CHAPTER VIII

HISTORY OF THE PERISSODACTYLA

In attempting to trace the evolutionary history of the various mammalian groups, it is necessary to bear in mind the inevitable limitations of work of this kind. Speaking of plants, Dr. D. H. Scott says: “Our ideas of the course of descent must of necessity be diagrammatic; the process, as it actually went on, during ages of inconceivable duration, was doubtless infinitely too complex for the mind to grasp, even were the whole evidence lying open before us. We see an illustration, on a small scale, of the complexity of the problem, in the case of domesticated forms, evolved under the influence of man. Though we know that our cultivated plants, for instance, have been developed from wild species within the human period, and often within quite recent years, yet nothing is more difficult than to trace, in any given instance, the true history of a field-crop or garden plant, or even, in many cases, to fix its origin with certainty.”[5] With some mammalian groups the task, though difficult enough, is not so hopeless, because of more complete records, yet in dealing with mammals a very troublesome complication is introduced by the existence within the families, and even within the genera, of two or more parallel phyla, or genetic series. Without complete and perfect material it is impossible to make sure that we are not confusing the different phyla with one another and placing in one series species and genera that properly belong in a different one. Thus, Osborn distinguishes no less than seven such phyla among the true rhinoceroses of the Old and New Worlds, which long followed parallel, but quite independent, courses of development, and five phyla among the American horses. While these phyla add so much to the difficulty of working out the genealogical series, it is possible to simplify the problem and treat it in a broad and comprehensive manner that will sufficiently establish the essential steps of change.

[Illustration: FIG. 145.—Left manus of Tapir (_Tapirus terrestris_). _S._, scaphoid. _L._, lunar. _Py._, pyramidal. _Pis._, pisiform. _Td._, trapezoid. _M._, magnum. _Un._, unciform. The metacarpals are erroneously numbered. _Mc. I._, second metacarpal. _Mc. II._, third do. _Mc. III._, fourth do. _Mc. IV._, fifth do. _Ph. 1_, first phalanx. _Ph. 2_, second do. _Ung._, ungual phalanx.]

In external appearance and general proportions the different families of existing perissodactyls have very little in common; that tapirs and rhinoceroses should be related is not surprising, but the horses would seem to be as far removed from both of the former as possible. Why, then, should they be included in the same order? A study of the skeleton, however, reveals the community of structure which obtains between the three families, a community which removes them widely from all other hoofed mammals. In all existing perissodactyls, though not in most of the Eocene genera, all the premolars, except the first, have the size and pattern of the molars. The foramina of the skull, or perforations by which blood-vessels and nerves enter and leave the cranium, are arranged in a way characteristic of the order and different from that seen in other hoofed mammals. The femur always has the third trochanter. The number of digits in each foot is usually odd, 1, 3 or 5, but four-toed forms occur, as the tapirs, which have four toes in the front foot, three in the hind; the important character is that the median plane of the foot bisects the third digit, which is symmetrical. The third and fourth, each asymmetrical, together form a symmetrical pair. Especially characteristic is the form of the astragalus and calcaneum (ankle and heel bones); the astragalus has but a single, deeply grooved and pulley-like surface, that for the tibia, the lower end is nearly flat and rests almost entirely upon the navicular, covering but little of the cuboid (see Figs. 146, 148). The calcaneum does not articulate with the fibula and its lower end is broad and covers most of the cuboid.

While the foregoing list includes the most important of the structural features which are common to all perissodactyls and differentiate them from other hoofed animals, there are many others which it is needless to enumerate.

[Illustration: FIG. 146.—Left pes of Tapir. _Cal._, calcaneum. _As._, astragalus. _N._, navicular. _Cn. 1_, _Cn. 2_, _Cn. 3_, first, second and third cuneiforms. _Mr. II_, _III_, _IV_, second, third and fourth metatarsals.]

The subjoined table gives the families and principal genera of the American Perissodactyla; extinct groups are marked †.

