc., etc.
Only one of the families of this suborder survived into the Pampean
stage, where it was represented by a single genus, _†Macrauchenia_. Like
all the other large Pampean mammals of distinctly South American type,
this was a grotesque creature, from the modern point of view. The genus
was first discovered by Darwin, who says of it: “At Port St. Julian, in
some red mud capping the gravel on the 90-foot plain, I found half the
skeleton of the Macrauchenia Patachonica, a remarkable quadruped, full as
large as a camel. It belongs to the same division of the Pachydermata
with the rhinoceros, tapir, and palæotherium; but in the structure of
the bones of its long neck it shows a clear relation to the camel, or
rather to the guanaco and llama.”[12] The views upon classification and
relationship here expressed have been superseded, but the passage is an
important one in the history of scientific opinion.
_†Macrauchenia_ (Fig. 120, p. 216), as Darwin says, was as large as
a camel; it had an unreduced dentition of 44 teeth and in each jaw
the teeth were arranged in continuous series and were quite decidedly
hypsodont. Both in the upper and the lower jaws the incisors formed
a nearly straight transverse row and have a “mark,” or enamel pit,
like that seen in the horses; the canines were but little larger than
the incisors and did not form tusks. The premolars were smaller and
simpler than the molars. The upper molars had two concave and crescentic
external cusps, connected by a median ridge, as in several families of
perissodactyls; two transverse crests and several accessory spurs and
enamel-pockets gave to the grinding surface, when somewhat worn, the
appearance of considerable complexity. The lower molars had the two
crescents, one behind the other, which recurred in almost all the South
American types of ungulates; the vertical pillar which so generally in
these types arose in the inner concavity of the posterior crescent was
wanting in the permanent teeth of _†Macrauchenia_, but present in the
milk-premolars.
No part of this remarkable animal was more curious than the skull, which
was quite small in proportion to the rest of the skeleton. It was long,
narrow and low, sloping and tapering forward to a blunt point at the end
of the muzzle, though there was a slight broadening here to accommodate
the transverse row of incisors. The sagittal crest was replaced by a
short, narrow and flat area; the cranium was shortened and the face
elongated, the orbits, which were completely encircled in bone, having
been shifted behind the line of the teeth, as in the modern horses.
The nasal bones were reduced to a minimum, a mere vestige of their
original length, the anterior nasal opening being directly over the
posterior, making the nasal passage vertical. Such an arrangement is an
almost positive proof that in life the animal had a flexible proboscis,
a conclusion which is confirmed by the presence, on the top of the
head and behind the nasal opening, of deep pits for the attachment of
the proboscis-muscles. A very curious feature of this skull was that
the bones of the upper jaw, the maxillaries and premaxillaries of the
opposite sides, united in the median line, making a long, solid, bony
rostrum in front of the nasal opening, a character not found in other
land mammals.
The neck was almost as long as in a camel and its vertebræ agreed with
those of the latter in the very exceptional character of having the canal
for the vertebral artery passing longitudinally through the neural arch,
instead of perforating the transverse process. As Darwin says in the
passage quoted above, “it shows a clear relation to the ... guanaco and
llama,” but this is founded on the postulate that such a likeness must,
of necessity, imply relationship. As was shown in the chapters on the
Artiodactyla and Perissodactyla, it is the general rule among long-necked
ungulates that the odontoid process of the axis assumes a spout-like
shape, but _†Macrauchenia_ was an exception and had an odontoid which
retained its primitive and peg-like shape; it was, however, relatively
very short and in cross-section was no longer circular, but oval. This
may be regarded as a step toward the assumption of the spout-like form,
but the extinction of the family put an end to further changes in that
direction.
The body was rather short and the limbs very long, giving the animal
a stilted appearance, while the feet were relatively short. The
proportionate lengths of the different limb-segments was unusual; the
upper arm was short, the fore-arm very long, the thigh long and the
lower leg quite short. The humerus was very heavy; the ulna and radius,
which were firmly coössified, formed a very long compound bone, which
was broad transversely and thin antero-posteriorly. The long femur had
only a small and inconspicuous third trochanter and the shaft was broad
and thin, being flattened, or “compressed” antero-posteriorly. The
tibia and fibula were united at both ends; the former was very heavy at
the upper end, but diminished downward in width and thickness, and the
fibula articulated with the calcaneum, as in the artiodactyls. The feet
were tridactyl and had mesaxonic symmetry; that is to say, the median
digit, or third of the original five, was symmetrical in itself and was
bisected by the middle line of the foot, while the lateral toes (second
and fourth), each of which was asymmetrical, formed a symmetrical pair.
It is this perissodactyl character of the foot to which Darwin refers
when he says that _†Macrauchenia_ “belongs to the same division of the
Pachydermata with the rhinoceros, tapir and palæotherium.” On the other
hand, the very significant structure of the ankle-joint was radically
different from that of the Perissodactyla; not only did the calcaneum
have a special facet for articulation with the fibula, but the lower end
of the astragalus was a convex “head,” resting only on the navicular, as
in the †Toxodontia, †Condylarthra, Hyracoidea and other very primitive
groups of hoofed animals and in clawed mammals generally. Such a
combination of characters is not known in any of the perissodactyls and
precludes the reference of the †Litopterna to that order, though such
a reference is strongly maintained by several authorities. The ungual
phalanges were small and appear to suggest the presence of pads on the
feet.
The appearance of _†Macrauchenia_ in life must have been sufficiently
strange. The small head with its proboscis and the long neck and legs
should probably be regarded as indicative of browsing habits, though
the hypsodont teeth show that grazing was at least an occasional mode
of feeding. The long limbs and short feet gave to the extremities an
appearance unlike that of any existing hoofed animal. The form and
size of the ears and the character of the hairy coat are, of course,
conjectural.
In the later Pliocene the family was represented by forms which differed
so little from the Pampean _†Macrauchenia_ as to call for no particular
notice, but in the presumably lower Pliocene of the Paraná stage,
occurred several genera, all unfortunately but imperfectly known, which
are of interest as being less specialized than _†Macrauchenia_ and as
showing the way in which some of the peculiarities of the latter were
acquired. In _†Scalibrinitherium_, which may be taken as an example of
these genera, the teeth were brachyodont; the upper molars were rather
less complex than those of _†Macrauchenia_, while the lower molars
had the pillar in the concavity of the posterior crescent, which the
Pampean genus retained only in the milk-teeth. As we have repeatedly
found, the milk-dentition is often conservative and retains primitive
or archaic features which have been lost in the permanent teeth, and
_†Macrauchenia_ is another illustration of the same principle. In the
skull of _†Scalibrinitherium_ the nasal bones, though very short, had not
suffered such extreme abbreviation as in the succeeding genus, the nasal
opening was farther forward and the maxillaries united in the superior
median line for only a short distance, while the premaxillaries were
fused together for their whole length. The orbit had not been shifted
entirely behind the teeth, but was above the third upper molar.
[Illustration: FIG. 244.—Santa Cruz †macrauchenid (_†Theosodon
garrettorum_) and predaceous marsupial (_†Borhyæna tuberata_). Restored
by C. Knight from skeletons in the museum of Princeton University.]
Next in the ascending series, to use the genealogist’s term, came
the genus _†Theosodon_ of the Santa Cruz, of which almost all the
skeletal parts are known and thus make possible a full comparison with
_†Macrauchenia_, which assuredly was its direct descendant. In view
of the great lapse of time involved, the differences between the two
genera were less than might have been expected, though the more ancient
animal was in all respects the more primitive. _†Theosodon_ was, in the
first place, considerably smaller, not much exceeding a llama in size;
the teeth had lower crowns than even those of _†Scalibrinitherium_ and
the incisors were arranged in line with the grinding teeth, not in
a transverse row, but curving inward slightly, so that those of the
opposite sides nearly met in front. The incisors, canine and first
premolar were simple, sharply pointed, conical teeth, which gave an
almost reptilian expression to the anterior part of the skull. The upper
molars were on the same fundamental plan as those of _†Macrauchenia_,
but in a less advanced stage of development, the transverse crests being
incomplete and the internal cusps had a certain degree of separateness
from the crests and from each other. It is evident that the upper
molars were derived from the quadritubercular type. The lower molars
had the vertical pillar in the concavity of the posterior crescent very
prominently developed.
The resemblance of the skull to that of _†Macrauchenia_ is obvious at
the first glance, but it was less specialized and departed less from the
ordinary ungulate type. The cranium was longer and the face shorter, the
orbit, which was incompletely closed behind, extending over the second
molar. There was a sagittal crest, the length of which differed much in
the various species; the nasal bones were already very short, though
decidedly longer than in the subsequent genus _†Scalibrinitherium_, and
the anterior nasal opening was extended forward as a long, narrow slit,
because the maxillaries did not come into contact with each other in the
superior median line, and the premaxillaries touched each other, but were
not coössified. The nasal canal, though very short, was horizontal, not
vertical. The skulls of the three genera thus displayed three successive
stages in the backward shifting of the orbit and of the anterior nasal
opening, in the shortening of the nasal bones and in the formation of a
solid rostrum by the fusion of the upper jaw-bones. No doubt also the
living animals exhibited a corresponding gradation in the development of
the proboscis.
[Illustration: FIG. 245.—Development of the skull in the †Macrauchenidæ,
side views. _A_, _†Theosodon_, Santa Cruz. _B_, _†Scalibrinitherium_,
Paraná. (After Ameghino.) _C_, _†Macrauchenia_, Pampean. (After
Burmeister.) _n._, nasal bones.]
[Illustration: FIG. 246.—Development of the skull in the †Macrauchenidæ.
_A_, _†Theosodon_. _B_, _†Scalibrinitherium_. (After Ameghino.) _C_,
_†Macrauchenia_. (After Burmeister.)]
The neck of _†Theosodon_ was even longer proportionately than in
_†Macrauchenia_ and the transference of the canal for the vertebral
artery from the transverse processes to the neural arch had already taken
place, except in the first, sixth and seventh vertebræ, and was thus
less complete than in the Pampean genus, in which all the vertebræ of
the neck, save the seventh, had the canal in its exceptional position.
The odontoid process of the axis was less modified than in the latter,
being relatively longer and more conical. The body was rather short, and
the spines of the trunk-vertebræ were proportionally higher and more
prominent. No caudal vertebræ have been found, but, from the shape of
the sacrum, it is evident that the tail was short.
The limbs were long, but more slender and less elongate than in
_†Macrauchenia_, in which the growth of the neck did not keep pace with
that of the limbs, the lengthening of the proboscis probably compensating
for this. The shoulder-blade had two conspicuous metacromia, very much as
in the contemporary †toxodont, _†Nesodon_, but shorter and more widely
separated. The humerus was short and quite slender and the fore-arm
bones, which were much longer, did not coössify. The femur had a more
slender and rounded shaft than in _†Macrauchenia_ and a much larger
third trochanter; the leg-bones were also separate from each other.
The tridactyl feet were so like those of the Pampean genus, that no
particular account of them is necessary, and the proportions of the limb
segments were similar in both genera, short upper arm and lower leg, very
long fore-arm and thigh, and short feet.
[Illustration: FIG. 247. Left manus of _†Theosodon_. _S._, scaphoid.
_L._, lunar. _Py._, pyramidal. _Tm._, trapezium. _Td._, trapezoid. _M._,
magnum. _U._, unciform. _V._, rudimentary fifth metacarpal.]
The appearance of the living animal, as shown in the restoration, was
no doubt somewhat like that of _†Macrauchenia_, but less bizarre. That
there must have been some sort of a proboscis or prehensile upper lip,
is indicated by the greatly shortened nasal bones, but this may not
have been longer than in the existing Moose or Saiga Antelope. The long
neck, short body and tail and long limbs suggest an animal not unlike a
Guanaco, but larger and heavier. The hair may or may not have had the
woolly character given to it in the drawing; upon such a point there can
be no certainty.
In the older formations preceding the Santa Cruz, the †macrauchenids are
known only from fragmentary material, though something of their history
may be made out even from these fragments. _†Protheosodon_, of the
Deseado stage, was considerably smaller than the Santa Cruz genus and had
more primitive upper molars, in that the internal cusps and intermediate
cuspules were isolated and conical, not forming transverse crests. Still
smaller were the several genera (_†Lambdaconus_, etc.) related to the
†macrauchenids found in the Casa Mayor Eocene, which have been referred,
perhaps correctly, to the †Condylarthra. In these the formation of the
external wall of the almost bunodont upper molars was in progress, by the
fore-and-aft extension and transverse thinning of the external cusps;
the internal pair of cusps and the cuspules were separate and conical.
With much confidence, it may be inferred that in these little animals
the skull was normal, the nasal bones were long and that the feet were
five-toed, but demonstration is lacking.
* * * * *
The second family of the †Litopterna, the †Proterotheriidæ, were
remarkable for their many deceptive resemblances to the horses. Even
though those who contend that the †Litopterna should be included in the
Perissodactyla should prove to be in the right, there can be no doubt
that the †proterotheres were not closely related to the horses, but
formed a most striking illustration of the independent acquisition of
similar characters through parallel or convergent development. The family
was not represented in the Pleistocene, having died out before that
epoch, and the latest known members of it lived in the upper Pliocene
of Monte Hermoso. In the still older Paraná formation more numerous
and varied forms occurred, but only from the Santa Cruz have materials
been obtained of sufficient completeness to furnish a full account of
the structure of these extraordinary animals. Not that this remarkable
character was due to grotesque proportions; on the contrary, they looked
far more like the ordinary ungulates of the northern hemisphere than
did any of their South American contemporaries; it is precisely this
resemblance that is so notable.
In Santa Cruz times the family was represented by a large number of
species, which have been grouped in four or five genera, which differed
sufficiently to require generic separation, yet were closely similar.
In all of them the dental formula was: _i_ 1/2, _c_ 0/1, _p_ 4/4, _m_
3/3, × 2 = 36. Except in one genus (_†Thoatherium_) a pair of small
tusks was formed by the enlargement of the second upper and third lower
incisors, as in the †toxodonts, but the first upper and lower and the
third upper incisors, which were retained in the †toxodonts, were lost
in this family, as was also the upper canine, and the lower canine was
very small, of no functional use. The teeth were brachyodont and, except
the small tusks, displayed no tendency at any time toward the acquisition
of high crowns. The premolars were less complex than the molars, though
the last one approximated the molar-pattern. The upper molars had two
crescentic outer cusps, meeting in a vertical ridge and together forming
the outer wall; the transverse crests were imperfect, especially the
hinder one which was often merely the intermediate cuspule, and did not
fuse with the external wall. The lower molars had the two crescents, one
behind the other, which recur in the †macrauchenids, all the suborders
of the †Toxodontia, except the †Pyrotheria, and other South American
ungulates, but the pillar in the posterior crescent, which was so
characteristic of the groups named, was reduced to very small proportions
and sometimes suppressed altogether. It should be noted, however, that
this was the loss of an element which was formerly present.
The skull had a long cranium and rather short face, with long, high
sagittal crest. The neck was short, the odontoid process of the axis
peg-shaped, and the canal for the vertebral artery was in its normal
position. The body was rather short, like that of a deer or antelope; the
number of trunk-vertebræ is not definitely known in any of the genera,
but was very probably 19 or 20, and the tail must have been short.
[Illustration: FIG. 248.—Skull of _†Diadiaphorus_, Santa Cruz. American
Museum.]
The limbs were slender and of moderate length; there was no
coössification between the bones of the fore-arm or the lower leg. The
feet were three-toed, except in one genus (_†Thoatherium_) in which they
were single-toed, and nearly or quite the whole weight was carried upon
the median digit, the laterals being mere dew-claws. The shape of the
hoofs and the whole appearance of the foot were surprisingly like those
of the three-toed horses, but there were certain structural differences
of such great importance as, in my judgment, to forbid the reference of
these animals, not merely to the horses, but even to the perissodactyls.
In studying the †Litopterna, one is continually surprised to note the
persistence of archaic and primitive characters in association with a
high degree of specialization.
[Illustration: FIG. 249.—Three-toed †proterothere (_†Diadiaphorus
majusculus_), Santa Cruz. Restored by C. Knight, from skeletons in the
American Museum and Princeton University.]
The largest Santa Cruz representatives of the family were the species
of _†Diadiaphorus_, animals considerably taller than a sheep and of
heavier build. Their appearance was not unlike that of a short-necked,
hornless antelope, but with the feet of the three-toed horses! These feet
were, however, merely superficially like those of the horses, differing
in points of fundamental significance. In the horses, the reduction of
the digits was accompanied by a readjustment of the carpal and tarsal
articulations, so that, in proportion as the median toe was enlarged
and the laterals reduced, the weight was shifted more entirely upon
the former. This is the method of digital reduction which Kowalevsky
called “adaptive” and is exemplified in all existing artiodactyls and
perissodactyls and by none more perfectly than by the monodactyl horses.
In “inadaptive reduction,” the method followed by _†Diadiaphorus_ and
the other genera of this family, there was no readjustment, or a very
imperfect one, of the articulations, the lateral digits, however small
and rudimentary, retaining the connections which they had when they were
of full size and function. This distinction may seem to be unimportant,
but its significance is shown by the fact that not a single ungulate with
inadaptively reduced feet has survived to the present time.
[Illustration: FIG. 250.—Left pes of _†Diadiaphorus_, from specimens in
Princeton University and the American Museum. _Cal._, calcaneum. _As._,
astragalus. _N._, navicular. _Cn. 3_, external cuneiform. _Cb._, cuboid.]
In still another respect the feet of _†Diadiaphorus_ deviated markedly
from those of the horses, viz. in the great proportionate length of
the phalanges, especially of the first one, and the shortness of the
metapodials, the three phalanges of the median digit together exceeding
in length the metacarpal or metatarsal, while in the horses this
proportion is reversed. The skull of this genus was short, deep and with
an anterior taper; it had a long sagittal crest, but a brain-chamber of
good capacity, considering its geological date. The nasals were quite
short, though the degree of shortening was not such as to suggest the
existence of a proboscis. In general appearance the skull recalls that of
one of the larger †oreodonts (p. 372) of the North American Oligocene.
To the genus _†Proterotherium_, the type of the family, belonged a
great number of Santa Cruz species, for at that time the genus was in
a state of most vigorous development and the species were so variable
that satisfactory discrimination of them is exceedingly difficult.
They were all much smaller and slighter animals than the species of
_†Diadiaphorus_, but did not differ from them in any important structural
character. The skull in this genus closely resembled that of the one
last named, save for its smaller size and lighter and more slender
proportions; the nasal bones were considerably longer and the occiput was
somewhat wider.
[Illustration: FIG. 251.—Skull of _†Thoatherium_, Santa Cruz. Princeton
University Museum.]
A more isolated position was held by the genus _†Thoatherium_, which
was very clearly demarcated from all of the other genera of the family.
Its species were the smallest of the commoner Santa Cruz members of the
order and were of very light and graceful form. The dental formula was
the same as in the other genera, but there were no tusks; the single
upper and two lower incisors were of nearly the same size and simple,
chisel-like form. The upper molars had the same elements as in the
preceding genera, but somewhat differently connected, the two internal
cusps and the anterior intermediate cuspule being united into a nearly
longitudinal ridge. The skull was light, slender and pointed; the nasals
were shortened, though less than in _†Diadiaphorus_; the sagittal crest
was shorter than in the latter and the occiput was far narrower. The neck
was short, the body of moderate length and the tail short. The limbs
and especially the feet were proportionately more elongate and slender
than in any other known genus of the family, giving quite a stilted
appearance to the skeleton. The fore-arm bones were not coössified, but
the ulna was much more reduced than in any of the other genera of the
family, and the same is true of the fibula, which, though very slender,
showed no tendency to unite with the tibia. The limb-bones, especially
the femur, had a decided resemblance to those of _†Mesohippus_, the lower
Oligocene tridactyl horse of North America, with the smaller species of
which, _†M. bairdi_, _†Thoatherium_ agreed well in size. Most remarkable
of all were the feet, _which were more strictly monodactyl than those
of any other known mammal_. The single functional digit, the third, had
on each side of its upper end a very small, scale-like nodule of bone,
the last vestiges of the lateral digits, corresponding to the immensely
larger splints of the horse. Despite the unrivalled completeness of
digital reduction which _†Thoatherium_ displayed, the mode of reduction
was inadaptive and the rudimentary metapodials retained the same carpal
and tarsal connections that they originally had in the pentadactyl manus,
a very great difference from the horses. The ankle-joint also was of
the same primitive character as in the other †Litopterna. The feet were
relatively longer and more slender than in the other †proterotheres and
the metapodial of the single functional digit longer in proportion to the
phalanges.
[Illustration: FIG. 252.—Single-toed †proterothere (_†Thoatherium
minusculum_), Santa Cruz. Restored by C. Knight from a skeleton in the
museum of Princeton University.]
The appearance of the living animal, aside from the character of the
hair, colour-pattern, etc., may be closely inferred from the skeleton.
It was a much smaller and more graceful animal than its contemporary and
relative _†Diadiaphorus_, as light and agile as a gazelle. The head had
some resemblance to that of a small horse, but the neck was much shorter
than in the horses; the body also was shorter than in the latter, and
the proportions of the trunk and limbs were quite as in the smaller
antelopes. But these likenesses to horses and antelopes were, it must
again be emphasized, superficial; the fundamental characteristics of
structure were more primitive than in the most ancient known artiodactyls
and perissodactyls.
[Illustration: FIG. 253.—Left pes of _†Thoatherium_. Princeton University
Museum. Letters as in Fig. 250.]
With the aid of the fragmentary material which alone represents the
†proterotheres in the formations preceding and following the Santa
Cruz in time, it is not practicable to trace the development of the
various phyla in a satisfactory manner. Two of the Santa Cruz genera,
_†Diadiaphorus_ and _†Proterotherium_, continued into the lower Pliocene
(Paraná), and two additional ones have been named, but little is known
about them. The latest known member of the family so far discovered is
a genus (_†Epitherium_) from the upper Pliocene of Monte Hermoso, a
tridactyl form like _†Diadiaphorus_. It is a noteworthy fact that the
most advanced and specialized genus of the entire family ended with
the Santa Cruz, while the less differentiated types survived till a
considerably later period. Possibly, it was the incoming of the highly
efficient Carnivora from North America that led to the extermination of
the last †proterotheres.
Turning backward from the Santa Cruz, the family may be traced without
any question to the Deseado stage of the Oligocene, though nothing but
teeth has yet been obtained, while in the Eocene it would appear to have
become merged in the same group of small, †Condylarthra-like animals with
quadritubercular molars, as those which are regarded as the probable
ancestors of the †macrauchenids. However likely this conclusion may seem
to be, its confirmation must await the discovery of much more complete
specimens than are now available.
ORDER †ASTRAPOTHERIA. †ASTRAPOTHERES
In the Santa Cruz another group of peculiar South American ungulates,
the †Astrapotheria, made its last recorded appearance. Though not at all
uncommon in that formation, no complete or even partial skeleton has
yet been found, but merely the skull and a few bones of the limbs and
feet. For this reason there is much doubt as to the systematic position
and relationships of these animals, which were among the most curious
of the many strange mammals which made up the Santa Cruz fauna. They
were mentioned in connection with the †Amblypoda (p. 456) as possible
representatives of that order in South America, but, as will be seen
later, this is an improbable conclusion, and the group appears to have
been indigenous in the southern continent, in which, at all events, it
had a very long history. It has not been found in any formation later
than the Santa Cruz, unless the Friasian fauna, which contains it, should
be removed from that stage, of which it apparently forms the latest
division.
I. †ASTRAPOTHERIIDÆ.
_†Astrapotherium_, Santa Cruz and Patagonian.
_†Astrapothericulus_, Patagonian.
_†Parastrapotherium_, Deseado. _†Astraponotus_,
Astraponotus Beds. _†Albertogaudrya_, Casa Mayor.
II. †TRIGONOSTYLOPIDÆ.
_†Trigonostylops_, Casa Mayor. _†Edvardocopeia_,
Astraponotus Beds.
The genus _†Astrapotherium_, which was the only well-defined
representative of its family and order in the Santa Cruz stage, contained
several species, some of them the largest animals of their time, as
well as the most grotesque in appearance. The dentition differed in
some important respects from that of all the other South American
ungulates, the formula being: _i_ 0/3, _c_ 1/1, _p_ 2/1, _m_ 3/3, × 2 =
28. The upper incisors had completely disappeared, but the lower ones
were large and, what was an exceptional character, they were partially
divided into two lobes, somewhat as in the Eocene †uintatheres of North
America (p. 446). The canines were very large and formidable tusks, which
grew throughout life and apparently formed no root; the upper tusk was
nearly straight and was obliquely truncated by the strongly curved and
sharp-pointed lower tusk. This arrangement was very unusual among South
American hoofed mammals, many of which had no tusks at all; and in those
which possessed them, such as the †toxodonts (p. 468), they were mostly
incisors. Only in the †astrapotheres and †homalodotheres were there
canine tusks, and in the latter group they were small and of limited
growth. All the teeth, except the canines, were brachyodont and, though
rather high-crowned, formed roots before coming into use. The premolars
were small and greatly reduced in number (2/1), and in pattern were
simpler than the molars. The upper molars were constructed on essentially
the same plan as in the †Toxodonta; indeed, the first specimen of this
genus collected was referred to a large species of _†Nesodon_ by Owen.
On the other hand, the resemblance to the rhinoceros teeth is very
decided, and has led several writers to postulate a relationship between
the †astrapotheres and the rhinoceroses. The lower molars were of the
bicrescentic pattern so frequently met with already; these teeth were
very narrow in proportion to their length and strongly suggest those of
_†Metamynodon_, the supposedly aquatic rhinoceros of the North American
Oligocene (p. 346). It may be confidently inferred that so small a
number of premolars was due to reduction from a full series, and this is
confirmed by the milk-dentition, in which the premolars were 4/3.
[Illustration: FIG. 254.—Head of _†Astrapotherium magnum_. Santa Cruz.
Restored from a skull in the museum of Princeton University.]
The skull was extremely peculiar, more so than in any other of the
contemporary genera of hoofed animals. The toothless premaxillaries were
quite small, but thick, and must have supported an elastic pad, against
which the lower incisors could effectively bite in cropping herbage. The
nasal bones were extremely short and there must have been a proboscis or
greatly inflated snout, probably the former; the immense development of
sinuses in the frontal bones elevated the whole forehead into a great,
dome-like convexity, a feature which is not equalled in any other known
mammal. The orbits were open behind and the brain chamber was small,
so that the sagittal and occipital crests were very high and strong,
to afford sufficient surface for the attachment of the great temporal
muscles. The horizontal portion of the lower jaw was shallow vertically,
but very thick and massive, and the symphyseal region was broad and
depressed.
Unfortunately, the skeleton is still very incompletely known. Of the
vertebræ, only the atlas and axis have been recovered, and these
resembled those of the Santa Cruz †toxodont _†Nesodon_, on a larger
scale. The scapula had a very thick spine, without the projections which
were found in most of the Santa Cruz ungulates. The limb-bones were long
and comparatively slender, and the processes for muscular attachment
were singularly small and weak; the bones of the fore-arm and lower leg
did not coössify and were proportionately elongate, the tibia being but
little shorter than the femur. The latter had the flattened shaft which
recurs in nearly all of the very heavy ungulates, but retained a remnant
of the third trochanter. If the feet found isolated in the Santa Cruz and
Deseado stages have been correctly referred to this order, then the genus
was five-toed and the feet were broad, short and heavy, quite elephantine
in appearance, especially the fore foot. The ankle-joint was very
peculiar and the calcaneum had no articulation with the fibula, which it
had in all the other indigenous South American ungulates.
Incomplete as the material is, it is yet possible to form some general
conception of this extraordinary animal when in life. The head was short,
broad and deep, rounded and very probably furnished with a proboscis; the
neck was of moderate length, so that the mouth could not reach the ground
without a straddling of the fore legs. The body was no doubt long, the
limbs long and rather slender, giving the animal a stilted appearance,
the feet very short, broad and columnar. Several species of the genus
are known, which differed much in size, the largest (_†A. giganteum_)
probably exceeding any modern rhinoceros in height and length, and the
smallest (_†A. nanum_) not much larger than a Wild Boar.
_†Astrapothericulus_, of the Patagonian stage, was smaller than the
average species of the Santa Cruz genus, and had teeth of the same
number, but the canines were not capable of indefinite growth, and the
lower molars had the pillar in the posterior crescent so characteristic
of the South American hoofed animals. In the Deseado stage, on the
contrary, the †astrapotheres were of larger size, and in the commonest
genus, _†Parastrapotherium_, the grinding teeth had lower crowns and the
premolars were more numerous, at least 3/2. In the still more ancient
_†Astraponotus_, which gives its name to the upper Eocene (or lower
Oligocene) of Patagonia, the premolars were present in full series.
In the Casa Mayor the order was abundantly represented by still more
primitive genera, which assuredly had an undiminished number of teeth,
though this has not been proved. One of these genera, _†Albertogaudrya_,
was the largest animal of its time and the highly probable ancestor of
the series leading to the Santa Cruz _†Astrapotherium_.
The second family of the order, the †Trigonostylopidæ, did not survive
beyond the Eocene and is so imperfectly known that any account of it
would be to small profit.
As stated above, the †Astrapotheria were an isolated group and their
relationships are problematical and are likely to remain so pending the
discovery of much more complete specimens of the various genera which
made up the series. I am inclined to the opinion, however, that all of
the indigenous groups of South American ungulates, which inhabited that
continent before the great immigration from the north, were derivatives
of the same stock and more nearly related to one another than to any of
the orders which lived in other regions.
* * * * *
In looking over the labyrinth of ungulate history, as recorded by the
fossils, certain facts stand out clearly, while others are still very
obscure. It is like trying to trace the plan of vast and complicated
ruins, which here are deeply buried in their own débris, there are fully
exposed and in another place are swept away so completely that hardly a
trace remains. But the problem is far more complex than any which can
be presented by buildings, for the factor of repeated migrations from
continent to continent comes in to obscure the evidence. Had each of the
great land areas received its original stock of early mammals and then
been shut off from communication with any other, many of the difficulties
would be removed, but the story would lose half its interest.
Within the limits of the family, giving to that group the broad and
elastic definition which has hitherto been employed, we have repeatedly
found it feasible to construct a phylogenetic series which very nearly
represents the steps of structural modification as they occurred in
time. Much less frequently is it possible to trace allied families to
their common starting point, and, so far as the hoofed animals are
concerned, in no case have we yet succeeded in doing this for the
separate orders. The obstacle lies in the fact that the ordinal groups
were already distinct, when they made their first appearance in the
known and accessible records, and the hypothetical ancestors common to
them all, or to any two of them, are to be sought in regions of which we
know little or nothing. Nevertheless, certain legitimate inferences may
be drawn from the available evidence. It remains to be proved whether
the assemblage of hoofed mammals, as a whole, was of single or multiple
origin. Have all ungulates been derived from a common stock, or did they
arise independently from several groups of clawed mammals? While the
records cannot be followed back to the point, or points, of origin of
the various orders, yet it is a noteworthy fact that, between several of
them, the differences grow less marked as the more ancient members are
reached, as though they were converging to a common term; others again
show little such approximation, and the most probable conclusion from
the evidence now at hand is that the ungulate assemblage is composed of
several independent series.
One such series is that of the Hyracoidea and Proboscidea, to which
Dr. Schlosser has given the name “Subungulata,” and has pointed out
its relationship to the †Condylarthra, which, however, is not a close
one and may be illusory. Another apparently natural group is that of
the peculiarly South American forms, the †Toxodontia, with its four
suborders, the †Litopterna and the †Astrapotheria, which all appear
to be traceable to closely allied families in the Eocene, whose teeth
strongly suggest derivation from the †Condylarthra; but the material does
not permit any positive statements. The Artiodactyla and Perissodactyla
have so many similarities that they have always been regarded as closely
related groups, but the distinction between them was almost as sharply
drawn in their most ancient known members as it is to-day, and there was
no distinct tendency to converge back into a common stem. Their mutual
relationships are thus obscure, but the Perissodactyla, at least, seem to
be derivable from a †condylarthrous ancestry.
The †Condylarthra, as a whole, were by far the most primitive of the
ungulates, which they connected with the clawed mammals. None of the
genera yet discovered can be regarded as ancestral to any of the higher
orders, but it is entirely possible that in the upper Cretaceous period
the †Condylarthra were spread over all the continents, except Australia,
and that from them the other ungulate orders arose in different regions.
At all events, the †Condylarthra show how the transition from clawed to
hoofed types may have occurred and perhaps actually did so, but it would
be premature to affirm this.
CHAPTER XIV
HISTORY OF THE CARNIVORA
The story of the hoofed mammals, as sketched in brief outline in the
preceding chapters (VIII-XIII), is a curious mixture of relatively full
and satisfactory paragraphs, with scanty, broken and unintelligible ones,
not to mention those which have not yet been brought to light at all.
With all its gaps and defects, which inhere in the nature of things, the
history of the various ungulate series is the best that the palæontology
of mammals has to offer and constitutes a very strong and solid argument
for the theory of evolution. For the Carnivora the story is less complete
and for obvious reasons. Individual abundance was a very large factor
in determining the chances of preservation in the fossil state for any
given species, and, as a rule, whole skeletons are found only when the
species was fossilized in large numbers. In any region the Carnivora are
less numerous than the herbivora upon which they prey, and while most
ungulates live in larger or smaller herds, the carnivores are mostly
solitary.
The Carnivora are divisible into three well-marked suborders, called
respectively the Pinnipedia, Fissipedia and †Creodonta. The Pinnipedia,
seals, walruses, etc., which are almost purely marine in habitat, are not
dealt with in this book, since so little can be learned of them from the
fossils, and the †Creodonta, an extremely ancient and primitive group,
will be treated separately. The Fissipedia are chiefly terrestrial,
though they include the otters, and their subdivisions, so far as the
American forms are concerned, are shown in the following table, which,
it should be observed, omits several genera. Unless otherwise noted, the
genera are North American.
Suborder FISSIPEDIA. Land Carnivora
I. CANIDÆ, Dogs, Wolves, Foxes, etc.
_Canis_, Wolves, Pleist. and Rec. _Vulpes_, Red
Fox, do. _Urocyon_, Grey Fox, do. _Cerdocyon_,
fox-like wolves, S. A., do. _Icticyon_, Bush-Dog,
S. A., do. _?Cyon_, Dhole, mid. and up. Mioc.
_†Dinocynops_, S. A., Pleist. _†Ælurodon_, up.
Mioc. and low. Plioc. _†Tephrocyon_, mid. Mioc. to
low. Plioc. _†Borophagus_, up. Mioc. to mid. Plioc.
_†Ischyrocyon_, up. Mioc. _†Amphicyon_, mid. Mioc. to
low. Plioc. _†Daphœnodon_, low. Mioc. _†Enhydrocyon_,
up. Oligo. _†Temnocyon_, up. Oligo. _†Mesocyon_, up.
Oligo. _†Cynodesmus_, low. Mioc. _†Daphœnus_, Oligo.
_†Cynodictis_, Oligo. _†Procynodictis_, up. Eoc.
II. PROCYONIDÆ, Raccoons, etc.
_Procyon_, Raccoons, N. and S. A., Pleist. and
Rec. _Nasua_, Coatis, S. A., Pleist. and Rec., now
extending to Calif. _†Cyonasua_, S. A., up. Plioc.
_Bassariscus_, Cacomistle, low. Plioc. to Rec.
_†Phlaocyon_, low. Mioc. _†Leptarctus_, up. Mioc.
_Potos_, Kinkajou, Neotropical, Recent.
III. URSIDÆ, Bears.
_Ursus_, true Bears, Pleist. and Rec. _Tremarctos_,
Spectacled Bear, S. A. _†Arctotherium_, †Short-faced
Bears, N. and S. A., Pleist.
IV. MUSTELIDÆ, Martens, Weasels, etc.
_Mustela_, Weasels, mid. Mioc. to Rec. _Grison_,
Grisón, S. A., Pleist. to Rec. _Tayra_, Tayra,
do. _Martes_, Martens, up. Mioc. to Rec. _Gulo_,
Wolverene, Pleist. and Rec. _†Canimartes_, mid.
Plioc. _†Brachypsalis_, up. Mioc. _†Megalictis_, low.
Mioc. _†Ælurocyon_, do. _†Oligobunis_, up. Oligo.
and low. Mioc. _†Bunælurus_, low. Oligo. _Mephitis_,
Skunk, Pleist. and Rec. _Spilogale_, Spotted Skunk,
do. _Conepatus_, S. A. Skunk, Pleist. and Rec., N.
A., Rec. _Taxidea_, Badger, Pleist. and Rec. _Lutra_,
Otters, up. Mioc. to Rec., S. A., Pleist. and Rec.
_Latax_, Sea-Otter.
V. FELIDÆ. Cats.
_Felis_, true Cats, N. A., low. Plioc. to Rec.,
S. A., Pleist. and Rec. _Lynx_, Lynx, Pleist.
and Rec. _†Pseudælurus_, mid. and up. Mioc.
_†Smilodon_, Sabre-tooth Tiger, N. and S. A., Pleist.
_?†Machairodus_, mid. Mioc. to Plioc. _†Nimravus_,
up. Oligo. _†Archælurus_, do. _†Hoplophoneus_, Oligo.
_†Dinictis_, do. _†Eusmilus_, low. Oligo.
Two families, the hyenas (Hyænidæ) and civet-cats (Viverridæ), are
omitted from the table because they apparently never reached the western
hemisphere. The bears, of Old World origin, invaded America at a very
late period and are not certainly known here before the Pleistocene. The
other four families were well represented in North American history,
though the great weasel tribe (Mustelidæ) went through the greater part
of its history in the Old World. None of the families is indigenous
in South America, and all of the five families which it now shares
with North America came in in the series of immigrations, of which the
first recorded effects are found in the Pliocene and continued into the
Pleistocene.
The Fissipedia are adapted to a great variety of habits and modes of life
and consequently there is considerable diversity of structure among them,
though they all form a homogeneous, natural group. The dogs (Canidæ)
are terrestrial, neither swimmers nor climbers; some, like the foxes,
are solitary, others, like the wolves, hunt in packs and nearly all are
strong, swift runners. The cats (Felidæ) which have a remarkable range of
size, are terrestrial or arboreal; they take their prey by stalking and
leaping upon it, not by running it down. The bears (Ursidæ) are mostly
omnivorous, not very often killing prey, and largely vegetarian in diet.
The raccoons (Procyonidæ) are chiefly arboreal and omnivorous. The very
large and varied weasel family (Mustelidæ) have different habits, though
nearly all are fierce and bloodthirsty. Otters and sea-otters are aquatic
and prey chiefly on fish; minks and fishers are semi-aquatic; martens are
arboreal, skunks terrestrial and badgers fossorial.
While there is thus much diversity of habit with corresponding
differences of structure among the Fissipedia, there is a certain
unity of plan recognizable among them all. With but few exceptions,
the incisors are present in full number and the canines are formidable
lacerating weapons. Especially characteristic of the dentition are the
“sectorial” or “carnassial” teeth, always the fourth upper premolar and
first lower molar, which form a pair of shearing blades, the premolar
biting outside. In the bears and most of the raccoons the teeth are
tuberculated, in adaptation to the omnivorous habit, and the carnassials
have lost the shearing form, though clearly derived from that type. The
skull has powerful jaws, and the crests and ridges for the attachment
of the jaw muscles are prominent except in very small animals, and the
stout, boldly outcurving zygomatic arches are very characteristic. The
face may be elongate, as in the dogs, or extremely short, as in the cats,
or of intermediate length; the brain-case is relatively capacious, and
the orbits, except in the cats, are widely open behind. The neck is never
very long, but the body often is, and the tail varies greatly in length,
as do also the limbs. There is great difference, too, between the various
families in the prominence of the processes on the limb-bones for the
attachment of muscles, as expressive of the muscular development of the
limbs, and also in the extent to which the fore foot can be rotated and
used for grasping. In all existing Fissipedia the femur has no third
trochanter, but many extinct genera possessed it. The bones of the
fore-arm and lower leg are always separate and uninterrupted.
In the wrist (_carpus_) there is always a large bone, the _scapho-lunar_,
which is made up by the coalescence of three elements, the scaphoid,
lunar and central, a feature which, though recurring in a few other
mammals, is essentially characteristic of the modern Carnivora. The feet
are armed with claws more or less sharp, which in some families, notably
the cats, are retractile and may be folded back into the foot. The gait
may be plantigrade, as in the raccoons and bears, or digitigrade, as in
the dogs and cats, or intermediate in character.
Throughout the Paleocene and most of the Eocene, there were no
Fissipedia, the flesh-eaters all belonging to the extinct †Creodonta, and
the first clearly recognizable fissipedes occurred in the upper Eocene or
Uinta.
1. _Canidæ. Dogs, Wolves, Foxes, etc._
This family, which may with convenience be called simply dogs, is at
present the most widely distributed of the families of Fissipedia,
occurring in every continent, even Australia, and ranging through all
climates almost from pole to pole. They are a singularly homogeneous
family and show few differences of structure; such differences as
there are affect chiefly the number and size of the teeth and external
characters, such as the size of the ears, length and colouring of the
hair, etc. The many domestic breeds are not here considered. Almost alone
among the Fissipedia the dogs capture their prey by running it down,
and they are endowed with remarkable speed and endurance. The entire
organism, especially the limbs and feet, are adapted to cursorial habits.
For the purpose of comparison with the extinct genera of the family, some
account of a wolf will suffice. The wolves, like most other members of
the family, have a larger number of teeth than is usual in the suborder,
as appears from the formula: _i_ 3/3, _c_ 1/1, _p_ 4/4, _m_ 3/3, × 2 =
42, that is to say, only the third upper molar has been lost from the
typical number, though the third lower is very small and seemingly on the
point of disappearance (Fig. 44, p. 93). The upper sectorial tooth, the
fourth premolar, has its shearing blade made up of two sharp-edged cusps,
one behind the other, and there is a small internal cusp carried on a
separate root; the upper molars are triangular and tritubercular and are
used for crushing. The lower sectorial, the first molar, has an anterior
blade of two shearing cusps, with the remnant of a third, and a low,
basin-like posterior “heel.”
The skull is characterized by the long face and jaws and by the structure
of the auditory region; the tympanic bones are inflated into large
oval bullæ, which are hollow and undivided, and the external opening
of each is an irregular hole, without tubular prolongation. There is
an alisphenoid canal for the passage of the internal carotid artery.
The neck, body and tail are of moderate length and the vertebræ of the
loins are not conspicuously large and heavy. There is no collar-bone.
The limb-bones have a distinct, though superficial, resemblance to those
of hoofed animals; the humerus has no very prominent ridges for the
attachment of muscles and no epicondylar foramen, and the femur no third
trochanter. The fore-arm bones are separate, but are so articulated
together and with the humerus as to give the fore foot no power of
rotation. The manus in all existing wild species has five digits, though
the pollex or first digit is very small, a mere dew-claw; the four
functional digits are arranged in two symmetrical pairs, very much as in
the artiodactyls, a longer median pair, of which the metacarpals have
a nearly square cross-section, and a shorter lateral pair (2d and 5th)
of more trihedral form. All the metacarpals are closely appressed and
almost parallel. The pes has four digits arranged in similar fashion.
The claws are blunt and non-retractile, and are of little use in seizing
or lacerating prey, but are useful in digging. The ungual phalanges have
no bony hoods reflected over the base of the claw. All modern forms are
digitigrade.
Materials are lacking for the construction of any such detailed
phylogeny of the dogs as has been accomplished for many ungulates.
Many of the extinct genera are known only from skulls, or even jaws,
and the well-preserved skulls are too few to form distinctly defined
and continuous series. On the other hand, there is every reason to
believe that the canine genera of the successive geological stages did
approximately represent the successive steps of development within the
family, though it is difficult to distinguish between the phyla.
The Pleistocene dogs, for the most part, differed little from the
Recent ones; there were some very large species like the _Canis †dirus_
(Frontispiece) of the Mississippi Valley and the Pacific Coast. Two very
peculiar genera have been reported. One (_†Pachycyon_), from a cave in
Virginia, had remarkably short, stout and strongly curved limb-bones,
which suggest otter-like habits; the other (_†Hyænognathus_), from
California, had a very short face and extremely massive lower jaw and
very heavy teeth; it was probably like a hyena in appearance.
[Illustration: FIG. 255.—Skull of _†Cynodesmus thoöides_, a lower Miocene
wolf. Princeton University Museum. Compare with Fig. 7, p. 62.]
As far back as the Blanco stage of the middle Pliocene, remains occur
which are assigned to the modern genus _Canis_, though better preserved
specimens would probably require their removal from that genus. In
the lower Pliocene the phylum of the true wolves was represented by
_†Tephrocyon_, which, so far as it is known, differed only in minor
details from _Canis_, and _†Tephrocyon_ went back to the middle Miocene.
What would appear to be its direct ancestor is _†Cynodesmus_, of the
lower Miocene, which, in view of the long lapse of time involved,
differed less from the modern wolves than one would have supposed, but
the differences are significant, as pointing back to a far more primitive
type of structure. _†Cynodesmus_ was a small animal, intermediate in size
between a Red Fox and a Coyote. The dental formula was the same as in
_Canis_, but the teeth were relatively smaller and more closely crowded,
as the face and jaws were shorter and the cranium, though longer, had
a less capacious brain-chamber. The cast of this chamber, which very
perfectly reproduces the form of the brain, shows that the latter was
not only smaller but less convoluted than in the modern animals, and
this, in turn, denotes a lower grade of intelligence. The limb-bones were
like those of wolves, but the feet were quite different. In the manus
the first digit, or pollex, was much less reduced, though considerably
shorter than the other digits, which were not in two symmetrical pairs,
but were all of different lengths, not closely appressed, but arranged
in radiating fashion; the metacarpals had not yet acquired the quadrate
or trihedral form, but were more oval in cross-section. The pes was
more modernized, but had five digits, which is not true of any existing
member of the family. The claws were thin and sharp and were slightly
retractile, a power which has been completely lost in all the modern
canids. Such an animal could hardly have been preëminently cursorial.
[Illustration: FIG. 256.—Skull of primitive “bear-dog” (_†Daphœnus
felinus_). White River stage. (After Hatcher.)]
Out of the crowd of dog-like creatures in the John Day Oligocene, it is
not yet practicable to select one which is to be taken as the ancestor
of the Recent wolves through _†Cynodesmus_, nor can this be done with
better assurance of success in the White River, though the beginning
(_†Daphœnus_) of the †bear-dogs in that formation probably closely
represents the ancestral stage sought for. It is likely that several of
the phyla into which the family was divided became blended in a common
stock at that stage.
[Illustration: FIG. 257.—Upper teeth of _†Daphœnus felinus_. _p. 4_ =
fourth premolar. (After Hatcher.)]
[Illustration: FIG. 258.—Right manus of _†Daphœnus felinus_. _Sl._,
scapho-lunar. _Py._, pyramidal. _Ps._, pisiform. _U._, unciform. (After
Hatcher.) Compare with Fig. 32, p. 82.]
A second phylum, now entirely extinct, is that of the †bear-dogs, which
is not certainly recorded later than the middle Pliocene, though some
have been doubtfully reported from the older Pleistocene of the Great
Plains and the remarkable Californian genus, _†Hyænognathus_, may have
been an offshoot of the same stock. The phylum was characterized by the
unusually large size of the molars and by certain other features, which,
however, are not known to have persisted through the entire series from
first to last. In the middle Pliocene lived some very large bear-dogs,
of the genus _†Borophagus_, the teeth of which had a strong likeness
to those of the hyenas and probably the animals had hyena-like habits,
feeding largely upon carrion and crushing the stoutest bones with their
massive teeth. The same, or a very similar, genus lived in the lower
Pliocene, but none of the species of that date is at all well known. In
the upper Miocene occurred several species which have been referred to
the European genera, _†Amphicyon_ and _†Dinocyon_. The latter was an
enormous canid, equalling in size the largest of living bears, the great
Kadiak Bear of Alaska, and, though probably having a long and heavy tail,
was much like a bear in appearance. The teeth indicate a more exclusively
carnivorous habit than that of the bears and these may well have been
savage and terrible beasts of prey.
[Illustration: FIG. 259.—Lower Miocene “†bear-dog” (_Daphœnodon
superbus_). Restored from a skeleton in the Carnegie Museum, Pittsburgh.]
_†Amphicyon_, which had three upper molars, continued down through the
middle Miocene, but was replaced in the lower by _†Daphœnodon_, which
may or may not have been its direct ancestor. The uncertainty as to
the exact relationship between the two genera will remain until more
complete material shall have been obtained from the middle Miocene.
_†Daphœnodon_ was the largest dog of its time, the contemporary wolves
(_†Cynodesmus_) having been hardly half so large, but was much inferior
in size to the huge †bear-dogs of the middle and upper Miocene. The skull
resembled that of a large wolf, but the tympanic bullæ were smaller and
more loosely attached and the molar teeth were relatively much larger, a
persistent characteristic of this phylum. The very long and heavy tail
was a cat-like feature. The limbs were comparatively short and stout;
the humerus had the epicondylar foramen and the femur retained a trace
of the third trochanter, both of which are lost in the modern members of
the family. The feet were not at all canine in type, but rather resembled
those of the ancient and unspecialized flesh-eaters. There were five
digits in manus and pes and were not arranged in parallel pairs, but
diverging; the metapodials were of oval cross-section, not squared, and
their lower ends, which articulated with the first row of phalanges, had
hemispherical surfaces, not semicylindrical. The claws were sharp and a
remnant of former retractility was to be observed. Such an animal could
hardly have been a strong and enduring runner and its structure suggests
that it captured its prey by stalking and leaping upon it. The wolf-like
head, with cat-like body, tail and limbs, made a strange combination, not
closely paralleled by any existing carnivore.
