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.