Suborder CHELODACTYLA. Normal Perissodactyls

I. EQUIDÆ. Horses.

_†Eohippus_, low. Eoc. _†Orohippus_, mid. Eoc. _†Epihippus_, up. Eoc. _†Mesohippus_, low. Oligo. _†Miohippus_, up. Oligo. _†Anchitherium_, up. Oligo. _†Parahippus_, low. Mioc. to low. Plioc. _†Desmatippus_, mid. Mioc. _†Hypohippus_, mid. Mioc. to low. Plioc. _†Merychippus_, mid. Mioc. to low. Plioc. _†Protohippus_, up. Mioc. _†Pliohippus_, up. Mioc. and low. Plioc. _†Neohipparion_, up. Mioc. and low. Plioc. _†Hipparion_, Plioc. _†Hippidion_, Pleist., S. Amer. _†Hyperhippidium_, Pleist., S. Am. _Equus_, Pleist., N. and S. Amer.

II. †TITANOTHERIIDÆ. †Titanotheres.

_†Lambdotherium_, low. Eoc. _†Eotitanops_, low. Eoc. _†Palæosyops_, mid. Eoc. _†Telmatherium_, mid. Eoc. _†Dolichorhinus_, up. Eoc. _†Diplacodon_, up. Eoc. _†Titanotherium_, low. Oligo.

III. TAPIRIDÆ. Tapirs.

_†Systemodon_, low. Eoc. _†Isectolophus_, mid. and up. Eoc. _†Protapirus_, Oligo. _†Tapiravus_, mid. Mioc. _Tapirus_, Pleist., N. Amer., Pleist. and Recent, S. Amer.

IV. †LOPHIODONTIDÆ. †Lophiodonts.

_†Heptodon_, low. Eoc. _†Helaletes_, mid. Eoc. _†Colodon_, low. Oligo.

V. RHINOCEROTIDÆ. True Rhinoceroses.

_†Trigonias_, low. Oligo. _†Cænopus_, Oligo. and low. Mioc. _†Diceratherium_, up. Oligo. and low. Mioc. _†Aphelops_, mid. Mioc. to low. Plioc. _†Teleoceras_, mid. Mioc. to low. Plioc.

VI. †HYRACODONTIDÆ. †Hyracodonts and †Amynodonts, cursorial and aquatic Rhinoceroses.

_†Hyrachyus_, low. and mid. Eoc. _†Triplopus_, mid. and up. Eoc. _†Colonoceras_, mid. Eoc. _†Hyracodon_, low. Oligo. _†Amynodon_, up. Eoc. _†Metamynodon_, low. Oligo.

Suborder †ANCYLOPODA. †Clawed Perissodactyls

VII. †CHALICOTHERIIDÆ. Chalicotheres.

_†Moropus_, up. Oligo. and low. Mioc. _?†Schizotherium_, low. Oligo. _†Eomoropus_, mid. Eoc.

The earliest perissodactyls of which we have any knowledge are found in the older part of the lower Eocene (Wasatch stage) of Europe and North America, into which they must have migrated from some other region yet unknown, for no probable ancestors of the group are found in the Paleocene of either continent.

I. SUBORDER CHELODACTYLA. NORMAL PERISSODACTYLA.

1. _Equidæ. Horses_

In order to make intelligible the evolutionary changes which have led up to the modern horses, it will be necessary to say something concerning the dental and skeletal features which characterize these animals. Using the term _horses_ in a broad sense to include all the existing members of the family Equidæ, true horses, asses, zebras and quaggas, we find a greater uniformity in the skeleton and teeth than would be expected from the external appearance. The differences in appearance are, however, largely due to colouring, growth of mane and tail and the size of the ears, which leave no record in the skeleton.

[Illustration: FIG. 147.—Asiatic Wild Horse (_Equus przewalskii_).—By permission of the N.Y. Zoölog. Soc.]