Through the Oligocene the phylum was carried back by the several species
of _†Daphœnus_, assuredly the ancestor of _†Daphœnodon_ and decidedly
more primitive in many respects. The Oligocene genus was a much smaller
animal than its lower Miocene successor, the larger species hardly
equalling a Coyote; the teeth were smaller and more closely set, but
the molars were proportionately large, while the carnassials were less
finished and effective shearing blades. The skull was less distinctively
dog-like and had a smaller brain-case, with very prominent sagittal and
occipital crests, a longer cranium and shorter face; the tympanic bones
were very small and so loosely attached to the skull that they are rarely
found, a very striking difference from all existing dogs. The backbone
was remarkable for the unusually large size of the lumbar vertebræ, a
point of resemblance to the cats and suggesting that _†Daphœnus_ had
great powers of leaping; there was a long, heavy, leopard-like tail,
and the caudal vertebræ were very like those of the long-tailed cats.
The limbs and feet were similar in character and proportions to those
of _†Daphœnodon_, but the astragalus was less grooved for the tibia,
the claws were rather more retractile and the gait was probably more
plantigrade. There were so many cat-like features in the skeleton of
_†Daphœnus_, that the observer cannot but suspect that these resemblances
indicate a community of origin, but, until the Eocene ancestors of the
cats are found, the question of relationship must remain an open one.
The most ancient member of the bear-dog phylum yet discovered appears to
be one of the †creodont family of the †Miacidæ, found in the Uinta Eocene.
A short-lived branch of the canine stock was that of the so-called
“†hyena-dogs,” a peculiar American type, which abounded in the upper
Miocene and lower Pliocene and then became extinct. Traced backward,
this brief series of species would appear to have sprung from the true
wolves (_†Tephrocyon_) of the middle Miocene. The upper Miocene and
lower Pliocene genus _†Ælurodon_ had several species, which differed
considerably in size; the commoner of these were large wolves with
very modern type of body, tail, limbs and feet, but having short and
massive heads. The premolars were extremely thick and heavy, with such
a resemblance to those of the hyenas, that these animals have sometimes
been mistakenly regarded as ancestral to that family. The especial
characteristic, however, of the series was in the form of the upper
sectorial tooth, which was much more feline than canine in construction
and has given occasion for the generic name which means “cat-tooth.”
A fourth phylum of the Canidæ, which would seem to be represented
in the modern world by the Indian Dhole, or Wild Dog (_Cyon_), and
perhaps by the Brazilian Bush-Dog (_Icticyon_), was characterized by
the lower sectorial molar, the heel of which was not basin-like, as in
the typical dogs, but trenchant and consisted of a single sharp-edged
cusp, the external one of the primitive basin. Although there is no
inherent improbability in the view that the Dhole and the Bush-Dog are
derivatives of this phylum, no positive statement can yet be made, for
the gap in the history is too great to be bridged with any assurance.
The fossil members of the series did not come down later than the middle
or upper Miocene and it is quite possible that the trenchant heel of the
carnassial was developed more than once. The middle and lower Miocene
members of the series are still very imperfectly known and it is only
from the upper Oligocene (John Day) that well-preserved skeletons have
been obtained. These pertain to an aberrant member of the phylum, the
genus _†Temnocyon_, in which not only does the sectorial have a trenchant
heel, but the second lower molar also was trenchant, having lost the two
inner cusps, while the upper molars were as large as in the †bear-dogs.
_†Temnocyon_ was a comparatively large animal and its skeleton had a
mixture of primitive and advanced characters, the latter predominating,
so that this genus was not only the largest but also the most specialized
canid of its time. There was the long, heavy tail, which all of the known
Oligocene carnivores possessed, but the limbs were long and the gait was,
it would seem, thoroughly digitigrade. While the epicondylar foramen was
retained by the humerus and the third trochanter by the femur, those
bones were otherwise very modern in form. The feet were five-toed, but
the functional metapodials were parallel, appressed and with something
of the quadrate shape. In very notable degree, therefore, the feet of
_†Temnocyon_ anticipated the characters which the true wolves acquired
considerably later. The less specialized _†Mesocyon_, which was smaller,
was the ancestor of the Miocene forms and was, in turn, very probably
derived from the White River _†Daphœnus_.
Still a fifth phylum, that of the †short-faced dogs (_†Enhydrocyon_),
is very imperfectly known and has, so far, been found only in the lower
Miocene and upper Oligocene. These also may have been descended from
_†Daphœnus_, but the connection is not clear, nor has the relationship
of the American genus to the extremely †short-faced dogs of the European
Pliocene been determined.
[Illustration: FIG. 260.—Small, fox-like dog (_†Cynodictis gregarius_)
of the White River. Restored from a skeleton in the American Museum of
Natural History.]
Finally, so far as North America is concerned, there was a phylum of very
small fox-like canids, which ranged from the lower Miocene to the upper
Eocene and were very abundant, relatively speaking, in the White River
and John Day. The dental formula was the same as in _Canis_ and the skull
was narrow and slender, though the brain-chamber was proportionately
capacious, and the face was quite short. The tympanic bullæ were large
and inflated. The body and tail were long and the limbs quite short and
weak. The humerus had no epicondylar foramen and the femur no third
trochanter. The five-toed feet had the spreading arrangement of the
metapodials seen in the more primitive fissipedes generally and the
claws were sharp. In proportions and appearance these animals must have
been more like civets or weasels than like dogs and it is evident that
they were not swift runners. The series had its earliest representatives
(_†Procynodictis_) in the Uinta and was doubtless derived from the
†creodont family †Miacidæ. The White River species are referred to the
European genus _†Cynodictis_, those of the John Day and lower Miocene to
_†Nothocyon_, and it has been suggested that this series gave rise to the
foxes, a suggestion which may prove to be true, but the very long gap in
time between these animals and the most ancient known foxes prevents any
conclusion.
To determine the mutual relationships of the six phyla of Canidæ which,
from the Eocene onward, inhabited North America in such numbers, is a
task of great difficulty and only a tentative solution of the problem can
be offered. The central stock would seem to be nearly represented by the
White River _†Daphœnus_, leading through _†Cynodesmus_ and _†Tephrocyon_,
of the Miocene, to the wolves. A short-lived series, apparently given
off from _†Tephrocyon_, was that of the †hyena-dogs, which flourished
greatly in the upper Miocene and lower Pliocene and then became extinct.
Another branch, that of the †bear-dogs, was derived from _†Daphœnus_,
through _†Daphœnodon_ to _†Amphicyon_, _†Dinocyon_ and _†Borophagus_, the
gigantic Miocene and Pliocene forms, ending perhaps in _†Hyœnognathus_ of
the California Pleistocene. A third branch, represented by _†Mesocyon_
and _†Temnocyon_, is believed to be continued to-day by the Asiatic Dhole
and the Brazilian Bush-Dog. The †short-faced dogs (_†Enhydrocyon_) are
still very obscure. The last phylum, that of _†Nothocyon_, _†Cynodictis_,
_†Procynodictis_, had become distinct in the upper Eocene and possibly
gave rise to the foxes, but this is highly conjectural.
2. _Felidæ. Cats_
The only other fissipede group whose development in North America may
be followed for a long period is that of the _†Sabre-Tooth Tigers_,
the subfamily _†Machairodontinæ_, which have been extinct since the
Pleistocene; the history of the True Cats (Felinæ) is much more obscure.
In most respects the two subfamilies agreed closely and, as they became
separate at least in the early Oligocene, they furnish instructive
parallel series. The †sabre-tooth cats were terrible beasts of prey,
which in most of the Tertiary period ranged over the whole northern
hemisphere and in the Pleistocene or late Pliocene extended throughout
South America.
[Illustration: FIG. 261.—Skull of the Pleistocene †sabre-tooth tiger
(_†Smilodon californicus_, after Matthew). _P. 4_, fourth upper premolar,
sectorial.]
The Pleistocene genus _†Smilodon_ (Frontispiece) belonged to nearly the
whole western hemisphere and its various species were distributed from
California and Pennsylvania on the north, to the Argentine Pampas on the
south. The most obvious and striking peculiarities of _†Smilodon_ were
in the teeth, which were much reduced in number, the formula being: _i_
3/3-2, _c_ 1/1, _p_ 2/2-1, _m_ 1/1. The upper canine was a great, curved,
scimitar-like blade, eight inches or more in length, with broad inner
and outer faces, but quite thin transversely, and with finely serrate
posterior edge. It is difficult to understand how these great tusks,
which would seem to have blocked the entrance to the mouth, could have
been effectively used, unless the creature could open its mouth much
more widely than any existing mammal, so as to clear the points of the
tusks, and would then strike with them as a snake does with its fangs.
There are great anatomical difficulties in the way of accepting this
explanation and the problem, which is the same as that presented by the
†uintatheres (p. 446), is still unsolved. It is, however, quite certain
that no arrangement which was disadvantageous, or even inefficient,
could have persisted for such vast periods of time. The lower canine
was much diminished and hardly larger than an incisor. The two upper
premolars were the third and fourth of the original series; the third
was small, but the fourth, the sectorial, was a very large and efficient
shearing blade. In addition to the two external trenchant cusps of the
blade, which are present in the Carnivora generally, the cats have a
third small, anterior cusp which in _†Smilodon_ was large; the internal
cusp had almost disappeared. The single upper molar was very small and
so overlapped by the great carnassial as to be invisible from the side.
The third lower premolar was small and unimportant and most specimens
had lost it, leaving only the fourth, which was larger and evidently of
functional value. The single molar was the sectorial, a large, thin,
flattened blade, consisting of only two cusps, one behind the other, the
trenchant edges of which met at nearly a right angle, and there was no
trace of a heel.
[Illustration: FIG. 262.—Upper teeth of _†Smilodon_, left side. _P. 4_,
fourth premolar. _m. 1_, first molar. (After Matthew.)]
The skull was in appearance closely similar to that of one of the
great modern cats, such as the Lion or Tiger; with extremely shortened
face, heavy and widely expanded zygomatic arches and very prominent
sagittal crest. The tympanic bullæ were large and inflated, each divided
by a septum into two chambers, but were not visible from the side,
being covered externally by very large processes, which served for
the attachment of some of the great muscles of the neck. The short,
rounded, bullet-head of the true cats was thus repeated, but there were
in the skull several interesting differences of detail, which it is not
worth while to enumerate here. Suffice it to say, that some of these
differences were due to the retention of primitive characters in the
skull of _†Smilodon_, which have been lost in the modern felines, and
others to special developments, in which the true cats did not share. The
lower jaw had on each side a small, descending flange for the protection
of the tusks, which, however, projected well below these flanges when the
jaws were shut. The neck was heavy and the structure of its vertebræ was
such as to suggest the presence of unusually powerful muscles; the back
and loins were also uncommonly stout, in the larger species heavier than
in the Lion or Tiger, but, in marked distinction from those modern forms,
the tail was short. The limbs were shorter and much heavier in relation
to the size of the body than in the great existing cats and must have
been extremely powerful. The humerus usually had no epicondylar foramen,
which all the true felines possess, though it was sometimes present.
The feet also were very stout and armed with large retractile claws;
the base of each claw was covered by a thin bony hood, an outgrowth of
the ungual phalanx, which is very characteristic of the entire family.
The hind foot had five digits, whereas no existing cat has more or less
than four. The appearance of these animals must have been very much like
that of the Lion or Tiger, aside from the unknown factors of mane and
colour-markings, but differed in the great tusks, the short tail and the
shorter and more massive legs and feet.
[Illustration: FIG. 263.—Skull of a †sabre-tooth tiger (_†Machairodus
palmidens_) from the Miocene of France. (After Filhol.) _P. 4_, fourth
upper premolar, sectorial tooth.]
On account of the very incomplete preservation of the material so far
collected, little is known of the †sabre-tooth series in North America
during the Pliocene and Miocene epochs. Remains of very large cats have
been found in the lower Pliocene and upper Miocene, but it is uncertain
whether they belong to the feline or the †machairodont subfamily. Some
of the species have been referred to the genus _†Machairodus_, which
ranged from the lower Pleistocene to the middle Miocene of Europe, and
the reference may be correct, but is uncertain. However, the European
representatives of that genus, which are much better known, will serve
to show the developmental stage from which _†Smilodon_ was undoubtedly
derived. The dental formula was the same as in the American genus,
though there were generally two premolars in the lower jaw and in
_†Smilodon_ generally but one; the individual teeth were formed on the
same plan as in the latter, but were relatively smaller, and the very
small, rudimentary upper molar was visible externally and was not
overlapped and concealed by the great carnassial; the sabre-like tusk
had not attained such great proportions. The skull of _†Machairodus_,
the only part of the skeleton which is definitely known, was like that
of _†Smilodon_ on a much smaller scale, but more primitive in several
respects. It was longer and had a less capacious brain-case and less
prominent sagittal and occipital crests. The large tympanic bullæ were
conspicuous in the side-view of the skull, as the processes for the
attachment of the neck-muscles had no such development as in _†Smilodon_.
The descending flanges of the lower jaw were larger than in the latter.
The upper Oligocene (John Day) contained a large variety of cat-like
forms, of which no less than five genera have been described; one of them
(_†Pogonodon_), nearly as large as a Lion, would seem to have died out
here without descendants, and two others, to which we shall return later,
so combined the characters of true felines and †machairodonts as to be of
uncertain reference. Two other genera, which are much commoner and better
known, from the White River, will be described from specimens of that
stage.
The White River, or lower Oligocene, had three highly interesting genera
of †machairodonts, two of them known from nearly or quite complete
skeletons. One of these (_†Hoplophoneus_), which was, it can hardly be
doubted, the direct ancestor of the later typical †machairodonts, had
several species, which are found in the various levels of the White
River beds. The largest of these species was considerably smaller than
_†Machairodus_, and the smallest and most ancient was inferior to the
modern Wild Cat. The number of teeth was variable, but normally greater
than in the genera above described, being _i_ 3/3, _c_ 1/1, _p_ 3-2/3-2,
_m_ 1/1, × 2 = 28-32. The foremost premolar in each jaw was very small
and often absent. The upper canine was a long and curved, but very
thin, scimitar, finely serrate on both edges, while the lower canine
was but little larger than the incisors. The carnassial teeth had a
significant likeness to those of other fissipede families; the upper
one, the fourth premolar, was relatively smaller than in _†Machairodus_
and its blade less effectively trenchant; the accessory antero-external
cusp was present, though extremely small, and the internal cusp, which
in _†Smilodon_ had almost disappeared, was quite large. The lower
sectorial, the first molar, though already cat-like and consisting of
two thin, broad and trenchant cusps in line, yet had vestiges of the
heel and sometimes of the inner cusp. These vestiges were a connecting
link between the highly specialized sectorial of the cats and the type
usual among the Fissipedia, which is exemplified by the dogs. The small
upper molar was less reduced than in the Miocene and Pliocene genera and
plainly consisted of a larger external and smaller internal cusp.
[Illustration: FIG. 264.—White River †sabre-tooth tiger (_†Hoplophoneus
primævus_). Restored from a skeleton in the American Museum. †Oreodonts
(_†Merycoidodon_) in the background.]
Compared with that of other Fissipedia, the skull was short and broad,
but in comparison with that of the modern cats and of _†Smilodon_, it
was decidedly longer and narrower and the face was less abbreviated;
the resemblance to _†Smilodon_ was very marked in the form of the
cranium, but, of course, the skull of _†Hoplophoneus_ was distinctly
more primitive in many respects. Thus, the orbit was much more widely
open behind, the tympanic bullæ were but imperfectly ossified, and
the perforations, or foramina, in the base of the skull, by which the
nerves and blood-vessels communicated with the brain-chamber, were quite
different and had more resemblance to those of the ancient dogs (_e.g._
_†Daphœnus_). In the classification of the Fissipedia much stress is
laid upon the number and arrangement of these cranial foramina, and it
is very significant to find the primitive dogs and cats agreeing so much
more closely than do the modern members of these families. The lower jaw
was relatively much stouter than in _†Smilodon_ and the anterior flanges
much more prominent, projecting downward so far that, when the jaws were
closed, the points of the tusks did not extend below the flanges. The
animal could have made no use at all of the sabre-tusks unless the mouth
could have been opened so widely as to clear their points.
With close general resemblance, allowing for the very inferior size, the
skeleton of _†Hoplophoneus_ had many significant differences from that
of _†Smilodon_. The neck was shorter and the body, especially the loins,
longer, lighter and more slender and the tail very much longer, equalling
that of the Leopard in relative length and surpassing it in thickness.
The limbs were much less massive and somewhat differently proportioned,
the upper arm being shorter and the fore-arm longer. The humerus, though
far more slender than that of _†Smilodon_, was remarkable for the great
development of the deltoid and supinator ridges, the latter, together
with the shape of the radius, indicating very free rotation of the fore
paw. The very prominent internal epicondyle was pierced by a foramen, and
the femur had a distinct remnant of the third trochanter. The five-toed
feet were comparatively small, but the claws were as completely
retractile and as fully hooded as in any of the subsequent genera.
That _†Hoplophoneus_ was a fierce destroyer, is made evident by every
part of its skeleton, and, like other cats, it no doubt subsisted upon
warm-blooded animals, which it killed for itself, the size of the prey
being determined by the size and power of the particular species of the
†sabre-toothed genus. In view of the probable extent of the Oligocene
forests, the restoration (Fig. 264) gives the animal a spotted coat and
the general aspect is that of one of the modern spotted cats, but the
protruding ends of the tusks and the relatively long head distinguish
it from any existing cat. “The presence of long, knife-like canines
is correlated with powerful grasping feet possessing highly developed
retractile claws. With its powerful feet the animal clung to its prey,
while it struck repeatedly with its thin, sharp sabres” (J. C. Merriam).
In the latter part of the White River stage lived one of the most highly
specialized of the †machairodonts, so far, at least, as the dentition
is concerned, for only the skull is known. This genus, _†Eusmilus_,
which also occurred in the Oligocene of Europe, was apparently an
example of premature specialization which led to nothing, for none of
the subsequent genera could have been derived from it. The teeth were
reduced to a minimum in number: _i_ 3/2, _c_ 1/1, _p_ 2/1, _m_ 1/1, ×
2 = 24, one lower incisor and at least one premolar less in each jaw
than had _†Hoplophoneus_. The canine tusk was very large and the flange
of the lower jaw for its protection correspondingly elongated, being
more prominent than in any other †machairodont. The American species,
_†E. dakotensis_, was the largest carnivore of its time and not greatly
inferior in size to the Lion.
Still another White River †machairodont, _†Dinictis_, differed in
many interesting ways from its contemporary _†Hoplophoneus_, being
more primitive and departing less from the ordinary fissipede type of
structure. This is shown by the greater number of teeth, which was
normally, _i_ 3/3, _c_ 1/1, _p_ 3/3, _m_ 1/2, × 2 = 34. The upper
carnassial had a considerably larger internal cusp and the trenchant
blade did not have the accessory anterior cusp, which is present in
almost all other cats and was thus more dog-like than cat-like. The lower
carnassial was more feline, but retained a remnant of the heel and of
the inner cusp, but the latter was variable, being sometimes present
in one side of the jaw and not in the other, a sign that it was on the
point of disappearance. The upper molar was plainly a reduced form of
the tritubercular tooth, in plan like that of the dogs, while the second
lower molar was a very small, single-rooted tooth. No other American cat
has such a primitive dentition as this, and, aside from the sabre-tusk,
which was not nearly so long as in _†Hoplophoneus_, and the lower
carnassial, it might almost as well have belonged to a dog or musteline.
[Illustration: FIG. 265.—Primitive †sabre-tooth (_†Dinictis felina_)
from the White River. Restored from specimens in the American Museum and
Princeton University.]
The skull was very like that of _†Hoplophoneus_, but was still longer
and somewhat different in shape, owing to the higher forehead and lower
occiput. The primitive features of the cranial base, such as the
foramina, the imperfectly ossified tympanic bullæ, etc., were repeated in
_†Dinictis_, but the lower jaw had much less prominent flanges for the
protection of the tusks. The limbs differed considerably from those of
_†Hoplophoneus_ in being relatively longer and more slender and retaining
more primitive features, such as the larger third trochanter of the
femur. The five-toed feet were decidedly small and weak, and the claws,
though retractile, were less so than in the other genus and were not
hooded. The gait was probably plantigrade or semi-plantigrade.
[Illustration: FIG. 266.—Skull of _†Dinictis squalidens_, White River.
(After Matthew.) _p. 4_ = fourth upper premolar, sectorial.]
The relationships of _†Dinictis_ and _†Hoplophoneus_ are rather puzzling;
none of the known species of the former could have been ancestral to
the latter, for the two genera were contemporaneous. _†Dinictis_ was
apparently the somewhat modified survivor of the ancestral stage and
represented very nearly the common starting point of both the feline and
†machairodont subfamilies. Dr. Matthew has propounded the bold theory
that this genus was the actual ancestor of the felines, continuing the
series through _†Archælurus_ and _†Nimravus_ of the John Day to the
unmistakable felines of the middle Miocene. This view runs contrary to
the supposed “law of the irreversibility of evolution,” a rule which
many authorities look upon as well established. The theory postulates
a different mode of development from anything that we have so far
encountered in the series previously described and supposes that the
upper canine first lost its original form, becoming a thin, elongate and
scimitar-like tusk, while the lower canine was reduced almost to the
proportions of an incisor and the lower jaw acquired a straight, flat
chin and inferior flanges for the protection of the tusks. Then, after
specialization had advanced so far, it was reversed and the original
condition regained. This interesting hypothesis may possibly turn out to
be true, though personally I cannot accept it, and, should it do so, it
would necessitate a thoroughgoing revision of current opinions as to the
processes of mammalian development.
[Illustration: FIG. 267.—Left pes of _†Dinictis felina_. _Cal._,
calcaneum. _As._, astragalus. _Cb._, cuboid. Princeton University Museum.]
The only John Day cat which was assuredly derived from _†Dinictis_ was
the large _†Pogonodon_, previously mentioned.
[Illustration: FIG. 268.—Skull of false †sabre-tooth (_†Nimravus
gomphodus_) from the John Day. (After Matthew.) _p. 4_ = fourth upper
premolar, sectorial.]
Also in the John Day stage lived _†Archælurus_ and _†Nimravus_, which,
as was noted above (p. 249), have been called the “false sabre-tooths,”
for in them the upper canine was not much larger than the lower and the
latter, though smaller than in the felines, was yet very much less
reduced than in the true †machairodonts. The skull closely resembled
that of _†Dinictis_, but the lower jaw was without flanges. The limbs
were long and slender and the feet long and digitigrade. The pes had
only four digits, of which the median pair was elongated and the lateral
pair shortened, so as to produce considerable resemblance to the pes of
the dogs, and the claws were partially retractile. The proportions of
the body, limbs and feet were suggestively like those of the Cheeta, or
Hunting Leopard (_Cynælurus jubatus_) of India, the generic name of which
means “dog-cat,” and it is quite possible that the Cheeta may have been
derived from some member of this “false †sabre-tooth” series, though
the connecting links are unknown. These cursorial cats quite displaced
the leaping †machairodonts of the _†Hoplophoneus_ type, at least in the
Oregon region at a time when, it will be remembered, that region had a
remarkable variety of dogs. In other parts of the continent, of which we
have no record, the true †machairodonts must have been thriving, as may
be inferred from their comparative abundance in the later formations.
Concerning the habits of these cursorial cats, Professor Merriam says:
“When the canines are not developed to the dagger-like form for stabbing,
the premolar teeth serve a more definite purpose in the destruction of
prey and would be less subject to reduction. The view suggested above
finds support in that such evidence as we have indicates that during
the deposition of the Middle John Day beds this region was in the main
a country of open plains, offering advantages to running types of
carnivores, and that during this epoch the _Archælurus-Nimravus_ type of
feline was by far the most common form [_i.e._ of cats].” The derivation
of these cats is still obscure, but their likeness to certain forms of
the European Oligocene suggests that they were immigrants.
The true cats of the subfamily Felinæ include the great variety of
living forms, large and small, from the Lion and Tiger at one extreme
to the Domestic Cat at the other. There is great difference among
naturalists with regard to the nomenclature of the Recent cats; some
make a considerable number of separate genera, while others include all
the species, except the lynxes and the Cheeta, in the genus _Felis_. For
the purposes of this book the latter practice is the more convenient and
will be followed. In _Felis_ the dental formula is: _i_ 3/3, _c_ 1/1, _p_
2-3/2, _m_ 1/1, × 2 = 28-30; the canines are large and strong, of oval
section, and the upper one is but little larger than the lower; there
are two large and functional premolars in each jaw, and an additional
very small one may or may not be present in the upper jaw. The upper
sectorial has a large shearing blade, with well-developed anterior
accessory cusp, and the inner cusp, which in _†Smilodon_ had almost
disappeared, is quite large and carried on a separate root. The lower
sectorial is composed of two cusps only, all traces of the heel and of
the inner cusp having disappeared. The single upper molar is very small
and usually concealed by the sectorial. The skull is very short and
broad, and the shortening of the jaws gives great power to the biting
muscles, because of the more favourable leverage. The zygomatic arches
are very stout and curve out boldly, contributing much to the rounded
shape of the head; the orbits are almost encircled in bone. The large
tympanic bullæ are two-chambered and there is no alisphenoid canal, but
in several other respects the base of the cranium differs markedly from
that of _†Smilodon_. The lower jaw is without flanges and there is no
angle between front and sides.
[Illustration: FIG. 269.—Dentition of Lynx (_L. rufus_), left side. _i.
3_, external upper incisor. _i. 1_, first lower incisor. _c._ = canine.
_p. 3_, _p. 4_, third and fourth premolars. _m. 1_, first molar.]
[Illustration: FIG. 270.—Upper teeth of Puma (_Felis concolor_), left
side. _p. 4_, fourth premolar. _m. 1_, first molar.]
The neck is short, the body long and the tail is long in most of the
species, but short in the lynxes. The limbs are relatively longer and
less massive than in _†Smilodon_, and there are five toes in the manus,
four in the pes; the claws are hooded and retractile.
The western hemisphere at the present day contains none of the very large
species, the Puma and Jaguar being the largest; but this was not true of
the Pleistocene, where a huge cat (_Felis †atrox_), surpassing the Lion
in size, ranged over the southern half of North America. Enormous cats
also lived in the lower Pliocene and upper Miocene of the Great Plains
region, but are not sufficiently well known for reference to either
subfamily.
[Illustration: FIG. 271.—Skull of Puma (_Felis concolor_). _p. 4_, upper
carnassial. The upper molar is concealed.]
The history of the true felines has been but partially deciphered,
and can, as yet, be traced back only to the middle Miocene, the genus
_†Pseudælurus_ representing the series both in Europe and North America.
In this genus the dental formula was nearly the same as in _Felis_, but
there was frequently an additional small premolar in the lower jaw and
the sectorials were more primitive, the upper one having the accessory
anterior cusp in a merely incipient stage and in the lower one there was
a vestige of the heel. The upper canine was considerably longer than the
lower, thinner and more blade-like than in _Felis_, which, so far as it
goes, is in favour of Dr. Matthew’s theory (p. 541). What little is known
of the skull and skeleton of _†Pseudælurus_ agrees with the modern cats.
[Illustration: FIG. 272.—Left manus of Domestic Cat (_Felis domestica_,
after Jayne). The horny claws are left in place, covering the ungual
phalanges.]
While it is not feasible to trace the series of true felines to an
earlier stage than the middle Miocene, there can be no doubt that the
subfamily was derived from the same stock as the †machairodonts and it is
probable that the White River _†Dinictis_ nearly represents the common
starting point for both series; the resemblances between _†Dinictis_ and
such primitive dogs as _†Daphœnus_ are suggestive of a common origin.
3. _Procyonidæ. Raccoons, etc._
An almost exclusively American family of Fissipedia is that of the
raccoons, which includes not only the latter (_Procyon_), but also the
coatis (_Nasua_), curious animals, with long, flexible, pig-like snouts,
the cacomistles (_Bassariscus_) and kinkajous (_Potos_). In addition
to these American forms, there is an outlying Asiatic genus, the Panda
(_Ælurus_) of the southeastern Himalayas, the last of a series which goes
back to the European Pliocene.
The Procyonidæ are animals of small and moderate size, largely arboreal
in habits and subsisting upon a mixed diet of fruit, eggs, insects and
the like; the teeth are adapted to this diet and the sectorials have
mostly lost their shearing form and the molars are tuberculated for
crushing and grinding. The species generally have long tails, except
in the raccoons proper, in which the tail is of medium length, and
five-toed, plantigrade feet, with naked soles. Fossil members of this
family are very rare in Tertiary formations and its history is therefore
but scantily known; in the lower Pliocene have been found fragmentary
remains with less specialized teeth, which appear to belong to the direct
ancestor of _Bassariscus_. The upper Miocene genus _†Leptarctus_ was an
undoubted member of the family, and, while it would seem not to have been
in the direct line of any of the modern forms, it was near to the common
ancestry of the American genera, so far as the imperfect specimens enable
us to judge.
[Illustration: FIG. 273.—Dentition of Raccoon (_Procyon lotor_), left
side. _i. 3_, external incisor. _c._, canine. _p. 4_, fourth premolar.
_m. 1_, first molar.]
By far the most primitive representative of the family yet discovered
is the lower Miocene genus _†Phlaocyon_, which connected the Procyonidæ
with the Oligocene genus of dogs, _†Cynodictis_ (p. 529). The dentition
resembled that of the latter, with several differences, which were all
changes toward the Procyonidæ. All the cusps were lower and blunter
than in _†Cynodictis_; the premolars were small, thick and closely
crowded together and the upper sectorial, while still trenchant, had a
postero-internal cusp, which is found in none of the Canidæ and was a
first step toward the tuberculated pattern of the raccoons, and the lower
sectorial had a very low cutting blade and large heel; the other molars
of both jaws were low, wide and of subquadrate shape. The skull was
short and broad, with the face as much shortened and the orbits as far
forward as in _Procyon_, but the brain-case was narrower, less capacious,
and the lower jaw had the curved form and much the same character as in
the modern genus. The limbs were relatively more slender than in the
latter and the five-toed feet were more canine than procyonine in the
proportions of the digits.
The discovery of _†Phlaocyon_ by Dr. Matthew was an event of capital
importance, as showing the highly probable derivation of the raccoons
from _†Cynodictis_ and thus bringing another fissipede family into
relationship with the dogs.
4. _Ursidæ. Bears_
The present distribution of the bear family is all but exclusively
northern, as there is but one African species, confined to the
northwestern corner of that continent, and one in the Andes of Peru and
Ecuador, all the others belonging to Eurasia and North America.
[Illustration: FIG. 274.—Dentition of Black Bear (_Ursus americanus_).
_i. 3_, external incisor. _c._, canine. _p. 1_, first premolar. _p. 4_,
fourth premolar. _m. 1_, first molar.—Below is a view, on a larger scale,
of the grinding surface of the fourth premolar and first molar, upper
jaw.]
Structurally, the family is very distinct and the dentition is quite
peculiar. The incisors and canines resemble those of other Fissipedia;
the three anterior premolars are very small, single-rooted and often
shed early; the carnassials have lost their trenchant character; and the
molars, which are usually longer than wide, are tuberculated, somewhat
resembling those of pigs. Almost all the bears live principally upon
vegetable food, and even the Polar Bear, which feeds upon fish and seals,
will eat grass and berries in the brief Arctic summer; thus, the shearing
teeth of the strictly carnivorous types are unnecessary to these animals.
The skull is not unlike that of the dogs in shape, but the tympanic bullæ
are much flattened and the entrances to them are long, bony tubes, while
the cranial foramina are nearly as in the dogs. The body is very heavy
and the tail always short. The limbs are short and thick; the humerus
has lost the epicondylar foramen in all existing species except the
South American Spectacled Bear (_Tremarctos ornatus_). The plantigrade
feet have naked soles (except in the Polar Bear) and each foot has five
well-developed and functional digits, armed with very long, sharp and
non-retractile claws.
[Illustration: FIG. 275.—Restored head of the †Short-faced Bear
(_†Arctotherium bonæerense_). From a skull in the National Museum, Buenos
Aires.]
The Pleistocene representatives of the family in America included species
of the true bears (_Ursus_) and of the very large †short-faced bears
(_†Arctotherium_) which ranged over both North and South America. In
_†Arctotherium_ the dentition was less modified; the larger premolars
were very closely crowded together and the molars were nearly square;
the lower jaw was almost as much curved as in the raccoons. The humerus
retained the epicondylar foramen. The family, which was of Old World
origin, may have reached America in the lower Pliocene, but was rare
until the late Pleistocene. _†Arctotherium_ has not been found in the
eastern hemisphere, but that, of course, is no proof that the genus
was not an immigrant from Asia. On the other hand, it may have been a
peculiar American development from Pliocene immigrants. In the Old World,
bears were first distinguishable in the upper Miocene, and may be there
traced back to forms which were unmistakably derivatives of the early
dogs.
5. _Mustelidæ. Mustelines_
The last fissipede family, which has, or has had, representatives in
the western hemisphere is that which includes a great variety of small
carnivores, such as minks, martens, skunks, badgers, otters, etc., and
was likewise of Old World origin, though now of universal distribution,
except in Australia and Madagascar. These are fierce and bloodthirsty
beasts of prey, most of them strictly carnivorous and often killing in
mere wantonness more than they can devour. Though now quite numerous and
varied in North and South America, they are decidedly less so than in the
eastern hemisphere and comparatively few peculiar types have originated
here. Owing to the small size and fragility of the skeletons, they have
not been well preserved as fossils, and little can be done as yet in
tracing out the genealogy of the various phyla.
The mustelines have shortened jaws and a reduced number of teeth, the
molars being 1/2 or even 1/1 and the premolars varying from four to two,
though three in each jaw is the usual number. The cranium is generally
very long and the facial part of the skull short, but the soft snout
may add considerably to the length of the face. The tympanic bullæ are
single-chambered and little inflated, and the lower lip of the entrance
is extended; the hard palate is usually continued well back of the teeth.
The body is very long and the tail variable and, in most of the genera,
is short rather than long. The limbs are short, the feet, except in one
genus, five-toed and plantigrade or semi-plantigrade, and the claws are
non-retractile. Terrestrial, arboreal, burrowing, aquatic and marine
forms are all represented in the family.
So far as North America is concerned, it is scarcely practicable to do
more than catalogue the genera of the successive geological epochs.
Pleistocene mustelines were very modern in character, differing
little from those now inhabiting the continent, though in some cases
with different ranges, according to climatic fluctuations. Badgers,
martens, skunks and others occurred then very much as they do now and
the Boreal Wolverene extended down to Pennsylvania. Little is known of
Pliocene mustelines, the Blanco having yielded fragments of only one
genus of uncertain affinities and though several genera occurred in
the lower Pliocene, but one, a marten (_Martes_), can be identified.
Unquestionably, North America had many more Pliocene members of the
family, but the conditions of preservation were unfavourable.
Much the same is true of the Miocene stages. In the upper Miocene
there were a marten (_Martes_), a weasel (_Mustela_) and two otters
(_†Potamotherium_ and the modern _Lutra_), of which the marten and the
more primitive otter went back to the middle Miocene. In the lower
Miocene were several mustelines quite different from any now existing.
One of those, _†Megalictis_, was truly gigantic, with a skull nearly as
large as that of a Black Bear and having heavy, pointed claws. This and
a similar genus, _†Ælurocyon_, were related to the Ratel (_Mellivora_)
of India and Africa and, more closely, to the Wolverene. _†Oligobunis_,
a much smaller animal, was apparently of the same group. This genus was
also in the upper Oligocene, but there represented by a larger species,
which was as large as a badger.
The White River beds have yielded but a single genus, _†Bunælurus_, which
was the most primitive of American mustelines and had four premolars
and two molars in each jaw, though the second upper molar was extremely
small. The face was much less shortened than in the modern weasels and
the tympanic bullæ were short and strongly inflated and had no tubular
entrance, and were thus canine rather than musteline in form. The
bony palate was not extended back of the teeth as it is in the modern
genera. The same primitive group was much more abundant in the European
Oligocene, migrating probably from Asia into Europe as well as into North
America.
SOUTH AMERICAN FISSIPEDIA
The history of the South American carnivores is a comparatively brief
one; the southern continent has representatives of the same five families
as the northern, but most of the genera are different, the time since the
great southward migration having been sufficient for the development of
peculiar forms in the new environment. Among the dogs, there are to be
noted the curious, close-haired, long-bodied and short-legged Bush-Dog
(_Icticyon_) and the fox-like wolves (_Cerdocyon_), but there are no
true foxes. Of the cats, the Puma differs little from that of North
America, and the Jaguar (_Felis onca_) and Ocelot (_F. pardalis_) also
range into the northern continent, but several small cats are confined to
South America, which has no lynxes. There is but one bear (_Tremarctos
ornatus_) of Andean range. Of the Procyonidæ, the northern _Procyon
lotor_ is replaced by the Crab-eating Raccoon, _P. cancrivorus_, while
the coatis (_Nasua_) and kinkajou (_Potos_) are chiefly Neotropical.
Except for the otters, the genera of Mustelidæ are nearly all different;
there are no badgers and a different genus of skunks (_Conepatus_)
replaces the northern _Mephitis_; the Grison (_Grison_), Tayra (_Tayra_)
and the Patagonian _Lyncodon_ are peculiar.
Even less can be done to trace the evolution of the South American
genera than for the forms of the northern continent, whence migrated
the more or less different ancestors of the former. The Pleistocene has
yielded most of the modern genera, both existing and extinct species. An
example of the latter was _Procyon †ursinus_ from the Brazilian caverns,
a truly gigantic Raccoon, as large as a bear. The †sabre-tooth tigers
(_†Smilodon_) and short-faced bears (_†Arctotherium_) were shared with
North America. In the Pliocene a bear, a raccoon and a dog were the only
known fissipedes, and in the Miocene none have been found, their place
being taken by flesh-eating marsupials.
* * * * *
While the history of the Fissipedia, as outlined in the preceding
pages, is sadly incomplete as compared with that of many ungulates,
it is nevertheless highly suggestive. In each family the advance of
specialization and adaptation to a narrow range of habits may be
followed; generally speaking, the teeth were diminished in number and
increased in size and were either simplified by the loss of parts, as in
the cats, or complicated by the addition of new elements, as in the bears
and raccoons. The brain grew larger and more convoluted and the cranium
more capacious; in most of the families, the face was shortened, notably
in the cats and mustelines, while in others, especially the dogs, it was
elongated. In all of the early types there was a long and heavy tail,
but in most series it underwent more or less reduction. There was little
reduction of digits, and no fissipede has less than four. In modern dogs
and cats there are five digits in the manus and four in the pes and the
hyenas have four in each, as has one genus of mustelines; other modern
genera throughout the suborder are pentadactyl.
It is significant that the more ancient members of the various families
differed less than do the modern ones; the various groups, as they are
traced back in time, would seem to be converging to a common ancestry, of
which the lower Oligocene dogs were the least changed representatives,
and it is probable that all the families of the Fissipedia were derived,
directly or indirectly, from a single Eocene group of primitive
flesh-eaters. The families, none of which is extinct, are not all of
equal antiquity. So far as now appears, the dogs and viverrines are
the most ancient, having become distinct in the upper Eocene; in the
Oligocene were added the mustelines and cats; the raccoons branched
off from the dogs in the lower Miocene, as did the bears in the upper
Miocene. Finally, the hyenas appeared in the lower Pliocene, seemingly
derived from the viverrines. The dogs passed through the greater part
of their development in North America, where, during the Oligocene and
Miocene, they were very abundant and varied, while at the same time they
were comparatively rare in Europe and belonged chiefly to the phylum of
the †bear-dogs. On the other hand, the remaining four families are of Old
World origin, the bears and mustelines migrating to America, while the
viverrines and hyenas did not.
SUBORDER †CREODONTA. †PRIMITIVE FLESH-EATERS
This group long preceded the Fissipedia in time, for they began their
recorded history in the Paleocene and became extinct in the Oligocene.
Through one family, the †Miacidæ, the †creodonts were broadly connected
with the fissipedes, and it seems probable that that family was the
ancestral stock from which all the fissipede families were derived. The
other †creodont families died out without leaving descendants.
There is some difference of practice as to the number of families to be
admitted; the table contains those listed in Professor Osborn’s book and
also adopted by Dr. Schlosser. I should prefer a somewhat larger number
of family groups, but the matter is one of secondary importance. Many
genera are omitted.
I. †OXYCLÆNIDÆ.
_†Oxyclænus_, Paleoc. _†Deltatherium_, do.
II. †ARCTOCYONIDÆ.
_†Clænodon_, Paleoc. _†Anacodon_, low. Eoc.
III. †MESONYCHIDÆ.
_†Triisodon_, Paleoc. _†Dissacus_, do. _†Pachyæna_,
low. Eoc. _†Mesonyx_, mid. Eoc. _†Dromocyon_, do.
_†Harpagolestes_, mid. and up. Eoc.
IV. †OXYÆNIDÆ.
_†Palæonictis_, low. Eoc. _†Oxyæna_, do.
_†Patriofelis_, mid. Eoc. _†Limnocyon_, do.
_†Machairoides_, do. _†Oxyænodon_, up. Eoc.
V. †HYÆNODONTIDÆ.
_†Sinopa_, mid. Eoc. _†Stypolophus_, low. and mid.
Eoc. _†Tritemnodon_, mid. Eoc. _†Pterodon_, low.
Oligo. _†Hyænodon_, do.
VI. †MIACIDÆ.
_†Didymictis_, Paleoc. and low. Eoc. _†Viverravus_,
mid. Eoc. _†Miacis_, low. Eoc. _†Uintacyon_, low.
to up. Eoc. _†Oödectes_, mid. Eoc. _†Vulpavus_, do.
_†Palæarctonyx_, do.
The †Creodonta were an extremely varied assemblage, of carnivorous,
omnivorous and presumably insectivorous habits, so that few statements,
not subject to exceptions, can be made of them all. Only seven genera
are known from skeletons, and several more from skulls, but most are
represented only by jaws and teeth; limb- and foot-bones, however, give
us a conception of the general structure of a considerable number. As
a rule, the dentition was complete, according to the formula, _i_ 3/3,
_c_ 1/1, _p_ 4/4, _m_ 3/3, × 2 = 44, but the first premolar or the last
molar may be lost. The canines were always large, as was befitting for
beasts of prey. In only one family, the Miacidæ, were the carnassial
teeth confined to a single pair and those the same as in the Fissipedia,
the fourth upper premolar and first lower molar; in all the other
families there were either no sectorial teeth, or else there was more
than one pair. In the Fissipedia the first is the largest of the lower
molars, while in the †Creodonta (except the †Miacidæ) it was usually the
smallest. The premolars were generally simple, compressed-conical teeth
and the molars, with all their great variety, may be reduced to a common
plan; those of the upper jaw were primitively tritubercular, with a
triangle of two external and one internal cusps, and those of the lower
jaw were in two distinct parts, an anterior, elevated triangle of three
cusps and a low heel of two.
The skull was almost always very large in proportion to the size of the
animal; the cranium, though long, was of small capacity and the face
varied much in length in the different families. Primitively, the face
and jaws were short in correlation with the small size of the teeth, and
this primitive condition was modified in two opposite directions; in
one the face and jaws were elongated, as the teeth enlarged, and in the
other they were shortened still further. The zygomatic arches were stout
and curved out strongly from the sides of the skull, making very wide
openings, and, in almost all cases, the sagittal and occipital crests
were very high, as would be necessary from the combination of powerful
jaws and small brain-case (see p. 63). The tympanic bullæ were not
ossified. The brain was extremely small, especially in the more ancient
genera, and the convolutions were almost always few and simple, which
indicates a low grade of intelligence and very marked inferiority to the
Fissipedia.
In all the genera of which sufficient material has been obtained the body
was long and had 19 or 20 trunk-vertebræ: in the lumbar and posterior
part of the dorsal regions the processes by which the successive
vertebræ were articulated together (zygapophyses) were cylindrical and
interlocking, as in the artiodactyl ungulates (p. 360). To this general
statement, the †Miacidæ formed a partial exception. The tail was very
long and heavy in all the forms of which the caudal vertebræ are known,
and this was probably true of all. The limbs were short and generally
heavy; the femur had the third trochanter and the humerus, save in a few
of the later genera, the epicondylar foramen, and the manus could, in
nearly all, be freely rotated. Except in the most advanced forms of one
family, the †Mesonychidæ, the feet were five-toed and plantigrade, or
semi-plantigrade, and of decidedly primitive structure. The scapho-lunar
bone of the Fissipedia (see p. 519) was not formed, its three elements,
with very few exceptions, remaining separate. The astragalus nearly
always had a shallow groove, or none at all. The claws were thick
and blunt and the ungual phalanges cleft at the end, except in the
†Arctocyonidæ and †Miacidæ, which had sharp claws and uncleft phalanges.
From this brief description, it is obvious that the †Miacidæ occupied
a very isolated position among the †creodonts and, in my judgment, it
would be better to transfer that family to the Fissipedia and include the
others in a separate order.
Throughout the Paleocene and Eocene epochs the †Creodonta were numerous
and varied, the first of the Fissipedia appearing in the upper Eocene.
Till then the †creodonts were the only predaceous mammals in North
America and Europe, and they were especially abundant in the former.
Most members of the suborder and all the Paleocene forms were of small
or moderate size, but some of the Eocene species were very large. In the
Uinta the †creodonts were greatly decreased in numbers and in the White
River there were only two genera of one family, the †Hyænodontidæ, and
since the Oligocene the suborder has been extinct.
1. _†Miacidæ. Fissipede-like †Creodonts_
It is unfortunate that no member of this family is known from a complete
skeleton, but the material collected is sufficient to give a fairly
adequate conception of these most interesting animals. These were the
only †creodonts with a single pair of carnassials, the fourth upper
premolar and first lower molar, but in some of the genera the carnassials
did not differ greatly from the other teeth. In the various genera the
skull differed considerably in length and in the proportions of cranium
and face; the brain-case was larger than in most other †creodonts and
the brain more advanced, though smaller than in the fissipedes, and the
sagittal and occipital crests were very prominent; the tympanic bullæ
were not ossified. The humerus had the epicondylar foramen and the femur
the third trochanter; in the wrist the scaphoid, lunar and central were
separate, almost the only important difference from the Fissipedia and
merely the primitive stage of the latter. The feet were pentadactyl and
the digits were arranged in spreading fashion; the claws were small,
sharp and partially retractile and the ungual phalanges not cleft at the
tip.
Within the family several different phyla may be distinguished, one
of which (_†Miacis_—_†Uintacyon_) led to the dogs, another to the
†bear-dogs, or †amphicyons. A third phylum (_†Didymictis_—_†Viverravus_)
is by several authorities regarded as ancestral to the civet family, or
viverrines, of the Old World, and a fourth (_†Oödectes_, _†Vulpavus_)
as the forerunner of the kinkajous (_Potos_). Except for the connection
with the dogs, the hiatus in time between the supposed ancestors and
descendants is too great to permit any confident statements. It seems
very probable, however, that the †Miacidæ represented the common
stock, from which the fissipede families were all derived, directly or
indirectly, though for most of them the details of the connection remain
to be learned.
We find thus a group separating itself from the other †creodonts in the
older Paleocene and gradually assuming fissipede characteristics, at the
same time dividing into several phyla. In the upper Eocene this group
passed almost imperceptibly into the Fissipedia, more obviously into
the dog family, which, as we have seen, represents the central line of
fissipede development.
2. _†Mesonychidæ_
This family displayed, in certain respects, the highest degree of
specialization attained by any †creodonts, for they were the only
ones which acquired cursorial limbs and feet. The †mesonychids were
prevailingly, but not exclusively, a North American family and their
range in time was through the Paleocene and Eocene.
The teeth, in the more advanced genera, had a curious mingling of
primitive and specialized characters and none were sectorial in the
proper sense of the word. The incisors were small, the canines large
and bear-like and the premolars simple. The upper molars were very
primitive, retaining the original tritubercular pattern, except that the
two outer cusps were joined together, but the lower molars had lost all
the internal cusps, which gave them a carnassial appearance; they were
not sectorial, however, for their cusps wore directly against the upper
teeth, not shearing past them, and were greatly blunted and worn down by
use.
The last of the family was _†Harpagolestes_, of the Uinta and Bridger,
one of the largest of the †creodonts. The skull, which was of
disproportionate size, exceeded that of the Grizzly Bear; the upper
profile of the skull had considerable resemblance to that of a bear in
the steep forward descent at the fore head. The teeth were more reduced
than in the other members of the family through the loss of the second
premolar and third molar of the upper jaw. The skeleton is little known,
but the humerus had a long and prominent deltoid crest and an epicondylar
foramen.
[Illustration: FIG. 276.—Upper teeth, right side, of _†Mesonyx
obtusidens_, showing the grinding surface.]
In the middle Bridger stage were closely allied and very similar genera,
_†Mesonyx_ and _†Dromocyon_ (Fig. 139, p. 269), which were like small,
big-headed wolves, for the skull was as long as that of a Black Bear.
Though the cranium was very long, the brain-chamber was very small and
the sagittal crest enormously high, to afford surface for the attachment
of the powerful jaw-muscles. The tympanic bullæ were ossified and had
quite long, tubular entrances, a feature which has been found in no other
†creodont skull. The face and jaws were also elongate, giving the head
quite a wolf-like appearance. The neck and body were of moderate length,
but the tail was extremely long, slender and whip-like.
The limbs and feet were more specialized than in any other †creodont and
the changes were all in the direction of adaptation to swift running. The
humerus was very smooth, with low ridges, and, alone among †creodonts,
had in these genera no epicondylar foramen, though the femur retained
the third trochanter. The radius was broad and so interlocked with the
humerus as to prevent any rotation of the manus. The feet were four-toed
and much resembled those of the modern dogs and hyenas. In each foot
the metapodials were closely appressed and parallel, not spreading, but
arranged in two symmetrical pairs, a longer median and shorter lateral
pair, much on the artiodactyl plan; the ankle-bone (astragalus) also
had an artiodactyl look, with its deeply grooved surface for the tibia
and pulley-like lower end. The ungual phalanges were so short and broad
as almost to suggest hoofs rather than claws. It is clear that the
gait was as fully digitigrade as in a modern wolf and these were the
only †creodonts of which this is known to be true. These were somewhat
puzzling animals; the whole structure of the limbs and feet was that of
cursorial types, but the broad, blunt claws do not suggest the running
down and capture of prey, nor were the teeth those of savage killers. The
speed may have been defensive, to escape from enemies, and the food may
have been largely vegetable.
Ancestors of these Bridger genera have not been found yet in the Wasatch,
a time when the family was represented by _†Pachyæna_, some of the
species of which were very large, rivalling _†Harpagolestes_, which was
descended from one or more of them. _†Pachyæna_ had extremely massive
teeth and was not improbably a carrion-feeder of hyena-like habits, and
it retained the epicondylar foramen of the humerus and pentadactyl feet.
Much more primitive was _†Dissacus_, of the upper Paleocene, which was
very probably the direct ancestor of both the Wasatch and the Bridger
genera. The upper molars were substantially as in the latter, but the
lower molars had the internal cusp of the primitive triangle, though
the heel was trenchant, and had lost its inner cusps. The feet had five
well-developed digits, which were arranged in spreading fashion, and the
gait was plantigrade. The claws were longer, more pointed and much less
hoof-like than in the Bridger genera. The Puerco genus _†Triisodon_ may
or may not have been directly ancestral to _†Dissacus_; at all events, it
was very nearly what the desired ancestor must have been. The teeth were
much less specialized than in _†Dissacus_; the tritubercular upper molars
were broader and their external cusps were more separated, while in the
lower molars the anterior triangle was made up of three nearly equal
cusps and the heel was low and basin-shaped. The skull had an extremely
narrow brain-case and a long, heavy sagittal crest.
The most interesting feature in the history of the †Mesonychidæ is the
demonstrable derivation of the cursorial, digitigrade, four-toed and
almost hoofed Bridger genera from the plantigrade, five-toed Torrejon
genus, which had sharp claws. To all appearances, this family was the
†creodont analogue of the hyenas.
3, 4. _†Arctocyonidæ and †Oxyclænidæ_
This second †creodont family which had no carnassial teeth has received
the not very happily chosen name of †Arctocyonidæ, or “bear-dogs,” though
they were not related to either bears or dogs. The family was a very
ancient one and has been found only in the Paleocene and lower Eocene
(Torrejon and Wasatch) of North America and Europe. The molar teeth were
very low-crowned and quadritubercular, with numerous small tubercles in
addition to the four principal cusps, a pattern which was rather pig-like
than bear-like. The Wasatch genus _†Anacodon_, known only from jaws and
teeth, had reduced premolars, both in size and number, while in the
Torrejon genus, _†Clænodon_, the premolars, though small, were present
in full number. The skull was like that of _†Mesonyx_ in the relative
lengths of cranium and face, the very small size of the brain-case and
the great prominence of the occipital and sagittal crests. The feet were
pentadactyl and plantigrade and the claws were long, thin and pointed,
and the ungual phalanges were not cleft at the tip, the only †creodont
family, except the †Miacidæ, of which this was true.
Of the †Oxyclænidæ, very little is known and they may not have been
†creodonts at all. They were quite small animals, with sharp-cusped
tritubercular upper molars and lower molars with high anterior triangle
and low heel. This is the type of dentition from which all the divergent
†creodont types were doubtless derived. The family was Paleocene.
5. _†Hyænodontidæ_
This was the last of the †creodont families to survive, being quite
common in the lower Oligocene of North America and Europe and in the
upper Eocene of the latter also. The family became extinct in the upper
Eocene of North America and the White River genera were not of native
origin, but migrants from the Old World. One of the more abundant
predaceous genera of White River times was the European _†Hyænodon_;
it was represented by several species which ranged in size from a fox
to a Black Bear. In this genus the dentition was somewhat reduced,
the incisors often numbering 2/2 and the molars constantly 2/3; there
were three pairs of carnassial teeth on each side, of which the pair
formed by the second upper and third lower molar was the largest and
most efficient, the other pairs being the first upper and second lower
molar, the fourth upper premolar and first lower molar, the latter the
smallest of the three. The upper molars had lost the internal cusp and
the remaining, external portion consisted of a flattened-conical anterior
cusp and a posterior trenchant ridge; the milk-teeth of _†Hyænodon_, as
well as the permanent dentition of the ancestral genera, show that the
anterior cusp was composed of the two external cusps of the primitive
tritubercular tooth fused into one and that the trenchant ridge was a
superadded element. The fourth upper premolar was a sectorial like that
of the Fissipedia, but of an unfinished, ineffective sort. The third
lower molar was very similar in shape to the carnassial of the cats and
was composed of only two large, thin and trenchant cusps, which made a
shearing blade, having lost the inner cusp of the primitive triangle and
the heel. The first and second molars were like the third except in size
and in retaining a vestige of the heel. The premolars were large and
massive, almost hyena-like, which suggested the generic name. The canines
were prominent and strong.
[Illustration: FIG. 277.—_†Hyænodon horridus_, a White River †creodont:
in the background, _†Leptomeryx evansi_. Restored from skeletons in the
American Museum of Natural History.]
The skull, as in almost all †creodonts, was relatively very large, but
in the various species there was considerable difference of shape; more
commonly it was long and narrow, with elongate jaws, and was quite
wolf-like in appearance, but in some of the species it was shorter
and wider. The brain-case was more capacious and the brain more richly
convoluted than in any other known †creodont, but the sagittal and
occipital crests were very prominent. The neck was rather short, not
equalling the head in length, the body elongate and the loins very
muscular; the tail was fairly long and thick, but much less so than in
most †creodonts. The limbs were short and, in most of the species, quite
slender, though in some they were much stouter; the primitive features,
such as the third trochanter of the femur, the epicondylar foramen of the
humerus, the separate scaphoid, lunar and central in the carpus, were
retained. The feet had five digits arranged in spreading fashion and were
probably semi-digitigrade; the claws were so thick and blunt that they
could hardly have served in seizing prey.
[Illustration: FIG. 278.—Skeleton of _†Hyænodon_. American Museum.]
The restoration gives the animal quite a near resemblance to the modern
hyenas and perhaps errs in making the likeness so close. From the whole
structure of the skeleton and the form of the claws, it may be inferred
that _†Hyænodon_ was not a swift runner or very efficient in the capture
of prey. While probably savage fighters, they doubtless subsisted chiefly
as carrion-feeders and scavengers.
[Illustration: FIG. 279.—Lower teeth, right side, of †hyænodontids. _A_,
_†Sinopa_. _B_, _†Tritemnodon_. _C_, _†Pterodon_. _D_, _†Hyænodon_. _X_,
_Oxyæna_. The dotted line connects the first molar of each, lost in
_†Pterodon_. See explanation of Fig. 280. (After Matthew.)]
Another doubtfully distinct genus, _†Hemipsalodon_, was so closely like,
if not identical with, the much better known European _†Pterodon_, that
the latter may be taken in place of it. _†Pterodon_ was similar in most
respects to _†Hyænodon_, but distinctly less advanced, and though not
the ancestor of the latter, serves to connect it with the older members
of the series. _†Pterodon_ did not, so far as we know, penetrate North
America south of the Canadian border, occurring in the lower White River
of Alberta. In this genus the upper molars retained a large internal
cusp, and the third molar, though small and not sectorial, had not
been lost; the two external cusps were connate, but not completely
fused together and the posterior ridge was not so well developed as in
_†Hyænodon_, nor was the fourth upper premolar so nearly a carnassial.
The lower molars were shearing blades, but distinct vestiges of the heel
remained. So far as they are known, the skull and skeleton resembled
those of _†Hyænodon_.
[Illustration: FIG. 280.—Upper teeth of †hyænodontids, right side,
showing the grinding surface. _A_, _†Sinopa_, Wasatch and Bridger.
_B_, _†Tritemnodon_, Bridger. _C_, _†Pterodon_, upper Eocene and lower
Oligocene of Europe. _D_, _†Hyænodon_, White River. The dotted line
connects the first molar of each. For comparison is added _X_, _†Oxyæna_,
one of the †Oxyænidæ. _C_ and _D_ are much larger than the others, but
all, except _X_, are reduced to the same length. (After Matthew.)]
_†Hyænodon_ and _†Pterodon_ were evidently derived from a group of small
†creodonts which, in the lower and middle Eocene, were spread all over
the northern hemisphere, but it is not yet possible to select from the
crowd of allied genera those which formed the actual steps of descent.
These small animals were numerous and varied and are far better known
in North America than in Europe and it is not at all improbable that
some of the lower Eocene genera migrated to the Old World and there gave
rise, among other forms, to _†Hyænodon_ and _†Pterodon_, which eventually
returned to the land of their earlier ancestry. If confirmed, this will
be an exceptionally interesting case of back and forth migration. However
that may be, the American Eocene genera, _†Sinopa_ and _†Tritemnodon_,
illustrate very well the ancestry of the Oligocene genera, as they must
have been similar to the actual progenitors.
[Illustration: FIG. 281.—_†Tritemnodon agilis_, a primitive †hyænodont,
Bridger stage. Restored from a skeleton in the American Museum.]
The first and most obvious difference from the Oligocene genera was the
very much smaller size of the animals, few of the Eocene forms equalling
a fox in height. The teeth were unreduced in number, and there were
three pairs of carnassials. The first and second upper molars were not
far removed from the primitive tritubercular form, but the two external
cusps were close together and a small posterior cutting ridge was
present; the third molar was progressively reduced in size. The three
lower molars were carnassials of a rather imperfect kind and the first
was the smallest of the series; the two outer cusps of the anterior
primitive triangle formed the shearing blade and there was a basin-shaped
heel. The skull was long, narrow and low and the cranial portion, despite
the very small brain-case, was especially elongate, though face and jaws
were also long; the sagittal crest was very prominent. The neck was of
moderate length, the body long and slender and the tail extremely long.
The short and delicate limbs were of very primitive character, but
the radius had already lost the power of rotation; the feet had five
spreading digits, armed with sharp claws. The †hyænodont relationships of
these small animals are obvious in every part of their structure and yet,
as would be expected, they were far less specialized. Probably, too, they
were more active and successful hunters of prey, the smaller mammals and
birds, less given to carrion-feeding. The line probably originated in the
†Oxyclænidæ of the Paleocene.
6. _†Oxyænidæ_
The genera of this family had such feline characters that more than one
writer has been misled into the belief that they were the ancestors of
the cats. In this family there were two pairs of sectorial teeth, of
which the larger pair was composed of the first upper and second lower
molar, the smaller pair of the fourth upper premolar and first lower
molar, as in the fissipedes. Of the three phyla within the family, the
most specialized one ran a brief career, through the Wasatch, Wind River
and Bridger, and then died out. The terminal member of this series, the
Bridger genus _†Patriofelis_, had a skull as large as that of a lion,
but the rest of the skeleton was not so large in proportion. The teeth
were considerably reduced in number, the formula being: _i_ ?/2, _c_
1/1, _p_ 3/3, _m_ 1/2, a loss of at least twelve from the primitive total
of 44. The single upper molar was a large sectorial, which was formed
much as in the †hyænodonts, the two external cusps connate, but not
indistinguishably fused together, and a long, trenchant ridge behind,
while the inner cusp had almost vanished. The second lower molar was very
cat-like; its cutting blade was formed of two shearing cusps; of the
inner cusp no trace was left, and of the heel merely a vestige. The first
lower molar was smaller and less specialized, since it retained a small
internal cusp and quite a large heel.
[Illustration: FIG. 282.—_†Patriofelis ferox_, Bridger stage. Restored
from a skeleton in the American Museum.]
The skull was very large and massive, with elongate cranium and
shortened face, the muzzle broad and abruptly truncate, not tapering;
the brain-case was exceedingly small, with very long and prominent
sagittal crest; the zygomatic arches were extremely heavy and curved
outward boldly, so that the head was very wide, notwithstanding the
absurdly small brain-case. The lower jaw was very deep and heavy and
the chin abruptly rounded, with almost vertical front. The very unusual
massiveness of the zygomatic arches and the great development of the
crests and ridges for the attachment of the jaw-muscles, and the short,
heavy lower jaw, all indicate a degree of power in the biting and
shearing apparatus such as occurred in no other known †creodont.
The neck was of medium length, while the body, though actually elongate,
was rather short as compared with most other †creodonts; the loins were
very heavy and must have been extremely powerful in the living animal;
in this region the articulations between the successive vertebræ were
more complex than in any other member of the suborder; resembling the
structure found in certain artiodactyls. The ribs were long and thick,
the chest deep and capacious. Even for a †creodont, the tail was long and
uncommonly thick.
[Illustration: FIG. 283.—Right pes of _†Patriofelis ferox_. _Cal._,
calcaneum. _As._, astragalus. _Cb._, cuboid. _N._, navicular. _Cn. 1, 2,
3_, internal, middle and external cuneiforms. (After Wortman.)]
The limbs, especially the anterior pair, were short and very stout; the
humerus had an immensely developed deltoid ridge, which extended down for
two-thirds the length of the shaft, and a very prominent supinator ridge;
the fore-arm bones, particularly the ulna, were heavy and the radius had
but a limited power of rotation. The feet were short and broad, with five
complete, spreading toes, ending in thick and blunt-pointed claws.
_†Patriofelis_ was by far the most formidable of the Bridger Carnivora
and, with the exception of _†Harpagolestes_, the largest. Its appearance
must have been very curious, judged from the modern standpoint, with its
disproportionately large, broad and rounded, leonine head, thick body
and long, extremely heavy tail. The short, powerful limbs and broad feet
must have given it something of the appearance of an otter. As in the
case of so many other †creodonts, the combination of characters in the
skeleton makes the question of habits a very puzzling one. The teeth
had a form suited only to seizing and devouring prey, but the short
legs and feet were not at all adapted to the swift movements, whether
by long-continued running, or by stealthy approach and sudden leap,
which are required in capturing agile prey, while the blunt claws could
have rendered no service in holding a struggling creature. The form of
the humerus and fore foot suggests burrowing habits, but it seems most
unlikely that so large an animal could have lived in any such fashion.
Terrestrial, arboreal and aquatic modes of life have all been suggested,
and, all things considered, perhaps the least improbable conclusion is
that _†Patriofelis_ was more or less aquatic and preyed chiefly upon
the fishes and turtles with which the Bridger waters abounded. This
hypothesis of Dr. Wortman’s is supported by the otter-like form of the
animal. Whatever the principal kind of food was, it must have been
something that greatly abraded the teeth, which in old animals were mere
stumps.
[Illustration: FIG. 284.—_†Oxyæna lupina_, Wasatch stage. Restored from a
skeleton in the American Museum.]
The Wind River representatives of the series are known only from
fragments, which, so far as they go, are not separable from
_†Patriofelis_. On the other hand, the Wasatch genus, _†Oxyæna_, is
fairly well understood. This genus was very like its Bridger successor,
but differed from it in just such ways as would be expected in an
immediately ancestral form, that is to say, in smaller size and less
advanced specialization. The number of teeth was not so far diminished:
_i_ 3/3, _c_ 1/1, _p_ 4/4, _m_ 2/2, × 2 = 40; the carnassial teeth were
the same, but they were less effective; the fourth upper premolar
and first upper molar had large inner cusps, and in the latter the
postero-external trenchant ridge was shorter. The second upper molar,
lacking in _†Patriofelis_, was a transversely placed ridge, which
engaged the heel of the second lower molar. The latter tooth, though
larger than the first molar, was much less completely trenchant than in
_†Patriofelis_ and retained a small internal cusp and quite large heel.
The skull resembled that of the Bridger genus, but the face was not so
much shortened, the zygomatic arches were not so widely expanded or so
massive, the lower jaw was not so heavy, nor the chin so steep. The body
was relatively longer and more slender, the ribs being thinner and the
chest shallower; the tail was even longer, but not nearly so thick. The
articulations of the lumbar vertebræ were less complex. Except for their
greater length and slenderness the limbs and feet were nearly identical
with those of _†Patriofelis_.
In appearance, _†Oxyæna_ must have been merely a smaller, lighter and
less powerful variant of the Bridger genus, and, no doubt, its habits
of life were substantially the same; but in the details of structure
were many minor differences, all of them in the direction of greater
primitiveness in the more ancient animal.
The second phylum of the family was represented in the Uinta and Bridger
stages by a group of small species, which were survivors of still more
ancient and primitive progenitors of the family. In the typical genus,
_†Limnocyon_, the dental formula was the same as in _†Oxyæna_: _i_ 3/3,
_c_ 1/1, _p_ 4/4, _m_ 2/2, but the first upper molar had its two external
cusps well separated and a much lower posterior cutting ridge, while the
inner cusp was much larger. The second upper molar, though transversely
placed, had all the elements of the primitive tritubercular tooth, the
pattern from which all the varied types of †creodont upper molars were
derived by the addition or suppression of parts. The two lower molars
were very primitive, having a high anterior triangle of three cusps,
forming an imperfect shearing blade, and a low heel. This dentition was
on nearly the same plan as that of the small, contemporary †hyænodonts,
but the emphasis of development, so to speak, was differently placed.
In the †hyænodonts there were three pairs of sectorials and the
best-developed pair was made up of the second upper and third lower
molar; while in _†Limnocyon_ the third molar was lost, and there were but
two pairs of sectorials, of which the largest pair was the first upper
and second lower molar, as was also true of _†Oxyæna_ and _†Patriofelis_.
The skull of _†Limnocyon_ had a much longer facial region, and more
elongate and slender jaws than in the last-named genera, and the feet
must have been quite different, with less spreading digits. _†Limnocyon_
thus tends to indicate a common origin for the †oxyænids and †hyænodonts,
though these common ancestors are still unknown.
A very interesting genus of this series, _†Machairoides_, of the
Bridger, shows another imitation of the cats, the flanges of the lower
jaw indicating sabre-like upper canines.
Another genus, _†Palæonictis_, of the Wasatch, found also in France,
is sometimes referred to the †Oxyænidæ and sometimes made the type of
a distinct family, but is too incompletely known for final reference.
It had the same number of teeth as _†Oxyæna_, but the principal pair
of carnassials was the fourth upper premolar and first lower molar, as
in the Fissipedia, the first upper and second lower molar forming the
subsidiary pair. The first upper molar was hardly sectorial at all; its
two outer cusps were long, sharp-pointed cones, and the posterior cutting
ridge was a mere tubercle. The skull had a short, cat-like face. The
genus left no successors.
* * * * *
This concludes the long story of the Carnivora, so far as it has been
recovered from the rocks. Incomplete as it is, and full of unsolved
problems, it yet enables us to follow, somewhat vaguely, but with a
general kind of accuracy, the development of the various modifications
which characterized the different families and genera of the group.
The more ancient and primitive suborder, the †Creodonta, made its first
recorded appearance in the lower Paleocene and was, no doubt, derived
from Mesozoic ancestors, which cannot yet be distinguished among the
very imperfectly understood mammals of that era. In the upper Paleocene,
if not before, the †creodonts had spread over the northern hemisphere
and had begun to diverge into a number of families, which continued to
diverge more and more widely throughout the Eocene epoch, as they became
more specialized and adapted to different habits of life. From the most
primitive group, represented more or less accurately by the †Oxyclænidæ,
may be traced the several lines of diverging adaptations incorporated in
the various families, some of which had become distinctly recognizable
in the lower Paleocene, others in the upper, while all were in existence
in the lower Eocene. In one series, the †Mesonychidæ, the upper teeth
underwent comparatively little change, while the lower ones lost the
inner cusps, but no carnassials were formed. The face and jaws were
elongated and the limbs and feet became adapted to cursorial habits, and
the more advanced genera had four-toed, completely digitigrade feet,
with blunt, almost hoof-like claws. A second series, the †Arctocyonidæ,
likewise failed to develop sectorial teeth, the molars becoming
quadritubercular, with many accessory tubercles, and assuming a bear-like
or pig-like pattern, while the premolars were reduced in size. The
pentadactyl feet had sharp claws.
In the †Oxyænidæ two pairs of carnassial teeth were formed, of which the
larger and more effective pair were the first upper and second lower
molar, the smaller pair the fourth upper premolar and first lower molar.
The teeth were diminished in number, first by the loss of the last molar,
then the suppression of the first premolar and finally by that of the
third incisor and second upper molar; the remaining teeth were enlarged.
The upper carnassial molar (the first) was formed by the approximation
and partial fusion of the two external cusps and the addition of a
trenchant ridge behind these, and by the reduction and eventual loss of
the internal cusp, thus becoming more exclusively shearing in function.
The second lower molar also lost the inner cusp and the heel, becoming
remarkably cat-like in form; the first was similar, but less simplified.
The face and jaws were greatly shortened, which, with the widely expanded
zygomatic arches, gave the head a very cat-like appearance. The body and
tail were long, the limbs short and thick, and the feet had spreading
toes and blunt claws. Save for a notable increase in size and muscular
power, the †oxyænids showed but little change within the family.
The †Hyænodontidæ differed from the †oxyænids in the retention of
all or nearly all the teeth and in having three pairs of sectorials,
of which the largest pair was the second upper and third lower
molar, but resembled them in the mode of forming these sectorials
and in the cat-like form of the inferior ones. Although the
actual line of descent was not through these genera, the series,
_†Sinopa_—_†Tritemnodon_—_†Pterodon_—_†Hyænodon_, extending from the
lower Eocene into the Oligocene, displays perfectly the successive steps
in the transformation of the teeth. The skull underwent a corresponding
series of changes, ending in long-faced, long-jawed, wolf-like forms,
with larger brain-case than in any other †creodonts. The elongated form
of body was retained, but the tail was reduced to moderate proportions.
The limbs and feet did not change greatly, except in size and in the
greater bluntness of the claws.
The †Miacidæ, if not actually referable to the Fissipedia, at least
anticipated them in the mode of carnassial development. The upper molars
changed very little from the primitive tritubercular plan, but the fourth
upper premolar was enlarged and acquired a trenchant ridge behind the
original single outer cusp. The lower molars were at first all alike,
except in size, the first being the largest; they had the primitive
pattern common to the earlier members of nearly all the †creodont
families, of an elevated anterior triangle of three subequal cusps and
low, basin-like heel. The first molar grew larger in the successive
genera and, by the enlargement of the two external cusps of the primitive
triangle and reduction of the inner one, gradually became an efficient
sectorial, the fourth upper premolar keeping pace with it. In proportion
as the first lower molar was elaborated, the second and third were
reduced in size and the anterior triangle was lowered to the level of the
heel, these teeth thus becoming tubercular. All the †Miacidæ were small
animals, none attaining the stature of a fox, though some had heads as
large. From this family, as was pointed out above, probably arose all of
the Fissipedia, the history of which it is needless to repeat.
CHAPTER XV
HISTORY OF THE PRIMATES
This order embraces the lemurs, monkeys, man-like apes and Man, though in
the general account Man will be omitted from consideration. The Primates
are clothed in dense fur or shaggy hair. The teeth are always low-crowned
and rooted and reduced in number, the incisors generally to 2/2 and the
premolars to 3/3-2/2; the molars are trituberculate or quadrituberculate.
The cranium is unusually capacious and the orbit is entirely encircled
in bone. The tail varies much in length and may be entirely wanting.
The bones of the fore-arm and lower leg are separate and the radius has
much freedom of rotation, in correspondence with the grasping power of
the hand. The pes is also a grasping organ and, with few exceptions,
the thumb and great toe are opposable to the other digits; the bones
of the wrist do not coössify and frequently the central is present.
The feet are plantigrade and almost always pentadactyl and, with a few
exceptions, have neither claws nor hoofs, but flat nails; the ungual
phalanges are correspondingly modified and do not taper toward the free
end, but expand at the tip. The Primates are characteristically arboreal
in habit, but a few, such as the baboons, have become secondarily adapted
to a terrestrial mode of life. They inhabit at present all the tropical
regions of both hemispheres, Australia excepted. Extra-tropical North
America has no existing member of the order and, so far as we know, has
had none since the Eocene epoch. The most important of the genera of the
western hemisphere are listed below.
Suborder LEMUROIDEA. Lemurs
I. †NOTHARCTIDÆ.
_†Pelycodus_, low. and mid. Eoc. _†Notharctus_, Eoc.
II. †ANAPTOMORPHIDÆ.
_†Anaptomorphus_, low. and mid. Eoc. _†Omomys_, mid.
Eoc. _†Hemiacodon_, do.
Suborder ANTHROPOIDEA. Monkeys, Apes, Man
Section PLATYRRHINA
III. HAPALIDÆ. Marmosets.
_Hapale._ Pleist. and Rec. _Midas_, Rec.
IV. CEBIDÆ. South American Monkeys.
_Cebus_, Pleist. and Rec. _Alouatta_, Howling
Monkeys, Pleist. and Rec. _Ateles_, Spider
Monkeys. _Pithecia_, Sakis. _Cacajao_, Uakaris.
_Nyctipithecus_, Douroucoulis. _†Eriodes_, Pleist.
_†Homunculus_, Santa Cruz. _†Pitheculus_, do.
The existing Primates are divided into two suborders, Lemuroidea and
Anthropoidea, which are quite clearly distinguished from each other, but
the fossil forms largely efface the distinction.
SUBORDER LEMUROIDEA. LEMURS
The name _Lemur_, which Linnæus gave to a genus of this suborder,
signifies in Latin a spectre or ghost and was probably suggested by the
very strange appearance and nocturnal habits of these curious creatures.
The term has been adopted as the English name for the group, as there
was no vernacular word for it. The lemurs are very obviously the more
primitive division of the Primates. Omitting for the present the extinct
forms, the dental formula is usually: _i_ 2/2, _c_ 1/1, _p_ 3/3, _m_
3/3, × 2 = 36; the upper canine is a long, sharp, dagger-like tooth,
but the lower one, in nearly all of the genera, is like an incisor and
its place is taken by the anterior premolar; the premolars are simple,
compressed and trenchant and the upper molars tritubercular. The skull
usually has a long and tapering facial portion, so that the living head
has some resemblance to that of a raccoon. The orbits almost always have
a more or less lateral presentation, instead of being directed forward,
as they are in the Anthropoidea; they are encircled in bone, but are not
walled in by a bony funnel; the lachrymal bone is extended on the face
and the foramen is outside of the orbit. The hind legs are longer than
the fore; the humerus retains the epicondylar foramen and the femur has
a third trochanter; the feet are plantigrade, almost always five-toed,
with opposable thumb and great toe, and having a varying proportion of
flat nails and sharp claws. The brain is of a primitive type and not much
convoluted.
All the existing and most of the fossil lemurs are small animals, some
quite minute, and only in the Pleistocene of Madagascar have large ones
been found. They are chiefly nocturnal and arboreal in habits, and
feed upon fruit and leaves, but vary their diet with insects, small
reptiles, birds and eggs. Their present geographical distribution is very
remarkable; more than two-thirds of the existing species are confined
to Madagascar; the others are in tropical Africa, southern Asia and the
Asiatic islands, as far east as Celebes and the Philippines. In the
Eocene epoch they extended all over the northern hemisphere, but have not
been found in any subsequent formation outside of their present range.
Lemurs occurred in the Uinta stage, but were much more abundant in the
Bridger, of which the best-known genus is _†Notharctus_. These Eocene
forms did not have the aberrant peculiarities of the modern lemurs, but
departed less from the primitive stock common to both of the suborders.
In _†Notharctus_ the dental formula was: _i_ 2/2, _c_ 1/1, _p_ 4/4,
_m_ 3/3, × 2 = 40, the dentition being reduced only to the extent of
losing one incisor on each side above and below; the lower canine was
not incisiform nor had the anterior premolar taken its place; the upper
molars were quadritubercular, and in the lower ones the anterior triangle
was hardly higher than the heel. The two halves of the lower jaw were
coössified at the symphysis, and the femur had lost the third trochanter.
It is not likely that _†Notharctus_ was ancestral to any of the existing
lemurs, but may have been to the numerous forms of the European upper
Eocene.
The Wasatch genera are known from very fragmentary material, but it
suffices to show that some of the genera, at least (_e.g._ _†Pelycodus_),
were decidedly more primitive than those of the Bridger. The incisors had
already been reduced to 2/2, the well-nigh universal formula among the
Primates; the upper molars were tritubercular, but with a minute fourth
cusp beginning to appear, and in the lower molars the anterior triangle
was elevated above the heel. The two halves of the lower jaw were
separate.
The Paleocene has yielded nothing that can be positively referred to
the Primates, but there was a group of genera (_e.g._ _†Indrodon_),
known only from jaws and teeth, which have been variously assigned to
the lemurs and the Insectivora and may have belonged to either order,
or have represented the transition between them. This very uncertainty
is in itself a significant fact, for it is another of the many examples
of the way in which, at that early period, the mammalian orders were so
approximated that it is often very difficult to distinguish them.
It was stated above that the distinction between existing lemurs and
anthropoids was a very clear one, but to this statement there is one
partial exception. The curious little Tarsier (_Tarsius spectrum_), an
animal about the size of the Grey Squirrel, an inhabitant of the Malay
Archipelago, is thus described by Mr. Beddard: “The ears are large and
the eyes are extraordinarily developed. The fingers and toes terminate
in large, expanded disks and are furnished with flattened nails, except
on the second and third toes, which have claws. The tail is longer
than the body and tufted at the end.... The Tarsiers are nocturnal and
particularly arboreal; they live in pairs, in holes in tree stems and
are mainly insectivorous in their food.... Like so many Lemurs, this
animal is held in superstitious dread, which is the result of its most
weird appearance.”[13] The skull is more anthropoid in character than is
that of any other lemur, the face being greatly shortened, the cranium
enlarged and the orbit not merely encircled in a bony rim, but with a
thin posterior wall of bone. There are also structural features in the
soft parts, which are more anthropoid than lemuroid.
[Illustration: FIG. 285.—Head of monkey-like lemur (_†Anaptomorphus
homunculus_) from the Wasatch. Restored from a skull in the American
Museum.]
The particular interest which _Tarsius_ possesses for the student
of American mammals is its resemblance to the Wasatch genus
_†Anaptomorphus_, the type of a family which was abundant and varied in
the lower and middle Eocene. This genus was remarkably advanced in view
of its great antiquity. The dental formula was: _i_ 2/2, _c_ 1/1, _p_
3/3-2, _m_ 3/3, × 2 = 34-36; in the upper jaw the premolars were bicuspid
and the molars tritubercular, while the lower premolars were simple. The
face was very much shortened; the orbits were very large and encircled in
bone, but without the posterior wall. This produces a decided likeness to
the Tarsier and is no doubt indicative of nocturnal habits. The cranium
was remarkably large, and no other Wasatch animal had a brain-case
so capacious in proportion to its size. A lemurine character was the
position of the lachrymal foramen outside of the orbit. The two halves
of the lower jaw were separate. It is hardly likely that these American
lemurs were the actual ancestors of the anthropoids, but they closely
represent what those ancestors must have been.
SUBORDER ANTHROPOIDEA. MONKEYS, APES, MAN
The specifically human characters will be omitted in defining the
suborder. The Anthropoidea are plantigrade, usually arboreal and
pentadactyl, with opposable thumb and great toe and thus the pes is
like a hand, hence the term “Quadrumana” formerly given to the apes and
monkeys. Except in the South American marmosets (Hapalidæ) all of the
digits have nails. The canines are generally more or less tusk-like,
projecting above (or below) the level of the other teeth; the premolars
mostly have two tubercles, like the human bicuspids, the upper molars
have three, or more commonly four, cusps and the lower, four or five.
Save in the baboons, the skull has a very short muzzle and a very large
cranium, the capacity of which is relatively greatest in the large apes;
the brain is large and complexly convoluted. The orbits present directly
forward and are deep, funnel-shaped cavities for the lodgment of the
eyeballs, a thin bony wall completely enclosing them externally and
posteriorly. The lachrymal bone and its foramen are within the edge of
the orbit; the nasal bones are short and have a nearly vertical position.
The two frontal bones are early fused into one and usually there is no
sagittal crest; the two halves of the lower jaw are coössified at the
symphysis. The tail is extremely variable in length and may be three
times as long as the body, or entirely absent. The fore and hind legs are
sometimes of nearly equal length, but far more frequently the anterior
pair are much the longer. The length of the legs in proportion to that of
the body is very different in the different families. The humerus is much
like that of Man and has no epicondylar foramen; the radius has a very
complete movement of rotation; the femur never has the third trochanter
and the lower leg bones are always separate. The thumb is more or less
opposable to the other digits, except in the marmosets, but never so
perfectly as in Man; the great toe is also opposable, but shorter than
the other digits.
The Anthropoidea are divisible into two sections, the Catarrhina,
characteristic of the Old World, and the Platyrrhina, confined to the
New. In the Catarrhina, or Old World apes and monkeys, the dental formula
is the same as in Man: _i_ 2/2, _c_ 1/1, _p_ 2/2, _m_ 3/3, × 2 = 32;
the nostrils are close together and the tympanic bullæ have tubular
entrances. Many, but not all, have cheek-pouches opening into the mouth.
The tail is never prehensile and, except in most of the large, man-like
apes (Simiidæ), there are naked callosities on the buttocks. With these
Old World forms we have no further concern, though it may be noted in
passing that Dr. Schlosser has discovered in the Oligocene of Egypt
certain monkeys (_†Parapithecus_) which he thus describes: “The number
and structure of the teeth, character of the jaws and bodily size make
complete the transition from the Anaptomorphids and Tarsiids to the
Simiids.”
_Section Platyrrhina. South American Monkeys_
In these animals the nostrils are separated by a broad septum, and
there are always three premolars above and below (_p_ 3/3). The tail
is frequently prehensile and serves as a fifth limb, being capable of
supporting the whole weight of the body. There are no cheek-pouches and
no callosities, and the tympanic bullæ have no bony tubes leading into
them. The thumb is but partially, or not at all, opposable and in some
genera is absent.
The New World monkeys are, in general, smaller and lighter than those
of the eastern hemisphere; there are no very large ones and they are
all arboreal and are confined to the forested parts of the Neotropical
region, except the West Indies, which have none. The marmosets
(Hapalidæ), the first of the two families into which the Platyrrhina
are divided, are little creatures, no longer than squirrels, with long,
non-prehensile tails. They are characterized by the dental formula: _i_
2/2, _c_ 1/1, _p_ 3/3, _m_ 2/2, × 2 = 32, and are the only Primates which
have no third molar above or below. The thumb is not opposable, though
quite long, and the hallux, or great toe, is very small; they are thus
deficient in grasping power. Instead of the flat nails common to all the
other Anthropoidea, they have long, sharp claws. All other South American
monkeys are included in the family Cebidæ which, in turn, is divided into
four subfamilies. It is not necessary to consider these or do more than
cite a few illustrative examples.
[Illustration: FIG. 286.—Common Marmoset (_Hapale_).—By permission of W.
L. Berridge, London.]
[Illustration: FIG. 287.—Sapajou (_Cebus_).—By permission of the New York
Zoölogical Society.]
Some twenty species of the genus _Cebus_ are distributed from Central
America to Paraguay; they have long, prehensile tails completely covered
with hair, and well-developed thumbs. The monkeys of this genus are
familiar to every one, as they are largely used by organ-grinders. The
spider-monkeys (_Ateles_) are so called because of the great length
and slenderness of their limbs; the tail is very long and perfectly
prehensile, naked on the lower side near the end, which improves its
grasping power. The hand has lost the thumb, but is used very effectively
as a hook. The species, ten or more in number, have a wider range than
those of _Cebus_ and extend from Uruguay to Mexico.
The howling monkeys (_Alouatta_, more commonly, but improperly, called
_Mycetes_) are gifted with most unusual vocal powers. Mr. Bates says of
them: “Morning and evening the howling monkeys make a most fearful and
harrowing roar.” “The brief evening chorus of animals then began, the
chief performers being the howling monkeys, whose frightful unearthly
roar deepened the feeling of solitude which crept on as darkness closed
around us.”[14] The tremendous volume of sound which these small
creatures are able to produce is due to a resonating apparatus, formed by
the great inflation of one of the hyoid bones (see p. 67), normally the
bony support of the tongue. The tail is long and prehensile, with the end
naked beneath; the thumb is well developed.
The sakis (_Pithecia_) have long and non-prehensile tail and complete
thumb. The species of this genus have a remarkable kind of distribution,
which is rare among mammals, though not infrequent for insects and birds.
Each species is limited to a definite area of forest along the Amazon and
its tributaries, which it occupies to the exclusion of the others. The
uakaris (_Cacajao_) are distinguished by the tail, which is much shorter
than in any other of the Cebidæ.
Finally, may be mentioned the nocturnal douroucoulis (_Nyctipithecus_),
which have long, non-prehensile tail and well-developed thumb. Mr. Bates
describes them thus: “A third interesting genus of monkeys, found near
Ega, are the Nyctipitheci, or night apes, called Ei-á by the Indians....
They sleep all day long in hollow trees and come forth to prey on insects
and eat fruits only at night. They are of small size, the body being
about a foot long and the tail fourteen inches, and are thickly clothed
with soft gray and brown fur, ... and the eyes are large and yellowish
in colour, imparting the staring expression of nocturnal animals of
prey.”[15]
The Brazilian caverns have preserved the remains of many Pleistocene
monkeys belonging to existing South American genera, and even several
modern species are represented, while others are extinct. There is also
one extinct genus (_†Eriodes_), a larger animal than any of the existing
Neotropical monkeys. The Pampean deposits of Argentina, on the other
hand, have yielded no remains of Primates, nor is this surprising, for
the Pampas would seem to have been open plains in the Pleistocene, as
they are to-day. Between the Pleistocene and the Santa Cruz Miocene
there is a long gap in the history. It is true that some bones have been
found in the Pliocene of Monte Hermoso which have been referred to the
Primates, but they are too few and imperfect to be of any real assistance
in the inquiry.
In the Santa Cruz beds fossil monkeys are very rare, but that they were
present in Patagonia at all, is strong evidence that the climate was then
far milder than it is at present. These were essentially members of the
modern family Cebidæ. The best-known genus, _†Homunculus_, retained a few
primitive characters, which the existing genera have lost. For example,
the premolars were relatively smaller and of simpler form and the humerus
had the epicondylar foramen, though the femur no longer had the third
trochanter. The radius was very modern in form and evidently could rotate
freely upon the humerus.
No monkeys have been found in the Deseado formation, though too much
stress should not be laid upon this fact, because of the general scarcity
of small animals in those beds. But the same is true of the still more
ancient stages; despite an abundant and varied fauna of small mammals,
they have yielded no Primates, nor anything which could be seriously
regarded as ancestral to them. The facts are essentially the same as we
have found them to be with reference to the South American rodents and
insectivores. All three of these orders appeared suddenly and unheralded
in the Deseado (Rodentia) or Santa Cruz (Insectivora, Primates), and all
of them were allied to African or European rather than to North American
types. If we may assume the existence of a land-connection with Africa
to account for the remarkable distribution of the hystricomorph rodents,
the same connection will equally well explain the introduction of the
Primates into South America.
Concerning the relations of the Old and New World monkeys, Mr. Beddard
remarks: “Not only are these two groups of the Primates absolutely
distinct at the present day, but they have been, so far as we know,
for a very long time, since no fossil remains of Monkeys at all
intermediate have been so far discovered. This has led to the suggestion
that the Monkeys are what is termed diphyletic, _i.e._, that they have
originated from two different stocks of ancestors. It is hard, however,
to understand on this view the very great similarities which underlie
the divergences that have just been mentioned. But, on the other hand,
it is equally hard to understand how it is that, having been separated
from each other for so long a period, they have not diverged further in
structure than they have.”[16]
The fossil monkeys of the Santa Cruz beds show that, as a matter of
fact, the South American Primates have undergone little change in the
essentials of structure since that remote period, and thus is removed
this objection to the conclusion that the Platyrrhina and Catarrhina
were derived from a common ancestry. In a certain sense also, the
discovery of _†Parapithecus_ in Egypt has diminished the gap between
these two sections of the Anthropoidea. The evidence, though by no means
conclusive, is distinctly in favour of the derivation of the South
American monkeys from Old World ancestors. The Catarrhina have developed
and advanced from the point of divergence far more than have the South
American forms, which have changed relatively little since their invasion
of the Neotropical region. So far as has been ascertained, South America
never had any of the lemurs.
MAN IN THE WESTERN HEMISPHERE
Though to most people this is undoubtedly the most interesting chapter
of all the mammalian history, little space can be given to it here, for
the reason that the subject belongs rather in the domain of Anthropology
and Ethnology than in that of Palæontology. There can be no question that
Man originated in the eastern hemisphere and at a very remote period;
abundant remains of his handiwork and of himself have been found in
Europe as far back as the early Pleistocene, and recent discoveries in
England have increased the already known length of the human habitation
of Europe. So primitive and ape-like were some of these ancient men
that they have been named as species (_Homo †neanderthalensis_ and _H.
†heidelbergensis_) distinct from the existing _H. sapiens_. Recently
discovered and very ancient remains in England have even been referred to
a separate genus, _†Eoanthropus_.
As has been repeatedly pointed out in the preceding chapters, America
received numerous successive waves of mammalian immigrants during the
Pleistocene epoch, at a time when there was a broad land-connection
between North America and Asia, where now is Bering Strait; and to this
late connection is due the fact that the Boreal zone of North America
(see p. 150) is zoölogically a part of the Old World and forms a division
of the Holarctic region. Now, there is no known reason why Man, whose
powers of dispersal are so superior to those of any other mammal, should
not have accompanied these migrations, and it is entirely possible that
he actually did so, but the fact has not been demonstrated. It is true
that discoveries of Pleistocene Man have been frequently reported from
both North and South America, but these have not stood the test of
critical examination, though such examination has by no means disproved
the presence of Pleistocene Man in America.
Dr. A. Hrdlička has recently concluded a series of exhaustive studies
of the bones of early Man in both North and South America and of the
localities where these bones were found. For both continents he has
reached a negative result. As to North America he says: “Thus far on
this continent no human bones of undisputed geologic antiquity are
known.”[17] For South America the result is the same. “A conscientious,
unbiased study of all the available facts has shown that the whole
structure erected in support of the theory of geologically ancient man
on that continent rests on very imperfect and incorrectly interpreted
data and in many instances on false premises, and as a consequence of
these weaknesses must completely collapse when subjected to searching
criticism.” “The conclusions of the writers with regard to the evidence
thus far furnished are that it fails to establish the claim that in South
America there have been brought forth thus far tangible traces of either
geologically ancient man himself or any precursor of the human race.
“This should not be taken as a categorical denial of the existence of
early man in South America, however improbable such a presence may now
appear.”[18]
On the other hand, the coexistence in North America of Man with several
extinct species of mammals has been made extremely probable, if not
certain. One of the most striking and best authenticated cases of this
was the discovery by Professor Williston in western Kansas of a flint
arrowhead beneath and in contact with the skeleton of the extinct _Bison
†occidentalis_. Professor Russell found in lake deposits of Nevada an
obsidian spear-head in association with the bones of an elephant or
†mastodon, and other such instances have been reported. In these cases
the doubt is as to the geological antiquity of the “finds,” for the
implements are of the type made by the pre-Columbian Indians.
In brief, there is no convincing evidence that either North or South
America was ever inhabited in prehistoric times by races of men different
from those first encountered by the European discoverers.
CHAPTER XVI
HISTORY OF THE EDENTATA
As here employed, excluding the so-called edentates of the Old World, the
Edentata form a highly variegated, but natural, assemblage of related
forms. The order is at present exclusively American and almost confined
to the Neotropical region, an armadillo which extends into Texas being
the sole exception. These animals are so peculiar and so isolated from
other mammals, that it has been proposed to treat them as a separate
subclass; and there is much to be said in favour of this procedure,
though it would perhaps be premature, until more is learned concerning
these most curious and exceptional animals. In the subjoined table only
the more important and better known genera are included.
Series PILOSA. Hairy Edentates
Suborder TARDIGRADA. Tree-Sloths
I. BRADYPODIDÆ.
_Bradypus_, Three-toed Sloth, Rec. _Cholœpus_,
Two-toed Sloth, Rec.
Suborder VERMILINGUA. Anteaters
II. MYRMECOPHAGIDÆ.
_Myrmecophaga_, Ant-Bear, Rec. _Tamandua_, Lesser
Anteater, do. _Cyclopes_, Tree Anteater, do.
Suborder †GRAVIGRADA. †Ground-Sloths
III. †MEGATHERIIDÆ.
_†Megatherium_, Plio. and Pleist., S. A.; Pleist., N.
A. _?†Prepotherium_, Santa Cruz. _?†Planops_, do.
IV. †MYLODONTIDÆ.
_†Mylodon_, Plio. and Pleist., S. A.; Pleist., N.
A. _†Paramylodon_, Pleist., N. A. _†Grypotherium_,
Pleist., S. A. _†Pseudolestodon_, Plio. and Pleist.,
S. A. _†Scelidotherium_, do. _†Nematherium_, Santa
Cruz. _†Analcitherium_, do.
V. †MEGALONYCHIDÆ.
_†Megalonyx_, Pleist., N.A. _†Nothrotherium_,
Pleist., S. A. _†Megalocnus_, Pleist., Cuba.
_†Hapalops_, Santa Cruz. _†Schismotherium_, do.
_†Pelecyodon_, do. _†Megalonychotherium_, do.
_†Protobradys_, Casa Mayor.
Series LORICATA. Armoured Edentates
Suborder DASYPODA. Armadillos
VI. DASYPODIDÆ.
_Dasypus_, 6-, 7- and 8-Banded Armadillos, Pleist.
and Rec., S. A. _Cabassous_, 11-Banded Armadillo,
do. _Priodontes_, Giant Armadillo, do. _Tolypeutes_,
Apar, Rec., S. A. _Zaëdyus_, Pygmy Armadillo, do.
_Scleropleura_, do. _Chlamydophorus_, Pichiciago, do.
_Tatu_, 9-Banded Armadillo, Pleist. and Rec., S. A.;
Rec., Texas. _†Eutatus_, Plio. and Pleist., S. A.
_†Chlamydotherium_, do. _†Proeutatus_, Santa Cruz.
_†Prozaëdius_, Deseado and Santa Cruz, _†Prodasypus_,
do. _†Stegotherium_, Santa Cruz. _†Meteutatus_,
Deseado. _†Sadypus_, do. _†Amblytatus_, do.
_†Prœuphractus_, do.
VII. PELTEPHILIDÆ.
_†Peltephilus_, Deseado and Santa Cruz.
VIII. INCERTÆ SEDIS.
_†Metacheiromys_, mid. Eoc., N. A.
Suborder †GLYPTODONTIA. †Glyptodonts
IX. †GLYPTODONTIDÆ.
_†Glyptodon_, Plio. and Pleist., N. and S. A.
_†Dœdicurus_, Pleist., S. A. _†Panochthus_,
do. _†Sclerocalyptus_, Plio. and Pleist.,
S. A. _†Glyptotherium_, mid. Plio., N. A.
_†Propalæohoplophorus_, Deseado and Santa Cruz.
_†Cochlops_, Santa Cruz. _†Eucinepeltus_, do.
_†Asterostemma_, do.
In the section Pilosa, which includes the sloths (Tardigrada), anteaters
(Vermilingua) and the extinct †ground-sloths (†Gravigrada), the skin
is thickly clothed with long hair, and in the Loricata, armadillos and
†glyptodonts, the head, body, tail and legs are more or less completely
encased in an armour of bony scutes covered with plates of horn, but with
some hairs also.
The name Edentata (toothless) is not very happily chosen, for only the
anteaters are quite toothless. Almost all the genera have no teeth in
the front of the mouth and the teeth are nearly always alike, so that
the distinction of regions among them is entirely a matter of position
in the jaws. In the tree-sloths and many †ground-sloths the foremost
tooth in each jaw is a more or less enlarged, canine-like tusk. The
teeth are always rootless, growing from permanent pulps, and are without
enamel, made up of dentine, which is sometimes homogeneous and sometimes
in layers of different hardness, and with a covering of cement, usually
thin and film-like. The number of teeth varies from 4/4 to 10/10 or more,
and their form usually approximates a simple cylinder, worn off flat at
the end, though the ends may be bevelled or grooved, differences which
are in no way due to pattern but simply to the mode of wear. In the
†glyptodonts the teeth were divided by deep vertical grooves into two
or three pillars, connected by narrow necks. In most of the edentates
there is no change of teeth, the milk-dentition having been completely
suppressed, but in the 9-Banded Armadillo (_Tatu_) each of the permanent
teeth is preceded by a two-rooted milk-tooth, and some other armadillos
have milk-teeth.
The skull varies much in form and proportions, according to the character
of the food and method of feeding. The tree-sloths and †ground-sloths
have short, rounded heads; in the †glyptodonts, the skull was short and
remarkably deep vertically; while the armadillos have long, shallow
heads, with tapering muzzle, the length and slenderness of which differ
in the various genera. In the anteaters the skull is extraordinarily
elongate and slender. The sagittal crest is seldom present at all and
never prominent. The zygomatic arch may be complete or interrupted;
in the tree-sloths, †ground-sloths, †glyptodonts and some extinct
armadillos, there is a descending, plate-like process given off beneath
the eye.
The backbone displays some of the most remarkable peculiarities of
the order. The neck in the tree-sloths has eight or nine vertebræ,
the only instances known among mammals in which the normal number of
seven is departed from. In the armadillos and †glyptodonts several of
the neck-vertebræ are coössified into a single piece, but the atlas is
always free, so as to permit the movements of the head. In the posterior
part of the dorsal and in the lumbar region the articulations between
the successive vertebræ are by far the most complex and intricate
known among mammals; in the tree-sloths these have degenerated, though
still plainly indicated. In the †glyptodonts, which were covered with
a huge, tortoise-like carapace, mobility of the backbone was needless,
and so all of the dorsal vertebræ were united into one long piece and
the lumbars were coössified with one another and with the sacrum.
The sacrum consists, throughout the order, of a very large number of
vertebræ and is attached to the hip-bones at two different points,
instead of only one, as in other mammals. The tail varies much in
length and thickness; in the tree-sloths it is extremely short and in
the anteaters very long and bushy, prehensile in the arboreal members
of the group; in the †ground-sloths, especially the gigantic forms, it
was of immense thickness; while in most of the †glyptodonts a varying
number of the terminal vertebræ were fused together. The sternal ribs
are better developed than in any other mammals, and in the anteaters and
†ground-sloths they articulate with the breast-bone by regular synovial
joints, and each rib has head and tubercle like a vertebral rib.
In the limbs and feet there is great variety, according to the manner
of their employment. The shoulder-blade has a very long acromion and
very large coracoid, which long remains separate from the scapula;
collar-bones are very generally present, though often in much reduced
condition. The hip-bones have in the tree-sloths, †ground-sloths and
†glyptodonts a much expanded anterior element, which in the other groups
is narrow. The humerus usually has very prominent deltoid and supinator
ridges and epicondylar foramen; the fore-arm bones are always separate,
and there is generally much freedom of rotation of the manus. In the
wrist there is no distinct central and usually there are the ordinary
eight separate bones. The tibia and fibula are frequently coössified.
The tree-sloths, which lead most strictly arboreal lives and are almost
helpless on the ground, are unique among mammals in that the body is
habitually _suspended_ from the limbs, not carried upon them; the feet
are curved hooks, which fit over the tree-branches and support the weight
without muscular exertion. The limb-bones are very long and slender,
the claws long, curved and sharp, and the metapodials of each foot, two
or three in number, are fused into a single mass. In the †ground-sloths
there was much change in foot-structure during the course of their
recorded development; they were usually five-toed and the feet were armed
with one or more great claws; the later and larger representatives of the
suborder walked upon the outer edge of the feet.
The armadillos, which are largely burrowers, have five-toed feet and
long, heavy, pointed claws, but in some of them the pes has a varying
number of flat, hoof-like nails. The immense †glyptodonts had very short,
broad feet, shod with hoofs, which, in some of the genera, were longer
and more claw-like in the manus.
The recorded history of the edentates was developed almost entirely
in South America. In the Casa Mayor formation there were numerous
armadillos, but as only scutes of the carapace have been found, little is
known of them. The †ground-sloths (_†Protobradys_) have been reported,
but from such imperfect material that the reference is uncertain. The
first assuredly determinable members of this suborder were in the
Astraponotus beds and, associated with them, the most ancient known
†glyptodonts. In the Deseado stage were many armadillos, some of them
extremely peculiar, several †glyptodonts and †ground-sloths, some species
of the latter very large. Edentates were far more numerous and varied
in the Santa Cruz than in any of the preceding stages. Tree-sloths and
anteaters have both been reported, but the evidence is insufficient,
though there can be little doubt that these suborders had begun their
separate existence in some part of South America other than Patagonia.
The three families of †ground-sloths were already distinguishable,
though much less clearly separated than they afterwards became; none
of them were large animals, smaller even than some of the Deseado
species and veritable pygmies in comparison with the giants of the
Pliocene and Pleistocene. The †glyptodonts were likewise far smaller
than their Pliocene and Pleistocene successors and in several respects
more primitive, approximating the armadillos more closely; nor was
there any such variety of forms as in the later stages. The armadillos
were extremely numerous and varied; they all belonged to extinct genera
and most of them apparently have no descendants at the present day.
The tropical forests of Brazil and the Guianas must then, as now, have
swarmed with mammals which did not extend their range to Patagonia and
of which we consequently have no record. No doubt, it was in these
forests that the ancestors of most modern armadillos, as well as of the
tree-sloths and anteaters, lived in Miocene times.
Pliocene edentates were of the same suborders as those of the Santa
Cruz, but far larger in size. Most of them are known only from very
incomplete specimens, but the Pleistocene has yielded an enormous mass
of beautifully preserved material. Of the tree-sloths and anteaters,
only questionable remains have been found. That these tropical and
forest-loving animals should not have occurred in the open Pampas of
Argentina is not surprising, but it is difficult to account for their
absence from the extremely rich cave-faunas of Brazil. Nearly all the
existing genera of armadillos have been obtained, and with these were
associated several extinct genera, some of them (_†Chlamydotherium_,
_†Eutatus_) relatively huge, as large as tapirs. There was a wonderful
variety of †glyptodonts, most of them enormous creatures, of which no
less than five genera have been collected in Argentina and Brazil, and
the †ground-sloths were even more numerous and varied. Nine genera,
with many species, of these great beasts, which ranged in size from an
elephant to a tapir, are already known and no doubt the list is still
incomplete. These †glyptodonts and †ground-sloths must have been among
the most conspicuous elements of the Pleistocene fauna.
Aside from certain problematical Eocene forms, the first North American
edentates, which were immigrants from the southern continent, appeared
probably in the middle Miocene of Oregon in the form of †ground-sloths,
but the specimen, as well as a similar one from the lower Pliocene of
Nebraska, is not sufficiently complete for positive reference. In the
middle Pliocene the †ground-sloths and †glyptodonts were unquestionably
present, and in the Pleistocene these two suborders were numerously and
conspicuously represented. Three or four genera of the huge, elephantine
†ground-sloths coexisted in Pleistocene North America. _†Megalonyx_
was abundant in the forested regions east of the Mississippi, from
Pennsylvania southward, and on the Pacific coast; _†Mylodon_ was
transcontinental in distribution; while _†Megatherium_ was apparently
confined to the southern states. While all three genera undoubtedly
originated in South America, _†Megalonyx_ has not yet been found in that
continent.
This genus was originally named by President Jefferson in 1805 from an
ungual phalanx found in a cave in Virginia, and he imagined that it
belonged to a colossal lion which must still be living in the mountains
of western Virginia. This was deduced from the assumption that no species
could become extinct, and the passage is of interest as showing the
prevalent belief of the time, although Cuvier had already demonstrated
that many species had actually been extinguished. The passage is as
follows: “The movements of nature are in a never ending circle. The
animal species which has once been put into a train of motion is still
probably moving in that train. For, if one link in nature’s chain might
be lost, another and another might be lost, till this whole system of
things should evanish by piecemeal.”
The †glyptodonts were also southern in distribution, and only very
imperfect remains of them have yet been recovered from the North American
Pleistocene, quite sufficient, however, to make the identification
certain.
There were several genera of rather small †ground-sloths in the
Pleistocene of Cuba. The best known of these, _†Megalocnus_, had
several peculiarities of structure, but was plainly a member of the
†Megalonychidæ. The ancestors of this genus probably invaded Cuba in the
Pliocene, when the island was joined to Central America.
SUBORDER †GRAVIGRADA. †GROUND-SLOTHS
As the †ground-sloths would appear to have had a more central position
within the order than any of the other groups, our study of development
may well begin with them. In the Pleistocene there were three families
of these gigantic brutes, which ranged through the western hemisphere
from Pennsylvania and California to Patagonia. Unfortunately our
knowledge of the developmental stages within the different families is
very unequal, and it is therefore impracticable to do more than sketch
the changes of the suborder as a whole and in a general way. In the
successive geological stages the proportionate representation of the
different phyla varied greatly; in the South American Pliocene and
Pleistocene the †Mylodontidæ and †Megatheriidæ were the abundant forms,
while the †Megalonychidæ were but scantily represented. In the Santa
Cruz Miocene, on the other hand, the overwhelming preponderance was with
the †Megalonychidæ, the other two families being comparatively rare and
incompletely known. From the still more ancient formations, the material
so far collected is so fragmentary that family distinctions have little
meaning. After all, there was no very wide range of variation among the
contemporary members of the three families, and the differences were
principally in size, in the form and number of the teeth, the shape of
the skull and the number of digits; in essentials they were all much
alike.
The genus _†Megatherium_ (Fig. 122, p. 220) included the largest and
most massive members of the suborder, _†M. americanum_ being as large as
an elephant, but very differently proportioned, as it was much longer
and lower in stature, owing to the shortness of the extraordinarily
heavy limbs; some of the skeletons measure 20 feet or more in length.
The teeth, which were 5/4 in number, formed an uninterrupted series on
each side; all had the same quadrate form and by abrasion were worn into
two transverse ridges, formed by the meeting of the harder dentine with
the thick coating of cement. The result was a form of tooth which much
resembled the lower molars of a tapir, but it was not a tooth-pattern in
any proper sense of the word, being due entirely to the mode of wear.
The skull was very small in proportion to the huge body and was low
and narrow in shape; the cranium had a broad, flat roof, without
sagittal crest; the orbit was completely encircled in bone, and the
descending process of the zygomatic arch beneath the eye was very long
and conspicuous. The nasals were short, and the slender, toothless
premaxillaries projected far in front of them, which makes the presence
of some sort of a proboscis likely. The lower jaw had a long, narrow,
spout-like symphysis, which was abruptly rounded at the free end,
not pointed; below the teeth, the lower margin of the jaw was very
strongly convex, descending in a great flange. The neck was short, the
body very long and enormously heavy, as was also the tail. The immense
shoulder-blade had a very long acromion, which curved forward and inward,
fusing with the coracoid and forming a bony loop or bridge. The hip-bones
had the anterior element (ilium) enormously expanded transversely, so as
to support the huge mass of viscera in the semi-erect position which the
animal, it is believed, frequently assumed in feeding. Collar-bones were
present.
The fore limb was very much more slender than the hind, but of nearly the
same length. The humerus had a comparatively slender upper portion and
extremely broad lower end, due to the great development of the internal
epicondyle and supinator ridge; there was no epicondylar foramen. The
radius evidently had the power of very free rotation upon the humerus.
The femur was short, flattened antero-posteriorly, but excessively
broad and heavy, and had no third trochanter. The tibia and fibula
were likewise short and very massive and were extensively coössified
at each end, leaving but a short interspace open between the bones.
The very peculiar feet were so connected with the limb-bones, that the
animal must have walked upon the outer edge of the foot, somewhat as the
existing Ant-Bear (_Myrmecophaga jubata_) uses the fore foot. The manus
had four functional digits, the first being a mere vestige; the fifth,
upon which the weight rested in walking, had two very small phalanges
and no claw, while the second, third and fourth had long, sharp claws.
The pes had but three functional digits, for the first and second were
reduced to rudiments; digit III had an enormous claw and of this digit
the metatarsal was short and very heavy and the first two phalanges were
fused together; the two external digits, Nos. IV and V, had no claws. The
astragalus had a very peculiar shape, made necessary by the application
of the external border of the foot to the ground and thus in both fore
and hind feet the great claws were turned inward and, in the case of the
pes, it must have been impossible to rest the sole upon the ground. The
heel-bone was enormous and club-shaped and formed the hinder portion of
the weight-carrying outer edge of the foot.
Almost all who have studied the structure of this extraordinary beast are
agreed as to its habits. That it fed principally, if not exclusively,
upon leaves, is indicated by the teeth. The general opinion as to its
manner of life is well summed up by von Zittel: “The hip-bones, hind legs
and tail are characterized by enormous strength. The entire structure
of the extremities proves that the gigantic sloth could move over the
ground but slowly and clumsily; on the other hand, the fore limbs
served as grasping organs and were presumably employed to bend down and
break off twigs and branches and even to uproot whole trees, while the
weight of the body was supported upon the hind legs and tail.”[19] It
would be quite absurd to suppose that such ponderous animals could have
been climbers or burrowers, hence the function of the enormous claws,
especially the single one of the pes, is not obvious, though they may
have been merely the weapons of the otherwise defenceless monsters. The
great claw in the fore foot of the Ant-Bear is a terrible weapon, with
which the creature vigorously and successfully defends itself against
dogs, and it may even be dangerous to men, if incautiously molested.
_†Megatherium_ had no bony scutes, or other ossifications in the skin, so
far as is known, and was probably covered with long and coarse hair, as
is known to have been the case in another †ground-sloth.
Less specialized in many respects than the †megatheres was _†Mylodon_,
type of a family which was numerously and variously represented in the
Pleistocene of South America, much less so in that of North America.
_†Mylodon_ was smaller and lighter, being from ⅓ to ¼ smaller in linear
dimensions than _†Megatherium_, and the contemporary _†Scelidotherium_
was no bigger than a tapir. The teeth numbered 5/4 and the anterior one
above and below had a somewhat tusk-like form; the others were worn off
evenly, with nearly horizontal grinding surface, but a vertical groove
on the inner side gave them a subtriangular, lobate form. The skull
was short and broad, with flat top, and orbit only partially enclosed
behind; the premaxillaries were very short and the muzzle very broad and
abruptly truncated, the nasal opening very large. The lower jaw had a
straight inferior border, a short, very wide and shovel-shaped symphysis
and square chin. Nothing indicates a proboscis, and the head must have
been very different from that of _†Megatherium_.
Within the family of the †mylodonts there was some variety in the
dentition and more in the shape of the skull. In _†Lestodon_, for
example, the first tooth in each jaw was a large, sharp-pointed tusk,
the muzzle was greatly broadened, and the whole animal was larger.
_†Scelidotherium_, the smallest Pleistocene member of the family,
had a much narrower and more elongate skull than the others. In
_†Glossotherium_, which also had an elongate skull, there was an arched
bony bridge connecting the anterior end of the nasal bones with the
premaxillaries and dividing the nasal opening into two parts.
The neck, body and tail of _†Mylodon_ did not differ materially from
those of _†Megatherium_, except in being smaller and less massive. The
fore limb was relatively somewhat shorter and much stouter, but otherwise
similar; the humerus had no epicondylar foramen and the femur no third
trochanter; the tibia and fibula were separate. The manus had five
digits, Nos. I, II and III carrying claws, that of III being especially
large; IV and V had no claws and the outer edge of the manus rested on
the ground in walking, the sole turned inward. The pes had lost the first
digit, the second and third had claws, but not the fourth and fifth; the
weight rested on the outer edge.
The skin is definitely known from large pieces belonging to the allied
genus _†Grypotherium_, found in a cavern near Last Hope Inlet, Patagonia,
where it had been preserved by burial in dry dust. Externally, the skin
was thickly covered with coarse hair and in the deeper layers was a
continuous armour of small ossicles, which were close set and in the
Last Hope specimens show like a cobble-stone pavement on the inner side
of the skin, the innermost layers of which have been destroyed; in life,
these small bones were not visible. Similar ossicles have been found
in association with several skeletons of _†Mylodon_. The habits, diet
and mode of feeding of the latter were no doubt essentially similar to
those of _†Megatherium_, but _†Scelidotherium_, which had a much shorter
and lighter tail, was probably more quadrupedal and browsed upon low
shrubbery.
[Illustration: FIG. 288.—Gigantic †ground-sloth (_†Mylodon robustus_),
Pampean. Restored from Owen’s figure of the skeleton.]
The third family, the †Megalonychidæ, was scantily represented in the
Pleistocene of South America, but relatively common in North America.
_†Megalonyx_ was, on the whole, less specialized than _†Mylodon_ or
_†Megatherium_, but had a strong resemblance to both of them. The
teeth, 5/4 in number, had the foremost one in each jaw separated by a
considerable space from the others and more or less tusk-like in form;
the grinding teeth were worn smooth, without ridges, and of somewhat
trihedral shape. The skull was short, broad and deep, resembling in shape
that of the tree-sloths; there was a long, but feebly developed sagittal
crest, and the orbits were widely open behind, with hardly a trace of
any posterior boundary. The muzzle was very short and broad and abruptly
truncated and the premaxillary bones were extremely small. The lower
jaw was short, thick and massive, with very broad symphysis and almost
vertical chin. Neck, body, tail, shoulder and hip-bones did not differ
sufficiently from those of _†Megatherium_ to require particular notice.
The fore limb was shorter and more slender than the hind; the humerus had
the epicondylar foramen and the very massive femur retained the third
trochanter; the tibia and fibula were separate. The feet had five digits,
three of which carried claws; the calcaneum was very peculiar, not at
all like the massive, club-shaped bone of _†Megatherium_ and _†Mylodon_,
but long, comparatively thin and sickle-shaped. Nothing in the skeleton
suggests that the creature’s habits differed in any important way from
those of the genera last named.
_†Megalocnus_, of the Cuban Pleistocene, a member of this family,
was apparently peculiar to the island and was probably derived from
ancestors which in the Pliocene migrated from Central America. Aside
from certain remarkable peculiarities of the teeth, this animal was more
primitive, as well as smaller, than any other of the Pleistocene genera.
Although remains of †Gravigrada are comparatively common in all of the
fossiliferous formations between the Pampean and the Santa Cruz, the
material is too imperfect to throw any useful light upon the development
of the various families. From the Santa Cruz beds, on the other hand, a
great wealth of specimens has been obtained, and it is possible to give
some fairly adequate account of the †ground-sloths of that time. These
animals were then extremely abundant individually and of extraordinary
variety; evidently, they were in a state of rapid expansion and divergent
evolution along many lines, for hardly any two specimens are alike
and therefore the satisfactory discrimination of species is well-nigh
impossible. Yet, with all this remarkable variability, the range of
structural differences was not great; the group was a very homogeneous
and natural one, and separation into families was not obvious. Two of the
three families were, however, unequivocally present in this fauna and
the third somewhat doubtfully so. The †Megalonychidæ, which in the South
American Pleistocene had dwindled to such insignificant proportions,
formed the overwhelming majority of the Santa Cruz †Gravigrada;
the †Mylodontidæ were quite rare in comparison and are still very
incompletely known; while the †Megatheriidæ, though probably present,
have not been identified beyond all doubt.
All of the Santa Cruz †ground-sloths were small animals, the largest not
approximating the smallest Pleistocene species, those of Cuba excepted,
and many of them were no larger than the modern tree-sloths. This was
a wonderful difference between the Santa Cruz and the Pampean, but a
difference which involved nearly all other groups of mammals. So far
as the skeleton is concerned, this is known with completeness only for
the †Megalonychidæ, especially the genus _†Hapalops_; but enough has
been learned of the others to show that there was far less difference
between the families than had arisen in the later epochs. This backward
convergence of the three groups towards a common term plainly indicates
their common origin, being exactly what might have been predicted in
advance of experience.
[Illustration: FIG. 289.—Santa Cruz †ground-sloth (_†Hapalops
longiceps_) and †glyptodont (_†Propalæohoplophorus australis_). Restored
by Knight from skeletons in Princeton University and the museum of La
Plata.]
In all the genera the teeth number 5/4; the teeth on each side were
sometimes in continuous series, sometimes the first one was isolated
and almost always more or less tusk-like, most so in _†Eucholœops_. The
other teeth were usually of transversely elliptical shape and worn into
two ridges, with a hollow between; the †mylodonts (_†Nematherium_, etc.)
already had the triangular, or lozenge-shaped, lobate form of teeth,
characteristic of the family.
The skull varied considerably in its proportions; generally, it was long
and narrow, with shortened face and elongate cranium; the sagittal crest
was seldom present, never prominent, and the orbit was always widely open
behind, without postorbital processes. The premaxillaries were always
short and toothless and in most of the genera they were slender rods, in
others (_e.g._ _†Hyperleptus_) broad and plate-like. The lower jaw had an
elongate spout-like symphysis, in which the two halves were coössified,
tapering forward to a blunt point and, though the length of this spout
differed greatly in the various genera, in none was there a broad, abrupt
chin such as _†Mylodon_ and _†Megalonyx_ had. In _†Prepotherium_, which
is believed to be referable to the †Megatheriidæ, the lower jaw had the
extremely convex inferior border, in less exaggerated degree, of its huge
Pampean successor; it would be premature to say descendant.
While the long, slender skull was the prevailing type among the Santa
Cruz †Gravigrada, there was a group of small animals in which the skull
was shorter and more rounded and had a very suggestive likeness to that
of the modern tree-sloths, as was likewise true of the teeth.
Despite innumerable variations of detail, the skeleton of the Santa Cruz
†ground-sloths may be described without distinction of genera, though
it should be added that the skeleton is but partially known in many of
the genera, and fuller knowledge might require modification of some of
the statements. The neck was of moderate length, the body long, the
tail long and heavy and, in some instances, very massive. The sternal
ribs were completely ossified and already had the same elaborate mode
of articulation with the breast-bone as in the great Pampean forms, and
the vertebræ the same intricate connections. The shoulder-blade also had
the same characteristics as in the latter, but the hip-bones had but a
moderate transverse expansion, having no huge mass of viscera to support.
The limbs were stout and short, fore and hind legs of nearly equal
length; the humerus had the epicondylar foramen and the broad, flattened
femur retained the third trochanter. The radius had a discoidal upper
end, which rotated freely upon the humerus; the tibia and fibula were
always separate. The feet were five-toed, all the digits complete and
functional and all provided with claws; there was no coössification
between the phalanges. The astragalus was little different from
the normal form, but in some genera (_e.g._ _†Prepotherium_) the
highly peculiar form of this bone characteristic of _†Mylodon_ and
_†Megatherium_ was distantly foreshadowed. The gait must have been simply
plantigrade, though some of the forms had probably begun to throw the
weight upon the outer edge of the foot.
[Illustration: FIG. 290.—Left pes of _†Mylodon_, Pampean (after Owen).
_Cal._, calcaneum. _As._, astragalus. _N._, navicular. _Cn. 2_, _Cn. 3_,
middle and external cuneiforms. _Cb._, cuboid.]
No dermal armour has yet been found in association with any of the
genera, and, so far as the predominant †Megalonychidæ are concerned, of
which so many skeletons have been collected, this negative evidence must
be allowed great weight. But the material of the other two families is
so rare and incomplete, that the failure to find dermal ossicles is of
no value in determining the question; probably, the †mylodonts possessed
them.
These small Santa Cruz †ground-sloths were not so clumsy and slow-moving
as their gigantic successors of the Pampean, and must have been
inoffensive plant-eaters, some of them perhaps more or less arboreal in
habits, but they could defend themselves with their long, sharp claws.
[Illustration: FIG. 291.—Left pes of _†Hapalops_, Santa Cruz. Princeton
University Museum. Letters as in Fig. 290 and scale of reduction the
same.]
It would require far too much space and lead us into a labyrinth of
anatomical technicalities to point out all the many resemblances to
other edentate suborders which are to be noted in the skeleton of the
Santa Cruz †Gravigrada, which thus justified their position as the most
nearly central group of the entire order. Not only was the skeleton of
these Miocene †ground-sloths very much less specialized than in their
Pleistocene successors, but they were much closer to the anteaters
than were the latter. Aside from the skull, all parts of the skeleton
displayed this resemblance in so marked a manner that the common
derivation of the two suborders seems hardly open to question. Different
as was the skull in the two groups, the differences were not such as
to preclude the origin of both from the same type. Even more closely
connected were the †ground-sloths and the tree-sloths; the resemblance
was most clear in the teeth and skull, but there were also many points
of likeness throughout the skeleton. In the tree-sloths the entire
bony structure has been profoundly modified in adaptation to their
altogether exceptional mode of life, in hanging _suspended_ from the
branches of trees; but, despite this modification, there are so many
notable resemblances between the Santa Cruz †Gravigrada and the existing
Tardigrada as irresistibly to suggest their community of origin, and thus
the former served to connect the anteaters, on the one hand, with the
tree-sloths, on the other. This must not be construed as meaning that
the Miocene †ground-sloths were the ancestors of the other suborders,
which were probably already in existence as distinct groups, but merely
that all three suborders had a common origin, from which the Santa Cruz
†Gravigrada had departed less than have the sloths and anteaters.
There is evidence that at least two of the †ground-sloth families, the
†Megalonychidæ and the †Mylodontidæ, were distinguishable in the Deseado
stage, but materials are still lacking to give us any real knowledge of
the suborder in that or the more ancient stages.
SECTION LORICATA. ARMOURED EDENTATES
SUBORDER DASYPODA. ARMADILLOS
Armadillos are still an important and characteristic element of the
Neotropical fauna, ranging from Texas to Patagonia and showing a
considerable variety of structure and appearance. Existing species are
all of small or moderate size, and the one which is by far the largest
(_Priodontes gigas_) may somewhat exceed three feet in length, exclusive
of the tail, and the smallest (_Chlamydophorus truncatus_) is hardly more
than five inches long. In most armadillos the hair is greatly diminished
in quantity and the animal is sheathed in a conspicuous armour of bony
scutes, covered with horny plates. There is a head-shield which covers
the top of the skull, and the tail is enclosed in a sheath; the back and
sides are protected by the great carapace and the limbs by irregular
scutes and scales, leaving only the under side of the body and the inside
of the legs uncovered. In most existing genera, the carapace is in
three parts, an anterior and posterior buckler, in which the plates are
immovably fixed together by their edges, and between a varying number
of transverse, overlapping bands, from 3 to 13, which permit sufficient
flexibility of the body. The tail-sheath is made up of a series of rings.
One genus (_Tolypeutes_) has the power of rolling itself into a ball, the
head-shield exactly closing the anterior notch of the carapace and the
tail-sheath filling the posterior notch. The animal is thus perfectly
protected against attack and does not seek refuge by digging, as other
armadillos do and with astonishing rapidity. In the little Pichiciago
(_Chlamydophorus_) the dermal ossifications are very thin and the
carapace is composed of twenty transverse bands of horny plates, without
bucklers; the rump is covered with a broad and heavy shield of bone,
overlaid with thin plates of horn, which is attached to the hip-bones
and notched below for the short tail. In certain rare and little known
genera there is a greater development of hair; in one (_Praopus_) the
whole carapace is covered with a dense coat of hair, and in another
(_Scleropleura_) the middle of the back has only a hairy skin and the
carapace is restricted to the sides.
The teeth vary in number and size in the different genera; in some (_e.g.
Dasypus_) there is one upper incisor on each side; the teeth are all
simple and of nearly cylindrical form. The skull is low and flattened,
with long tapering snout and orbits widely open behind; the zygomatic
arches are uninterrupted. Most of the vertebræ of the neck are fused into
a single piece; in the lumbar and posterior dorsal regions there are not
only the usual highly intricate articulations between the vertebræ, but
also high processes on each side for the support of the carapace. The
fully ossified sternal ribs have movable joints with the breast-bone, but
not the double articulations found in the anteaters and †ground-sloths.
The shoulder-blade has a very long acromion, which does not form a bony
loop with the coracoid, and the clavicles are complete. The anterior
element (ilium) of the hip-bone is narrow, very different from the
broad plate of the †Gravigrada. The humerus has prominent deltoid and
supinator ridges and an epicondylar foramen, and the femur has the third
trochanter. Though the fore-arm bones are separate, the radius has no
freedom of rotation; tibia and fibula are coössified at both ends.
In the hind foot there is no great variety of character; it is
five-toed and usually has claws, but may have broad, flat nails (_e.g.
Priodontes_), but the manus, which is a burrowing organ, displays
different degrees of specialization, which is carried farthest in the
Giant Armadillo (_Priodontes_). _Tatu_ has the fore foot of quite
different type. The armadillos feed chiefly upon insects and worms, but
they are omnivorous and eat roots and carrion and sometimes even capture
and devour small rodents and lizards.
As in the case of the †ground-sloths, the fossil armadillos so far
available are insufficient for tracing the history of the various phyla,
or for doing more than making a very brief sketch of the development
of the suborder as a whole. Nearly all of the modern genera have been
found in the Pleistocene together with several that are extinct, some of
the latter of very large size. One of these, _†Eutatus_, had a carapace
without bucklers and made up of 33 movable, transverse bands. Another,
_†Chlamydotherium_, as large as a rhinoceros and the largest known
armadillo, had anterior and posterior bucklers, with several movable
bands between; it was especially characterized by the teeth, which were
divided by a vertical groove into pillars or lobes, thus approximating
the teeth of the †glyptodonts. The genus went far back into the Pliocene,
and the more ancient species were successively smaller.
Though remains of armadillos abound in the formations between the
Pampean and the Santa Cruz, they are for the most part so fragmentary
as to be of no service in deciphering the history of the group. In the
Santa Cruz beds also they are very abundant and varied, and several of
the genera are very completely known. As a whole, this assemblage of
armadillos was very different from that of the Pleistocene, and only a
few direct ancestors of the latter have been found in the Miocene of
Patagonia; no doubt, like the ancestral tree-sloths and anteaters, they
were then living in the warmer regions of the north. Most of the Santa
Cruz armadillos belonged to aberrant types, of which no descendants
have survived; but, nevertheless, they throw welcome light upon the
developmental stages of the suborder.
[Illustration: FIG. 292.—Skull of _†Peltephilus_, Santa Cruz. Ameghino
collection.]
These armadillos had the complete armour of head-shield, carapace and
tail-sheath, but the carapace had no anterior buckler in any of the Santa
Cruz genera, and in some there was no posterior buckler, the carapace
consisting entirely of transverse, movable bands, as in the Pleistocene
_†Eutatus_. In one especially peculiar genus, _†Peltephilus_, the
head-shield was remarkable; it was made up of large, polygonal plates,
the two anterior pairs of which were elevated into high, sharp points,
which must have supported horns, that were quite large in proportion to
the size of the animal. A 4-horned armadillo, like a tiny rhinoceros in
armour, must have been a sufficiently bizarre object.
As a rule, the teeth of the Santa Cruz armadillos were of the same
simple, cylindrical form as in the modern genera and arranged in the same
way, but there were some exceptions. In the horned _†Peltephilus_, the
teeth of each jaw were so inserted as to form a continuous series around
the sides and front of the mouth; and, at first sight, it would seem that
this genus differed from all other known edentates in having a full set
of incisors, but actually it had but one on each side above and below, as
has the modern _Dasypus_, with the difference that, in the latter, the
incisors of the opposite sides are widely separated and in _†Peltephilus_
were brought close together. The anterior upper teeth were long and sharp
and passed outside of the lower ones, when the jaws were closed, and
all the teeth had an external layer of hard and shining dentine, which
had almost the appearance of enamel. Another variant in dentition was
_†Proeutatus_, which was the largest of Santa Cruz armadillos and larger
than any existing forms except _Priodontes_ and _Cabassous_. It had teeth
like those of the huge Pliocene and Pleistocene _†Chlamydotherium_, of
which it was a probable ancestor; the five posterior ones in each jaw
were of trihedral shape, and the two kinds of dentine, of which they were
composed, were so arranged as to form a rough grinding surface. Probably
this animal subsisted largely upon vegetable food; at all events, the
food was of such a nature as to keep the teeth worn down more than in
any of the associated genera. A fourth type of dentition was displayed
by _†Stegotherium_ (Fig. 243, p. 480); the teeth were so few and small
that they can have had no functional value and were merely minute points
almost level with the gums. In all probability, _†Stegotherium_ was more
exclusively insectivorous than the other genera.
Among the Santa Cruz armadillos may be distinguished four well-marked
types of skull. (1) That which agrees closely with the modern form,
especially as exemplified by the genus _Dasypus_. (2) _†Proeutatus_ had
a higher and less flattened cranium and a very long, cylindrical muzzle.
(3) In the horned _†Peltephilus_ the face was very short and broad,
and the lower jaw was horseshoe-shaped, the two halves coössified at
the symphysis, which is not true of any other armadillo. (4) Quite the
opposite extreme was displayed by _†Stegotherium_, in which the face was
drawn out into a very long, slender and tapering muzzle; the lower jaw
was extremely weak and thin, the posterior, ascending portion low and
ill-defined, the condyle and coronoid process much reduced. No other
known armadillo has such fragile jaws, and there was a distinct likeness
in the skull to that of the Ant-Bear.
[Illustration: FIG. 293.—Skull of _†Proeutatus_, Santa Cruz. Princeton
University Museum.]
[Illustration: FIG. 294.—Skull of _†Stegotherium_, Santa Cruz. Princeton
University Museum.]
Aside from carapace and skull, the skeleton of the Santa Cruz armadillos
was surprisingly modern. The vertebræ of the neck were coössified,
those of the lumbar and posterior dorsal regions had the extremely
complex articulations and the high processes for the support of the
carapace, just as in the Recent genera. The limb-bones did not differ
in any significant way from those of the latter, and the feet closely
resembled those of the modern _Dasypus_; none of the genera displayed
the specialization of the manus seen in _Cabassous_, _Priodontes_ or
_Tolypeutes_. Whether these specializations have all been acquired since
Santa Cruz times, or whether they had already appeared in some other
region of the continent, is a question that remains to be determined.
Little can yet be done in the way of tracing the history of the
armadillos through the stages preceding the Santa Cruz times, because of
the fragmentary character of the material. The suborder was abundantly
represented in the Deseado stage, in which some of the Santa Cruz genera
existed. Even in the most ancient of the Patagonian Tertiary formations
are found scutes of the carapace essentially like those of the modern
armadillos. The group is thus of very high antiquity, older than any
other of the suborders is known to be.
In addition to the typical armadillos of South America, there were, in
other continents, certain more or less doubtful forms, concerning which a
word should be said. In the Bridger Eocene of North America was a genus
(_†Metacheiromys_) of armadillo-like animals, the true relationships
of which are far from clear. The teeth were mostly lost, leaving but
one on each side of each jaw, and this _was covered with enamel_, which
is not true of any unquestioned edentate. However, this is not an
insuperable objection to the inclusion of these animals in the edentates,
for there can be no doubt that these were derived from ancestors with
enamel-covered teeth. Even in modern armadillos the enamel-organ is
formed in the embryo, though it does not perform its functions. The skull
of _†Metacheiromys_ had something of the armadillo-shape, but was not
especially characteristic. The vertebræ of the neck were all separate,
and those of the dorsal and lumbar regions did not have the complex
articulations common to all known edentates, fossil and Recent; the
sacrum had on each side but one point of contact with the hip-bones, and
the sternal ribs were not ossified. The shoulder-blade, hip-bones and
humerus were all armadillo-like. The plantigrade feet were five-toed
and the metapodials were very edentate in form. No indication of bony
armour has been found. While these curious animals may very possibly have
been referable to the Edentata and, at all events, had several features
suggestive of relationship to that order, it can hardly be maintained
that they were unequivocal members of it. In the Oligocene of France have
been obtained some very fragmentary fossils which were classified and
described as armadillos, but their character is quite problematical. It
is thus possible, though far from certain, that in the early Tertiary,
armadillo-like edentates were spread all over the northern hemisphere.
SUBORDER †GLYPTODONTIA. †GLYPTODONTS
In the Pliocene and Pleistocene these huge armoured creatures ranged
from the southern United States to Patagonia. That they were nearly
related to the armadillos is clear, but they were so greatly modified and
specialized as to demand recognition as a distinct suborder.
Aside from their enormous size, the most striking feature of the
†Glyptodontia is the extraordinary development of their defensive
armour, which was far more complete and massive than in the armadillos.
The top of the head was protected by a thick head-shield, or _casque_,
composed of several coössified plates; the body and much of the limbs
were enclosed in the immense carapace of elongate-oval, domed shape,
which covered the neck and trunk and on the sides almost reached to
the ground. This tortoise-like carapace was composed of very thick,
polygonal plates of bone (no doubt covered externally with horny plates)
immovably fixed together by their rough edges, and ornamented with an
elaborate pattern of sculpture, which varied according to the genus.
With one or two exceptions, the plates of the carapace were not arranged
in transverse rows, but formed a mosaic without discernible banding. In
the exceptions noted, the sides of the carapace were made up of bands,
and near the margins were two or three overlapping transverse bands which
permitted a minimal degree of flexibility. The tail-sheath was remarkable
and differed much in appearance and make-up in the various genera. In
_†Glyptodon_ the tail was comparatively short and the tail-sheath was
made up of a series of overlapping rings, each ring consisting of two
rows of plates; those of the second row were ornamented, on the top and
sides of the tail, with very prominent, conical projections, capped, in
the living animal, with still longer and sharper spines of horn, so that
the tail must have bristled with spikes. A more usual type of tail-sheath
was exemplified by _†Sclerocalyptus_, in which there were several
overlapping rings at the root of the tail, but for much the greater part
of its length the plates of the sheath were fused together into a long,
transversely oval tube, tapering very gently to the free end, where
it was bluntly rounded. A modification of this type was the very long
tail-sheath of _†Panochthus_, in which there were seven overlapping rings
at the root, followed by a long, massive tube, the sides of which were
set with three or more large and heavy, horn-like spines. In _†Dœdicurus_
was reached the maximum specialization of this type; the very long tube
had its free end greatly expanded and thickened into a huge, club-shaped
mass, on the top and sides of which were fixed long and sharp horns.
The teeth, which in all the known genera numbered 8/8, were all very much
alike; each was divided by two broad and deep vertical grooves on each
side into three pillars, connected by narrow necks. Harder dentine in
the centre and on the periphery of the tooth, with a softer intermediate
layer, kept the grinding surface rough through differential wear. Teeth
of this character are indicative of a vegetable diet and these great
creatures were, no doubt, as harmless and inoffensive as possible.
[Illustration: FIG. 295.—Pampean †glyptodonts, _†Dœdicurus
clavicaudatus_ and _†Glyptodon clavipes_. Restored from skeletons in the
museums of La Plata and Buenos Aires.]
The skull was remarkably short, broad and high, the facial region being
especially abbreviated; the cranium, though forming the greater part
of the skull, was yet small in comparison with the size of the animal;
it had a distinct, though not prominent, sagittal crest. The occipital
surface was inclined forward and had a very elevated position, the
condyles being near the top of the head and raised very far above the
level of the teeth. The orbits were relatively small, more or less
completely encircled with bone and as near to the top of the head as
they could be brought; this was to make room for the extremely high
teeth, which required a great depth of jaw; the elevation of the whole
cranium left unlimited space for the jaws beneath it. The zygomatic
arches were strong and curved out widely from the sides of the skull;
beneath each eye was given off a very long descending process which
projected downward, outside of the lower jaw. In most of the species the
upper profile of the skull was nearly straight, but in _†Panochthus_
it descended very steeply from the forehead to the nose. The forehead
was dome-like and the nasals extremely short. Sinuses were extensively
developed, especially in the frontals, and in _†Sclerocalyptus_ the
bones around the nostrils were grotesquely inflated. The two halves
of the lower jaw were fused together, and the symphysis was prolonged
into a short, wide spout, which projected considerably in advance of
the upper jaw, showing that the soft parts of the muzzle must have had
a corresponding extension. The horizontal portion of the lower jaw,
carrying the teeth, was short and very deep; the posterior, ascending
portion had a forward inclination and was very high.
The skeleton of the Pleistocene †glyptodonts was unique among mammals,
though evidently a modification of the armadillo type. The extreme
modification was conditioned by the enormous weight of the carapace,
which the skeleton had to support. The neck was very short, made up of
short vertebræ, which were extensively coössified; the atlas was always
free, but the axis was fused with a varying number of the succeeding
vertebræ; usually, the axis and the third to the sixth formed one mass,
while the seventh was fused with the dorsals. The joint between the sixth
and seventh vertebræ was such as to permit at least a partial downward
bending of the head beneath the carapace, closing its anterior opening
with the head-shield. The seventh neck vertebra and all the dorsals,
except the last one, were coössified into a heavy curved rod, the “dorsal
tube”; the conjoined neural arches formed a tunnel for the spinal cord
and the spines made a continuous ridge. As the hind legs were very much
longer than the fore, the back was strongly arched upward from the neck
to the hips. The last dorsal, the lumbars and the sacrum were all fused
together to form the “lumbo-sacral tube,” of which the coössified neural
spines made a very prominent ridge, the principal support of the carapace
in the median line; the anterior half of the trunk skeleton, comprising
the short, deep thorax, was free from the carapace, which in that region
must have rested upon the muscles of the back and shoulders. The number
of neck and trunk vertebræ combined varied in the different genera from
26 to 28, but fusion had reduced the number of separate parts to 4, or
at most 5. Such greatly diminished flexibility of the back was rather
an advantage. The tail differed much in length in the various genera,
but was always massive; the anterior vertebræ, usually 7 in number, were
free, the others were fused into a heavy, tapering rod; but for nearly
its whole length the processes of the vertebræ were very prominent, each
vertebra touching the tail-sheath at five points and thus giving it very
effective support. In _†Glyptodon_ the tail-vertebræ were all free.
In most of the genera the scapula was very broad and had the very long
acromion common to all the edentates; there were no clavicles. The
hip-bones were very peculiar; the anterior element (ilium) stood almost
vertically, at right angles to the backbone, and formed a broad plate,
facing forward, the top of which was roughened and thickened to support
the carapace. The posterior element (ischium) was also much expanded,
but faced outward, and its hinder end, curved upward and thickened,
was another point of strong support for the carapace. The two elements
together formed an inverted arch, the crown of which rested on the head
of the femur.
Though less massive than those of the hind leg, the bones of the fore
limb were yet very heavy. The humerus was short and had reduced deltoid
and supinator ridges and no epicondylar foramen; the short fore-arm bones
were separate and heavy, the ulna especially so. The femur was much the
longest of the limb-bones and was extremely strong, especially in its
great breadth, the antero-posterior flattening, common to nearly all very
heavy mammals, being well marked. A very unusual feature was the position
of the third trochanter near the lower end of the shaft. The tibia and
fibula were much shorter than the femur, extremely heavy and coössified
at both ends. The very short and broad feet retained five digits; in the
manus the claws were sometimes comparatively long and sharp, sometimes
blunt and hoof-like; those of the hind foot were always broad hoofs.
Among all the many strange and grotesque mammals which the study of
fossils has brought to light, none can have been more remarkable than the
Pleistocene †glyptodonts; slow-moving hillocks they must have seemed, the
larger species 12 to 14 feet long and 5 feet or more in height. Those
that had claws on the fore feet probably used them to dig for roots and
tubers, but all were plant-feeders. When attacked by the †sabre-tooth
tigers (_†Smilodon_) or the great bears (_†Arctotherium_) they needed
only to squat down, bringing the edges of the carapace to the ground, and
draw in the head, to be perfectly protected, while a sweep of the spiny
or club-like and horned tail would have been fatal to anything in its
path.
As in the case of so many other groups, little has yet been learned
regarding the history of the †glyptodonts during the interval between
the later Pliocene and the Santa Cruz; the intermediate formations have
yielded many †glyptodonts, but not in such preservation as to be of any
service in this connection. We find, as might be expected, many and very
great differences between the Pampean and the Santa Cruz representatives
of the suborder, the latter being in all respects less modified and less
widely removed from the armadillos.
(1) The most obvious and striking distinction was in size, the Santa Cruz
forms being all small and some of them very small.
(2) In all cases the carapace was made up of transverse bands, which
permitted a slight degree of flexibility, and near the anterior end, at
the margins of the shell, were two or three overlapping bands. The plates
were thin and were but rarely coössified; the ornamentation was made by
shallow grooves.
(3) The tail-sheath, which was of very uniform character, consisted
of two quite distinct portions; the anterior region consisted of 5 or
more freely movable, overlapping rings, each of two rows of plates, and
in the posterior region the rings were closely fitted together, less
distinctly marked and not movable. This posterior portion was sometimes
thick and ended abruptly, sometimes slender and tapering and in one genus
(_†Asterostemma_) it was very armadillo-like. In none of the genera
were there any spines or horns, nor were the separate plates ever fused
together to form a tube.
(4) There was considerable variety in the head-shield, which was usually
made up of many separate plates, but in one genus (_†Eucinepeltus_) they
were coössified into a single heavy casque.
(5) The teeth had a less extreme height and the four anterior ones of
each jaw were much simpler than in the Pampean forms. An interesting
survival was the retention of two minute incisors in each premaxillary
bone, in one genus (_†Propalæohoplophorus_), but these were of no
functional value and were early lost.
(6) The skull was much longer, narrower and lower and had a relatively
longer facial portion; the occiput was higher and more erect, and the
condyles had no such elevation above the level of the teeth; the orbit
was widely open behind and the descending process given off from the
zygomatic arch beneath the eye had no such exaggerated length; the bones
were not conspicuously inflated by sinuses. The lower jaw was shallower,
the symphysis and anterior spout shorter and the ascending portion far
lower.
(7) The backbone had a greater number of separate parts; the atlas, as
always, was free, the axis was fused with two or three of the following
vertebræ; the sixth was free and the seventh fused with the first and
second dorsals to form one piece, which was succeeded by two or three
separate vertebræ: the other dorsals, except the last one, were united in
the dorsal tube, and the lumbo-sacral tube was already complete. Thus,
instead of four or five, there were eight or nine distinct parts. None of
the tail-vertebræ were fused together.
(8) There was the same disparity in the length of the fore and hind
limbs, but the bones were far more slender and armadillo-like; this
was especially true of the radius and humerus, the latter having
well-developed deltoid and supinator ridges and epicondylar foramen; the
ulna was more massive and glyptodont-like. The femur was very much more
slender and rounded and the third trochanter was placed higher up the
shaft; tibia and fibula were coössified at both ends and resembled those
of the Pampean genera, except for their much greater slenderness.
(9) The feet were much as in the latter, but relatively narrower, and the
manus had longer claws.
In short, the Santa Cruz †glyptodonts departed much less widely from
the armadillos than did the Pliocene and Pleistocene genera, and, to a
certain extent, bridged over the gap between the two suborders. Such
backward convergence in time is very strong evidence for the community of
origin of the two groups.
The †glyptodonts of the more ancient formations, so far as they are
known, teach us little concerning the stages of modification in these
extraordinary animals, because of their fragmentary condition. The oldest
stage in which representatives of the suborder have been detected is the
Astraponotus beds, which may be Oligocene or upper Eocene. On the face
of the records, therefore, the †glyptodonts had no such antiquity as the
armadillos.
* * * * *
It has long been recognized that the Edentata occupy a very isolated
position among the placental mammals; their relationships to other orders
and their point of departure from the main stem are unsolved problems.
The South American fossils have so far thrown little light into these
dark places, but they bear very cogent witness to the unity of origin of
the five suborders, which were most probably all derived from a single
early Eocene or Paleocene group.
In the Paleocene and through most of the Eocene of North America there
lived an order of mammals called the †Tæniodontia (or †Ganodonta)
which many of the foremost palæontologists regard as an ancestral type
of the Edentata, and Dr. Schlosser actually includes them in that
order. That the †tæniodonts had certain striking resemblances to the
edentates, especially to the †ground-sloths, is not to be denied, but
the interpretation of these resemblances is a very complex and difficult
question. Unfortunately, no member of the order is known from an even
approximately complete skeleton, and therefore a discussion of the matter
here would be unprofitable. My own conclusion, however, may be stated,
to the effect that the supposed relationship of the †tæniodonts to the
edentates is illusory and not real. Definite decision must await the
finding of more complete material both of the †tæniodonts and the most
ancient South American edentates.
CHAPTER XVII
HISTORY OF THE MARSUPIALIA
The marsupials are a group of more primitive structure and greater
antiquity than any which we have yet considered, so primitive, indeed,
that they are referred to a separate infraclass, the Didelphia or
Metatheria. The order is one of very great variety in size, form,
appearance, diet and habits, and mimics several of the higher orders in
quite remarkable fashion. Herbivorous, insectivorous and carnivorous
forms are all numerous, as well as arboreal, terrestrial, cursorial,
leaping and burrowing genera. Some are like hoofed mammals in appearance
and the Rodentia, Carnivora and Insectivora are also closely imitated in
externals. With all this diversity, most unusual within the limits of a
single order, there is such a unity of structure, that a division of the
group into two or more orders is impracticable.
At the present time, marsupials are very largely confined to Australia
and adjoining islands, where they constitute nearly the whole mammalian
fauna, and it is in the Australian region that the remarkable diversity
already mentioned is to be observed. There are found the phalangers,
kangaroos, bandicoots, Tasmanian “devil” and “wolf,” and banded
anteaters, not to mention many other curious creatures. In the western
hemisphere only the opossums (_Didelphis_, _Chironectes_) and one very
interesting relic of a long vanished assemblage, _Cænolestes_ of Ecuador
and Colombia, are in existence to-day. The opossums, of which some
twenty-three species are recognized, have their headquarters in South
America, to which nearly all of the species are confined, North America
having but two or three.
The more important American marsupials are given in the table below:
Suborder POLYPROTODONTA
I. DIDELPHIIDÆ. Opossums.
_Didelphis_, Opossum, Pleist. and Rec., N. and
S. A. _Chironectes_, Water Opossum, Rec., S. A.
_†Peratherium_, low. Eoc. to low. Oligo., N. A.
_†Microbiotherium_, Santa Cruz. _†Eodidelphys_, do.
_†Ideodidelphys_, Casa Mayor. _†Proteodidelphys_,
†Cretaceous, S. A.
II. THYLACYNIDÆ. Predaceous Marsupials.
_†Cladosictis_, Santa Cruz. _†Amphiproviverra_,
do. _†Prothylacynus_, do. _†Borhyæna_, do.
_†Proborhyæna_, Deseado. _†Pharsophorus_, do.
_†Procladosictis_, Casa Mayor. _†Pseudocladosictis_,
do.
Suborder DIPROTODONTA
III. CÆNOLESTIDÆ.
_Cænolestes_, Rec., S. A. _†Zygolestes_, Paraná.
_†Palæothentes_, Santa Cruz. _†Abderites_, do.
_†Palæpanorthus_, Deseado.
IV. †GARZONIIDÆ.
_†Garzonia_, Santa Cruz. _†Halmarhiphus_, do.
_†Cladoclinus_, do.
Suborder †ALLOTHERIA
V. †PLAGIAULACIDÆ.
_†Polymastodon_, up. Cretac. and Paleoc., N. A.
_†Ptilodus_, do. _†Neoplagiaulax_, Paleoc., N. A.
VI. †POLYDOLOPIDÆ.
_†Propolymastodon_, Casa Mayor. _†Polydolops_, do.
_†Amphidolops_, do.
Despite all their diversity of appearance and habits, the unity of
structure among the marsupials is such that the formation of groups of
higher than family rank is very difficult, and it is by no means certain
that the suborders currently accepted correspond to the facts of actual
relationship.
Except in certain extinct South American genera, there is very little
change of teeth, only the last premolar in each jaw being replaced.
Sometimes the temporary tooth is long retained in function and, more
rarely, it is shed very early; while in several genera no replacement
of teeth has been observed. There is a difference of opinion among
naturalists as to the proper interpretation of the marsupial dentition.
According to one view, all except the replaced premolar belong to the
milk-series and the permanent series has been lost; the alternative
and more probable belief is that the milk-dentition has been almost or
completely suppressed. Whichever one of these interpretations be the
right one, there is strong reason to maintain that the very limited
amount of change is not a primitive condition, but a secondary one,
for a series of rudimentary teeth is formed before those which are to
become functional. The only reasonable explanation of such a condition
is that it has been derived from one in which the normal succession and
replacement of the teeth took place. Something of the same sort has been
observed in the simplicidentate rodents. The marsupial dentition differs
from the placental one in the usual number of four molars, instead of
three, and frequently also in exceeding the normal total number of 44.
The incisors are almost always of a different number in the upper and the
lower jaw and are frequently more numerous than in the placentals.
The skeleton has several diagnostic characters, which are present
throughout the order, though one or other of these features may be absent
in particular instances. The skull has a very small brain-capacity and
elongate face and jaws. In the placental mammals, the sutures between
adjoining bones of the skull tend to close by coössification, and the
separate bones are clearly distinguishable only in young animals; but
in the marsupials the sutures remain open for a much longer period.
The lachrymal is expanded on the face and the foramen is outside of
the orbit. The tympanic is a mere ring and permanently separate from
the other bones of the cranium, while a false bulla is formed by the
inflation of part of the alisphenoid. In almost all marsupials there
are large openings or vacuities in the bony palate. One of the most
characteristic and constant features of the marsupial skull is in the
conformation of the angle of the lower jaw, which is turned inward, or
_inflected_, at nearly a right angle with the body of the jaw. It is true
that one existing Australian genus has lost this character; and in some
of the placental orders, especially the Rodentia, a somewhat similar
structure may occasionally be found, but it is never quite the same as in
the marsupials, in which it goes back to a remote antiquity.
There are very constantly 19 trunk-vertebræ, of which usually 13 are
dorsals. The tail differs greatly in length in the various genera, but
most of them have well-developed tails. An additional pair of elements,
besides the three which are found in the placentals, enter into the
composition of the hip-bones; these are the _marsupial bones_, slender,
flattened rods, directed forward in the abdominal wall and diverging
in V-shape. Save in a few genera, clavicles are present and of full
size. The humerus may or may not have the epicondylar foramen, but the
femur never has the third trochanter. The feet vary greatly in form and
structure, in accordance with the habits, but there is a very widespread
adaptation to an arboreal life, and even in terrestrial and burrowing
forms more or less distinct traces of this arboreal adaptation may be
noted. This fact has led to a generally accepted inference that all
existing marsupials had an arboreal ancestry.
The soft parts and more especially the organs of reproduction are
likewise very characteristic, and one or two of these peculiarities may
be mentioned. (1) In the female, the vagina is double and on the abdomen
is the pouch, or _marsupium_ (which gives its name to the order), a
hair-lined bag, opening either forward or backward, which serves to carry
the young and into which the teats open. A considerable number of species
have lost the marsupium, while other species of the same genera retain
it, and there can be little question that its absence is a secondary
condition. (2) Except in one modern Australian genus, the marsupials
have no true placenta, and the young are born in a very immature state,
incapable of even swallowing. The new-born young are transferred to the
nipples of the mother and are attached to these and fed by the pumping
of milk into their mouths by muscular action of the mother. A special,
though temporary, arrangement of the gullet and windpipe is provided, so
that the helpless young animal shall not be suffocated by the entrance of
milk into the lungs.
SUBORDER POLYPROTODONTA
This suborder, as is indicated by its name, is characterized by its
numerous incisors, which are 5/4, or 4/3, and none of them is especially
enlarged; by the large canines in both jaws, simple premolars and
tritubercular upper molars. The members of this group are carnivorous
or insectivorous in habit, and all the existing ones are of small or
moderate size, though some very large extinct forms are known. Except
in one Australian family, the feet are not “syndactyl,” a term which
means the enclosure of two or more digits in one fold of skin. The only
existing American representatives of the suborder are the opossums, the
great majority of which are Neotropical in distribution.
1. _Didelphiidæ. Opossums_
In this family the dental formula is: _i_ 5/4, _c_ 1/1, _p_ 3/3,
_m_ 4/4, × 2 = 50. The incisors are small and closely crowded
together, the canines large and tusk-like, the premolars simple and
of compressed-conical form; in existing species, the upper molars are
triangular, each of the three main cusps is V-shaped and there are
additional minute cusps along the outer border; the lower molars have a
high anterior triangle of three pointed cusps and a low heel with several
distinct cusps. The humerus has an epicondylar foramen and the feet are
five-toed; in the manus all the digits are armed with claws and the thumb
is but partially opposable, while in the pes the hallux is without a claw
and completely opposable, making the foot much like that of a monkey.
The division of the existing opossums into genera has caused much
difference of opinion and practice among naturalists; there are five
groups, which by some are regarded as genera, and by others as subgenera,
all modern members of the family being very much alike. The species
_Didelphis marsupialis_, which is common in the eastern United States
and extends through temperate North America, Central America and
tropical South America, has a complete pouch and is chiefly arboreal
and insectivorous in habit. In the woolly opossums (_Caluromys_) there
is no pouch, and the young, when sufficiently advanced, are carried on
the mother’s back, winding their tails around hers. In both of these
genera the tail is long, naked and prehensile, but in the tiny species
of _Peramys_ the tail is short and hairy. Another Neotropical genus,
_Chironectes_, the Yapock or Water Opossum, is the only existing instance
of an aquatic marsupial. It has light grey fur, striped with brown, and
webbed hind feet; living chiefly in the water, it feeds upon crayfish,
water-insects and small fish.
The derivation of the modern North American opossums is a matter of
great uncertainty. The present distribution of the family, with by far
the greater number of its species confined to the Neotropical region, is
certainly suggestive of a South American origin, but such considerations
are very untrustworthy guides in tracing the history of animal groups.
No opossum has been found in any North American formation between the
Pleistocene and the lower Oligocene, though in the case of such small
animals, negative evidence must be accepted with caution. In the White
River Oligocene many minute opossums have been found and referred to
the European genus _†Peratherium_, though it so closely resembles the
modern _Didelphis_ that many systematists do not make the distinction.
In the Eocene, Paleocene and upper Cretaceous, opossums were represented
doubtfully; the material is too incomplete for assured determination; in
Europe they existed in the Oligocene and upper Eocene. In South America
the family went back uninterruptedly to the oldest mammal-bearing beds
of Patagonia, which may be upper Cretaceous. The opossums are thus the
remnants of an exceedingly ancient group, whose beginnings are to be
sought in the Mesozoic era and which was probably spread over all the
continents. To all appearances, the whole group vanished completely from
the northern hemisphere, but reëntered North America from the south
at some time during the Pliocene or early Pleistocene and permanently
established itself here.
The opossums are the most primitive of existing marsupials, especially
the little South American genus, _Marmosa_, and are regarded, by some
of the most competent students of the order, as closely representing
the ancestral type of all the Recent families and genera, both of the
Polyprotodonta and Diprotodonta.
2. _Thylacynidæ. Predaceous Marsupials_
By many naturalists this group of flesh-eating forms is included in the
Dasyuridæ. The family never entered North America, but played a very
important part in the Tertiary of South America. Three existing genera of
the Australian region throw considerable light upon the South American
types, and therefore some account of them will not be out of place here.
The largest of modern predaceous marsupials is the animal (_Thylacynus
cynocephalus_) erroneously, but very naturally, called the “Tasmanian
Wolf,” now confined to Tasmania, but occurring also in the Pleistocene
of Australia. As “wolf” applied to a marsupial is misleading, it will
be less confusing to employ the anglicized form of the generic name
“Thylacine.” This animal is of the size of the small Prairie Wolf or
Coyote (_Canis latrans_) and has very wolf-like appearance and habits.
The muzzle is long and pointed, the ears erect and rather small, the tail
long, very thick at the base and tapering to the end, not bushy, but
covered with short, close-set hairs; the colour is greyish brown, with
dark, transverse stripes on the posterior half of the back and base of
the tail. Apparently the creature is in process of losing its stripes and
acquiring the solid body-colour. The dental formula is: _i_ 4/3, _c_ 1/1,
_p_ 2/2, _m_ 4/4, × 2 = 46; the incisors are small, the canines large
fangs, and the premolars simple; the upper molars are tritubercular,
with large inner cusp and postero-external cutting ridge, and the lower
molars are trenchant, with low heel. The whole dentition is remarkably
like that of many Eocene †creodonts, such as _†Sinopa_ and _†Tritemnodon_
(see p. 566). The milk-premolar is small and functionless and is shed
very early. The skull is very wolf-like in appearance, but thoroughly
marsupial in structure, and has the large palatal vacuities common in
the order. The marsupial bones do not ossify and are evidently on the
point of disappearance. There are five digits in the manus, four in the
pes, the hallux being completely suppressed. In habits, the Thylacine
is carnivorous and so destructive to sheep that the farmers have nearly
exterminated it.
[Illustration: FIG. 296.—Thylacine, or “Tasmanian Wolf” (_Thylacynus
cynocephalus_).—By permission of W. S. Berridge, London.]
The other forms to be mentioned belong to the closely allied family of
the Dasyuridæ. The “Tasmanian Devil” (_Sarcophilus ursinus_) is now, like
the Thylacine, confined to Tasmania, but remains of it have been found in
the Australian Pleistocene; it has one less premolar in each jaw, giving
the formula: _i_ 4/3, _c_ 1/1, _p_ 2/2, _m_ 4/4, × 2 = 42; there is no
milk-tooth. The premolars are closely crowded and the molars resemble
those of the Thylacine in construction, but are broader and heavier. The
skull is disproportionately large, with shorter and wider muzzle and jaws
than in the Thylacine; the tail is of only moderate length and somewhat
shaggy; the hallux is wanting. In size and build, the Tasmanian Devil
resembles a badger and has long and heavy fossorial claws on the fore
feet; the hair is rough and shaggy, black in colour with white patches.
The animal has received its name from its fierce and savage disposition
and is almost as destructive to sheep as the Thylacine.
The five species of _Dasyurus_ are distributed through Tasmania,
Australia and New Guinea and are called “Native Cats”; they are much
smaller animals than the two preceding genera, not exceeding a domestic
cat in size. As the Thylacine imitates a wolf and the Tasmanian Devil
a badger, the dasyures resemble the civets. In them the dental formula
is the same as in _Sarcophilus_, but the teeth have higher and sharper
cusps. The head has a narrow, tapering muzzle and narrow ears; the body
is long and the tail of moderate length. The limbs are short and slender
and a small hallux is present in some of the species. The fur is grey or
brown, with numerous white spots, and the tail is covered with long hair,
but not bushy. The dasyures are largely arboreal and prey upon small
mammals, birds and eggs.
Until the arrival of the true Carnivora from the north, their rôle was
taken in South America by predaceous marsupials, which persisted as
late as the presumably Pliocene beds of Monte Hermoso. Little is known
of them in that stage, however, or in the older Paraná, but abundant
material representing those of the Santa Cruz has been collected.
Among these there was a considerable range of size and some variety of
structure, and they all differed in certain respects from the modern
Australian genera, differences which have led some authorities to
deny the marsupial character of all these South American forms. The
differences are of three kinds: (1) there are no vacuities in the bony
palate; (2) the milk-dentition is less reduced, the canines and one or
two premolars being changed; (3) the enamel of the teeth, in the only
genus (_†Borhyæna_) which has been examined microscopically, resembles
in its minute features that of the placentals and lacks the marsupial
characters. Though by no means unimportant, these differences are
altogether outweighed by the thoroughly marsupial nature of all other
parts of the skeleton, and I cannot but agree with Dr. Sinclair[20] in
including them in the same family with the Tasmanian Thylacine.
The genus _†Prothylacynus_ was especially like the latter and must have
had a very similar appearance, though in the restoration (Fig. 297)
the colour-pattern is changed to one of longitudinal stripes, as more
probably pertaining to so ancient and primitive a form. The humerus had
the epicondylar foramen, and a large vestige of the hallux was retained,
though it could not have been visible in the living animal.
A more specialized Santa Cruz genus was _†Borhyæna_ (Fig. 244, p. 494),
an animal of about the same length and height as _†Prothylacynus_
and the Thylacine, but much more massive and powerful. The skull was
remarkable for the small size of the brain-case and the great spread of
the zygomatic arches, which gave a rounded and almost cat-like appearance
to the head, as is shown in the restoration (Fig. 244). In this genus
the upper incisors were reduced to three, a very unusual thing among
the Polyprotodonta, and the humerus had lost the epicondylar foramen.
_†Prothylacynus_ and _†Borhyæna_ were the largest of the Santa Cruz
flesh-eaters and no doubt pursued the smaller and more defenceless
ungulates, but were hardly sufficiently powerful to attack successfully
the larger hoofed animals, which were probably well able to defend
themselves.
[Illustration: FIG. 297.—Santa Cruz predaceous marsupial (_†Prothylacynus
patagonicus_) and †typothere (_†Interatherium robustum_). Restored by C.
Knight from skeletons in the museum of Princeton University.]
[Illustration: FIG. 298.—Skull of _†Borhyæna_, Santa Cruz. (After
Sinclair, Reports Princeton University Expeditions to Patagonia, Vol.
IV.)]
[Illustration: FIG. 299.—Skull of small predaceous marsupial
(_†Amphiproviverra manzaniana_), showing the punctured wound from a bite.
Princeton University Museum.]
Associated with these larger predaceous marsupials were several much
smaller kinds, ranging in size from a fox to a weasel, which must have
preyed upon the abundant rodents and other small mammals and birds.
One of these (_†Amphiproviverra_) had an opposable hallux, somewhat as
in the opossums, and was therefore probably arboreal. An interesting
specimen in the museum of Princeton University illustrates the pugnacity
of these small creatures; it is a skull in which the left upper canine
was completely torn out, the circular puncture of the enemy’s bite being
unmistakable and the healed edges of the wound proving that the loss of
the tooth was suffered during life. In structure, these smaller animals
differed so little from the larger ones, that no particular description
of them is needed. In the restoration of _†Cladosictis_ (Fig. 300) the
spotted pattern of the Australian dasyures, or native cats, has been
taken as a model.
In the Deseado formation the predaceous marsupials have been less
abundantly found than in the Santa Cruz and there can be little doubt
that the group is very inadequately represented by the material so far
collected. Only two genera, known from lower jaws, have been described,
but one of these (_†Proborhyæna_) is of interest because of its enormous
size, far surpassing any of the Santa Cruz forms and equalling the
largest modern bears. This is another illustration of the unusual
relationship between the Deseado and Santa Cruz faunas, the older stage
so frequently having the larger animals.
Predaceous marsupials of small size may be traced back to the Casa Mayor
formation, but very little is yet known of them. There is no obvious
difficulty in the way of their derivation from opossum-like forms, such
as are found in the Cretaceous of North America and probably of South
America also.
The relation of the South American to the Australian marsupials
offers problems of unusual interest, a discussion of which would be
impracticable here. Several alternative solutions of the problem have
been offered and great differences of opinion exist with regard to it.
To my mind the most probable suggestion is that a land-connection, by
way of the Antarctic continent, existed in early Tertiary times, by
means of which the ancestors of the Australian marsupials migrated, from
South America, though this explanation is rejected by several eminent
authorities.
[Illustration: FIG. 300.—Small predaceous marsupial (_†Cladosictis
lustratus_) and rabbit-like †typothere (_†Pachyrukhos moyani_), Santa
Cruz stage. Restored by C. Knight from skeletons in Princeton University
and the American Museum of Natural History.]
SUBORDER DIPROTODONTA
North America never had any representatives of this suborder, but South
America possessed many of them in the Santa Cruz Miocene and one genus
(_Cænolestes_) has survived to the present time. Australia, on the other
hand, has three well-defined families of the suborder, the kangaroos,
phalangers and wombats, but no member of any of these has ever been found
outside of the Australian region. So far as we know, therefore, the
suborder is and always has been confined to the southern hemisphere.
[Illustration: FIG. 301.—Skull of _Cænolestes obscurus_, enlarged. (After
Sinclair.)]
The modern South American genus _Cænolestes_ is a small, rat-like animal
and very rare; it has been found only in Ecuador and Colombia. Its
dentition is not at all typically diprotodont, but rather intermediate in
character between the latter and the Polyprotodonta. The dental formula
is: _i_ 4/3, _c_ 1/1, _p_ 3/3, _m_ 4/4, × 2 = 46. The upper incisors are
small and of subequal size, though the second is somewhat the largest
of the series, and the canine is considerably larger and more prominent
than any of them. The foremost lower incisor is long and pointed and
directed almost straight forward; the other lower incisors and the canine
are minute and can have little or no functional value. The premolars are
small and simple and the upper molars quadritubercular, the third one
triangular, and the fourth very small and apparently about to disappear.
Such teeth would seem to indicate a vegetable diet, but it is reported
that the animal subsists chiefly upon small birds and their eggs. The
skull, which is typically marsupial in all its characters, is most like
that of the smaller Australian native cats (Dasyuridæ) and the feet show
no signs of the syndactyly which all the other diprotodonts display so
clearly. Dr. Gregory is “inclined to regard _Cænolestes_ and its allies
as an independent suborder, an offshoot of primitive Polyprotodonts
which has paralleled the Diprotodonts in certain characters of the
dentition.”[21]
[Illustration: FIG. 302.—Lower jaws of Santa Cruz cænolestids, enlarged.
_A_, _†Garzonia patagonica_. _B_, _†Abderites crassignathus_. _C_,
_†Callomenus ligatus_. (After Sinclair, in Reports Princeton University
Expeditions to Patagonia, Vol. IV.)]
Evidently, the animals of this series were extremely rare or absent in
the areas where the known South American deposits of the Pleistocene
and Pliocene were laid down, for there is a very long hiatus in their
history from the Recent to the Santa Cruz, during which none has yet
been found, except one genus (_†Zygolestes_) in the Paraná. In the Santa
Cruz, however, there was a great abundance of these little marsupials,
to which various generic names have been given and which displayed
considerable variety in the forms of the teeth. Some (e.g. _†Garzonia_)
agreed with _Cænolestes_ in having no trenchant shearing teeth; behind
the large, procumbent lower incisor, followed four or five very minute
teeth, which must have been nearly or quite functionless, succeeded
by the well-developed molars. Other genera (_e.g._ _†Abderites_) had
a similar dentition, with the important exception that the last upper
premolar and first lower molar were enlarged and trenchant, together
forming a shearing pair; these teeth were vertically fluted or ribbed in
very characteristic fashion. The Australian phalangers have very similar
trenchant and fluted teeth, but in that family the lower one of the pair
is the last premolar, not the first molar. Marsupials of this type have
not been found in formations older than the Deseado.
The relationship of these South American genera to the Australian
phalangers is a very interesting question from the standpoint of
mammalian distribution, but is not likely to receive a positive answer
until something is learned regarding the history of the Australian family.
SUBORDER †ALLOTHERIA
[Illustration: FIG. 303.—Skull of Paleocene †allothere (_†Ptilodus
gracilis_), enlarged, Fort Union stage. (After Gidley.)]
This extinct suborder is still very imperfectly understood, for it is
known almost exclusively from jaws and teeth; so far, the skull of one
genus and most of that of another have been obtained, but hardly anything
of the skeleton. The †Allotheria were small or minute marsupials,
herbivorous or omnivorous, which had lost all trace of the canines and
had one pair of incisors above and below, which grew from persistent
pulps and had a scalpriform, rodent-like character. The molars were
composed of numerous tubercles (whence the name “†Multituberculata,”
often applied to the group) arranged in two or three longitudinal rows,
and the premolars were either like the molars, but of simpler pattern,
or compressed, sharp-edged and trenchant. The †Allotheria were among the
most ancient of mammals and have been found in the Triassic of Europe,
the Jurassic of Europe and South Africa, the Jurassic and Cretaceous
of North America and the Paleocene of both northern continents, while
the South American Eocene (Casa Mayor) had certain problematical genera
(†Polydolopidæ), which may be referable to the †Allotheria or to the
_Cænolestes_ series. The suborder was thus preëminently a Mesozoic one
and, with the doubtful exception of South America, it is not known to
have passed beyond the limits of the Paleocene. There is not the least
likelihood that any existing mammals were derived from the †Allotheria.
[Illustration: FIG. 304.—Head of _†Ptilodus gracilis_, about natural
size. Restored from a skull in the United States National Museum.]
While the †Allotheria have an antiquity at least equal to that of
any other mammals known, there were other groups in the Jurassic and
Cretaceous, which, so far as may be judged from teeth alone, would seem
to have been ancestral to the other marsupials and to the placentals. It
would serve no useful purpose to describe these minute creatures, which
are so very incompletely known, though to the specialist they are of the
highest interest. The genera found in the Triassic of North Carolina may
or may not represent the primitive mammalian stock.
* * * * *
The question of the origin of the Mammalia is still involved in great
obscurity, and the most divergent opinions are held concerning it. It
remains an unsolved problem whether the mammals were all descended from
a common stock, or have been derived from two independent lines of
ancestry, or, in technical phrase, whether the class is monophyletic or
diphyletic. Assuming, as seems most probable from present knowledge,
that the mammals are monophyletic, the question next arises: From what
lower vertebrates are they descended? A great controversial literature
has grown up around this problem, one party regarding the Amphibia
and the other the Reptilia as the parent group. The palæontological
evidence, while not conclusive, is decidedly in favour of the latter
view. In the Triassic of South Africa is found a group of reptiles which
approximated the mammals very much more closely than do any other known
representatives of the lower vertebrates. While it is not believed that
any of these Triassic reptiles were directly ancestral to the mammals,
they did, to a very great extent, bridge the gap between the two classes
and show us what the reptilian ancestors of the mammals were probably
like.
With perhaps the exception of certain Insectivora, the Paleocene faunas
contained few, if any, ancestors of modern mammals. These originated
in some region which has not been identified, but may be plausibly
conjectured to be central Asia, whence they migrated westward to Europe
and eastward to North America, reaching both of those continents in
the lower Eocene. From that time onward they increased and multiplied,
becoming more and more differentiated through divergent evolution, until
the existing state of things was attained. From the lower Eocene we
are on firm ground, and, though very much remains to be learned, much
has already been accomplished in the way of tracing the history and
development of many mammalian orders. It has been my endeavour in the
body of this book to sketch the better established and more significant
parts of this marvellous story.
CHAPTER XVIII
MODES OF MAMMALIAN EVOLUTION
Throughout this book the theory of evolution has been taken for granted,
as it seemed superfluous to present an outline of the evidence upon
which that theory rests. “Descent with modification” is now accepted
among naturalists with almost complete unanimity, but, unfortunately
or otherwise, this general agreement does not extend beyond the point
of believing that the present organic world has arisen by descent from
simpler and simpler forms. The application of the theory to concrete
cases is beset with grave difficulties and gives rise to the most
divergent views. The uninitiated reader who takes up a treatise upon
some animal group may well be surprised to see the apparently minute
accuracy with which the genealogy of the series is set forth and the
complex relationships of its members marshalled in orderly array. Another
treatise on the same subject, however, while agreeing perfectly with the
first as to the facts, will contradict its conclusions in almost every
particular. Indeed, so notorious did this become, that “phylogenetic
trees” were rather a laughing-stock, and most naturalists lost interest
in the problems of phylogeny and turned to fields that seemed more
promising.
To some extent, this almost hopeless divergence is inherent in the very
nature of the problem, which deals with the value of evidence and the
balancing of probabilities, as to which men must be expected to differ;
but there is another and more potent cause of the discrepancy. When the
contradictory schemes are analyzed, it is seen that each is founded upon
certain assumptions regarding the evolutionary process, assumptions
which are generally implicit and often apparently unconscious. In the
present state of knowledge, these postulates are, for the most part,
matters of judgment, incapable of definite proof, and they appeal with
very different force to different minds; what to one seems almost
self-evident, another regards as all but impossible. It will, however,
be of service to examine such of these postulates as are involved in
mammalian history.
It is quite impracticable to construct a genetic series without making
certain assumptions as to the manner in which the developmental process
operated and the kinds of modification that actually did occur. In the
preceding chapters, which deal with the evolutionary history of various
mammalian groups, it was repeatedly stated that, of two contemporary
genera, one was to be taken as the ancestor of some later form and the
other regarded as a collateral branch, but it was also pointed out that
in certain cases, palæontologists differed more or less decidedly as to
the proper interpretation of the facts; it is just this lack of agreement
as to the modes and processes of change that forms the root of the
difficulty.
There are instructive analogies between the history, aims and methods of
comparative philology, on the one hand, and zoölogy, on the other. In
both sciences the attempt is made to trace the development of the modern
from the ancient, to demonstrate the common origin of things which are
now widely separated and differ in all obvious characteristics, and to
determine the manner in which these cumulative modifications have been
effected. At the present time zoölogy is still far behind the science of
language with regard to the solution of many of these kindred problems
and has hardly advanced beyond the stage which called forth Voltaire’s
famous sneer: “L’étymologie est une science ou les voyelles ne font rien
et les consonnes fort peu de chose.” Many of the animal genealogies
which have been proposed have no better foundation than the “guessing
etymologies” of the eighteenth century, and for exactly the same reason.
Just as the old etymologists made their derivations upon the basis of
a likeness of sound and meaning in the words compared, so the modern
zoölogist, in attempting to trace the relationships of animals, must
proceed by balancing their similarities and differences of structure. The
etymologist had no sure test for distinguishing a true derivation from
a plausible but false one, and the zoölogist finds himself in the same
predicament. How much weight should be allowed to a given likeness and
how far it is offset by an accompanying difference, there are no certain
means of determining, and we are still in search of those laws of organic
change which shall render such service to zoölogy as Grimm’s law did to
the study of the Indo-European languages. Doubtless, the analogy may be
pushed still farther, and it may be confidently assumed that, just as
sound principles of etymology were established by tracing the changes of
words step by step from their modern forms to their ancient origins, so
the existing animal forms must be traced back through the intermediate
gradations to their distant ancestors, before the modes of organic
development can be deduced from well-ascertained facts.
The evolutionary problem has been attacked by the aid of several distinct
methods, each of which has its particular advantages and its peculiar
limitations and drawbacks. Most of the methods suffer from the fact that
they deal only with the present order of things, and thus resemble the
attempt to work out the derivations of languages that have no literature
to register their changes.
(1) Of necessity, the oldest of these methods is Comparative Anatomy,
which had made great advances in pre-Darwinian days. It is the
indispensable foundation of the whole inquiry, for an accurate knowledge
of Comparative Anatomy is absolutely necessary to the use of the other
methods; in the hands of the great masters it has registered many notable
triumphs in determining the mutual relationships of animal groups; but
finality cannot be reached by this method, because it deals only with
existing forms and possesses no sure criterion for determining the value
of similarities. It is thus unable to distinguish with certainty between
those resemblances which are due to inheritance from a common ancestry
and those which have been independently acquired. It is a very frequent
fallacy to assume that, because two allied groups, B and C, possess a
certain structure, their common ancestor, A, must also have possessed it.
This may or may not have been the case, and Comparative Anatomy offers no
assured means of deciding between those alternatives or of confidently
distinguishing primitive characters from degenerative or retrograde
changes.
(2) Embryology, which is the study of the development of the individual
animal from the unfertilized egg to the adult condition, was long
regarded as the infallible test of theoretical views in zoölogy. This
was on the assumption that individual development (_ontogeny_) is a
recapitulation in abbreviated form of the ancestral history (_phylogeny_)
of the species, and was called by Haeckel “the fundamental biogenetic
law.” It was soon learned, however, that the “recapitulation theory” was
not to be implicitly trusted, for structural features which could not
possibly be a part of ancestral history were imposed upon or substituted
for those due to phylogenetic inheritance. Now the whole theory is
strongly questioned, and the absence of any universally accepted rules
of interpretation, by which the contradictory embryological data may
be harmonized into a consistent whole, has deprived the method of that
authoritative character once so generally ascribed to it. It is like
dealing with a literature which has been vitiated with many forgeries,
only the grossest of which can be readily detected. Embryology has
rendered many great services in the solution of zoölogical problems and
will no doubt render many more, but it cannot, of itself, reach final
conclusions.
(3) Experimental Zoölogy, especially that part known as “Genetics,” one
of the newest and most promising provinces of the science, has already
taught us much concerning the laws of inheritance and the manner in
which new characters arise, and no one can venture to fix the limits of
its possible results. On the other hand, it does not seem likely that
the larger problems of relationship and classification can be solved by
this method, because of the brief time which the shortness of human life
allows for the experiments.
(4) Palæontology suffers from the drawback that much of the past
history of life is irretrievably lost, and even when the record is
remarkably complete, as it is for certain chapters of the history, the
material is but partially preserved. With such rare exceptions as are of
little practical importance, only the hard parts, bones, teeth, etc.,
are retained and the soft parts completely destroyed. Nevertheless,
Palæontology has the preëminent advantage of offering to the student
the actual stages of development, and thus, to recur to the simile of
language, has preserved original documents and in the true order of
succession. It is true that it is well-nigh impossible to reconstruct a
phylogenetic series of ancestor and descendant, unaffected by theoretical
preconceptions, and the differences which arise in the interpretation of
undisputed facts are caused by divergent beliefs concerning the actual
course of the evolutionary process. If final and definitive results are
ever to be reached, it must be through the coöperation of all the methods
of research, and such results must be able to stand the tests applied by
every sound method. On the other hand, the study of those phylogenetic
series which are generally accepted as well established, should furnish
us with some fairly definite information as to the modes in which
development has operated in the past, since the order of succession in
time fixes a limit to the rearrangement of related series. Some of the
conclusions thus suggested may be stated here.
I. One of the most fundamental problems concerning the course of
development is that which deals with _parallel_ and _convergent
evolution_. The term _parallelism_ implies that forms having a common
origin may independently run through a similar course of development and
arrive at similar results. Illustrations of this principle are given by
the many phyla of horses, rhinoceroses and camels, which persisted side
by side through several geological stages, following independent, but
parallel, courses of change. An even more striking case is that of the
two subfamilies of the cats, the true felines and the †sabre-tooths.
Whatever view may be taken of the relationships of these two groups, it
is clear that, at least from the upper Oligocene to the Pleistocene, they
were separate, but kept remarkably even pace with each other in their
advance and specialization.
By _convergence_ is meant a similar result which is reached by two or
more independent lines having different starting points, so that the
descendants are more alike than were the ancestors, and is thus the
opposite of _divergence_, the result of which is to make the descendants
of common ancestors less and less alike with each succeeding stage.
Either parallelism or convergence may be involved in the independent
acquisition of similar characters, of which these are so many examples.
It is obvious that this problem is fundamental and that little real
progress is possible until a solution is reached. As to the correct
solution, there is much difference of opinion among naturalists. Some
deny altogether the reality and importance of these modes of development,
but such are almost exclusively concerned with the modern world; others
go to the opposite extreme, and looking upon every large group as
polyphyletic, consider parallel and convergent development to be the
rule of evolution. Few palæontologists are disposed to doubt that these
modes of evolution are very frequent; their difficulty is to determine
what limits can be drawn, and this difficulty can be removed only by much
wider and more exact knowledge than we now possess.
So far as single structures are concerned, the fossils demonstrate
unequivocally that they have been independently acquired in a great many
cases. The resultant similarity may be attained through the loss, the
acquisition or the modification of parts. The reduction of toes from the
primitive number of five to four, three, two, or even one, has happened
over and over again in the most diverse groups. There is good reason to
believe that all the early and primitive placental mammals had the third
trochanter on the femur and the epicondylar foramen on the humerus, but
in most of the modern groups these structures are lost; and the list of
such similar reductions of parts might be almost indefinitely extended.
Of much greater significance is the independent similar modification
of parts and acquisition of new structures. Innumerable examples of
this kind of parallel and convergent development might be given, but a
few will be sufficient to illustrate the principle. (1) The odontoid
process of the axis (second vertebra of the neck) was primitively
a bluntly conical peg, a form which is still retained in the great
majority of mammals, but in the true ruminants, the camels, the horses
and the tapirs, the process is spout-shaped, concave on the upper side,
convex on the lower. By tracing the development of those groups, it
has been conclusively demonstrated that the change of form took place
independently in each of the four. (2) The ruminants have molar teeth
composed of four crescentic cusps arranged in two transverse pairs,
the pattern called _selenodont_. The evidence is very strong that this
highly characteristic molar pattern has been several times independently
repeated, as in the true ruminants, the camels, the †oreodonts and
probably other groups also. (3) The family †Macrauchenidæ of the extinct
†Litopterna shares with the camel tribe the remarkable peculiarity of
having the canal for the vertebral artery running through the neural
arches of the neck-vertebræ. (4) A very striking instance is afforded by
the three widely separated groups of hoofed animals, members of which had
their hoofs transformed into claws; the †chalicotheres arose from the
normal perissodactyls (p. 356), the †agriochœrids from the †oreodonts
and the †Entelonychia from the †toxodonts. From time to time attempts
have been made to unite two or more of these groups, but in each case
better material and fuller knowledge have demonstrated the unnatural
character of such association and the separate origin of the peculiar
structure.
Admitting the reality and frequency of these modes of development, a
far more difficult problem is to determine the extent to which such
independent acquisition of similar structures has actually been carried,
and it is at this point that the widest divergences of opinion are to be
found. As yet, our knowledge is far too imperfect to permit the making
of positive statements, but there is no evidence which would justify the
conclusion that the same genus, family or order of mammals ever arose
independently from radically different ancestors. We have no reason to
believe that identical groups of mammals were ever separately developed
in land areas which through long periods of time had no means of
intercommunication. If such a thing ever happened, it must have been the
rarest of exceptions. On the other hand, parallelism, by which _related_
forms pass through similar stages of development, would seem to have
been so exceedingly common, as fairly to deserve being called a normal
method of evolution. As more and better material has been gathered, it
has grown increasingly clear that almost every large group of generic,
family or higher rank, whose history is known in any adequate measure,
consists of several distinct, though related phyla, which pursued more or
less closely parallel courses of modification, though diverging from one
another sufficiently to make the distinction of them comparatively easy.
The parallelism was thus not exact, however perfect it may have been
in particular structures, and the longer the phyla persisted, the more
distinctly did they diverge.
A typical problem, which involves these principles, is afforded by the
very curious and interesting group of South American hoofed animals
known as the †Litopterna (Chap. XIII). The many remarkable resemblances
between these ungulates and the perissodactyls and, more specifically,
between the family †Proterotheriidæ and the horses, have been very
differently interpreted by palæontologists. Some have insisted that the
†Litopterna should be merged in the Perissodactyla, on the ground that
such a degree of likeness could not have been independently acquired.
Others hold that this is a remarkable case of parallelism or convergence,
and the latter is, in my opinion, much the more probable view. Until
the ancestry of both groups, Perissodactyla and †Litopterna, shall have
been definitely ascertained, it will not be practicable to make a final
decision between these alternatives, nor, if the similarities were really
independently acquired, to determine whether parallel or convergent
evolution is involved. It is quite possible that both groups were rooted
in the common ground of the †Condylarthra, and, if so, their relation
is one of parallelism; but no such common ancestry has been proved, and
it is equally possible that their ancestry was totally distinct. In the
latter case the resemblances were due to convergence.
Assuming that the remarkable resemblances between the †Proterotheriidæ
and the horses were separately acquired, it should be emphasized that
these similarities nowhere amount to identity. The likenesses are not
confined to a few structures, but are general throughout the skeleton and
may be noted in the teeth, skull, trunk, limbs and feet, but in every
single one of these parts the similarities are offset by differences of
great significance. No competent anatomist would mistake any of the bones
of the †proterotheres for the corresponding parts of the horses, whatever
view he might hold as to the relationship between the two groups. The
case is thus one of a very instructive kind, as tending to show that
identity of structure in so highly complex creatures as mammals is not
independently attained by widely separated or entirely unrelated forms.
Probable as this conclusion is made by all the available evidence, it
cannot be regarded as demonstrated; it is proverbially impossible to
prove a negative.
On the other hand, it is equally probable that nearly related forms do
very frequently, perhaps normally, pass through separate, but closely
similar, courses of development. It is likely that a new species is
usually formed through similar and simultaneous modification of many
individuals, rather than from a single individual or pair. It may be the
general rule, as almost certainly has often happened, that a new genus
arises by the separate assumption of the new character by several species
of the ancestral genus, rather than through the rapid diversification of
a single species, though, no doubt, parallel and divergent modification
are both very frequent and important processes. Dr. Eigenmann concludes
from his study of South American fresh-water fishes that a certain
new genus is even now in process of origin through the transformation
of several species of an older genus, which in different parts of the
continent are simultaneously, but independently, taking on the new
character.
Sometimes it is possible to assign a definite reason for the independent
origin of similar structures in different groups of mammals. Except
for the head, there is much similarity of appearance among the very
massive hoofed animals, such as the elephants, rhinoceroses, tapirs
and hippopotamuses of the present time, a fact which induced Cuvier
to unite them in one order, the “Pachydermata,” a term which has
passed into vernacular, if metaphorical, usage. No doubt also, several
extinct groups, such as the †Amblypoda and the perissodactyl family
of the †Titanotheriidæ, would have been included, had they been
known in Cuvier’s day. In the largest and heaviest of these animals,
the elephants, †amblypods and †titanotheres, there are many close
correspondences in all parts of the skeleton, which are clearly due to
the mechanical necessities imposed by the support of immense weight,
and the developmental history of each group shows that the smaller and
lighter ancestors were less similar than the larger and more massive
descendants. Such subsequently acquired likenesses are thus obvious
examples of convergence and were caused by adaptation to similar needs.
Fürbringer has shown that among birds size and weight of body determine
many resemblances between unrelated families, the largest forms
displaying a more advanced grade of specialization.
It is thus extremely probable that evolution is a highly complex process,
in which divergent, parallel and convergent modes of development are
normally concerned. This complexity greatly increases the difficulty of
determining phylogenies, which would be very much easier could every
notable resemblance be at once accepted as proof of relationship. It
often renders impossible the classification of some isolated group,
which seems to have several incompatible affinities. It emphasizes the
necessity of founding schemes of classification upon the totality of
structure and the importance of determining the value of characters,
whether they are primitive or advanced, divergent, parallel or
convergent, before attempting to use them in classification.
In looking over the field of mammalian evolution, so far as that is
recorded by the fossils, the general impression received is that the
most important process is divergent development, one line branching out
into several. This process became especially vigorous and rapid at times
of important change in the character of the environment, what Osborn
has called “adaptive radiation.” As we have repeatedly observed in the
history of particular groups, _e.g._ the rhinoceroses, horses and camels,
numerous parallel phyla of the same family existed together in certain
geological stages, but as these phyla were traced back in time, they were
found to draw together and display themselves as branches of a single
stem. This favours the inference that the mammalian orders, so far as
they are truly natural groups and not arbitrary assemblages, are each
of single, or monophyletic, origin, and that the parallel and convergent
modes of development, while very frequent and important, are subordinate
to divergence.
II. A second problem is whether development among mammals is always
by means of reduction in the number of parts, or whether that number
may not be increased. With this is involved the so-called law of the
“irreversibility of evolution,” according to which organs once lost, or
reduced to a vestigial condition, are never regained, or reëstablished
in function. There can be no question that the usual mode of mammalian
development is by reduction in the number of parts and the enlargement
and elaboration of those which are retained, as, for example, in the
reduction of five toes to one in the series of the horses; but there are
cases which require a different explanation. The very numerous teeth of
the porpoises and dolphins and of the Giant Armadillo are not a primitive
feature, but must have arisen by a process of multiplication. In the
very curious Large-eared Wolf (_Otocyon_) of South Africa the number
of molar teeth 3/4 exceeds that found in any other placental mammal.
This feature has been interpreted as a proof of marsupial relationship,
but, as the creature is a typical dog in all other respects, such a
relationship would involve a degree of convergence in development that
is quite inadmissible without the most cogent evidence. Until something
is learned regarding the descent of _Otocyon_, no positive statement can
be made as to the significance of its exceptional dentition, but much
the most likely supposition is that additional teeth have been developed
in an otherwise normal canid. However that may be, the testimony of the
fossils is unequivocally to the effect that the usual mode of development
among mammals is by a reduction in the number of parts, accompanied by
enlargement and specialization in those which are retained.
It is equally clear that the “law of irreversibility” holds good in
a very large number of cases, but whether it is always valid is very
doubtful. In the Guinea Pig, as in all its family (Caviidæ), there are
four toes in the front foot, three in the hind; but Professor Castle has
lately succeeded in producing a race with four toes in the hind foot. To
call this a “monstrosity” or “abnormality” explains nothing; the fact
remains that the four-toed race has been established and no reason can be
assigned why the same thing might not happen in nature. If Dr. Matthew’s
view concerning the origin of the American deer from _†Leptomeryx_ (p.
409), should prove to be well founded, another example of the same
kind would be furnished. In _†Leptomeryx_ of the Oligocene the upper
canine was reduced to minute, almost vestigial proportions, while in the
ancestral deer, _†Blastomeryx_ of the lower Miocene, it was a large,
scimitar-like tusk. While I am unable to accept this derivation of the
deer, it may be true nevertheless and, if so, will be a most interesting
example of the rehabilitation of a vestigial organ. Decision must await
the discovery of the intermediate forms. Many such cases and instances
of the addition of parts may be so far undetected, but the phylogenetic
series, as we have them before us, point decidedly to the conclusion that
such rehabilitation or new addition is exceptional.
III. So far as we are able to follow it by the aid of the fossils,
development among the mammals would appear to be a remarkably direct
and unswerving process. When any long-lived phylum, made up of numerous
well-preserved members, is studied, the observer cannot fail to be
impressed by the straightforward course of the evolutionary process,
as though the animals were consciously making for a predetermined
goal, which, needless to say, they were not. A minute cusp makes its
appearance on a tooth, enlarges steadily in each succeeding genus, and
ultimately becomes a very important element in the pattern; and in this
series of changes there is no oscillation backward and forward. In the
perissodactyls and a few other groups, the premolars in each family
gradually and steadily assumed the size and complexity of molars;
beginning at the hinder end of the series, these teeth one by one become
molariform, not in irregular and haphazard fashion, but by perfectly
graded stages. The same gradual and direct process was maintained in the
oft-recurring reduction of digits among the hoofed animals, differing
for each group according to the symmetry of the foot. In the horses, for
example, the first digit became vestigial and disappeared, and then the
fifth followed, leaving a three-toed foot, in which the median digit
was notably the largest and bore most of the weight. Throughout the
Oligocene and Miocene epochs the horses were all tridactyl, but there was
a gradual enlargement of the median digit and dwindling of the laterals,
until these became mere dew-claws, not touching the ground, and the
weight was carried entirely upon the median one. Finally, the laterals
lost their phalanges and were farther reduced to splints, which is the
modern condition. In the same gradual and unswerving manner the higher
artiodactyls went through a process of digital reduction from five to
two, and numberless other instances of similar sort might be adduced.
On the other hand, the direction of change long followed may be departed
from, the deviation being due to the introduction of a new factor. In
the earliest deer the males were hornless, but they developed effective
weapons of defence by the enlargement of the upper canine teeth into
long and sharp, sabre-like tusks. When antlers appeared, the work of
defence was transferred to them, and the tusks began to dwindle, being
eventually suppressed in those deer which had large and complex antlers,
though persisting to the present time in the hornless Musk Deer and in
the small-antlered Muntjaks, which can defend themselves with their sharp
tusks.
It would be inaccurate to say that fluctuations in the size and
effectiveness of parts never occurred; on the contrary, there is evidence
that such fluctuations in details were not infrequent, and may have been
even more common than we suppose. To give one instance, the very early
camels of the upper Eocene and lower Oligocene had small canines, which
though not at all functionless or vestigial, were yet little larger than
incisors. Though the ancestral camels of the middle and lower Eocene are
not yet definitely known, there is strong reason to believe that in them,
as in all of their contemporaries among the ungulates, the canines were
enlarged and fang-like. If so, the canine teeth in the camels underwent
decided fluctuations in size, being first larger, then smaller and again
enlarging. If Dr. Matthew’s interesting theory as to the origin of
the true felines from primitive †sabre-tooth cats (see p. 540) should
be confirmed, it would furnish a very striking example of fluctuating
development. The acceptance of the theory involves the admission of the
following changes: (1) The upper canine was enlarged and changed into
a thin, recurved, scimitar-like tusk; (2) the lower canine was much
reduced, becoming little larger than the incisors; (3) the lower jaw
developed a flange on each side from its inferior border, against which
the inner side of the upper canine rested, when the mouth was closed, and
the chin was nearly flat, meeting the outer surface of the jaw at a right
angle. After these peculiarities had been fully established, the stock
divided into two series; in one, the †machairodonts, the specialization
continued along the same lines, assuming more and more exaggerated forms,
while in the true cats it was reversed. The upper canine grew shorter and
thicker, the lower canine was very greatly enlarged, the lower jaw lost
its flange, and its external and anterior surfaces no longer met at a
right angle, but curved gradually into each other. As previously stated,
such a reversal strikes me as improbable and not to be accepted without
very much more complete evidence than we now have, but it is perfectly
possible that such evidence may be forthcoming.
Making the fullest allowance for all such cases of fluctuation, it
remains true that in the great majority of the phyla whose history may
be followed in some detail, development has been remarkably direct and
unswerving. Plasticity of organization and capacity for differentiation
of structure in widely different directions would seem to be limited in
the mammals, especially among the more advanced groups.
IV. A question that has been much debated and is still a centre of
controversy deals with continuity and discontinuity in development. In
other words, does evolution proceed by the cumulative effects of minutely
graded modifications, or is it a succession of leaps and sudden changes?
The difference is illustrated by many breeds and races of animals and
plants under domestication, the history of which is known. Some have
arisen from “sports,” sudden and marked deviations from the parent stock,
which “breed true” from the beginning. Of this character was the Ancon
breed of sheep, which was derived from a single short-legged ram that
was born of normal parents in 1791 and transmitted his peculiarities
to his offspring. Professor Castle’s race of four-toed Guinea Pig
originated from one four-toed individual, which suddenly appeared in a
litter of normal ones. Other breeds have been formed by the careful and
long-continued selection of minute individual variations. Which of these
methods is the one that has been followed under natural conditions? or
has now one method been used and now another, according to circumstances?
The problem is one that has a profound and far-reaching importance for
the whole of evolutionary philosophy, which largely hinges upon it.
Unfortunately, palæontology is not well fitted to give a decisive answer
to these questions, for, however complete the record of any given series
may be, we never can be sure that it actually is so, and interruptions
in the continuity of development might be due either to progress by
abrupt changes, or to a failure to preserve all the gradations. For
that reason different observers have put divergent interpretations upon
the facts as we have them. The general impression that is made by the
study of a well-preserved mammalian phylum is that of continuity, but a
closer analysis reveals numerous small breaks, and suggests, so far as
the record may be trusted, that the advance was made by separate steps,
though very short ones. Indeed, it has been objected that so completely
recorded a phylum as that of the horses must be illusory, because there
is not perfect continuity between the successive genera, it being taken
for granted that such continuity is the normal mode of development.
Dr. Schlosser, on the other hand, is a disbeliever in perfect continuity.
“I am of the opinion that we must reckon with development _per saltum_
more frequently than is usually done. We have been decidedly spoiled
by the phylogenetic series of quiet successive development, such as
we meet with in the Oligocene and Miocene of North America in the
titanotheres, oreodonts, camels, etc., and in the upper Eocene of
Europe in _Palæotherium_, _Paloplotherium_, etc., as well as from the
Oligocene into the Pleistocene, _e.g._, in the rhinoceroses, cervids,
suillines, amphicyonids. Even here we often make for ourselves artificial
difficulties by balancing, with an exaggerated scrupulousness, the
individual forms one against another, to see whether they really are
exactly fitted to fill up any gaps. It is not the lack of suitable
intermediate forms which so often renders difficult the establishment of
genetic series, but, quite on the contrary, the abundance of the forms
at our disposal, among which we must make a choice. If, however, the
development of phyla did not take place in the same region and under
constant climatic and topographical conditions, we must necessarily
find apparent gaps, for adaptation to a new environment occasions rapid
changes of organization, so that the immediate descendant will often
deviate considerably from its ancestor. But that must not mislead us into
denying the connection between such forms.”[22]
Better adapted to a solution of this problem than mammals are the fossil
shells of Mollusca, the development of which may often be traced through
a thick series of strata, each step of modification being represented
by innumerable individuals. In very many instances it appears that each
species in a series of successive modifications had many contemporary
fluctuating variations, but the change from one species to the next
succeeding one was by a small though abrupt mutation. The difference
between two successive species may be no greater than that between two
contemporary variants of the same species, but it was a constant and not
a fluctuating difference. There is much reason to believe that such is
at least a frequent mode of development, namely, that from species to
species and genus to genus the transition has been by slight and sudden
changes. The possibility that such abrupt changes, however slight, are
illusory and due to small gaps in the record, must be admitted, and
though this does not seem to be a very likely explanation, it is given
plausibility by the almost perfect continuity between successive species
which may sometimes be observed.
The extremely important and significant distinction between contemporary,
fluctuating variations and successive, constant mutations was first drawn
by Waagen, who says of them: “One must therefore distinguish strictly
between varieties in space and those in time. To describe the former,
the long-used name ‘variety’ will suffice, for the latter, on the other
hand, I would propose, for the sake of brevity, a new term, ‘mutation.’ A
species as such, with reference to its connection with earlier or later
forms, may be conceived and regarded as a mutation. But also in regard to
the value of these two concepts, just established (variety and mutation),
an entirely different value is displayed on closer consideration. While
the former appears extremely vacillating, of small systematic value, the
latter, even though in minute characteristics, is extremely constant and
always to be recognized with certainty.”[23]
The same conception was adopted and elaborated by Neumayr: “Still other
characteristics appear, which mark mutations as something different from
varieties, especially that, as a rule, there is a definite direction of
mutation in each series, the same characters changing in the same sense
through a considerable succession of strata.”[24]
Whether development was continuous or discontinuous, there is no reason
to suppose that the amount and rate of modification were always constant.
On the contrary, there is strong evidence that at times of great climatic
or geographical changes, or when a region was invaded by a horde of
immigrants, widespread readjustments were accomplished with comparative
rapidity. Indeed, such periods of relatively quick changes have long
seemed to be implied by the facts of the palæontological records.
* * * * *
It is only too clear that the principles as to the modes of mammalian
development which can be deduced from the history of the various groups
must, for the most part, be stated in a cautious and tentative manner,
so as not to give an undue appearance of certainty to preliminary
conclusions, which should be held as subject to revision with the advance
of knowledge. Much has, however, been already learned, and there is every
reason to hope that Experimental Zoölogy and Palæontology, by combining
their resources, will eventually shed full light upon a subject of such
exceptional difficulty.
FOOTNOTES
[1] Memoirs of the University of California, Vol. I, pp. 209-211.
[2] Voyage of a Naturalist, Amer. ed., pp. 133-134.
[3] J. W. Gregory, The Great Rift Valley, p. 268.
[4] Voyage of a Naturalist, Am. ed., 1891, p. 82.
[5] D. H. Scott, Studies in Fossil Botany, London, 1900, pp. 524-525.
[6] The names, Javan and Sumatran rhinoceroses, are somewhat misleading,
since both of these species are also found on the mainland of India.
[7] This plausible and no doubt correct explanation was suggested to me
by my colleague, Professor C. F. Brackett.
[8] Flower and Lydekker, Mammals Living and Extinct, p. 332.
[9] Flower and Lydekker, _op. cit._, pp. 307-308.
[10] Flower and Lydekker, _op. cit._, pp. 355 and 357.
[11] The Woodland Bison of Canada is now regarded as a distinct species.
[12] Darwin, Voyage of a Naturalist, p. 172.
[13] F. E. Beddard, Mammals, London, 1902, pp. 550, 551.
[14] Bates, Naturalist on the Amazons, London, 1875, pp. 32, 140.
[15] Bates, Naturalist on the Amazons, London, 1875, pp. 332, 333.
[16] Beddard, _op. cit._, pp. 555, 556.
[17] A. Hrdlička, Smithsonian Institution, Bureau of Ethnology, Bulletin
33, 1907, p. 98.
[18] _Ibid._, Bulletin 52, 1912, pp. 385, 386.
[19] K. von Zittel, Handbuch der Palaeontologie, Bd. IV, p. 132.
[20] Reports of the Princeton University Expeditions to Patagonia, Vol.
IV, Pt. 3.
[21] W. K. Gregory, The Orders of Mammals; Bull. Amer. Mus. Nat. History,
Vol. XXVII, p. 211.
[22] M. Schlosser, Beiträge zur Kenntniss der Oligozänen Landsäugethiere
aus dem Fayum, Vienna, 1911, p. 165.
[23] W. Waagen, Die Formenreihe des Ammonites subradiatus, _Benecke’s
Geognost.-Palæont. Beitr._, Bd. I, pp. 185-186.
[24] M. Neumayr, Die Stämme des Thierreiches, Bd. I, p. 60.
GLOSSARY
=Acetabulum=, the deep socket in the hip-bone for the head of the
femur.
=Acromion=, the projecting lower end of the spine of the
shoulder-blade.
=Alisphenoid canal=, canal in the base of the skull for the external
carotid artery.
=†Allotheria=, an extinct suborder of Mesozoic and Paleocene
Marsupials.
=†Amblypoda=, an extinct order of hoofed mammals.
=Anconeal fossa=, a deep pit on the posterior side of the humerus,
near the lower end.
=Anconeal process=, _see_ Olecranon.
=†Ancylopoda=, an extinct suborder of Perissodactyla.
=Angle=, of the lower jaw, the postero-inferior corner.
=Angular process=, a hook-like projection from the angle of the lower
jaw.
=Anterior nares=, the forward opening of the nasal passage.
=Anthropoidea=, Monkeys, Apes, Man; suborder of Primates.
=Appendicular skeleton=, bones of the limbs and limb-girdles.
=Araucanian=, Pliocene of Argentina, including the Catamarca and
Monte Hermoso.
=Artiodactyl=, _see_ Artiodactyla.
=Artiodactyla=, Cattle, Deer, Camels, Pigs, etc., etc., order of
hoofed mammals.
=Ascending ramus=, posterior, vertical portion of the lower jaw.
=Astragalus=, the ankle-bone.
=Astraponotus Beds=, upper Eocene or more probably, lower Oligocene
of Patagonia.
=†Astrapotheria=, an extinct order of hoofed mammals.
=Atlas=, the first vertebra of the neck.
=Auditory bulla=, one of a pair of inflated bony capsules at the base
of the skull; the tympanic bone.
=Auditory meatus=, the entrance to the bulla.
=Axial skeleton=, the skull, backbone, ribs and breast-bone.
=Axis=, the second vertebra of the neck.
=†Barytheria=, an extinct order of elephant-like mammals.
=Biceps muscle=, the large flexor muscle of the front of the upper
arm; its contraction bends the elbow.
=Bicipital groove=, a groove between the tuberosities of the humerus
for the upper tendons of the biceps.
=Brachyodont=, low-crowned teeth, with early-formed roots.
=Bridger stage=, middle Eocene of N. W. America.
=Bunodont=, teeth composed of conical tubercles.
=Calcaneum=, the heel-bone.
=Cannon-bone=, a compound bone formed by the coössification of two or
more long bones of the foot.
=Cape Fairweather=, marine Pliocene of Patagonia.
=Carnassial=, a shearing, sectorial tooth in a flesh-eater.
=Carnivora=, Wolves, Bears, Cats, etc., etc.; an order of placental
mammals.
=Carnivorous=, flesh-eating, predaceous.
=Carpal=, one of the elements of the carpus.
=Carpus=, the wrist-bones.
=Casa Mayor stage=, terrestrial formation of Patagonia, probably
Eocene.
=Catamarca=, a Pliocene formation of Argentina.
=Caudal vertebræ=, those of the tail.
=Central=, a small carpal, wedged in between the two rows.
=Centrum=, the body of a vertebra.
=Cervical vertebræ=, those of the neck.
=Cetacea=, Whales, etc.; a cohort of marine mammals.
=Chelodactyla=, suborder of Perissodactyla.
=Chevron-bones=, Y-shaped bones attached to the under side of the
caudal vertebræ.
=Chevrotains=, “Mouse Deer,” of the suborder Tragulina.
=Chiroptera=, Bats, an order of placental mammals.
=Class=, a group of the fifth order in classification.
=Clavicle=, the collar-bone.
=Cnemial crest=, a massive prominence on the front face of the tibia,
near the upper end.
=Cohort=, division of infraclass, containing a series of related
orders.
=†Condylarthra=, an extinct order of hoofed mammals.
=Condyle=, a knob-like, articular protuberance.
=Convergence=, or =Convergent Evolution=, similar forms resulting
from two or more independent lines of descent.
=Coracoid=, a hook-like bone, fused with the shoulder-blade in the
higher mammals.
=Coronoid process=, a projection in front of the condyle of the lower
jaw, to which the temporal muscle is attached.
=Cotyles=, concavities on the atlas to receive the occipital condyles
of the skull.
=Cranium=, the part of the skull above and behind the eyes, which
lodges the brain and higher sense-organs.
=†Creodonta=, an extinct suborder of the Carnivora.
=Cretaceous=, third and last of the Mesozoic periods.
=Crown=, the exposed part of a tooth.
=Deltoid crest=, a ridge on the anterior face of the humerus for the
attachment of the deltoid muscle.
=Dental formula=, an arithmetical expression of the number and kinds
of teeth.
=Dermoptera=, Flying Lemur, order of placental mammals.
=Deseado stage=, terrestrial formation of Patagonia, probably
Oligocene.
=Didelphia=, lower infraclass of the Eutheria.
=Digit=, a finger or toe.
=Diprotodonta=, Kangaroos, etc., a suborder of Marsupials.
=Dorsal vertebræ=, those which carry ribs.
=Duplicidentata=, Hares and Rabbits, suborder of Rodentia.
=Edentata=, Sloths, Anteaters, etc., an order of placental mammals.
=Edentates=, _see_ Edentata.
=†Embrithopoda=, an extinct order of elephant-like mammals.
=Embryo=, young animal in early stages of development within the
uterus.
=†Entelonychia=, extinct suborder of the †Toxodontia.
=Eocene=, second of the five Tertiary epochs.
=Epicondylar foramen=, perforation of the internal epicondyle for
transmission of the ulnar nerve.
=Epicondyle=, a rough prominence on each end of the humeral trochlea.
=Epiphysis=, the ends of the long bones, which ossify separately and
do not coalesce with the shaft until growth ceases.
=Equus Beds=, _see_ Sheridan stage.
=Eutheria=, the higher subclass of mammals; viviparous.
=Family=, group of the third order in classification, typically
containing several genera.
=Fauna=, the totality of animals of a given time or place.
=Femur=, the thigh-bone.
=Fibula=, the external bone of the lower leg.
=Fissipedia=, land-carnivores; suborder of the Carnivora.
=Flora=, the totality of plants of a given time or place.
=Fœtus=, young animal in the later stages of development within the
uterus.
=Foramen=, a perforation in a bone for the passage of a nerve or
blood-vessel.
=Foramen magnum=, the opening in the occiput for the passage of the
spinal cord to the brain.
=Formation=, a general term for a group of strata, laid down
continuously and under uniform conditions.
=Frontal=, one of a pair of bones which form the anterior part of the
cranial roof; the forehead.
=Genus=, group of the second order in classification, typically
containing several species.
=Glenoid cavity=, (of the squamosal) the articular surface for the
condyle of the lower jaw; (of the scapula) the socket for the
head of the humerus.
=Hallux=, the first digit of the pes, or great toe.
=Herbivorous=, plant-eating.
=†Homalodotheres=, _see_ †Entelonychia.
=Horizontal ramus=, the tooth-carrying part of the lower jaw.
=Humerus=, the bone of the upper arm.
=Hyoid arch=, a series of bony rods, attached to the base of the
cranium, for support of the tongue.
=†Hyopsodonta=, an extinct suborder of the Insectivora.
=Hypsodont=, high-crowned teeth, with late-formed roots.
=Hyracoidea=, Klipdases, an order of hoofed mammals.
=Ilium=, the anterior element of the hip-bone.
=Inferior maxillary=, the lower jaw.
=Infraclass=, division of subclass.
=Insectivora=, Moles, Shrews, etc., an order of placental mammals.
=Ischium=, the postero-superior element of the hip-bone.
=John Day stage=, upper Oligocene of N. W. America.
=Jugal=, the cheek-bone. _See_ Malar.
=Jurassic=, the second of the Mesozoic periods.
=Lachrymal=, a small bone on the front edge of the orbit.
=Lachrymal foramen=, a canal for the tear-duct piercing the lachrymal
bone.
=Lemuroidea=, Lemurs, suborder of the Primates.
=Lemurs=, _see_ Lemuroidea.
=Limb-girdles=, the bones which attach the limbs to the body.
=Lipotyphla=, suborder of the Insectivora.
=†Litopterna=, extinct order of hoofed mammals.
=Loricata=, Armadillos and Glyptodonts; the armoured Edentates.
=Lumbar vertebræ=, those of the loins.
=Lunar=, the middle bone in the upper row of the carpus.
=Magnum=, the middle bone in the lower row of the carpus; supports
the third digit or middle finger.
=Malar=, cheek-bone. _See_ Jugal.
=Malleolar bone=, the lower end of the fibula, persisting as a
separate bone after loss of the shaft.
=Malleolus, external=, the lower end of the fibula.
=Malleolus, internal=, process from the lower end of the tibia.
=Mammal=, a warm-blooded vertebrate, which suckles its young.
=Mandible=, the lower jaw.
=Manubrium=, the anterior segment of the breast-bone.
=Manus=, the hand or fore foot.
=Marsupial=, _see_ Marsupialia.
=Marsupialia=, Opossums, Kangaroos, etc., etc.; only order of the
infraclass Didelphia.
=Marsupium=, the hairy pouch in which the young Marsupials are
carried.
=Masseter muscle=, a muscle of mastication, attached to the lower jaw
and inferior border of the zygomatic arch.
=Mastoid=, that part of the periotic bone which is exposed on the
surface of the skull.
=Mastoid process=, a spine-like outgrowth of the mastoid.
=Maxillary=, the upper jaw-bone.
=Medullary cavity=, the marrow cavity of a long bone.
=Mesozoic=, the middle era of geological time.
=Metacarpal=, a member of the metacarpus.
=Metacarpus=, the long bones of the manus, or fore foot.
=Metapodial=, a metacarpal or metatarsal.
=Metatarsal=, a member of the metatarsus.
=Metatarsus=, the long bones of the pes, or hind foot.
=Miocene=, the fourth of the Tertiary epochs.
=Monodelphia=, placental mammals; the higher infraclass of the
Eutheria.
=Monophyletic=, derived from a single line of ancestry.
=Monotremata=, Duck-billed Mole and Spiny Anteaters; the only
existing order of the Prototheria.
=Monte Hermoso stage=, upper Pliocene of Argentina.
=Mouse Deer=, chevrotains; suborder Tragulina.
=Mystacoceti=, Whalebone Whales; order of the Cetacea.
=Nasal=, one of a pair of bones, forming the roof of the nasal
passage.
=Navicular=, central bone of the tarsus.
=Neural arch=, the bony arch of a vertebra.
=Neural canal=, the cavity in the arch, lodging the spinal cord.
=Neural spine=, or spinous process, the projection arising from the
summit of the neural arch.
=Notostylops Beds=, _see_ Casa Mayor stage.
=Occipital condyles=, a pair of knob-like protuberances from the
occiput for articulation with the first vertebra.
=Occipital crest=, an elevated bony ridge around the margin of the
occiput.
=Occiput=, the posterior surface of the skull.
=Odontoceti=, Toothed Whales; order of Cetacea.
=Odontoid process=, a peg-like projection from the body of the second
vertebra, which fits into the ring of the first.
=Olecranon=, the heavy projection from the upper end of the ulna,
forming the point of the elbow.
=Oligocene=, the third of the Tertiary epochs.
=Opposable=, used of the thumb and great toe, when they can be
opposed to the other digits.
=Orbit=, the bony eye-socket.
=Order=, a group of the fourth rank in classification, typically
including many families.
=Oviparous=, egg-laying.
=Palate, hard=, the bony roof of the mouth.
=Palatine=, one of a pair of bones which form the hinder part of the
hard palate.
=Palatine process=, a shelf-like projection of the maxillary, which
forms most of the hard palate on each side.
=Paleocene=, the oldest of the five Tertiary epochs.
=Palmate=, form of antler in which the tines are fused into large
plates.
=Pampean=, Pleistocene, perhaps including the uppermost Pliocene, of
Argentina.
=Parallelism=, or =Parallel Evolution=, similar development of
related, but separate series.
=Paraná stage=, lower Pliocene (or perhaps upper Miocene) of
Argentina.
=Parietal=, one of a pair of large, vaulted bones, which form most of
the sides and roof of the cranium.
=Paroccipital process=, a bony projection from the infero-external
angle of the occiput.
=Patagonian stage=, marine lower Miocene of Patagonia.
=Patella=, the knee-cap.
=Pecora=, true Ruminants, suborder of Artiodactyla.
=Pelvic girdle=, _see_ Pelvis.
=Pelvis=, the hip-bones.
=Periotic=, a small, dense bone, which lodges the internal labyrinth
of the ear.
=Pes=, the hind foot.
=Petrosal=, _see_ Periotic.
=Phalanx=, one of the joints of the fingers or toes.
=Pholidota=, Pangolins or Scaly Anteaters; order of placental mammals.
=Phylum=, a genetic series of ancestors and descendants within a
family.
=Pilosa=, Sloths, Anteaters, etc.; suborder of Edentata.
=Pinnipedia=, Marine Carnivores; suborder of Carnivora.
=Pisiform=, an accessory bone attached to the postero-external angle
of the carpus.
=Placenta=, a temporary structure connecting mother and fœtus, by
means of which the fœtus is nourished in the womb.
=Placental=, having a placenta; the Monodelphia.
=Pleistocene=, the older of the two Quaternary epochs.
=Pliocene=, the fifth and last of the Tertiary epochs.
=Pollex=, the first digit of the manus, or thumb.
=Polyphyletic=, derived from two or more distinct lines of ancestry.
=Polyprotodonta=, Opossums, etc.; suborder of Marsupials.
=Posterior nares=, the hinder opening of the nasal passage.
=Postglenoid process=, a bony ridge behind the glenoid cavity of the
squamosal to prevent backward dislocation of the jaw.
=Postorbital process=, a bony projection from the frontal or jugal,
bounding the eye-socket behind.
=Premaxillary=, the anterior bone of the upper jaw, carrying the
incisor teeth.
=Primates=, Lemurs, Monkeys, Apes and Man; cohort and order of
placental mammals.
=Proboscidea=, Elephants, etc.; order of hoofed mammals.
=Process=, a distinct prominence or projection of bone for the
attachment of muscle or ligament.
=†Proglires=, an extinct suborder of the Insectivora.
=Prototheria=, most primitive subclass of mammals; oviparous.
=Pubis=, the postero-inferior element of the hip-bone.
=Pyramidal=, the external bone in the upper row of the carpus.
=†Pyrotheria=, an extinct suborder of †Toxodontia.
=Pyrotherium Beds=, _see_ Deseado stage.
=Radius=, the internal bone of the fore-arm.
=Rodent=, _see_ Rodentia.
=Rodentia=, Gnawers; order of placental mammals.
=Rotular groove=, a broad, shallow groove on the anterior face of the
femur, near the lower end, in which the knee-cap glides.
=Round ligament=, the ligament between the head of the femur and a
pit in the acetabulum of the hip-bone.
=Sacral vertebræ=, those of the sacrum.
=Sacrum=, a bony mass of fused vertebræ, for the support of the
hip-bones.
=Sagittal crest=, a ridge of bone in the median line of the cranial
roof, running forward from the occipital crest.
=Scaphoid=, the inner bone in the upper row of the carpus.
=Scapho-lunar=, a compound bone made up of the coalesced scaphoid,
lunar and central.
=Scapula=, the shoulder-blade.
=Section=, primary division of a suborder.
=Sectorial=, a carnassial or shearing tooth of a flesh-eater.
=Selenodont=, teeth composed of crescent-shaped cusps.
=Shaft=, the body of a long bone, comprising most of its length.
=Sheridan stage=, older Pleistocene of the Great Plains.
=Shoulder-girdle=, the bones to which the fore limb is attached.
=Simplicidentata=, Squirrels, Rats, Porcupines, etc., etc.; suborder
of Rodentia.
=Sinus=, an air-cavity in one of the skull-bones.
=Sirenia=, Sea Cows and Dugong; order of marine mammals.
=Species=, the unit group in classification, made up of individuals
which are most closely similar.
=Spine=, (of the scapula) a bony ridge on the outside of the
shoulder-blade; (of the tibia) a single or double prominence
from the upper end of the shin-bone; (of a vertebra) the neural
spine.
=Squamosal=, a bone forming the posterior side-wall of the cranium.
=Sternal ribs=, the inferior segments of the ribs, which articulate
with the breast-bone.
=Sternum=, the breast-bone.
=Stratum=, a layer of bedded rock.
=Subclass=, primary division of class.
=Subfamily=, a group of related genera within the family.
=Subgenus=, a group of related species within the genus.
=Suborder=, primary division of order.
=Subspecies=, a definite subdivision of a species.
=Suina=, swine-like animals; suborder of Artiodactyla.
=Superfamily=, a group of related families.
=Superorder=, a group of related orders.
=Supinator ridge=, a crest on the outer side, near the lower end of
the humerus, for attachment of the supinator muscle.
=Symphysis=, the line of junction of the two halves of the lower jaw.
=Synonym=, a name improperly given to a genus or species already
named.
=†Tæniodontia=, an extinct order of clawed mammals.
=Tarsal=, an element of the tarsus.
=Tarsus=, the bones of the ankle-joint.
=Temporal muscle=, a muscle of mastication attached to the side of
the cranium and the coronoid process of the lower jaw.
=Tertiary=, the more ancient of the two Cenozoic periods.
=Thoracic vertebræ=, _see_ Dorsal.
=Thorax=, the bony frame-work of the chest.
=Tibia=, the shin-bone, internal bone of the lower leg.
=†Tillodontia=, an extinct order of clawed mammals.
=†Toxodonta=, an extinct suborder of the †Toxodontia.
=†Toxodontia=, an extinct order of hoofed mammals.
=†Toxodonts=, _see_ †Toxodonta.
=Tragulina=, “Mouse Deer”; suborder of Artiodactyla.
=Transverse processes=, projections from the sides of a vertebra.
=Trapezium=, internal bone in the lower row of the carpus; supports
the first digit, or thumb.
=Trapezoid=, second bone in the lower row of the carpus; supports the
second digit, or index finger.
=Triassic=, first of the three Mesozoic periods.
=†Triconodonta=, an extinct suborder of Mesozoic Marsupials.
=†Trituberculata=, an extinct order of Mesozoic mammals.
=Trochanter=, a projection from the femur.
=Trochanter, third=, a hook-like process on the outer side of the
shaft of the femur, near the middle of its length.
=Trochlea=, the pulley-shaped lower end of the humerus for
articulation with the fore-arm bones.
=Trunk vertebræ=, those of the body, the dorsals and lumbars.
=Tubercle=, an articular projection on a rib, connecting with the
transverse process of a dorsal vertebra.
=Tuberosities= (of the humerus), heavy projections from the upper end
of the bone, in front of the head.
=Tubulidentata=, the Aard Vark; an order of placental mammals.
=Tylopoda=, Camels and Llamas; suborder of Artiodactyla.
=Tympanic=, a bone forming the support of the ear-drum and usually
inflated into a hollow capsule.
=†Typotheres=, _see_ †Typotheria.
=†Typotheria=, an extinct suborder of the †Toxodontia.
=Ulna=, the external bone of the fore-arm.
=Unciform=, the external bone in the lower row of the carpus;
supports the fourth and fifth digits, or ring and little
fingers.
=Unconformity=, the relation between two groups of strata, one of
which was deposited upon the worn surface or upturned edges of
the other.
=Ungual phalanx=, the terminal joint of a digit, which supports the
claw, nail or hoof.
=Unguiculata=, clawed mammals; cohort of Monodelphia.
=Ungulata=, hoofed mammals; cohort of Monodelphia.
=Ungulates=, _see_ Ungulata.
=Uterus=, the womb.
=Vagina=, the genital canal of the female.
=Variety=, a more or less constant group within a species.
=Vertebra=, a joint of the backbone.
=Vertebral column=, the backbone.
=Viviparous=, producing living young.
=†Zeuglodontia=, an extinct order of Cetacea.
=Zygapophyses=, the projecting processes, by means of which
successive vertebræ are articulated together.
=Zygomatic arch=, a bony bridge from the eye-socket to the hinder
part of the cranium.
INDEX
N. B.—The most important references are in =heavy-faced type=; technical
names of genera and species are italicized, though most of the specific
names are omitted as unnecessary. Extinct groups are indicated by a
dagger (†).
Aard Vark, 60
_†Abderites_, 627, =641= (jaw fig.)
_†Achænodon_, 273, 361, 369 (skull fig.), =370=
†Achænodonts, Bridger, 369;
Uinta, 369;
Wasatch, 370
†Acœlodidæ, 477
Adaptive radiation, 655
_†Adinotherium_, 462, =473=, 474 (restoration)
_†Adpithecus_, 462
_†Ælurocyon_, 517, 551
_†Ælurodon_, 517, =527=
_Ælurus_, 546
Africa, 184, 245, 328, 332, 417, 419, 421, 422, 426, 442, 458, 481,
551, 579, 642, 656;
elephants of, 138;
mammals of, 145;
zoölogy of, 146
AGASSIZ, L., 129
Age, geological, 15
Agouti, 185 (fig.)
_Agouti_, 183 (fig.), 185
Agoutis, Pleistocene, 218
†Agriochœridæ, 247, 250, 361, =383=, 484, 652;
Eocene, 383;
John Day, 250, 383;
Oligocene, 383;
Uinta, 267, 385;
White River, 268, 383
†Agriochœrids, _see_ †Agriochœridæ
_†Agriochœrus_, 252 (restoration), 361, =383= (skull fig.), 384
(restoration), 385 (manus fig.)
Alachua stage, 127, 225
Alaska, 103, 106, 197, 199, 202, 203, 332, 418, 419, 420, 427, 433;
†Mammoth in, 40;
Miocene of, 118;
Oligocene of, 113;
Pleistocene glaciation in, 131;
volcanoes, 133;
Pliocene of, 125
_†Albertogaudrya_, 509, =512=
_Alce_, 65, 151, 156 (fig.), 202, 208, 362, 411, 412
ALLEN, J. A., 141, 161
Alligators, 102
†Allothere, Paleocene., 642 (skull fig.)
†Allotheria, 59, 627, =642=
_Alouatta_, 578, =585=
Alps, Arctic animals and plants of, 193;
Eocene, 104
_†Alticamelus_, 224, 362, 388, =391=;
restoration, 236
Amazon, 585;
as barrier to species, 137
†Amblypoda, 60, =443=, 508;
Bridger, 269, 445;
Eocene, 443;
Puerco, 286, 454;
Torrejon, 285, 453;
Wasatch, 277, 452;
Wind River, 274, 450, 452
_†Amblytatus_, 592
AMEGHINO, F., 228, 263, 467, 471, 476, 496, 497, 613
America, connections of North and South, 123
American †Mastodon, 196;
restoration, 195
Americas, marsupials of, 138
Amherst expedition, 487
Amphibia, 55;
as ancestral to mammals, 643
_†Amphicyon_, 517, 524, =525=, 530
†Amphicyons, 558
_†Amphidolops_, 627
_†Amphiproviverra_, 627, =637=;
skull fig., 637
_†Amynodon_, 272, 291, 340, =348=, 349
†Amynodontinæ, 291, 340, 341, =346=, 350, 351, 353;
Bridger, 272, 350;
Oligocene, 339;
Uinta, 266, 348;
White River, 255, 346.
†Amynodonts, _see_ †Amynodontinæ
_†Anacodon_, 277, 554, =561=
_†Analcitherium_, 592
†Anaptomorphidæ, 578, 583
_†Anaptomorphus_, 281, 578, =581=;
head restored, 581
_†Anchitherium_, 290, 299
Ancon sheep, 660
†Ancylopoda, 60, 291, =353=;
Bridger, 357;
Miocene, 238, 355;
Pliocene, 224, 355
Andes, 178, 179, 180, 185, 189, 211, 213, 322, 548;
Eocene, 112;
Miocene, 124;
Pleist. glaciation, 133, 134;
Plioc. 128, 129
ANDREWS, C. W., 422, 435
Antarctic continent, 103, 123, 638
Ant-Bear, 91, 187, 188 (fig.), 206, 591, 355, 600, 601, 615
Anteater, Collared, 187, 188 (fig.);
Lesser, 591;
tree, 591;
Two-toed, 188
Anteaters, 60, 75, 94, 187, 189, 591, 593, 596;
Pleistocene, 218, 596;
Santa Cruz, 245, 596;
scaly, 60, 353;
spiny, 57, 59
Antelope, 202;
bones of, 35;
Mioc. restored, 237;
Prong-horned, 5, 162 (fig.)
Antelopes, 54, 60, 222, 312, 362, 409, =416=, 418;
flat-horned, 417;
goat-horned, 417;
Miocene, 235, 417;
Old World, 202;
Pleistocene, 202;
Pliocene, 224;
S. Amer., 213, 215, 221, 418, 466;
strepsicerine, 225, 417;
Tertiary, 419;
twisted-horned, 417
†Anthracotheres, _see_ †Anthracotheriidæ
†Anthracotheriidæ, 259, 266, 361, =370=, 381, 384, 386
_†Anthracotherium_, 259, 361, 371
Anthropoidea, 60, 578, 579, 580, =582=
Anthropoids, _see_ Anthropoidea
Antigua, 134
Antilles, Eocene, 112;
Miocene, 123;
Oligocene, 117;
Pliocene, 128
Antillia, 112;
Oligocene, 117
_Antilocapra_, 162 (fig.), 202, 225, 362, =416=, 417
Antilocapridæ, 362, =416=
Antilopidæ, 416
Antler, 411
Antwerp, 37
Apar, 592
Apes, 60, 577, 578, 582, 583;
night, 585
_†Aphelops_, 291
_Aplodontia_, 153, 233 (_see_ Sewellel)
Aplodontiidæ, 249
Appalachian Mts., 101, 150, 153
Aquatic habits, 2
Araucanian stage, 128
Arboreal animals, 2, 77, 84
Archæan period, 15
_†Archælurus_, 249, 517, 541, 543
†Archæohyracidæ, 462
_†Archæohyrax_, 462
†Archæopithecidæ, 462, 477
_†Archæotherium_, 259, 361, =367=;
manus fig., 367;
restoration, 252, =260=;
skull fig., 367;
teeth fig., 368
Arctic, archipelago, 125;
islands, 210;
fauna in Pleisto., 128;
mammals, 109;
regions, 128;
Cretac. climate of, 26;
Sea, 106;
shells, Pleisto., 27;
species, distribution of, 141;
zone, 147 (map), 148
†Arctocyonidæ, 554, 557, =561=, 575;
Torrejon, 285;
Wasatch, 561
_†Arctotherium_, 211, 517, =549=, 553;
head restor., 549
Argentina, 180, 185, 211, 213, 215, 218, 219, 245, 324, 391, 418,
436, 463, 466, 531, 586, 596, 597;
drought in, 33;
plains of, 133;
Pliocene of, 20, 128;
spread of horses and cattle in, 142
_†Argyrohippus_, 476
Arid province, 164
Aridity, evidences of, 24
Arikaree age, or stage, 17, =120=, 235, 259, 356
Armadillo, 5, 162, 591;
6-Banded, 189 (fig.), 592;
7-Banded, 592;
9-Banded, 190 (fig.), 592, 593;
11-Banded, 592;
Bridger, 268, =616=;
Giant, 190, 592, 612, 656;
Pygmy, 592;
restoration of Santa Cruz, 243, =480=
Armadillos, 60, 97, 141, 185, 189, 592, 593, 594, 595, =610=, 623,
624, 625;
Araucanian, 226;
Casa Mayor, 282, 595;
Deseado, 262, 595, 616;
Paraná, 228;
Pleistocene, 218, 596, 612, 613;
Santa Cruz, 245, 596, 612.
(_See also_ Dasypoda _and_ Dasypodidæ)
Artiodactyl, †primitive, restoration, 252
Artiodactyla, 54, 55, 60, 69, 247, 284, 310, 355, =358=, 402, 459,
460, 491, 507, 514;
Araucanian, 226, 227;
Blanco, 222;
Bridger, 273;
classification, 361;
John Day, 250;
Miocene, 231, 235, 239;
Neotropical, 176;
North American, 176;
Old World, 176, 362;
Pleisto. N. Amer., 201;
S. Amer., 213;
Pliocene, 224;
†Primitiva, 60, 361, =370=;
Uinta, 266;
Wasatch, 281;
White River, 255, 257;
Wind River, 275
Ash, volcanic, 29
Asia, 106, 239, 254, 258, 280, 317, 321, 328, 332, 352, 355, 369,
386, 390, 408, 413, 414, 417, 418, 419, 422, 426, 546, 550,
552, 579, 644;
circumpolar area, 148;
elephants of, 138;
hyracoids of, 138;
Minor, 458;
Pleisto. glaciation of, 130;
zoölogy of, 146
_†Asmodeus_, 462
Asphalt, 31
Ass, 52
Asses, 213, 292, 308
_†Asterostemma_, 592, 623
Astragalus, 88
_†Astraponotus_, 509, 512;
Beds, 20, =281=, 282, 476, 479, 487
†Astrapothere, Santa Cruz, restoration of head, 243
†Astrapotheres, _see_ †Astrapotheria
†Astrapotheria, 60, 489, =508=, 514;
of _†Astraponotus_ Beds, 282;
Casa Mayor, 283, 512;
Deseado, 264, 512;
Patagonian, 512;
Santa Cruz, 247, 508
_†Astrapothericulus_, 509, =512=
†Astrapotheriidæ, 509
_†Astrapotherium_, 243 (restor. of head), =509=, 510 (restor. of head)
_Ateles_, 578, =584=
Atlantic coast, Eocene, 104, 111, 117;
Miocene, 117, 120;
Oligocene, 113, 116;
Paleocene, 101;
Pliocene, 125;
Tertiary mammals of, 369
Atlantic Ocean, 106, 109;
connection with Pacific, 104
Atlas, 70 (fig.)
Auditory bulla, 66
Australia, 14, 21, 57, 58, 138, 140, 307, 340, 426, 461, 520, 550,
634;
marsupials of, 626;
Miocene, 123;
Permian glaciation, 25;
Pleistocene, 632, 634;
rabbits introduced, 142;
zoölogical peculiarity of, 145
Australian region, 640
Axis, 71 (fig.)
_Axis_, 46, 412
AZARA, 34
Baboons, 577, 582
Bad Lands, 107 (fig.)
Badger, 153, 162, 163, 168 (fig.), 517
Badgers, 174, 213, 518, 550, 551, 552;
Pleistocene, 203, 204, 205
Bahia Blanca, 129
Bandicoots, 626
Barriers to spread of mammals, 139
†Barytheria, 60
Basal Eocene, 99
_Bassariscus_, 517, 546, 547
Bat, 89
BATES, H. W., 585
_†Bathyopsis_, 275, 443, =450=, 451, 455
Bats, 59;
absence from Amer. Tertiary, 39;
in European Tertiary, 39;
West Indian, 191
Bear, Alaska Brown, 156 (fig.);
African, 548;
Black, 90 (pes fig.), 548 (teeth fig.);
Pampean, 622;
Polar, 148 (fig.), 548;
†Short-faced, 549 (restor. of head);
South American, 552;
Spectacled, 172 (fig.), 176, 517, 548
†Bear-dog, 222;
Miocene, 525 (restoration);
primitive, 523 (skull fig.)
†Bear-dogs, 523, =524=, 530, 554, 558;
John Day, 249;
Oligocene, 526;
Pliocene, 222;
Pleistocene, 524
Bears, 4, 59, 90, 152, 163, 517, 518, 519, =548=, 553, 554;
Old World, 204;
Old World origin of, 518, 549;
Paraná, 227;
Pleistocene, 203, 204, 549;
Pliocene, 223;
polar, 141;
†Short-faced, 210, 211, 517, =549=;
true, 211, 527, 549.
(_See also_ Ursidæ)
Beast, 1
Beasts of prey, 59, 92
Beaver, 2, 44, 157 (fig.);
dentition, 96 (fig.);
†Giant, 195 (restoration), =205=, 311, 222
Beaver Creek, Wyo., 12 (fig.)
Beavers, 60, 95, 153, 182;
John Day, 249;
Miocene, 238;
Pliocene, 222;
White River, 254
BEDDARD, F. E., 580, 587
Bedded rocks, 6
Bering, Sea, 100, 101;
Strait, 197, 588;
opening and closing of, 23;
Pliocene, 125
BERRIDGE, W. L., 160, 171, 174, 175, 181, 183, 184, 185, 189, 320,
584, 633
Bicuspids, 93
Big Horn Basin, 107, 108, 109
Bighorn, 419
Binomial system of nomenclature, 42
Biogenetic law, 648
Birds, 655;
distribution of, 141;
migrations of, 143;
Santa Cruz, 244
Bison, 4, 152, 162, 358;
American, 154 (fig.);
entombment of, 36;
European, 152, 154 (fig.);
Wood, 162, 419
_Bison_, 202, 362, =420=;
_B. bison_, 152, 154 (fig.), =419=;
_B. bonasus_, 152, 154 (fig.), 420;
_B. †crassicornis_, 203, 420;
_B. †latifrons_, 203, 420;
_B. †occidentalis_, 589
Bisons, 409, 416, 418, =419=
Blanco age and stage, 17, =127=, 221, 388, 413, 551
_Blarina_, 163, 173
_Blastoceros_, 180 (fig.)
_†Blastomeryx_, 224, 241, 362, =414= (restoration), 657
Boar, Wild, 45 (fig. of sow and young), 46, 363
Bogs, burial of mammals in, 33
Bolivia, 178, 184, 215, 225, 436;
Pleistocene, 20, 211;
Pliocene, 129
Bones, gnawed, 36;
Pleistocene, 40;
preservation of, 36;
Tertiary, 40
_†Boöchœrus_, 361, 367
Boreal, fauna, 178;
region, 150;
subregion, 150;
zone, 147, 148 (map), 162, 164, 551, 588
_†Borhyæna_, 244, 494 (restoration), 627, =635=, 637 (skull fig.)
Borneo, 137, 327
_†Borophagus_, 517, 524, 530
_Bos_, 70
_†Bothriodon_, 252 (restoration), 259, 361, =370=, 371 (restoration)
Bovidæ, 362, =418=
†Bow-Tooth, 463
Brachyodont teeth, 95
_†Brachypsalis_, 517
BRACKETT, C. F., 368
_Bradypus_, 186 (fig.), 187, 591
Brain-casts, fossil, 41
Brazil, 118, 181, 190, 201, 213, 215, 218, 219, 221, 245, 324, 391,
436, 527, 530, 552;
caverns of, 19, 30, 133, 211, 218, 221, 586, 596;
Miocene, 596;
Pleistocene, 20
Brazilian subregion, 164, 170 (map), 191
Bridger age and stage, 17, 30, 109, =110=, 340, 380, 386, 568;
restorations of mammals, 271
British Columbia, Miocene, 118;
Oligocene, 113;
Pleistocene glaciation, 131;
Pliocene, 125
Brocket, Wood, 181 (fig.)
Brockets, 181
BROWN, B., 210
Brown-tailed Moth, 143
_Budorcas_, 418
Buffalo, 36, 152
Buffaloes, 409, 416
Bulgaria, 316
_†Bunælurus_, 517, =551=
Bunodont teeth, 360
_†Bunomeryx_, 361
Buno-selenodont teeth, 371
Buried valleys, 132
BURMEISTER, H., 496, 497
Burrowers, 45, 79
Burrowing mammals, 77
Bush-Dog, 174, 212, 527, 530, 552
_Cabassous_, 592, 614, 616
_Cacajao_, 578, =585=
Cacomistle, 162, 517, 546
_Cænolestes_, 58, 190, 284, 626, =640= (skull fig.), 641, 642
Cænolestidæ, 627
_†Cænopus_, 238, 252 (restoration), 256 (do.), 291, 333, =336= (molar
and skull fig.), 339 (front teeth fig.), 342, 351
_†Calamodon_, 274
Calcaneum, 88
California, Eocene, 104, 111;
marine Pleisto., 132;
Mesozoic, 23;
Miocene, 118, 121, 127;
Pliocene, 125
_Callithrix_, 218
_Caluromys_, 631
Cambrian period, 15;
glaciation in, 25
Camel, 48, 54, 60, 70, 79, 358, 490;
distribution, 138;
family, 178;
Miocene, 232 (restoration);
tribe, 13;
True, 178;
White River, 252 (restor.)
Camelidæ, 362, =386=;
distribution, 138
Camel-like animals, 386
Camels, 56, 81, 84, 87, 90, 257, 258, 312, 362, 373, =386=, 421, 461,
651, 655;
Bridger, 273, 398;
browsing, 388, 393;
Eocene, 397, 398, 402, 659;
grazing, 393;
John Day, 250, 394;
Miocene, 231, 232, 235, 241, 391;
Old World, 231;
Oligocene, 394, 402, 659;
phyla of, 650;
Pleistocene, 196, 202;
Pliocene, 224, 388;
true, 13, 386, 387, 390, 391;
Uinta, 267, 397;
White River, 257, 394
_Camelus_, 70, 138, 362, 387
Canada, 257, 357, 565;
Eocene climate, 111;
Paleocene, 102;
White River, 113;
zoölogy, 146
Canadian fauna, 151;
subregion, 147, =150=
Canidæ, 173, 223, 517, 518, =520=;
fox-like, 529
(_See also_ Dogs)
_†Canimartes_, 517
Canine teeth, 93
_Canis_, 152, 517, 522, 529;
_C. †dirus_, restor., frontispiece, 204, 521;
_C. †indianensis_, 204;
_C. latrans_, 162, 165 (fig.), 632;
_C. nubilis_, 159 (fig.);
_C. occidentalis_, 62 (skull fig.), 64 (skull fig.), 162.
(_See_ Wolves)
Cannon-bone, 84, 91 (fig.), 410 (fig.)
Cape Fairweather stage, 128
_†Capromeryx_, 362, 417
_Capromys_, 184
Capybara, 205.
(_See also_ Carpincho _and_ Water Hog)
Capybaras, Pleistocene, 218
Carboniferous period, 15
Caribbean, region, Miocene, 123;
Sea, Oligocene, 113
Caribou, 4, 181, 202, 207, 208, 210, 412, 413;
Barren-ground, 148;
Pleistocene, 27, 413;
Woodland, 152, 157 (fig.)
Carnivora, 43, 59, 83, 90, 244, 268, 282, 284, 285, 459, =516=, 634;
Araucanian, 226;
Blanco, 222;
Boreal, 152;
distribution, 138;
Eocene, 554;
John Day, 249, 528;
marine, 59;
migration to S. Amer., 508, 518;
Miocene, 229, 233, 238;
Neotropical, 173;
Pleistocene, N. Amer., 203, 210;
S. Amer., 211;
Plioc., 222;
Sonoran, 163;
Uinta, 265;
White River, 254, 312
Carnivores, _see_ Carnivora
_†Carolozittellia_, 462, =488=
Carpincho, 183 (fig.), 185.
(_See also_ Capybara _and_ Water Hog)
Carpus, 82
Casa Mayor age and stage, 20, =112=, 281, 488, 499, 512
Cascade Mts., 121;
Oligocene craters of, 116
CASTLE, W. E., 657, 660
_Castor_, 96, 153, 157 (fig.)
_†Castoroides_, 195, =205=
Cat, 222;
Domestic, 546 (manus fig.)
Catamarca age and stage, 20, =129=, 226
Catarrhina, 583, 587, 588
Cats, 54, 59, 90, 176, 517, 518, 519, =530=, 532, 553, 568;
cursorial, 543;
Miocene, 545;
Native, 634, 638, 640;
Oligocene, 530;
Pleistocene, 545;
Pleisto. S. Amer., 211, 212;
Pliocene, 223, 545;
South America, 552;
true, 249, 517, 530, =543=.
(_See_ Felidæ)
Cattle, 95;
spread of, 142
Caves as sources of fossil mammals, 30
_Cavia_, 183 (fig.), 185
Cavicornia, 328, 411, 412, =416=, 421
Cavies, _see_ Caviidæ
Caviidæ, 185, 657;
Araucanian, 226;
Pleistocene, 218;
Santa Cruz, 245
Cavy, Rock, 183 (fig.)
Caxomistle, _see_ Cacomistle
Cebidæ, 172, 578, =584=, 585
_Cebus_, 218, 578, =584= (fig.), 585
Celebes, 579
Cement, 96
Cenozoic era, 15, 16, 17, =18=, 99;
South America, 19
_Centetes_, 173
Central, 83
Central America, 123, 164, 178, 179, 320, 585;
Eocene, 104, 112;
geology, 120;
mammals, 141;
Oligocene, 113, =117=;
Paleocene, 103;
tapirs, 137;
Tertiary, 22;
zoölogy, 146
Central American subregion, 164, 170 (map), =191=
_Cerdocyon_, 171 (fig.), 174, 517, 552
_†Cervalces_, 195 (restoration), 208, 209 (restoration), 362, =413=
Cervicornia, =411=, 421
Cervidæ, 362, =411=, 661;
Neotropical, 179.
(_See also_ Deer)
_Cervulus_, 412
_Cervus_, 208, 362;
_C. canadensis_, 151, 155 (fig.), 202, 208, 411, 412;
_C. elaphus_, 151;
_C. eustephanus_, 151.
(_See_ Deer)
Cetacea, 60, 442;
Miocene, 123, 125
_Chætomys_, 184
†Chalicothere, 240 (restoration), 356 (manus fig.)
†Chalicotheres, _see_ †Chalicotheriidæ
†Chalicotheriidæ, 60, 247, 291, =354=, 383, 385, 458, 484, 651;
Bridger, 357;
John Day, 250, 357;
Miocene, 231, 235, 238, 356;
White River, 257, 357
_†Chalicotherium_, 354
CHAMBERLIN, T. C., 130
Chamois group, 202, 417;
subfamily, 152
_†Champsosaurus_, 102
Cheeta, 542, 543
Chelodactyla, 60, 290
Chevron-bones, 73
Chevrotains, 54, 60, 408
(_see also_ Mouse-Deer _and_ Tragulina)
Chili, 124, 184, 436;
marine rocks, 112;
Pleistocene, 20;
Pleisto. glaciation, 133
Chilian subregion, =164=, 170 (map)
_Chinchilla_, 184 (fig.), 185
Chinchilla-family, Araucanian, 226
Chinchillas, 185;
Santa Cruz, 245
Chipmunks, 141, 153
_Chironectes_, 626, 627
Chiroptera, 59
_Chlamydophorus_, 190, 592
_†Chlamydotherium_, 218, 592, 596, 612, 614
_Cholœpus_, 74, 187 (fig.), 591
Chronology, geological, 10;
of rocks, 6
Civet cats, 518, 558
(_see_ Viverridæ)
_†Cladoclinus_, 627
_†Cladosictis_, 243 (restoration), 627, =638=, 639 (restoration)
_†Clænodon_, 554, =561=
Classification of mammals, 50
Clavicle, 77 (fig.)
Clawed mammals, 59, 74, 456, 459, 460, 492, 514
Climate, as barrier to species, 140;
determining distribution, 24;
Cretaceous, 26;
Eocene, 109, 448;
Miocene, 122;
Mioc. of Patagonia, 124, 244, 586;
Oligocene, 116;
Paleocene, 102;
Pleistocene, 116, 134, 192;
Pliocene, 127;
vicissitudes of, 100
Climatic changes, 14;
affecting distribution, 140;
evidences of, 24;
Pleisto., effects on migrations, 207
Coast Range, elevation, 122;
Miocene, 113, 125
Coati, 162
Coatis, 76, 213, 517, 546, 552
_†Cochlops_, 592
_Coendou_, 182 (fig.), 184
_†Colodon_, 257, 291, =327=
Colombia, 626, 640
_†Colonoceras_, 272, 291, =347=, 350
Colouration, animal, 45
_†Colpodon_, 462
Columbia River valley, Miocene, 118
Comparative Anatomy, 647
Conard Fissure, 30, =210=
†Condylarth, 278 (restoration), 457 (skeleton fig.), 459 (restoration)
†Condylarthra, 60, 443, =456=, 484, 492, 499, 508, 514, 515, 653;
Puerco, 286, 460;
Torrejon, 285, 459;
Wasatch, 277, 457;
Wind River, 274, 456
_Condylura_, 152
_Conepatus_, 174 (fig.), 213, 517, 552
Conies, 60, 458, 481
Conifers, 103
Continental deposits, Eocene, 106, 112;
Miocene, 120;
Oligocene, 113, 117;
Paleocene, 101;
Pliocene, 127, 128
Continental islands, 140
Continuity of development, 660
Convergence, 650, 653, 655, 656
COPE, E. D., 306, 343, 399, 400, 401
Coracoid, 76
_†Coryphodon_, 275, 277, 279 (restoration), 285, 443, =452=, 454, 456
†Coryphodontidæ, 285, 443, 454;
lower Eocene, 456
†Coryphodonts, _see_ †Coryphodontidæ
Costa Rica, 181;
Pliocene, 128
Cotton-rats, 163
Coyote, 162, 165 (fig.)
Coyotes, Pleistocene, 218
_†Cramauchenia_, 489
†Creodont, 252 (restoration), 563 (restoration)
†Creodonta, 59, 516, 519, 527, 529, =554=, 574;
Bridger, 268, 271 (restoration);
Eocene, 633;
Paleocene, 633;
Puerco, 286;
Torrejon, 285;
Uinta, 265;
Wasatch, 276;
White River, 253;
Wind River, 274.
(_See_ Flesh-eaters)
Cretaceous period, 15, 16, 103, 112, 117, 261, 281, 443, 460, 514,
642, 643;
climate, 26
Crocodiles, 122, 244;
absent from John Day, 116;
Eocene, 111;
Paleocene, 284;
White River, 116
Crown of tooth, 95
Crustal movements, Miocene, 122
_Ctenomys_, 184
Cuba, 173, 185;
junction with Central America, 128, 598;
Miocene, 123;
Pleistocene, 134, 604;
Pliocene, 128, 605
Cuboid, 89
Culebra Cut, Tertiary rocks, 22
Cuneiform, 83, 89
CUVIER, G., 44, 654
_Cyclopes_, 591
_†Cyclopidius_, 361, =376=
_†Cynodesmus_, 517, =522= (skull fig.), 523, 530
_†Cynodictis_, 254, 517, 529 (restoration), =530=, 547
_Cyon_, 213, 517, 527
_†Cyonasua_, 517
_Dama_, 412
_†Daphœnodon_, 517, =525= (restoration), 526, 530
_†Daphœnus_, 254, 517, 523 (skull fig.), 524 (manus and teeth fig.),
=526=, 528, 530, 537, 546
DARWIN, C., 33, 35, 52, 136, 137, 143, 193, 217, 463, 489, 490, 491,
492
Dasypoda, 189, 592, =610=.
(_See also_ Armadillos)
Dasypodidæ, 592
_Dasyprocta_, 185 (fig.)
_Dasypus_, 189 (fig.), 592, 611, 614, 616
Dasyures, Australian, 638
Dasyuridæ, 632, =634=, 640
Deep River age and stage, 17, =121=, 233
Deer, 46, 54, 60, 95 (molar fig.), 222, 312, 319, 360 (molar fig.),
362, 409, =411=, 461;
American, 153, 162, 202, 208, 409, 412, 414, 420, 657;
Axis, 412;
Barking, 412;
Black-tailed, 5, 202;
Chinese Water-, 412;
earliest, 658;
Fallow, 412;
Florida, 179 (fig.);
Hog, 412;
hornless, 414;
Marsh, 179, 180 (fig.);
Miocene, 232, 235, 414 (restoration);
Mule, 46 (fawns fig.), 167 (fig.);
Musk-, 224, 412, 658;
Neotropical, 179;
North American, 179;
Old World, 151, 179, 181, 202, 412, 415;
Pampas, 180;
Patagonian, 91 (pes fig.), 410 (manus and pes fig.);
Pleistocene, 202, 208, 412;
Pleisto., S. Amer., 213, 215;
Pliocene, 224, 226;
South American, 415, 418, 466;
southern, 412, 413;
Tertiary, 412, 419;
Virginia, 4, 166 (fig.), 179, 202, 412
†Deer-Antelopes, 202, =224=, 362, 417;
Miocene, 232, 235, 414, 415 (restoration);
Pleistocene, 417
Degu, 184
_†Deltatherium_, 554
Dental formula, 93
Dentine, 96
Deposits, continental (_see_ Continental deposits);
lake, 37;
river, 36
Dermoptera, 59
Deseado age and stage, 20, =117=, 282, 283, 474, 475, 477, 479, 481,
485, 486, 487, 508, 511, 512, 586, 587
Desiccation, Miocene and Pliocene, 128
_†Desmathyus_, 361
_†Desmatippus_, 290
_†Deuterotherium_, 489
Development, convergent, 446, 499;
parallel, 499;
per saltum, 661.
(_See also_ Evolution)
Devonian period, 15;
glaciation in, 25
Dhole, 213, 249, 517, 527, 530
_†Diadiaphorus_, 248, 489, 501 (skull fig.), 502 (restoration), =503=
(pes fig.), 505, 507, 508
†Diceratheres, _see_ _†Diceratherium_
_†Diceratherium_, 238, 239 (restoration), 250, 256, 291, 333, 334,
350, 444
_Dicerorhinus_, 327, 329
†Dichobunidæ, 361, 398
Didelphia, =57=, 59, 626
Didelphiidæ, 627, =630=
_Didelphis_, 161, 626, 627, 631;
_D. marsupialis_, 161 (fig.), 631
†Didolodidæ, 489
_†Didolodus_, 489
_†Didymictis_, 555, 558
Digit, 90
Digital reduction, 658
Digitigrade, 90
_†Dinictis_, 254, 517, =538=, 539 (restoration), 541 (pes fig.), 542,
546
†Dinocerata, 443
_†Dinocynops_, 517
_†Dinocyon_, 524
_†Dinohyus_, 239, 361, =366=
†Dinosaurs, 103, 284
†Dinotheres, _see_ _†Dinotherium_
_†Dinotherium_, 435, 438, 486
_†Diplacodon_, 266, 291, =313=, 317 (head restored)
_Dipodomys_, 163 (fig.)
Diprotodonta, 59, 627, =640=;
Deseado, 642;
Paraná, 641;
Pleistocene, 641;
Pliocene, 641;
Santa Cruz, 640, 641;
South American, 640
Discontinuity of development, 660
Dispersal of species, 143
_†Dissacus_, 554, =560=
Distribution, discontinuous, 127, 138, 193;
geographical, of mammals, 135
Divergence, _see_ Evolution
_†Dœdicurus_, 212 (restoration), 219, =618=, 619 (restoration)
Dog, 90, 553;
family, 558;
fox-like, 529 (restoration)
Dogs, 90, 173, 517, 519, =520=, 548, 553, 554, 558;
Blanco, 522;
early, 550;
John Day, 249, 523, 528, 529;
Miocene, 229, 234, 238, 522, 527, 528, 529;
Oligocene, 523, 547, 553;
Paraná, 227;
Pleistocene, 521;
Pleisto., S. Amer., 212;
Pliocene, 522;
Plioc. S. Amer., 226;
†primitive, 537;
†short-faced, 530;
South American, 552;
Uinta, 265;
White River, 254, 529.
(_See also_ Canidæ)
_†Dolichorhinus_, 272, 291
_Dolichotis_, 185
Dolphins, 37, 60, 94, 656;
Miocene, 123
Domesticated plants, history of, 288
Douroucoulis, 578, =585=
Drainage, the Pleistocene changes of, 132
Drift-sheets, 25, 132
_†Dromocyon_, 269 (restoration), 271 (restoration), 554, =559=
_†Dromomeryx_, 235, 237 (restoration), 362, =417=
Drought, effects of on mammals, 33
Duck-billed Mole, 57, 59
Dugong, 60, 442
Duplicidentata, 59
Dust, volcanic, 29;
wind-blown, 33
East Indian Archipelago, 191
_Echidna_, 57
_Echimys_, 184
Ecuador, 178, 284, 391, 548, 626, 640;
Pleistocene, 20, 211;
Pliocene, 129
Edentata, 60, 72, 75, 91, 97, 120, 185, 267, 355, =591=;
Araucanian, 226;
armoured, 60, 592, =610=;
Casa Mayor, 283, 592, 595;
Deseado, 261, 595;
distribution, 138;
Eocene, N. Amer., 597, 616;
hairy, 60, =591=;
Old World, 185, 591;
Paraná, 227;
Pleisto., N. Amer., 205;
Pleisto., S. Amer., 218, 596;
Plioc., N. Amer., 225, 597;
Plioc., S. Amer., 226, 596;
Santa Cruz, 245, 596;
South American, 276, 625
Edentates, _see_ Edentata
_†Edvardocopeia_, 509
Egg-laying mammals, 59
Egypt, 254, 370, 422, 432, 442, 450, 587;
Eocene, 234;
Oligocene, 234, 264, 583
Ei-á, 585
EIGENMANN, C. H., 654
_†Elachoceras_, 443, =449=, (skull fig.), 450, 451, 455
_†Elasmotherium_, 350, 351
Elephant, 590;
African, 423 (molar fig.);
†Columbian, 195 (restoration), =197=, 198 (restoration), =427=, 430;
East African, 425;
†Imperial, 199, =427=, 485;
Indian, 97 (section of tooth fig.), 197, 423, 425 (manus fig.), 426
(section of fore foot fig.);
tribe, 82;
West African, 425
Elephantidæ, 432
Elephants, 45, 60, 73, 91, 92, 95, 97, 215, 264, 312, 436, 446, 448,
465, 487, 654;
American, 430;
cranial bones of, 63;
distribution, 138;
hairy, 448,
proboscis of, 65;
Pleistocene, 196, 211, 426;
Siberian Pleisto., 39;
true, 423, 438, 439;
tusks of, 97
_Elephas_, 436, 437 (head and tooth fig.);
_E. †columbi_, 195 (restoration), =197=, 198 (restoration), =427=;
_E. †imperator_, =199=, =427=, 485;
_E. maximus_, 97 (section of molar fig.), 197, 423, 425 (manus
fig.);
_E. †primigenius_, =196=, 207, 332, =426=
Elk, 50, 141, 151, 155 (fig.);
Scandinavian, 151
Elms, 102
†Embrithopoda, 60
Embryology, 648
Emigrants from N. Amer. to Old World, 255, 256, 456
Enamel, 96
England, early Man in, 588;
Paleocene flora, 103;
Pliocene, 127
_†Enhydrocyon_, 517, =528=, 530
_†Entelodon_, 369
†Entelodontidæ, 250, 361, =366=, 445;
Wasatch, 281
†Entelodonts, _see_ †Entelodontidæ, _also_ †Giant Pigs
†Entelonychia, 60, 247, 462, =482=, 652;
Casa Mayor, 282;
Deseado, 263.
(_See also_ †Homalodotheres)
Entrerios, 128
_†Eoanthropus_, 588
_†Eobasileus_, 443, 449, 451, 455
_†Eocardia_, 243
Eocene epoch, 17, =104=;
climate of, 26;
close of, 111;
Europe, 262, 370, 452, 562, 661;
North America, =104=, 105 (map), 201, 250, 251, 253, 273, 287, 291,
325, 369, 421, 519, 529, 554, 557, 574, 644;
South America, 20, =112=, 261, 281, 477, 481, 482, 485, 487, 488,
508, 509, 512, 514, 625, 642
_†Eodidelphys_, 627
_†Eohippus_, 280, 290, =302=, 303 (restoration), 304, 305 (skull
fig.), 307 (manus and pes fig.), 308
_†Eohyus_, 281
_†Eomoropus_, 291, =357=
_†Eotitanops_, 275, 291, =315=
_†Eotylopus_, 257, 362
_†Epigaulus_, 223 (restoration)
_†Epihippus_, 290, =301=, 302
_†Epithelium_, 227, 489, =508=
Epoch, geological, 15
_†Eporeodon_, 361, 375, 379
Equidæ, 290, =291=.
(_See also_ Horses)
_Equus_, 95, 199, 213, 223, =291=, 295, 305 (skull fig.), 306 (manus
and pes fig.);
American species, 296;
_E. asinus_, 52;
_E. burchelli_, 200;
_E. caballus_, 52, 199, 213, 295;
_E. †fraternus_, 199;
_E. †giganteus_, 200, 201, 295;
_E. †occidentalis_, 200;
_E. †pacificus_, 201;
_E. †pectinatus_, 200;
_E. przewalskii_, 52, 292 (fig.);
_E. †scotti_, 195 (restoration), =200= (do.);
_E. †semiplicatus_, 200;
South American species, 307;
_E. †tau_, 199, 295
Equus Beds, 33, =131=, 133, 200, 205.
(_See_ Sheridan)
Era, geological, 15
_Erethizon_, 151 (fig.), 153, 182, 184, 205
_†Eriodes_, 578
Ermine, 152, 159 (fig.)
Ethiopian region, 146
_†Euceratherium_, 202, 362, 418
_†Eucholœops_, 607
_†Eucinepeltus_, 592, =623=
_†Euprotogonia_, 457, =459=
Eurasia, 110, 548
Europe, 253, 254, 255, 267, 272, 276, 277, 280, 281, 284, 287, 291,
303, 323, 324, 325, 340, 350, 351, 354, 356, 357, 369, 370,
380, 417, 418, 419, 421, 422, 432, 435, 452, 456, 486, 534,
538, 543, 545, 546, 552, 554, 557, 561, 642, 644;
caverns of, 30;
circumpolar area, 148;
Eocene, 104;
Eoc. separation from Asia, 104;
human habitation of, 588;
loess of, 133;
mammals of, 145;
†Mammoth in, 197;
pre-Eocene immigration into, 108;
Miocene, 235;
Mioc. climate, 122;
Pleisto. glaciation, 133;
tapirs in, 138;
Triassic, 642;
zoölogy of, 146
_†Eusmilus_, 254, 517, =538=
_†Eutatus_, 592, 596, =612=, 613
Eutheria, =57=, 59
_†Eutrachytherus_, 263, 462, =477=
Extinction of species, 13, 211
Evolution, of †Amblypoda, 454;
of camels, 400;
convergent, 649, 650, 655;
of †Creodonta, 574;
divergent, 18, 139, 650, 655;
of Fissipedia, 553;
of horses, 305, 325, 400;
irreversibility of, 541, 656;
modes of Mammalian, 645;
of †oreodonts, 381;
parallel, 393, 649, 655;
of Proboscidea, 436, 437 (diagram);
of rhinoceroses, 351;
of tapirs, 324;
of †titanotheres, =316=, 325
Fallow Deer, 46
Families, distribution of, 138
Fauna, 56;
Araucanian, 226;
Bridger, 265, =267=, 273, 315;
Deseado, =261=, 638;
mid. Eocene, 267;
John Day, 249;
low Miocene, 237;
Neotropical, =283=, 610;
Oligocene, 237;
Paraná, 227;
Pleistocene, N. America, =193=, 207;
Pleisto., S. Amer., =211=, 226, 597;
Puerco, 285;
Santa Cruz, 26, 124, =242=, 638;
Torrejon, 284;
Uinta, =265=, 273;
Wasatch, 276;
White River, =251=, 265, 266;
Wind River, =274=, 275, 315
Faunas, Casa Mayor, 281, 283;
Eocene, N. Amer., 265;
Eoc., S. Amer., 281;
Miocene, 229;
Oligocene, N. Amer., 249;
Oligo., S. Amer., 261;
Paleocene, =283=, 286, 644;
Pliocene, N. Amer., 221;
Plioc., S. Amer., 225;
Quaternary, N. Amer., 193;
Quat., S. Amer., 211;
successive mammalian, 192;
Tertiary, 221;
Tertiary, S. Amer., 461
Fawns, 46 (fig.)
Fayûm, 432
Felidæ, 54, 517, 518, =530=
Felinæ, 54, 254, 535, 542, =543=, 650;
Miocene, 229, 234, 238, 541, 545;
origin of, 659;
Pleistocene, 204, 545;
Pliocene, 545
_Felis_, 54, 517, =543=, 545, 546;
_F. †atrox_, 204, 545;
_F. concolor_, 168 (fig.), 544 (skull fig.), 545 (dentition fig.);
_F. domestica_, 546 (manus fig.);
_F. †imperialis_, 204;
_F. leo_, 204;
_F. onca_, 176, 177 (fig.), 552;
_F. pardalis_, 176 (fig.), 552
Femur, 84, 85 (fig.)
Ferret, Black-footed, 160 (fig.)
_Fiber_, 153
Fibula, 86, 87 (fig.)
Field-mice, 141
FILHOL, H., 534
Fisher, 152
Fishers, 141, 518
Fishes, Florissant, 121;
Green River, 109;
Panama marine, 23;
South American fresh-water, 652;
teeth of, 92
Fissipedia, 59, 516, =517=, 553, 554, 555, 556, 557, 558, 563, 576
Flesh-eaters, †primitive, 59, =554=;
Santa Cruz, 637
Florida, island, 122;
Miocene, 117;
Oligocene, 113;
Paleocene, 101;
Pliocene, 125, 127
Florissant formation, 121
FLOWER, W. H., 389, 390, 411, 412, 419
Flying Lemur, 59
Forests, Oligocene, 538;
Paleocene, 102;
petrified, 122
Fort Union stage, 17, 99, =102=, 642
Fossils, 7, 29;
classification, 55;
entombment, 29;
evidence of climate, 25;
mammals, 61
Fossorial habits, 2
Fox, 191;
Arctic, 148, 149 (fig.), 150 (fig.);
Grey, 165 (fig.), 517;
Red, 158 (fig.), 517
Foxes, 141, 173, 518, 520, 530, 552;
grey, 162;
Pleistocene, 204;
red, 152;
White River, 254, 529
France, 256, 333, 364, 441, 574;
Eocene, of, 108;
Oligocene, 617
Frankstown Cave, 30
Friasian fauna, 509
FÜRBRINGER, M., 655
Gait, varieties of, 90
Galapagos Archipelago, 136
†Ganodonta, 625
_†Garzonia_, 627, =641= (jaw fig.)
†Garzoniidæ, 627
Gazelle, bones of, 35
†Gazelle-Camel, 241, 242 (restoration), =393=, 394, 408
Genera, origin of, 654
Generic area, 137
Genetic series, 56
Genetics, 648
Genus, 53
Geographical changes affecting distribution, 139
Geology, 5
_Geomys_, 163
Geomyidæ, 265
†Giant Pig, 252 (restoration), 260 (do.)
†Giant Pigs, 250, 259, 266, 361, =366=;
Bridger, 273, 370;
John Day, 259, 367;
Miocene, 239, 366, 369;
Oligocene, 281, 368;
Uinta, 369;
Wasatch, 281, 370;
White River, 259, =367=
GIDLEY, J. W., 33, 202, 642
Giraffe, 70, 79, 358, 389
†Giraffe-Camel, 236 (restoration), =391=, 392 (restor.)
†Giraffe-camels, 235, =388=, 394;
Miocene, 231, 241, 394;
Pliocene, 224, 388
Giraffes, 54, 389, 409, 411
Glacial, accumulations, 25;
climate, 25, 26;
periods, _14_, 25;
stages, 17, 130;
theory, 129
Glaciation, Pleistocene, 25, 130;
causes of, 134
Glaciers, Pleistocene, 131
_†Glossotherium_, 602
_†Glyptodon_, 212 (restoration), 219, 592, =618=, 619 (restor.), 621
†Glyptodont, Santa Cruz, 243 (restoration), 606 (do.)
†Glyptodontia, 60, 245, 246, 592, 593, 594, 595, =617=;
Araucanian, 226;
Astraponotus, 281, 595, 625;
Deseado, 262, 595;
Pampean, 212 (restorations), 619 (restorations), 623;
Paraná, 227;
Pleistocene, N. Amer., 205, 206, 211, 597, 598;
Pleisto., S. Amer., 218, 221, 596, 597, 620, 624;
Pliocene, N. Amer., 221, 225, 596;
Plioc., S. Amer., 596, 622, 624;
Santa Cruz, 245, 596, =622=, 623
†Glyptodontidæ, 592
†Glyptodonts, _see_ †Glyptodontia
_†Glyptotherium_, 221, 592
Gnawing mammals, 59
Goat, Rocky Mt., 152, 158 (fig.), 202, 416
Goats, 362, 409, 416
_†Gomphotherium_, 229, =430=, 431 (head restored), 434, 436, 437
(head and molar fig.), 438, 439
Gopher, †Horned, 223 (restoration)
Grasses, 273;
Paleocene, 284
Grassy plains, spread of, 233
†Gravigrada, 91, 120, 355, 591, 592, =598=, 612;
Pleistocene, N. Amer., 205, 597;
Pleisto., S. Amer., 218, 598;
Santa Cruz, 605, 607, 609, 610.
(_See also_ †Ground-Sloths)
Great Basin, 322;
Pleistocene of, 131
Great Britain, 21, 140, 418
Great Plains, 33, 200, 229, 235, 322, 386, 432;
Miocene, 121;
Oligocene climate, 116;
Pleistocene, 131
Greenland, 101, 103, 210;
Pliocene, 125
Green River stage, 109
GREGORY, J. W., 35
GREGORY, W. K., 641
_Grison_, 175 (fig.), 517, 552
†Ground-Sloth, giant, 195 (restoration), 603 (restoration);
Pleistocene of Cuba, 598;
Santa Cruz, 243 (restor.), 606 (restor.);
skin of, 40, 602
†Ground-Sloths, 75, 91, 120, 267, 355, 591, 592, 593, 594, 595, =598=;
Araucanian, 226;
Astraponotus, 595;
Casa Mayor, 284, 595;
Deseado, 262, 595;
Miocene, 609;
Mioc., N. Amer., 597;
Pampean, 212 (restoration), 220 (do.), 605, 608, 609;
Paraná, 227;
Pleistocene, N. Amer., 205, 206, 211, 219, 597;
Pleisto., S. Amer., 218, 219, 221, 596, 598, 604, 605;
Pliocene. N. Amer., 221, 225, 597;
Plioc., S. Amer., 596, 598;
Santa Cruz, 245, 246, 596, 598, 605, 608, 609.
(_See also_ †Gravigrada)
Ground-squirrels, 164, 181
_†Grypotherium_, 592, 602
Guanaco, 60, 139, 178, 389 (fig.), 399 (skull and tooth fig.), 400
(manus fig.), 401 (pes fig.), 490, 491;
destruction by cold, 36;
distribution, 138
Guiana, 179
Guianas, Miocene, 596
Guinea-Pig, 185;
four-toed race, 657, 660
Gulf-coast, Eocene, 104, 111, 117;
Miocene, 117;
Pliocene, 125
Gulf of Mexico, Eocene, 106, 113;
Oligocene, 113, 117;
Paleocene, 101
Gulf Stream, Oligocene, 113
_Gulo_, 152, 155 (fig.), 237, 517
Gypsy Moth, 143
HAECKEL, E., 648
Hairless skin, 45
_Halicore_, 442
_†Halmarhiphus_, 627
Handwriting, development of, 9, 13, 14
_Hapale_, 578
Hapalidæ, 172, 578, 582, 583
_†Hapalops_, 243 (restoration), 592, =605=, 606 (restor.), 609 (pes
fig.)
Hare, Arctic, 150
Hares, 59, 181, 245, 249;
Miocene, 229, 238;
Plioc., N. Amer., 222;
Plioc., S. Amer., 226;
tailless, or whistling, 153
_†Harpagolestes_, 554, =559=, 560, 571
Harrison stage, 120, 235
HATCHER, J. B., 337, 523, 524
Hayti, 173, 185;
junction with Centr. Amer., 128;
Miocene, 123;
Pliocene, 128
Hedgehogs, 59, 276;
White River, 253
†Hegetotheriidæ, 462, 472
_†Hegetotherium_, 462, 479
_†Helaletes_, 272, 291
_†Helohyus_, 273, 361, =365=
_†Hemiacodon_, 578
_†Hemipsalodon_, 253, =565=
_†Henricosbornia_, 462
_†Heptodon_, 275, 291, =327=
Herbivora, 516
Herbivorous mammals, 45;
large, 44
_†Hipparion_, 291
_†Hippidion_, 212 (restoration), 213, 214 (restor.), 291, 296, =307=,
308 (skeleton fig.)
_Hippocamelus_, 91 (pes fig.), 180, 410 (manus and pes fig.)
Hippopotamus, 45, 54, 60, 70, 92, 358, 654
Hogs, ruminating, 372
Holarctic region, =146=, 147, 150, 588
_†Homacodon_, 273, 361, =398=
†Homalodontotheriidæ, 462;
Casa Mayor, 283
_†Homalodontotherium_, 462, =482=
_†Homalodothere_, 482
_†Homalodotheres_, 462, =482=, 509
_†Homo heidelbergensis_, 588;
_H. †neanderthalensis_, 588;
_H. sapiens_, 588
_†Homunculus_, 578, =586=
Hoofed animals, 74, 77, 81, 83, 89, 312, 313, 461;
Araucanian, 227;
Bridger, 269, 273;
Casa Mayor, 282;
clawed, 651;
Deseado, 262, 264;
massive, 654;
Miocene, 229, 234;
Paraná, 228;
Pleistocene, N. Amer., 199;
Pleisto., S. Amer., 213;
†primitive, 492;
Santa Cruz, 246;
Torrejon, 285;
Uinta, 273;
Wasatch, 277;
Wind River, 274
(_see_ Ungulata)
Hoofed mammal, clawed, 484
Hoofed mammals, 60, 456, 459, 460;
even-toed, 54, 60;
odd-toed, 60;
White River, 255
(_see_ Ungulata)
HOOKER, J., 193
_†Hoplophoneus_, 252 (restoration), 517, =535=, 536 (restoration),
539, 540, 543
Horn-cores, 416
Horse, 44, 48, 52, 62, 76 (scapula fig.), 79 (humerus fig.), 81
(fore-arm bones fig.), 85 (femur fig.), 87 (leg-bones fig.), 95
(molar fig.), 294, (manus and pes fig.), 359;
Asiatic Wild, 52, 292 (fig.);
†Dawn, 302, 303 (restoration);
†forest, 200;
†Pampas, 212 (restoration), 214 (restoration), 308 (skeleton fig.);
†Texas, 195 (restoration), 200 (restoration);
†three-toed grazing, 298 (restoration);
True, 199, 213, 295;
†White River, 252 (restoration), 300 (restoration).
(_See also_ _Equus_)
Horses, 56, 60, 81, 95, 97, 289, 290, =291=, 312, 319, 330, 353, 360,
382, 397, 458, 461, 499, 504, 651, 653, 655, 656, 658, 661;
Blanco, 222;
bones of, 33;
Bridger, 272, 302;
browsing, 223, 231, 235, 297, 298;
Eocene, 304, 307;
grazing, 223, 231, 235, 297, 298;
John Day, 299;
Miocene, 295, 297, 298, 231, 232, 234, 238, 301;
North American, 39;
Oligocene, 299;
phyla of, 289, 650;
Pleistocene, N. Amer., 199, 208, 211, 213, 221, 295, 304, 307;
Pleisto., S. Amer., 213, 215, 307;
Pliocene, 223, 295, 307, 331;
South American, 307;
spread of, 142, 143;
three-toed, 33, 501;
tridactyl, 658;
true, 292, 308;
Uinta, 301;
Wasatch, 280, 302;
White River, 257, 299, 300;
Wind River, 275, 302, 303, 396.
(_See also_ Equidæ)
HORSFALL, R. B., 42
HRDLIČKA, A., 589
Hudsonian fauna, 151
Hudson’s Bay slope, interglacial forests, 131
Huemul, 180
Humerus, 78 (fig.)
Humid province, 164
Humidity, effect on distribution, 141
Hungary, 316
Hutias, 184
HUXLEY, T. H., 28
Hyænidæ, 518
_†Hyænodon_, 252 (restoration), 253, 555, =562=, 563 (restoration),
564 (skeleton fig.), 565 (teeth fig.), 566 (teeth fig.), 567,
576
†Hyænodont, primitive, 567 (restoration)
†Hyænodontidæ, 253, 555, 557, =562=, 565 (teeth fig.), 566 (teeth
fig.), 569, 573, 575;
Bridger, 268;
Eocene, 254, 566, 576;
Wind River, 274
†Hyænodonts, _see_ †Hyænodontidæ
_†Hyænognathus_, 522, 524, 530
_Hydrochærus_, 183 (fig.), 185, 205
_Hydropotes_, 412
Hyena, bones of, 35
†Hyena-dogs, 222, 249, 527, 530
Hyenas, 518, 527, 553, 554
Hyoid arch, 67
†Hyopsodonta, 59
_†Hyperhippidium_, 213, 291, =307=
_†Hyperleptus_, 607
†Hypertragulidæ, 267, 362, 386, =402=, 414;
Eocene, 408;
John Day, 251, 404, 407;
Miocene, 241, 258, 404;
White River, 258, 406, 408
†Hypertragulids, _see_ †Hypertragulidæ
_†Hypertragulus_, 241, 258, 267, 362, =407=, 408
_†Hypisodus_, 258, 362, =408=
_†Hypohippus_, 291, 297, 300
Hypsodont teeth, 95 (fig.);
prevalence of, 232
_†Hyrachyus_, 271 (restoration), 272, 291, 339, =344= (restor.), 345
(skull fig.), 346, 349, 350
_†Hyracodon_, 252 (restoration), 255, 266, 291, =341= (restor.), 343
(manus fig.)
†Hyracodontidæ, 291, =403=
†Hyracodontinæ, 291, 340, =341=, 346, 350, 351, 352;
Bridger, 272, 343;
Eocene, 342;
Uinta, 266, 343;
White River, 255, 256, 341;
Wind River, 275, 276, 344
†Hyracodonts, _see_ Hyracodontinæ
Hyracoidea, 60, 458, 481, 492, 514;
distribution, 138
Hystricomorpha, 245, 262
Ice Age, 25
_Ichthyomys_, 182
_Icticyon_, 174, 212, 517, 527, 552
_†Ideodidelphys_, 627
IHERING, H. VON, 124
_†Ilingoceros_, 362
Ilium, 77
Immigrants from Old World to N. America, 229, 276, 279, 316, 365,
370, 386, 416, 417;
artiodactyls, 201, 202, 259;
bison, 420;
Carnivora, 203;
felines, 258;
†hyænodonts, 254;
insectivores, 253;
mustelines, 238, 254;
otters, 234;
Proboscidea, 422;
rhinoceroses, 234;
sheep, 419;
from North to South America, 171, 211, 226, 227, 242, 461;
from South to North America, 205, 206, 233
Immigration, 266;
Eocene, 324;
Miocene, 233;
Pleistocene, 151;
Pliocene, 151
Incisors, 93
India, 14, 213, 327, 390, 412, 418, 430, 527, 542, 551;
Permian glaciation of, 25
Indian Ocean, 442
Indians, pre-Columbian, 590
_†Indrodon_, 580
Insect-eaters, 92
Insectivora, 59, 191, 249, 459, 580;
Bridger, 268;
Miocene, 238;
Neotropical, 172;
Paleocene, 284;
Puerco, 286;
Santa Cruz, 245, 587;
Torrejon, 285;
Uinta, 265;
Wasatch, 276;
White River, 253;
Wind River, 274
Insectivores, _see_ Insectivora
Inserts, 141;
Florissant, 121;
Green River, 109
Interglacial stages, 17, 130, 207;
climate of, 134;
mammals of, 131
†Interatheriidæ, 462, 476, =479=
_†Interatherium_, 462, =481=, 636 (restoration)
Irreversibility of evolution, 541, 656
_†Ischyrocyon_, 517
†Ischyromyids, Bridger, 270;
Uinta, 265;
Wasatch, 280
_†Ischyromys_, 254
_†Isectolophus_, 291
†Isotemnidæ, 462, =485=
_†Isotemnus_, 462
Isthmian region, geology, 120;
Pliocene, 128
Isthmus of Panama, 170;
geology, 21, 22;
Miocene, 123;
Oligocene, 117, 123;
Pleistocene, 122, 134
Jackal, bones of, 35
Jaguar, 176, 177 (fig.), 212, 545, 552
Jamaica, Miocene, 123;
mongoose introduced, 142
Japan, 135
Java, 21, 140, 327
JEFFERSON, T., 206, 597
Jerboas, 90
John Day age and stage, 17, 30, =116=, 375, 543
Jumping Mouse, 153, 160 (fig.);
mice, 182;
shrews, 59
Jurassic period, 15, 16, 642, 643
Kangaroo-rats, 163 (fig.), 182;
Miocene, 238
Kangaroos, 59, 626, 640
Kinkajou, 175 (fig.), 517, 546, 552
Klipdasses, 458, 481
KNIGHT, C. R., 42, 470, 478, 480, 481, 494, 502, 506, 606, 636, 639
KOWALEVSKY, W., 233, 503
Kudu, 225
Labrador, Pliocene, 125
_Lagidium_, 185
Lake, Argentine, 36;
Bonneville, 131;
Callabonna, 34;
Lahontan, 131;
Ontario, invasion by sea, 132
Lakes, relation to glaciation, 132;
sediments of, 37
_Lama_, 138, 362, =388=;
_L. huanacus_, 178, 389 (fig.);
_L. vicunia_, 178 (fig.)
_†Lambdaconus_, 489
_†Lambdotherium_, 275, 291, =315=
Land-bridges, 18
Land-connections, how ascertained, 20;
Cuba and Centr. Amer., 128;
Hayti and Centr. Amer., 128;
N. Amer. and Asia, 18, 125, 588;
N. Amer. and Europe, 18, 106, 108, 109, 118, 120;
N. Amer. and Old World, 21, 23, 109, 115, 249, 251, 267, 276, 287;
N. and S. Amer., 100, 120, 123, 233;
S. Amer. and Africa, 103, 112, 124, 587;
S. Amer. and Antarctica, 112, 124;
S. Amer. and Australia, 103, 123, 638;
S. Amer. and Old World, 262;
West Indies and Mediterranean lands, 120
La Plata, estuary, 34
Last Hope Inlet, 60
_Latax_, 517
Lava-fields, the Columbia River, 121, 127
Lavas, Miocene, 118, 121, 122;
Pleistocene, 133;
Pliocene, 127
LECHE, W., 63
LEIDY, J., 372
Lemming, 148
Lemmings, 141, 153
Lemur, †monkey-like, 581 (head restored)
_Lemur_, 578
Lemuroidea, 60, 284, 459, 577, =578=, 588;
Bridger, 270, 578;
Eocene, 579;
Wasatch, 281, 580;
Wind River, 275
†Leontiniidæ, 462, =475=
_†Leontinia_, 263 (head restored), 462, =475=
Leopard, 45;
Hunting, 543
_†Leptarctus_, 517, 547
_†Leptauchenia_, 258, 361, =377= (skull fig.), 378 (restoration), 381
†Leptochœridæ, 361
_†Leptochœrus_, 361
_†Leptomeryx_, 258, 267, 362, =407= (skull fig.), 409, 563
(restoration), 657
_†Leptoreodon_, 362
_†Leptotragulus_, 267, 362
_Lepus_, 164
_†Lestodon_, 602
_†Limnocyon_, 555, =573=
Linnæan system, 51, 56, 57
LINNÆUS, C., 1, 51, 52, 55, 578
Lion, 45, 48, 92, 204;
cubs, 46
Lions, 210, 212
Lipotyphla, 59
_†Listriodon_, 364
†Litopterna, 60, 469, =489=, 514, 651, 653;
Araucanian, 227;
Casa Mayor, 283;
Deseado, 264;
Pampean, 212 (restoration), 216 (do.);
Paraná, 228;
Pleistocene, 215, 221;
Santa Cruz, 243 (restorations), 247
Lizards, 102;
Santa Cruz, 244
Llama, 54, 60, 490, 491;
distribution, 138
Llama-like animals, 386
Llamas, 13, 90, 241, 257, 362, 386, =388=, 390, 391, 421, 461;
Pleistocene, N. Amer., 196, 202;
Pleisto., S. Amer., 213, 215;
Pliocene, 224;
South American, 231
Loess, 133
_Loncheres_, 184
LOOMIS, F. B., 487
†Lophiodontidæ, 257, 272, 291, 319, 325, =326=, 341, 343, 348;
Eocene, 326;
Oligocene, 339;
Wasatch 280, 326;
White River, 257, 326;
Wind River, 275, 315
†Lophiodonts, _see_ †Lophiodontidæ
Loricata, 592, =610=
Loup Fork age and stage, 17, =121=
Loup River stage, 127
Lower Sonoran zone, 148, 164
Lowest Eocene, 99
_Loxodonta_, 423 (molar fig.)
LUCAS, F. A., 337
LULL, R., 437
Lunar, 83
_Lutra_, 152, 160 (fig.), 164, 175, 213, 517, 551
_Lutreola_, 152 (fig.)
LYDEKKER, R., 150, 181, 389, 390, 411, 412, 419
_Lyncodon_, 175, 552
_Lynx_, 153, 163, 169 (fig.), 517, 544 (dentition fig.)
Lynxes, 141, 176, 543, 544, 552;
Pleistocene, 204
†Machairodontinæ, 54, =530=, 535, 542;
cursorial, 543;
Oligocene, 535
†Machairodonts, _see_ Machairodontinæ, _also see_ †Sabre-tooth tigers
_†Machairodus_, 517, =534= (skull fig.), 536
_†Machairoides_, 555, =573=
_†Macrauchenia_, 212 (restoration), 215, 216 (do.), 217, 227, 248,
=489=, 493, 495, 496 (skull fig.), 497 (do.), 498
†Macrauchenid, Santa Cruz, 494 (restoration)
†Macrauchenidæ, 248, =489=, 496 (skull fig.), 497 (do.), 651;
Deseado, 264, 499;
Eocene, 499;
Paraná, 228, 496;
Pleistocene, 489;
Pliocene, 493;
Santa Cruz, 248, 493
_†Macrotherium_, 354
Madagascar, 173, 530;
Pleistocene, 579;
zoölogy of, 146
Magnum, 83
Malagasy region, 146
Malay Archipelago, 146, 191, 580;
islands, 281, 327, 408;
Peninsula, 137, 281
Malleolar bone, 87
Mammal, defined, 1
Mammalia, classification, 50;
evolution of, 645;
geographical distribution, 135;
skeleton and teeth of, 61
†Mammoth, 39, =196=, 207, 332, =426=, 427, 429;
Siberian, 44
Man, 60, 62, 66, 76 (scapula fig.), 77 (clavicle fig.), 79 (humerus
fig.), 80 (fore-arm bones fig.), 82 (manus fig.), 84, 88 (pes
fig.), 90, 93, 577, 578, 582;
American Pleistocene, 589;
European Palæolithic, 197;
European Pleistocene, 39, 588;
origin of, 588;
in Western Hemisphere, 588
Manatee, 207, 442
_Manatus_, 442
_†Manteoceras_, 272, 317 (head restored)
Manus, 82 (fig.)
Maples, 102
Mara, 185
Marine, fauna, Miocene, 117;
Oligocene, 117;
Pliocene, 127;
habit, 2;
mammals, 37, 45;
rocks, 37;
shells, Pleistocene, 132;
Pliocene of England, 127
_Marmosa_, 632
Marmoset, 584 (fig.)
Marmosets, 172, 578, =582=, 583
Marmot, 150, 152 (fig.)
_Marmota_, 152 (fig.), 153
Marmots, 60, 141, 153, 181, 245;
Miocene, 229;
Pliocene, 222
MARSH, O. C., 318
Marsupial, †allotherian, 286 (head restored), predaceous, Santa Cruz,
243 (restoration), 494 (do.), 636 (do.), 639 (do.)
Marsupialia, 43, 57, 59, 459, =626=;
Araucanian, 226, 634;
Australian, 145, 632, 638;
Bridger, 268;
carnivorous, 59;
Casa Mayor, 282, 638, 642;
Deseado, 261, 638, 642;
distribution, 138;
flesh-eating, 553;
herbivorous, 59;
insectivorous, 59;
Miocene, S. Amer., 226;
Paleocene, 284;
Paraná, 227, 634, 641;
predaceous, 627, =632=;
Puerco, 286, 642;
Santa Cruz, 244, 635, 640;
South American, 190, 638;
Torrejon, 285, 642;
Wasatch, 276;
White River, 251
Marten, 551
Martens, 152, 231, 517, 550, 551;
Miocene, 229;
Pleistocene, 204
_Martes_, 517
†Mastodon, 207, 426, 590;
American, 195 (restoration), 196, 207, 229 (molar fig.), 428,
(restoration), =429=, 437 (head fig.), 438, 439, 448;
Miocene, 431 (head restored)
_†Mastodon_, =429=, 430, 437 (head and molar fig.);
_†M. americanus_, _see_ †Mastodon, American;
_†M. andium_, 436
†Mastodons, 60, 264, 430, 438;
Blanco, 222;
early, 432;
Miocene, 229, 234;
Pleistocene, N. Amer., 196, 211;
Pleisto., S. Amer., 215, 221, 436;
Pliocene, 225;
Tertiary, 429
MATTHEW, W. D., 241, 257, 407, 409, 414, 531, 532, 540, 542, 546,
547, 565, 566, 657, 659
_Mazama_, 180, 181 (fig.), 362
Meadow-mice, 153, 182, 218
Mediterranean, Eocene, 104, 106
_†Megalictis_, 517, =551=
_†Megalocnus_, 592, =604=
†Megalonychidæ, 592, =598=, 610
_†Megalonychotherium_, 592
_†Megalonyx_, 195 (restoration), 206, 219, 221, 592, 597, =604=, 607
_†Megamys_, 226
_†Megatheriidæ_, 591, =598=, 607
_†Megatherium_, 206, 212 (restoration), 220 (do.), 591, 597, =599=,
602, 604, 608
_Mellivora_, 551
†Meniscotheriidæ, 457, =458=
_†Meniscotherium_, 457, =458=, 459 (restoration)
Menotyphla, 59
_Mephitis_, 153, 167 (fig.), 517, 552
MERRIAM, C. H., 140, 141, 147, 148, 150, 161
MERRIAM, J. C., 31, 32, 538, 543
_†Merychippus_, 291, =297=, 298
_†Merychyus_, 232, 361, 372, =373=, 374, 377, 381, 382
_†Merycochœrus_, 241, 361, 372, =373= (head restored), 374, 376, 381,
382 (manus fig.)
†Merycodontidæ, 362, =414=
_†Merycodus_, 224, 362, =414=, 415 (restoration), 417
_†Merycoidodon_, 252 (restoration), 258, 259 (do.), 361, =379= (skull
fig.), 382 (manus fig.), 536 (restoration)
_†Mesatirhinus_, 271 (restoration), 314 (do.)
Mesaxonic symmetry, 359
_†Mesocyon_, 517, =528=, 530
_†Mesohippus_, 252 (restoration), 290, =300= (restor.), 302, 305
(skull fig.), 308 (manus and pes fig.), 326, 342, 343, 396,
397, 505
†Mesonychid, 269 (restoration), 271 (do.)
†Mesonychidæ, 554, 556, =558=, 574;
Bridger, 268, 559;
Torrejon, 285, 560;
Uinta, 265, 559;
Wasatch, 277, 560;
Wind River, 274
_†Mesonyx_, 554, =559= (teeth fig.), 561
_†Mesoreodon_, 361, 372, =378=
Mesozoic era, 15, 16, 18, 23, 103, 284, 574, 632, 643
Metacarpal, 84
Metacarpus, 83
_†Metacheiromys_, 592, =616=
_†Metamynodon_, 255, 291, =346=, 347 (restoration), 352, 510
Metapodial, 90
Metatarsal, 89
Metatarsus, 89
Metatheria, 626
_†Meteutatus_, 592
Mexico, 33, 179, 181, 199, 200, 207, 229, 419, 427, 585;
Eocene, 104;
lowlands, 142, 146, 164;
mammals, 135, 141, 142;
Miocene, 118, 121;
plateau, 142;
Pliocene, 125
_†Miacidæ_, 527, 530, 554, 555, 556, =557=, 562, 576;
Bridger, 268;
Torrejon, 285;
Uinta, 519, 558;
Wasatch, 277, 279;
Wind River, 274
_†Miacis_, 555, 558
Mice, 60, 244;
groove-toothed, 182;
John Day, 249;
jumping, 182;
Miocene, 229;
Pleistocene, S. Amer., 218;
vesper, 182;
white-footed, 153, 164, 182;
White River, 254
_†Microbiotherium_, 627
_Microtus_, 153, 218
_Midas_, 578
Migration, of birds, 143;
of mammals, 18, 19, 143;
of †hyænodonts, 567;
between N. and S. Amer., 129;
Oligocene, 254;
Pleistocene, 207, 211;
pre-Wasatch, 108;
of Proboscidea, 441;
White River, 116
Milk-dentition, 94
Mink, 152 (fig.)
Minks, 213, 518, 550;
Pleistocene, 204
Miocene epoch, 17, 33, 112;
North America, =117=, 119 (map), 233, 249, 251, 284, 386, 420, 421,
438, 554, 658, 661;
European, 235, 364, 435, 441, 550;
South American, 20, =123=, 242, 261, 553, 640
_†Miohippus_, 290, 299
_†Miolabis_, 362, =391=
Mississippi, Embayment, 104, 117;
Valley, loess of, 133
Missouri River, drowning of bison in, 36
MITCHELL, P. CHALMERS, 52
_†Mœritherium_, =434=, 437 (head and molar fig.), 438, 439, 440, 441,
442, 450
Molars, 93
Mole, 2;
Star-nosed, 152
Moles, 59, 77, 89;
American, 163;
Bridger, 268;
golden, 245;
White River, 253
Mole-shrews, 153
Mongoose, 142
Monkeys, 2, 60, 141, 282, 283, 284, 577, 578, =582=;
Bridger, 270;
eastern hemisphere, 172;
howling, 578, =585=;
Neotropical, 172, =586=;
New World, 583, 587;
Old World, 583, 587;
Pleistocene, 218, 586;
Santa Cruz, 245, 586, 587;
South American, 578, =583=, 587;
spider, 578, =584=;
Wind River, 275
Monodelphia, =58=, 59, 145
Monotremata, 59;
distribution, 138
Monte Hermoso age and stage, 20, =129=, 226, 479, 499, 508, 634
Moose, 4, 65, 141, 151, 156 (fig.), 181, 202, 208, 411, 412, 413
Moraine, Great Terminal, 131
Moraines, 25
_†Moropus_, 238, 240 (restoration), 291, =356= (manus fig.)
_†Morphippus_, 462
_Moschus_, 412
Mt. Hood, 121;
Kenya, 134;
Tacoma, 121
Mountain Lion, 153, 168 (fig.)
Mountain ranges, as barriers to mammals, 142;
history of, 23
Mouse, Jumping, 153, 160 (fig.)
Mouse-Deer, 54, 60, 358, 408.
(_See also_ Chevrotains _and_ Tragulina)
†Multituberculata, 642
Mummies of Pleistocene rodents, 40
Muntjac, Indian, 412
Muntjacs, 412, 414, 658
Musk-Ox, 148, 149 (fig.), 202, 207, 211, 418
Musk-Oxen, 27, 141, 208, 210
Muskrat, 2, 151, 153, 182
_Mustela_, 159 (fig.), 160 (fig.), 517
Mustelidæ, 174, 222, 265, 517, 518, =550=, 553, 554;
John Day, 249;
Miocene, 238, 551;
Old World origin, 550;
Pleistocene, 551;
Pleisto., S. Amer., 213;
Pliocene, 223, 551;
South American, 552;
White River, 254, 551
Mustelines, _see_ Mustelidæ
Mutation, 662
_Mycetes_, 585
†Mylagaulidæ, 222, 229, 233
†Mylagaulids, _see_ †Mylagaulidæ
_†Mylodon_, 206, 212 (restoration), 219, 592, 597, 601, =602=, 603
(restoration), 604, 607, 608 (pes fig.)
†Mylodontidæ, 206, 592, 598, 602;
Deseado, 610;
Santa Cruz, 605, 607, 609
†Mylodonts, _see_ †Mylodontidæ
_Myocastor_, 184
_Myodes_, 153
_Myrmecophaga_, 91, 187, 188 (fig.), 206, 355, 591, 600
Myrmecophagidæ, 591
Mystacoceti, 60
_Nasua_, 162, 176, 213, 517, 546, 552
Nasuas, 141
Navicular, 88
Navidad formation, 124
†Necrolestidæ, 245
_†Nematherium_, 592, 607
Neogæa, 145
Neogæic realm, 146, 164
_†Neohipparion_, 33, 291, 298 (restoration), 299 (skeleton fig.)
_†Neoplagiaulax_, 627
_Neotoma_, 153, 164
_†Neotragocerus_, 362
Neotropical region, 146, 147, =164=, 170 (map), 322, 363, 418, 436,
461, 552, 583, 591, 630
_†Nesodon_, 243 (restoration), 462, =467= (skull fig.), 470
(restoration), 473 (pes fig.), 474, 475, 478, 482, 483, 498,
510, 511
NEUMAYR, M., 663
New Guinea, 634
New York Zoölogical Society, 148, 149, 150, 151, 152, 154-169,
176-180, 182, 183, 186, 188, 189, 190, 292, 389, 584
Newfoundland, Pliocene, 125
New Zealand, 284;
Miocene, 123
Nicaragua, 218
_†Nimravus_, 249, 541, =542= (skull fig.), 543
Nomenclature, 50
North America, the circumpolar area, 148;
mammals of, 145;
zoölogical divisions, 146, 147 (map)
†Notharctidæ, 578
_†Notharctus_, 578, =579=
_†Nothocyon_, 530
_†Nothrotherium_, 592
†Notohippidæ, 262, 462, =475=
_Notohippus_, 462, =476=
Notopithecidæ, 462
_†Notopithecus_, 462
†Notostylopidæ, 282, 462, =485=
_†Notostylops_, 462
Notostylops Beds, 20, 281
Notoungulata, 461, =489=
Nova Scotia, Pliocene, 125
_Nyctipithecus_, 578, =585=
Oaks, 102
Ocelot, 176 (fig.), 212, 552
_Ochotona_, 153
_Octodon_, 184
Octodontidæ, 184
_†Octodontherium_, 262
_Odocoileus_, 95 (molar fig.), 153, 162, 202, 208, 360 (molar fig.),
362, 413;
_O. hemionus_, 46 (fawns fig.), 167 (fig.);
_O. virginianus_, 166 (fig.), 179, 412;
_O. virg. osceola_, 179 (fig.)
Odontoceti, 60
Okapi, 45
Old World, 101, 258, 266, 295, 327, 331, 332, 335, 341, 351, 353,
358, 386, 390, 413, 420, 426, 518, 550, 554, 558, 562, 583;
camels, 138;
horses, 201;
mammals, 120, 121, 142;
separation from N. A., 146
_†Oligobunis_, 517, =551=
Oligocene epoch, 17;
Europe, 324, 352, 370, 543, 552, 661;
North America, =113=, 114 (map), 204, 224, 265, 287, 378, 576, 658;
South America, 20, =117=, 282, 456, 485, 508, 512, 625
_†Omomys_, 578
_†Onohippidium_, 307
Ontogeny, 648
_†Oödectes_, 555, 558
Opossum, 161 (fig.), 627;
Water-, 627, 631
Opossum-like forms, Cretaceous, 638
Opossums, 2, 58, 59, 141, 161, 249, 626, 627, =630=;
Araucanian, 226;
Bridger, 268;
Casa Mayor, 282;
Cretaceous, 631;
Eocene, 631;
European, 631;
North American, 631;
Oligocene, 631;
Paleocene, 631;
Paraná, 227;
Pleistocene, 221;
Santa Cruz, 244;
South American, 190, 221, 631;
White River, 251;
Wind River, 274;
woolly, 631
_Opsiceros_, 327, 329, 330, 332, 350, 351
Orders, distribution of, 138
Ordovician period, 15
_Oreamnos_, 152, 158 (fig.), 202, 416
_†Oreodon_, 379
†Oreodont, White River, 252 (restoration), 259 (do.), 536 (do.)
†Oreodontidæ, 250, 361, =372=, 383, 384, 385, 404, 436, 652, 661;
Eocene, 372, 381;
grazing, 232;
John Day, 250, 375, 377, 379;
Miocene, 231, 235, 241, 372, 374;
Pliocene, 225, 373;
Uinta, 267, 380;
White River, 258, 377
Oriental region, 146
_Ornithorhynchus_, 57
_†Orohippus_, 272, 290, =302=
OSBORN, H. F., 18, 59, 102, 193, 194, 199, 207, 235, 241, 265, 273,
288, 297, 331, 340, 341, 345, 357, 406, 409, 414, 427, 450,
554, 655
_Otocyon_, 656
Ottawa valley, marine invasion of, 132
Otter, 2, 160 (fig.), 175, 213
Otters, 152, 164, 516, 517, 518, 550, 551;
Miocene, 229, 234;
Pleistocene, 204;
South American, 552
_Ovibos_, 149 (fig.), 202, 208, 362, 418
_Ovis_, 152, =419=
Ox, 70
Oxen, 54, 60, 362, 409, 416, 418
_†Oxyæna_, 274, 277, 555, 565 (teeth fig.), 566 (do.), =571=, 572
(restoration), 573
_†Oxyænidæ_, 555, =568=, 575;
Bridger, 268, 568, 573;
Uinta, 265, 573;
Wasatch, 277, 571;
Wind River, 274, 571
_†Oxyænodon_, 555
†Oxyclænidæ, 554, 561, =562=, 568, 574
_†Oxyclænus_, 554
_†Oxydactylus_, 241, 362, =391=, 392 (restoration), 393 (skeleton
fig.)
OWEN, R., 217, 463, 467, 510, 603, 608
Paca, 183 (fig.), 185
_†Pachyæna_, 274, 277, 554, =560=
_†Pachycyon_, 522
Pachydermata, 44, 490, 492, 654
_†Pachyrukhos_, 227, 462, =478=, 479, 639 (restoration)
Pacific Coast, Eocene, 104, 111;
mingling of mammals, 140;
Miocene, 117, 120;
Oligocene, 113;
Paleocene, 101;
Pleistocene, 132;
Pleisto. volcanoes, 133
_†Palæarctonyx_, 555
_†Palæomastodon_, =432=, 434, 435, 436, 437 (head and molar fig.),
438, 439, 440, 441, 450
_†Palæonictis_, 277, 555, =574=
Palæontological method, 9, 11, 29
Palæontology, 29, 649, 660, 663
_†Palæosyops_, 272, 291, 314 (molar fig.), 317 (head restored), 318
(manus fig.)
_†Palæothentes_, 627
_†Palæotherium_, 490, 492, 661
Palæozoic era, 15
_†Palæpanorthus_, 627
Paleocene epoch, 17, =99=, 108, 253, 273, 276, 291, 443, 453, 454,
456, 459, 460, 519, 554, 557, 558, 560, 561, 562, 568, 578,
580, 625, 642
Palms, 103, 111, 116, 122
_†Paloplotherium_, 661
Pamir, 419
Pampas, 133, 142, 211, 213, 218, 219, 596
Pampean Beds, 19, 133, 136, 228, 248, 463, 471, 478, 489, 493, 496,
498, 586;
mammals, =212= (restorations), 489
Panda, 546
Pangolins, 60, 353
_†Panochthus_, 592, =618=, 620
†Pantodonta, 443, =451=
_†Pantolambda_, 285 (restoration), =453=, 454
Paraguay, 164, 178, 189
_†Parahippus_, 290, =297=
_†Parahyus_, 281, 361, =370=
Parallelism, 397, =649=, 652, 653
_†Paramylodon_, 592
_†Paramys_, 270, 271 (restoration), 280
Paraná age and stage, 20, =128=, 242, 493, 499, 507, 635
Paraná River, 34
_†Parapithecus_, =583=, 587
_†Parastrapotherium_, 509, =512=
Paraxonic symmetry, 359
Patagonia, 30, 40, 139, 178, 180, 184, 185, 189, 190, 191, 242, 263,
463, 467, 477, 586, 596;
Cretaceous, 117, 632;
Eocene, 112, 117;
marine rocks, 112;
Miocene, 123, 613;
Oligocene, 117;
Pleistocene glaciation, 133;
Pliocene, 128;
Tertiary, 20
Patagonian age and stage, 20, =123=, 474, 475, 479
Patella, 86 (fig.)
_†Patriofelis_, 271 (restoration), 274, 555, =568=, 569
(restoration), 570 (pes fig.)
_†Paulogervaisia_, 462, =488=
Peace Creek stage, 127, 221, 322
Peccaries, 141, 178, 361, =363=, 461;
Bridger, 273, 365;
John Day, 250, 365;
Miocene, 232, 235, 239, 365;
Oligocene, 365;
Pleistocene, 201;
Pleisto., S. Amer., 213, 215;
Pliocene, 224, 226, 364;
Uinta, 266, 365
Peccary, 33, 60, 161, 177 (fig.), 222, 360 (molar fig.)
Pecora, 54, 60, 362, 387, 402, =409=, 420, 421;
Neotropical, 179;
Oligocene, 421;
Pleistocene, 201
_†Pelecyodon_, 592
†Peltephilidæ, 592
_†Peltephilus_, 592, =613= (skull fig.), 615
Pelvis, 77
_†Pelycodus_, 578, =580=
PENCK, A., 134
_†Peraceras_, 332
_Perameles_, 58
_Peramys_, 631
_†Peratherium_, 627, 631
_†Perchœrus_, 361, =365=
†Periptychidæ, 443
_†Periptychus_, 443, =454=
Perissodactyla, 60, 247, 248, 284, =288=, 310, 353, 354, 358, 359,
360, 383, 402, 450, 458, 484, 485, 491, 499, 507, 514, 651, 653;
Bridger, 270, 344;
†Clawed, 60, =353=;
Eocene, 289, 338, 339, 352, 354, 359;
John Day, 250;
Miocene, 230, 234, 238;
Neotropical, 176;
normal, =291=, 355;
Pleistocene, N. Amer., 199;
Pleisto., S. Amer., 213;
Uinta, 266;
Wasatch, 280;
of western hemisphere, 322;
White River, 255;
Wind River, 275
Perissodactyls, _see_ Perissodactyla
Permian period, 15;
climate of, 24, 25;
glaciation, 25
Peru, 178, 179, 180, 184, 356, 393, 548
Petrifaction, 40
Petrified forests, 122
_Phacochœrus_, 363
Phalangers, Australian, 244, 626, 640, 641, 642
Phalanges, 84
_†Pharsophorus_, 627
†Phenacodontidæ, 457
_†Phenacodus_, 277, 278 (restoration), 285, =457= (skeleton fig.),
458, 459
Philippines, 579
Philology, 646
_†Phlaocyon_, 238, 517, =547=
Pholidota, 60, 353
Phylogeny, 648
Phylum, 56
Pichiciago, 190, 592, 611
Pig, 359 (fore-arm bones and manus fig.), 360 (pes fig.);
Wild Texas, 161
Pigs, 281
Pikas, 59, 153, 181
Pilosa, 60, 591, =592=
Pinnipedia, 59, 516
Pisiform, 83
_Pithecia_, 578, =585=
_Pitheculus_, 578
Placenta, 58
Placental mammals, 58, 59, 145
_†Plagiarthrus_, 481
†Plagiaulacidæ, 627
_Plagiodontia_, 185
_†Planops_, 591
Plant-feeders, 92, 95
Plantigrade, 90
Plants, distribution, 141;
Florissant, 121, 122;
Green River, 109;
Miocene, 122;
Miocene of Andes, 124;
Mioc. of Europe, 122;
Oligocene of Alaska, 116;
Oligo. of Europe, 116;
Pliocene, 127
Plateau region, 101, 111, 122;
Pleistocene upheaval of, 133
Plateaus as affecting spread of mammals, 142
_†Platygonus_, 33, 202, 222, 361, =364=
Platyrrhina, 578, =583=, 587
Pleistocene epoch, 17, =129=, 130, 172, 229, 239, 245, 246, 263, 264,
299, 324, 332, 335, 336, 350, 351, 354, 364, 365, 386, 391,
412, 413, 415, 416-419, 426-429, 436, 438, 439, 448, 485, 499,
518, 524, 530, 531, 545, 549, 551, 552, 586, 588, 614, 631, 632;
climate, 25;
effects of climate on animal distribution, 192;
glaciation, 25;
European, 661;
lowest, 127;
mammals, 195 (restorations);
South American, 19, 20, 133, 296, 465, 476, 479
_†Pleurocœlodon_, 462
_†Pliauchenia_, 362
PLINY, letter on eruption of Vesuvius, 30
Pliocene epoch, 17, 112, =125=;
North American, 126 (map), 199, 201, 202, 229, 233, 238, 242, 245,
246, 248, 258, 263, 282, 295, 298, 299, 324, 327, 333-336, 340,
353, 354, 356, 357, 364, 365, 370, 372, 373, 386, 388, 390,
391, 413, 414, 416, 417, 421, 427, 429, 430, 435, 436, 485,
486, 493, 499, 507, 508, 524, 527, 530, 531, 534, 536, 545,
546, 547, 549-552, 554, 598, 612, 614, 632;
South American, 20, =128=, 466, 467, 479
_†Pliohippus_, 291, =296=, 307
Pocket-gophers, 163, 164, 182;
John Day, 249;
Miocene, 229, 238;
Pliocene, 222;
Uinta, 265
Pocket-mice, 191
_†Poëbrotherium_, 252 (restoration), 257, 362, =394= (restor.), 397,
399 (skull and tooth fig.), 400 (manus fig.), 401 (pes fig.)
_†Pogonodon_, 535, =541=
†Polydolopidæ, 627, =642=
_†Polydolops_, 627
_†Polymastodon_, 286 (head restored), 627
Polyprotodonta, 59, 627, =630=, 640, 641
Pompeii, 30
Poplars, 102
Porcupine, Brazilian Tree, 182 (fig.);
Canada, 5, 151 (fig.), 153, 182, 205;
Short-tailed, 150, 182, 205
Porcupine group, 182, 262;
suborder, 245
Porcupines, 59, 184;
short-tailed, 141;
tree, Pleistocene, 218;
tree, Santa Cruz, 245
Porpoises, 3, 37, 45, 60, 94, 442, 656
Port Kennedy Cave, 30, 210
Port St. Julian, 489
Portugal, caverns, 40
_Potos_, 175 (fig.), 517, 546, 552, 558
Pouched mammals, 57, 59.
(_See also_ Marsupialia)
_†Prœuphractus_, 592
Prairie-Dog, 169 (fig.)
Prairie-dogs, 164, 181
_Praopus_, 611
Pre-Cambrian eras, 15
Premolars, 93
Pre-occupation, 142
_†Prepotherium_, 591, 607, 608
_†Preptoceras_, 202, 203 (restoration), 362, =418=
Primates, 60, =577=;
Bridger, 270;
Eocene, 577;
Santa Cruz, 587;
South American, 587;
Wasatch, 281, 580
_Priodontes_, 190, 592, 610, 612, 614, 616
_†Proadinotherium_, 262, 462
_†Proasmodeus_, 462
_†Proborhyæna_, 627, =638=
Proboscidea, 60, 230, 254, 264, 269, =422=, 442, 446, 448, 449, 454,
455, 469, 487, 488, 514;
African origin of, 234;
American, 485;
Eocene, 434;
Miocene, 234, 238, 430;
Oligocene, 432, 441;
Pleistocene, N. Amer., 196;
Pleisto., S. Amer., 436.
(_See also_ Elephants _and_ †Mastodons)
Proboscis, 65
_†Procamelus_, 232 (restoration), 362, =391=, 399 (skull and tooth
fig.), 400 (manus fig.), 401 (pes fig.)
_†Procladosictis_, 627
_†Procynodictis_, 517, 529, 530
_Procyon_, 163, 175, 213, 517, =546=, 547;
_P. cancrivorus_, 552;
_P. lotor_, 153, 166 (fig.), 547 (dentition fig.), 552;
_P. †ursinus_, 552
Procyonidæ, 517, 518, =546=;
Miocene, 238, 547;
South American, 552
_†Prodasypus_, 592
_†Proectocion_, 489
_†Proeutatus_, 592, =614=, 615 (skull fig.)
_†Proglires_, 59
_†Promerycochœrus_, 235, 251, 361, =375=, 376 (restoration)
_†Pronesodon_, 262, 462
Prong Buck, 202, 225, 416, 417, 420. (_See_ Antelope, Prong-horned)
_†Pronomotherium_, 231, 361, =374=, 375 (head restored), 376, 381
_†Propalæohoplophorus_, 243 (restoration), 592, 606 (restor.), =623=
_†Propolymastodon_, 627
_†Propyrotherium_, 462, =487=
_†Prosthennops_, 361
_†Protagriochœrus_, 267, 361, 383, =385=
_†Protapirus_, 257, 291, =323= (skull fig.), 324 (head restored), 325
(teeth fig.)
_†Proteodidelphys_, 627
†Proterothere, single-toed, 506 (restoration);
three-toed, 502 (restor.)
†Proterotheres, _see_ †Proterotheriidæ
†Proterotheriidæ, 227, 248, 489, =499=, 507, 653;
Araucanian, 227, 508;
Deseado, 264, 489;
Paraná, 228, 499;
Santa Cruz, 248, =501=
_†Proterotherium_, 248, 489, =504=
_†Protheosodon_, 489, =499=
_†Prothoatherium_, 489
_†Prothylacynus_, 243 (restoration), 244, 627, =635=, 636 (restor.),
637
_†Protitanotherium_, 266, 313
_†Protobradys_, 592, 595
_†Protoceras_, 252 (restoration), 258, 362, 405 (restor.), =406=
(skull fig.), 407, 445
†Protodonta, 59
_†Protogonodon_, 457
_†Protohippus_, 291, 305 (skull fig.), 306 (manus and pes fig.)
_†Protolabis_, 362, =391=
_†Protomeryx_, 241, 251, 362, 391
_†Protopithecus_, 218
_†Protoreodon_, 267, 361, =380= (skull fig.), 381
Prototheria, =57=, 59, 76
_†Protylopus_, 267, 362, =397=, 399 (skull and tooth fig.), 400
(manus fig.), 401 (pes fig.)
_†Protypotherium_, 243 (restoration), 462, =479=, 480 (restor.)
Province, zoölogical, 145
_†Prozaëdius_, 592
_†Pseudælurus_, 517, =545=
_†Pseudocladosictis_, 627
_†Pseudolabis_, 362
_†Pseudolestodon_, 592
_†Pterodon_, 253, 555, =565= (teeth fig.), 566 (do.), 567, 576
_†Ptilodus_, 627, 642 (skull fig.), 643 (head restored)
Pudu, 180
_Pudua_, 180
Puerco age and stage, 17, 99, 101, 454, 460, 561
Puma, 168 (fig.), 212, 544 (dentition fig.), 545 (skull fig.);
South American, 552
Pumas, 153, 163, 176;
Pleistocene, 204
Pyramidal, 83
Pyrenees, 104
†Pyrotheres, _see_ †Pyrotheria
†Pyrotheria, 60, 462, =485=, 500;
Casa Mayor, 283, 488;
Deseado, 262, 485
_†Pyrotherium_, 264, 462, =485=, 486 (head restored)
Pyrotherium Beds, 20, 117, 261, 486
Quadrumana, 582
Quadruped, 1
Quaggas, 292
Quaternary period, 15, 17, 61, 100, =129=, 267, 319;
South America, 19
Quicksands, burial of mammals in, 37
Rabbit, 218
Rabbits, 59, 141, 142, 164, 245;
White River, 254
Raccoon, 153, 162, 163, 166 (fig.), 175, 547 (dentition fig.), 553;
Crab-eating, 552
Raccoon-family, Miocene, 238;
Pliocene, S. Amer., 226
Raccoons, 5, 90, 213, 517, 518, 519, =546=, 553;
Miocene, 229, 547;
Paraná, 227;
Pleistocene, 204;
Tertiary, 547
Race, geographical, 52
Radius, 80
Raised beaches, 113, 134
Rancho La Brea, 31
_Rangifer_, 70, 152, 157 (fig.), 202, 208, 362, 412
Ratel, 551
Rats, 60, 245;
fish-eating, 182;
Pleisto., S. Amer., 218;
spiny, 184
Rattlesnake stage, 127
RAY. J., 51
Realm, zoölogical, 145
Recent epoch, 17, 132, 335, 336;
South American, 19
Reduction of parts, 656
Region, zoölogical, 145
Reindeer, 70, 141, 412;
Lapland, 152;
Pleistocene, 27
Reptiles, _see_ Reptilia
Reptilia, 55;
as ancestral to mammals, 643;
distribution, 141;
Mesozoic, 284;
Oligocene, 117;
Paleocene, 284;
Santa Cruz, 244;
teeth of, 92;
Triassic of S. Africa, 644
Republican River age, 17, =127=
Restorations, how made, 42
_Rheithrodon_, 182
Rhinoceros, 350, 490, 492;
African, =327=, 328, 329, 337;
†aquatic, 347 (restoration);
Bornean, 44;
Broad-lipped, =329=, 351, 448;
†cursorial, 252 (restor.), 341 (do.), 343 (manus fig.), 344
(restor.);
†hornless, 252 (restor.), 256 (do.), 335 (skull fig.);
Indian, 44, 327, 328, 329;
Javan, 327, 328 (skull fig.), 473;
†paired-horned, 239 (restor.);
†primitive, 271 (restor.);
†small-horned, 230 (restor.);
Sumatran, 327, 329;
White, 329;
†Woolly, 332
_Rhinoceros_, 327;
_R. sondaicus_, 327, 328 (skull fig.), 473;
_R. unicornis_, 329
Rhinoceroses, 45, 56, 60, 63, 91, 289, 312, 382, 461, 510, 654, 655,
661;
African, 346;
†aquatic, 291, =340=;
†aquatic, Bridger, 272;
†aquatic, Uinta, 348;
†aquatic, White River, 346;
bones of, 35;
†cursorial, 291, =340=;
†cursorial, Bridger, 272, 343;
†cursorial, Uinta, 266;
†cursorial, White River, 255, =340=;
†cursorial, Wind River, 275;
Eocene, 338, 339;
hairy, 448;
John Day, 250, 256, 333;
Miocene, 230, 234, 238, 256, 332, 333;
North American, 39, 199;
Oligocene, 333;
Pliocene, 224, 331;
†paired-horned, 256, 444;
phyla of, 289, 650;
Siberian, 39;
true, 291, =326=, 340, 346, 350, 351;
true, Uinta, 266;
true, White River, 255, 333;
White River, 255, 333
Rhinocerotidæ, 291, =326=, 340, 350
_†Rhynchippus_, 462
Ribs, 74 (fig.);
sternal, 74
Rio de La Plata, 128
River deposits, 36
Robin, 50
Rocky Mts., 101, 150, 153;
Pleistocene glaciers, 131
Rodent, †primitive, 271 (restoration);
Santa Cruz, 243 (do.)
Rodentia, 5, 59, 282, 283, 284, 459, 629;
Araucanian, 226;
Boreal, 153;
Bridger, 270;
Deseado, 587;
distribution, 138;
John Day, 249;
jumping, 90;
Miocene, 229, 233, 237;
Neotropical, 183 (figs.);
Paraná, 227;
Pleistocene, 134, 205;
Pleisto., S. Amer., 218;
Pliocene, 222;
Santa Cruz, 245;
simplicidentate, 628;
Sonoran, 163;
South American, 181;
Uinta, 265;
Wasatch, 280;
White River, 254;
Wind River, 275;
West Indian, 191
Rodents, _see_ Rodentia
Roots of teeth, 95
Rootless teeth, 96
Rosebud stage, 120, 235
Ruminants, 81, 84, 87, 281, 373, 651;
hollow-horned, 328;
Miocene, 232;
true, 54, 201, 362, 387, 402, =409=, 446
RUSSELL, I. C., 589
Sables, 141
†Sabre-tooth, 32;
cat, 252 (restoration);
cats, 659;
false, 542 (skull fig.);
primitive, 539 (restor.);
Tiger, frontispiece (restor.), 195 (restor.), 517, 531 (skull
fig.), 534 (do.), 536 (restor.);
tigers, 54, 210, =530=, 552;
Miocene, 229, 234, 534;
Oligocene, 535;
Pleistocene, 204;
Pleisto., S. Amer., 211;
Pliocene, 223
†Sabre-tooths, 265, 650;
false, 249, 541;
John Day, 249, 535, 541, 542;
Miocene, 238;
White River, 254.
(_See also_ †Machairodontinæ)
Sacramento Valley, Miocene, 118
Sacrum, 73 (fig.)
_†Sadypus_, 592
Sagittal crest, 63
_Saiga_, 65
Saiga Antelope, 65
St. Elias Alps, 101
St. Lawrence Valley, invasion of, by sea, 133
Sakis, 578, =585=
Saline water, 34
SALISBURY, R. D., 130
Salt Lake, Utah, 131
Salt lakes, 24
Sand, wind-blown, 33
Santa Cruz age and stage, 20, 30, =124=, 262, 263, 264, 282, 283,
467, 470, 473, 474, 475, 477, 479, 481, 482, 485, 493, 499,
500, 501, 504, 508-512, 586, 587;
mammals, 243 (restorations)
Santa Cruz Mts., Calif., 118
Santa Cruz River, as barrier to armadillos, 139
Sapajou, 584 (fig.)
_Sarcophilus_, 634
SARMIENTO, 143
_†Scalibrinitherium_, 489, =493=, 495, 496 (skull fig.), 497 (do.)
_Scalops_, 163
Scalpriform teeth, 96 (fig.)
_Scapanus_, 163
Scaphoid, 83
Scapho-lunar, 83
Scapula, 76 (fig.)
_†Scelidotherium_, 592, 601, =602=, 604
_†Schismotherium_, 592
_†Schizotherium_, 291, 357
SCHLOSSER, M., 262, 380, 514, 555, 583, 625, 661
SCHUCHERT, C., 105, 114, 119, 126
_†Sciuravus_, 280
Sciuromorpha, 270
_Sciuropterus_, 164
_Sciurus_, 164 (fig.)
_†Sclerocalyptus_, 219, 592, =618=, 620
_Scleropleura_, 592, =611=
SCOTT, D. H., 288
Sea-Cow, 60, 207, 442
Sea-Otter, 517, 518
Seals, 1, 2, 3, 37, 56, 59;
Pleistocene, 132
Seas, barriers to land mammals, 139
Section, geological, 7, 9 (diagram)
Sedimentary rocks, 6
Sediments, 6
Selenodont tooth, 360 (fig.);
origin of, 651
Sewellel, 153, 233
Sewellels, 249;
Miocene, 238
Shales, Florissant, 129;
Green River, 109
Sheep, 54, 60, 93, 362, 409, 416, 418, =419=, 420;
Rocky Mt., 152, 419
Shells, fossil, 662
Sheridan stage, 33, =131=, 133, 200
Shrews, 59, 141, 173, 191;
American, 163;
jumping, 59;
Old World, 152;
tree, 59
Siberia, 197, 207, 332, 350, 426;
frozen carcasses in, 39
Sierra Nevada, 101, 122, 150, 153;
Miocene, 118;
Pleistocene glaciers, 131
_Sigmodon_, 163
Silurian period, 15
Simiidæ, 583
Simplicidentata, 60
SINCLAIR, W. J., 107, 437
_†Sinopa_, 565 (teeth fig.), 566 (do.), 633
Sirenia, 60, =442=
_Sitomys_, 153, 164, 182
Skeleton, 61;
significance of, 42
Skull, 61
Skunk, 163, 167 (fig.), 213, 517;
Argentine, 174 (fig.);
Little, 174 (fig.);
Spotted, 517
Skunks, 153, 163, 174, 210, 518, 550, 551;
Pleistocene, 204;
South American, 552
Sloth, Three-toed, 186 (fig.), 591;
Two-toed, 74, 187 (fig.), 591
Sloths, 2, 60, 97, 186, 189, 592;
Araucanian, 226;
Pleistocene, 218;
Santa Cruz, 245
_†Smilodon_, frontispiece (restoration), 195 (do.), 204, 211, 517,
=531= (skull fig.), 532 (teeth fig.), 535, 536, 537, 544, 553,
622
SMITH, PERRIN, 23
SMITH, WILLIAM, 7, 9
Smith River stage, 121
Snake Creek age and stage, 17, =127=, 222, 388
Snakes, 244;
Paleocene, 284
_Solenodon_, 173 (fig.)
Solitary species, 38
Sonoran region, 146, 147 (map), 152, 153, 161, 176, 178, 191, 363
_Sorex_, 152
South Africa, 14;
Permian glaciation, 25;
Triassic reptiles, 644
South America, Eocene separation from N. Amer., 104;
Miocene junction with N. Amer., 120;
Permian glaciation, 25;
Pleistocene Man, 589;
zoölogical divisions, 173 (map);
zoölogy, 146
South Australia, dry lakes of, 34
South Shetland Islands, 124
Sparnacian stage, 108
Species, definition, 51;
distribution, 136;
origin, 20
Specific area, 136
_Spermophilus_, 163 (fig.), 164
_†Sphenophalus_, 362
_Spilogale_, 174 (fig.), 517, 552
Spiny rats, Pleistocene, 218
Sports, 660
Squirrel, Grey, 164 (fig.);
suborder, 270
Squirrels, 2, 60, 245;
flying, 164;
John Day, 249;
Miocene, 229, 238;
true, 164, 181;
White River, 254
Stag, 358;
European, 151;
Thian Shan, 151
†Stag-Moose, 195 (restoration), 208, 209 (restor.), =413=
Stage, geological, 15
Stalagmite, 30
Stations, 136
_†Stegodon_, 430, 439
†Stegodonts, 438
_†Stegotherium_, 243 (restoration), 480 (do.), 592, =614=, 615 (skull
fig.)
_†Stenomylus_, 241, 242 (restoration), 362, =393=, 408
_Sternum_, 75 (fig.)
_†Stibarus_, 361
STIRLING, E. C., 34, 35
Straits, of Lombok, 135;
of Magellan, 143
Stratified rocks, 6
Stream-channels, White River, 113
_†Stylinodon_, 274
_†Stypolophus_, 555
Subregion, zoölogical, 145
Subsidences, Pleistocene, 132
Subungulata, 514
Suillines, 661
Suina, 54, 60, 361, =362=
Sumatra, 21, 140, 327
Superposition of beds, 7, 8 (diagram)
_Sus_, 359 (fore-arm bones fig.), 363
Swamps, burial of mammals in, 33
Swan, 70
Swine, 54, 60;
American, 363;
Old World, 363, 364;
Pleistocene, 201;
true, 364, 365.
(_See also_ Peccaries)
Swine-like animals, 361, =362=
Sycamores, 102
_†Symbos_, 208, 362, =418=
_Synaptomys_, 153
_†Syndyoceras_, 241, 258, 362, 403 (restoration), =404=, 406, 407
Syria, 481
_†Systemodon_, 280, 291, =324=
_†Tæniodontia_, 60, 276, =625=;
Bridger, 267;
Puerco, 286;
Wind River, 274
_Tagassu_, 161, 177 (fig.), 178, 360 (tooth fig.), 361, 363
(dentition fig.), 364
Tagassuidæ, 361, =363=
Takin, 418
†Taligrada, 443
_Tamandua_, 187, 188 (fig.), 591
_Tamias_, 153
Tapir, 47 (fig. of young), 81 (fore-arm bones fig.), 87 (leg-bones
fig.), 289 (manus fig.), 290 (pes fig.), 320 (adult fig.),
=321= (skull fig.), 471, 490, 492;
Asiatic, 321;
Pinchaque, 322;
White River, 323 (skull fig.), 324 (head restored)
_†Tapiravus_, 291
Tapiridæ, 60, 65, 89, 141, 176, 289, 291, 312, 315, =319=, 330, 341,
348, 359, 461, 651, 653;
American, 322;
Bridger, 272;
distribution, 137;
Eocene, 323;
John Day, 250;
Miocene, 231, 234, 322;
North American, 39;
Oligocene, 323, 339;
Pleistocene, 199, 201, 208, 210, 322;
Pleisto., S. Amer., 213, 215;
Pliocene, 223, 322;
South American, 324;
Uinta, 266;
Wasatch, 280, 324;
White River, 257, 322
Tapiroid, 272, 315
Tapiroids, 321
Tapirs, _see_ Tapiridæ
_Tapirus_, 176, 291;
_T. †haysii_, 201, 322;
_T. roulini_, 322;
_T. terrestris_, 47 (young fig.), 87, 201, 289 (manus fig.), 290
(pes fig.), 320 (adult fig.), =321= (skull fig.), 322, 325
(upper teeth fig.)
Tardigrada, 186, 591, 592, 610;
Araucanian, 226.
(_See also_ Sloths)
Tarija Valley, Pliocene, 129, 225
Tar-pools, 31;
Pleistocene, 32
Tarsier, 281, =580=
Tarsiids, 583
_Tarsius_, 281, =580=
Tarsus, 88
Tasmania, 138, 632, 634
Tasmanian Devil, 627, =634=;
Wolf, 43, 226, 244, 626, =632=, 633 (fig.)
_Tatu_, 160, 190 (fig.), 592, 593, 612
_Taurotragus_, 202
_Taxidea_, 153, 162, 168 (fig.), 517
_Tayra_, 175 (fig.), 213, 517, 552
Teeth, 92;
importance of fossil, 38
_†Teleoceras_, 291, =331=, 332, 333, 350
_†Telmatherium_, 291
_†Temnocyon_, 517, =528=, 530
Temperature as a barrier to species, 140, 141
Tenrecs, 173
_†Tephrocyon_, 517, 522, =527=, 530
Tertiary period, 15, 17, 19, =99=, 267, 319, 369, 413, 460, 531;
Central America, 22;
Culebra, 22;
Great Plains, 36;
Patagonia, 20;
South America, 20, 248, 461, 463;
Tierra del Fuego, 20
Terrestrial habit, 2
_†Tetrabelodon_, 430, 437
_Thalarctus_, 148 (fig.)
_†Theosodon_, 243 (restoration), 248, 489, =493=, 494 (restor.), 496
(skull fig.), 497 (do.), 498 (manus fig.)
Thian Shan, 419
_†Thinohyus_, 361
_†Thoatherium_, 243 (restoration), 248, 489, 500, 501, =504=, 505
(skull fig.), 506 (restor.), 507 (pes fig.)
_†Thomashuxleya_, 462, 485
_Thomomys_, 164
Thorax, 74
Thousand Creek age and stage, 17, 127
Thylacine, 43, =632=, 633 (fig.), 634, 635.
(_See also_ Tasmanian Wolf)
Thylacynidæ, 627, =632=
_Thylacynus_, 43, 226, 244, =632=, 633 (fig.)
Tibet, 224, 418
Tibia, 86, 87 (fig.)
Tierra del Fuego, 20, 178
Tiger, 45
†Tillodontia, 59, 276;
Bridger, 267;
Wasatch, 276;
Wind River, 274
Time, geological, 16
†Titanothere, 253 (restoration), 271 (do.), 309 (do.), 314 (restor.
and fig. of molar)
†Titanotheriidæ, 291, =308=, 317 (heads restored), 334, 352, 353,
357, 366, 445, 446, 458, 465, 654, 661;
Bridger, 270, =313=;
Oligocene, =310=, 314, 315, 339;
Uinta, 266, 313;
White River, 255, =310=, 313, 315;
Wind River, 275, 276, 315
_†Titanotherium_, 253 (restoration), 291, 309 (restor.), 310 (molar
fig.), 311 (skull fig.), 317 (head restored), 318 (fig. of
manus)
_Tolypeutes_, 189, 592, 611, 616
Toronto, interglacial beds near, 130
Torrejon age and stage, 17, 99, =101=, 286, 453, 561
Tortoises, 244;
Paleocene, 244
_†Toxodon_, 212 (restoration), 215, 217 (restor.), 462, =463=, 466
(skull fig.), 467, 468, 469, 471, 472 (pes fig.), 473, 477, 487
†Toxodont, 498;
horned, 228 (head restored), 263 (do.);
Pampean, 212 (restoration), 217 (do.);
Santa Cruz, 243 (restor.), 467 (skull fig.);
Santa Cruz horned, 474 (restor.)
†Toxodonta, 60, 282, 462, =463=, 477, 482, 483, 487, 500, 509, 511,
652;
Araucanian, 227;
Deseado, 262, 264, =474=;
Paraná, 228;
Santa Cruz, 246, =467=
†Toxodontia, 60, 355, =461=, 478, 485, 489, 492, 500, 514;
Pleistocene, 215, 221
†Toxodontidæ, 462, 474
†Toxodonts, _see_ †Toxodonta
Tragulina, 54, 60, 408, 409, 410. (_See_ Mouse-Deer)
Transition zone, 147 (map), 153
Trapezium, 83
Trapezoid, 83
Tree-sloths, 591, 593, 594, 595, 596, 609;
Pleistocene, 596;
Santa Cruz, 596.
(_See also_ Sloths)
_Tremarctos_, 172, 176 (fig.), 517, 548, 552
Trèves, 10
Triassic period, 15, 16, 642, 643;
climate, 24
†Triconodonta, 59
Trier, cathedral of, 10
_†Trigodon_, 227, 228 (head restored), 462, =466=, 473, 474
_†Trigonias_, 256, 291, =336=, 337 (skull and front teeth fig.), 338,
339 (manus fig.), 351
_†Trigonolestes_, 281, 361, =398=
†Trigonolestidæ, 361
†Trigonostylopidæ, 509, 512
_†Trigonostylops_, 509
_†Triisodon_, 554, =561=
_†Trilophodon_, 229
Trinidad, 170
Trinomial system of nomenclature, 52
_†Triplopus_, 266, 272, =343= (manus fig.), 345
_†Tritemnodon_, 271 (restoration), 555, 565 (teeth fig.), 566 (do.),
=567= (restor.), 576, 633
†Trituberculata, 59
Tropical species, distribution, 141
Tse-tse Fly, 142
Tuatara, 284
Tubulidentata, 60
Tuco-tuco, 184
Tuff, Miocene, 112, 122;
Santa Cruz, 124
Turkestan, 419
Turtles, 102
Tusks, 92
Tylopoda, 54, 60, 362, =386=, 409, 410;
Pleistocene, 202
†Typothere, 243 (restoration), 480 (do.), 636 (do.), 639 (do.)
†Typotheres, _see_ †Typotheria
†Typotheria, 60, 215, 372, 462, =476=;
Araucanian, 227;
Casa Mayor, 282, 479;
Deseado, 263, 264;
Paraná, 228;
Pleistocene, 215, 221, 476;
Santa Cruz, 246, =479=;
Tertiary, 215
†Typotheriidæ, 462, =476=
_†Typotherium_, 215, 217, 263, 462, =476=, 477
Uakaris, 578, =585=
Uinta age and stage, 11, 17, =110=, 270, 271, 272, 301, 339, 345,
349, 365, 369, 370, 380, 383, 385, 386, 397-400, 409, 443, 519,
527, 529, 557, 559, 573, 579
Uinta Mts., 106, 108;
Pleistocene glaciers, 131
_†Uintacyon_, 555, 558
†Uintatheres, _see_ †Uintatheriidæ
†Uintatheriidæ, 285, =443=, 444, 445 (skull fig.), 451, 452, 454,
465, 509, 532;
Bridger, 269, 443;
Wasatch, 279, 451;
Wind River, 274, 450
_†Uintatherium_, 51, 271 (restoration), 443, =444=, 445 (skull fig.),
447 (restoration)
Ulna, 80
Unciform, 83
Unconformity, 312
Ungual phalanx, 84
Ungulata, 60, =513=, =516=;
primitive, 460;
Santa Cruz, 481, 511;
†short-footed, 443;
South American indigenous, 461, 466, 469, 486, 489, 490, 500, 509,
511, 513, 514;
White River, 258
Ungulates, _see_ Ungulata
Unguligrade, 91
University of California, 31, 32
Upheavals, Pleistocene, 132, 133;
Pliocene, 132
Upper Sonoran zone, 148, 164
Ural, Mts., 106;
Sea, 106, 108
_Urocyon_, 162, 165 (fig.), 517
_†Urotrichus_, 153
Ursidæ, 517, 518, =548=
_Ursus_, 90 (pes fig.), 156 (fig.), 163, 517, 549
Uruguay, 585
Variant, 53
Varieties, 52, =662=
Vegetation, Eocene, 111;
Paleocene, 283.
(_See also_ Flora _and_ Plants)
Vermilingua, 187, 591
Vertebra, 68;
caudal, 73 (fig.);
cervical, 70 (fig.);
dorsal, 69 (fig.), 72 (fig.);
lumbar, 72, 73 (fig.);
sacral, 73 (fig.);
thoracic, 69
Vertebral column, 67
Vertebrata, 55
Vesuvius, 30
Vicuña, 178 (fig.)
Virgin Valley stage, 127
_Viscaccia_, 183 (fig.), 185
_†Viverravus_, 555, =558=
Viverridæ, 518, 553, 554, 558
Viverrines, _see_ Viverridæ
Viviparous mammals, 59
Vizcacha, 183 (fig.), 185
Vizcachas, Pleistocene, 218
Volcanic ash, 29;
Bridger, 110, 115;
John Day, 116;
Santa Cruz, 124;
White River, 115
Volcanic dust, 29
Volcanic material, 6;
Florissant, 121;
Miocene, 118;
Pliocene, 125
Volcanoes, 133
Voles, 182
VOLTAIRE, 646
Vulcanism, Miocene, 118, 121;
Pliocene, 127
_†Vulpavus_, 555, 558
_Vulpes_, 149 (fig.), 150 (fig.), 158 (fig.), 517
WAAGEN, W., 662
WALLACE, A. R., 136, 139, 150, 170, 171
Walnuts, 102
Walruses, 1, 45, 207, 210, 516;
Pleistocene, 27, 132
Wapiti, 50, 151, 155 (fig.), 181, 202, 208, 411, 412, 413
Warm Temperate region, 161
Wart Hog, 363
Wasatch age and stage, 17, =106=, 273, 274, 275, 285, 316, 325, 370,
398, 400, 451, 452, 453, 455, 457, 459, 560, 561, 566, 568,
571, 572, 580, 581
Wasatch Mts., Pleistocene glaciers, 131
Wasatch-Sparnacian stage, 115
Water Hog, 183 (fig.), 185, 205, 211.
(_See also_ Capybara _and_ Carpincho)
Weasel, 551;
family, 174;
Miocene, 238;
Pleisto., S. Amer., 213;
tribe, 518
Weasels, 59, 152, 517;
Miocene, 229, 238;
Pleistocene, 204, 205
WEBER, M., 426
Western Hemisphere, marsupials, 626
West Indian, islands, 164, 191;
shells on N. J. coast, 113;
subregion, 170 (map)
West Indies, 583;
Eocene, 112;
Oligocene, 113;
Paleocene, 103;
Pleistocene, 134;
zoölogy, 146
Whale, Right, 48
Whales, 1, 2, 3, 37, 45, 60, 74, 442;
Miocene, 123;
Pleistocene, 132;
toothed, 60;
whalebone, 60, 94
White Mts., Labrador plants of, 193
White River age and stage, 11, 12, 17, =113=, 250, 266, 267, 270,
271, 272, 312, 325, 326, 340, 341, 346, 350, 357, 365-371, 375,
377-380, 382-385, 394-396, 399, 405, 407, 408, 523, 528-530,
535, 538-541, 546, 557, 562, 563, 565, 566, 631;
mammals, 252 (restorations)
Wild-cats, 141
Willamette Valley, Miocene, 115
WILLISTON, S. W., 33, 589
Willows, 102
Wind River age and stage, 17, =109=, 273, 315, 316, 326, 339, 350,
400, 450, 452, 456, 457, 460, 568, 571
Windward Islands, Pleistocene, 134
Winter, destruction of mammals by, 36
Wisent, 152
Wolf, 32, 62 (skull fig.), 64 (do.), 69 (dorsal vertebra fig.), 70
(atlas fig.), 71 (axis fig.), 72 (cervical and dorsal vertebræ
fig.), 73 (lumbar and caudal fig.), 74 (ribs fig.), 75 (ribs
and sternum fig.), 76 (scapula fig.), 77 (hip-bone fig.), 78
(humerus fig.), 80 (fore-arm bones fig.), 82 (manus fig.), 85
(femur fig.), 86 (femur and patella fig.), 87 (leg-bones fig.),
88 (pes fig.), 92, 93 (dentition fig.);
Fox-like, 171 (fig.);
Grey, 152, 159 (fig.);
Large-eared, 656;
Miocene, 522 (skull fig.);
Timber, 159 (fig.), 162
Wolverene, 141, 152, 155 (fig.), 213, 238, 517, 551;
Pleistocene, 204
Wolves, 59, 164, 173, 249, 517, 518, 520, 523, 525, 530;
fox-like, 173, 212, 552;
Pleistocene, 204;
Pliocene, 222;
White River, 254
Wombats, 640
Woodchuck, 152 (fig.), 153
Wood-rats, 141, 153, 164
WORTMAN, J. L., 383, 385, 399, 570, 571
_†Xotodon_, 462
Yapock, 631
Yellowstone Park, petrified forests, 122;
Miocene lava, 122;
Pliocene lava, 127
Young animals, colour pattern of, 46
Yucatan, 128
Yukon Valley, Miocene, 118
_Zaëdyus_, 190, 592
_Zapus_, 153, 160 (fig.)
Zebra, 44;
bones of, 35;
Burchell’s, 200
Zebras, 213, 292, 308
†Zeuglodontia, 60
ZITTEL, K. VON, 601
Zoölogy, Experimental, 648, 663
_†Zygolestes_, 627, 641
Zygomatic arch, 65
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An Introduction to Geology
BY WILLIAM B. SCOTT, PH.D., LL.D.
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The Age of Mammals in Europe, Asia, and North America
BY HENRY FAIRFIELD OSBORN
A.B., SC.D. PRINCETON, HON. LL.D. TRINITY, PRINCETON, COLUMBIA, HON.
D.SC. CAMBRIDGE UNIVERSITY, HON. PH.D. UNIVERSITY OF CHRISTIANIA,
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“The Age of Mammals” is not written for the palæontologist only. No
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to find it of value. The geologist finds here the clearest exposition
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the Western States. The anthropologist finds in the closing chapter on
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succession and evolution of flora.
To the general reader it offers the first connected account of the
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contains glimpses into the remote past of the continental outlines, the
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R. Knight, many of which are published here for the first time. Moreover,
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geologic and geographic distribution and the popular names of all the
important genera of mammals, both living and fossil.
Comments on The Age of Mammals
“Students of palæontology have awaited impatiently the past few years
a promised work on extinct mammals by Professor Osborn. In his ‘Age of
Mammals,’ expectations have been more than realized.”—S. W. WILLISTON, in
_Science_, Feb. 17, 1911.
“Dr. H. F. Osborn is a great palæontologist; in this book he has gathered
together the work of a life-time, and that work, besides being original
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Field_, Jan. 7, 1911.
“It is in the best sense a source book, for it gives at first hand, from
the original material, the ideas of an acknowledged master in mammalian
palæontology.” “It has the clarity and directness of style so welcome,
and rare, in such a book.”—E. C. CASE, in _Bulletin American Geographical
Society_, July, 1911.
“A book of the utmost value to the student and teacher of mammalian life
and likewise to the serious reader.”—_American Journal of Science_, Feb.,
1911.
“M. Osborn ... devait s’attacher a nous présenter le tableau aussi
complet et aussi fidèle que possible des faunes de Mammifères fossiles
qui se sont succédé dans l’hémisphère Nord pendant l’ère tertiaire. Et
j’ai plaisir à dire tout de suite qu’il y a parfaitement réussi.”—M.
BOULE, in _Mouvement Scientifique_, 1911, p. 569.
“Professor Osborn has produced a book which will appeal to the learned
specialist and to the thoughtful general reader as well.” “The
work is well adapted to school and college use, and is abundantly
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“One of the most notable books on evolution since the appearance of
Darwin’s ‘Origin of Species.’”—_Forest and Stream_, Dec. 10, 1910.
“Nejlepší současný palæontolog americký, Henry Fairfield Osborn, vydal
nedàvno s titulem tuto citovaným znamenitě psanou a pěkně vypravenou
knihu o ‘věku ssavců.’”—F. Bayer in _Věstníku Ceské Akademie císaře
Františka Josefa pro vědy, slovenost a umení_.—Ročník XX, 1911.
“Written with clearness and vivacity, most admirably illustrated,
especially by the ‘restorations’ of Mr. Knight, and illuminated by maps,
Professor Osborn builds, page after page, his story-mosaic.... The reader
will soon discover that he is a brilliant generalizer, possessed of
material gathered from all around the globe, fructifying his knowledge
by the exercise of a constructive imagination, and expressing his facts
and ideas in a literary style, clear, vigorous, and entertaining.”—_The
Literary Digest_, Feb. 4, 1911.
The Cambridge Natural History
EDITED BY
S. F. HARMER, Sc.D., F.R.S.
Fellow of King’s College, Cambridge; formerly Superintendent of the
University Museum of Zoölogy; Keeper of the Department of Zoölogy in the
British Museum (Natural History)
AND
A. E. SHIPLEY, M.A., HON. Sc.D., Princeton, F.R.S.
Master and Fellow of Christ’s College, Cambridge; formerly Reader in
Zoölogy in the University; Chairman of Council of Marine Biological
Association
_In Ten Volumes_ _Fully Illustrated_ _Medium 8vo_ _Gilt Tops_ _Each
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=CONTENTS=
VOL. I. Protozoa, by M. Hartog; Porifera, by Igerna Sollas;
Coelenterata and Ctenophora, by S. J. Hickson;
Echinodermata, by E. W. MacBride.
VOL. II. Flatworms and Mesozoa, by F. W. Gamble; Nemertines,
by L. Sheldon; Threadworms and Sagitta, by A. E.
Shipley; Rotifers, by M. Hartog; Polychaet Worms,
by W. B. Benham; Earthworms and Leeches, by F. E.
Beddard; Gephyrea, etc., by A. E. Shipley; Polyzoa,
by S. F. Harmer.
VOL. III. Molluscs, by H. A. Cooke; Recent Brachiopods, by A. E.
Shipley; Fossil Brachiopods, by F. R. C. Reed.
VOL. IV. Spiders, Mites, Scorpions, etc., by C. Warburton;
Trilobites, etc., by M. Laurie; Pycnogonids, by
D’Arcy W. Thompson; Lingulatulidæ and Tardigrada,
by A. E. Shipley; Crustacea, by Geoffrey Smith.
VOL. V. Peripatus, by A. Sedgwick; Myriopods, by F. G. Sinclair;
Insects, Part I, by D. Sharp.
VOL. VI. Insects, Part II, by D. Sharp.
VOL. VII. Hemichordata, by S. F. Harmer; Ascidians and Amphioxus,
by W. A. Herdman; Fishes (exclusive of systematic
account of Teleostei), by T. W. Bridge; Fishes
(systematic account of Peleostei), by G. A. Boulanger.
VOL. VIII. Amphibia and Reptiles, by Hans Gadow.
VOL. IX. Birds, by A. H. Evans.
VOL. X. Mammalia, by F. E. Beddard.
WHAT THE CRITICS SAY OF
THE
CAMBRIDGE NATURAL HISTORY
New York _Evening Post_.
Its editors may well be congratulated upon the completion of
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_American Journal of Science._
The most convenient and generally useful work of reference on
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_Book Review Digest._
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_Field._
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_Times._
There are very many, not only among educated people who take
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History by men who are familiar with, and competent to deal
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_Academy._
The editors have aimed very high, and they have succeeded....
Well conceived, carefully coördinated, and executed with the
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_Athenæum._
The series certainly ought not to be restricted in its
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_Science Gossip._
Every library, school, and college in the country should
possess this work, which is of the highest educational value.
_Daily News_ in a Review of Vol. X.
A volume which, for the interest of its contents and for its
style and method of treatment, is not only worthy of its
predecessors, but may be regarded as one of the most successful
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_A GREAT EXPLORER’S STORY_
My Life With the Eskimo
BY VILHJÁLMUR STEFÁNSSON
_ILLUSTRATED WITH HALF-TONE REPRODUCTIONS OF PHOTOGRAPHS TAKEN BY THE
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_Decorated cloth, 8vo_
A fascinating book of description and adventure has been written by the
famous traveler and explorer, who has passed years of his life within
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which to draw and he has made his selection wisely. He has lived with
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