The teeth (Figs. 45, p. 95; 154, p. 306) are extremely high-crowned, or hypsodont, and do not form roots till an advanced age; the incisors have a deep, enamel-lined pit, the “mark” in the centre of the grinding surface; the first premolar in each jaw is very small and of no functional importance; the other premolars have the same pattern as the molars, which is excessively complex in the arrangement of the enamel ridges and the areas of dentine and cement.

The skull (Fig. 154, p. 306) is long, especially the facial portion, the eye-socket (orbit) being shifted behind the teeth, which otherwise, on account of their great height, would press upon the eye itself; the orbit is completely encircled in bone. The lower jaw is deep vertically and the ascending ramus (see p. 66) very high, on account of the hypsodont character of the teeth, which thus necessitates a remodelling of the skull in several respects. The neck is long, each of its seven vertebræ being elongate; except in the atlas and axis, the anterior face of each centrum is strongly convex and the posterior of all except the atlas is deeply concave; the odontoid process of the axis (see p. 71) is spout-shaped, concave on the upper and convex on the lower side, lodging and protecting the spinal cord. The spines of the anterior dorsal vertebræ are very high, making a low hump at the withers between the shoulder-blades; the trunk-vertebræ are so arranged as to make the back almost straight and horizontal. The limbs and especially the feet are very long. The two bones of the fore-arm, the ulna and radius, are coössified into a single piece (Fig. 30, p. 81), but the limits of each are still plainly to be seen, especially in a young animal; and it is evident that the ulna is greatly reduced in size and has lost its middle portion, while all the weight is borne by the radius. Similarly, in the hind leg the enlarged tibia, or shin-bone, alone supports the weight; and only the two ends of the fibula are preserved (Fig. 38, p. 87), and these are indistinguishably fused with the tibia in the adult animal, but may be made out in the colt. The thigh-bone has a very characteristic shape, which is difficult to describe without an undue use of technical terms, but the unusual prominence of the great trochanter (Fig. 35, p. 85) and of the rotular groove is an important factor in producing this appearance.

[Illustration: FIG. 148.—Left pes of Horse. _Cal._, calcaneum. _As._, astragalus. _N._, navicular. _Cn. 3_, third cuneiform. _Mt. III_, functional (third) metatarsal. _Mt. II_ and _Mt. IV_, splints.]

[Illustration: FIG. 149.—Left manus of Horse, front side; to the right, rear view of the metacarpus. _S._, scaphoid. _L._, lunar. _Py._, pyramidal. _Pis._, pisiform. _Td._, trapezoid. _M._, magnum. _U._, unciform. _Mc. II_, _Mc. IV_, rudimentary second and fourth metacarpals, or splints.]

The very long and slender feet are so raised from the ground that the animal walks upon the very tips of the toes, the wrist being what horsemen call the “knee” and the heel is the “hock,” and the gait is thoroughly unguligrade. Each foot has but a single functional toe, the third or middle one of the primitive five-toed foot; and, as this toe has to carry the whole weight supported by its leg, it is necessarily much larger than in animals which distribute the weight among several digits. The horses are therefore said to be _monodactyl_, or single-toed, but the term is not strictly accurate, for on each side of the functional digit is a rudimentary or vestigial one, the 2d and 4th of the original five. These rudimentary digits, which are not visible externally, have no phalanges and are merely “splint-bones,” metapodials (see p. 90) which have very slender shafts and end below in blunt points. The single functional metapodial has encircling its lower articular end a prominent ridge or keel, which fits into a corresponding groove on the upper end of the first phalanx and serves to prevent lateral dislocation. In most mammals this keel is merely a projection from the lower articular surface and is confined to the posterior side, so as not to be visible from the front. The terminal or ungual phalanx is much enlarged to carry the great weight which it supports and is enclosed in the characteristic hoof, unlike that of any other mammal.

In brief, the whole structure of the horses is pre-eminently adapted to swift running; they are admirable “cursorial machines,” as they have been called, and every part of the skeleton has been modified and specialized to that end; the narrow, rigid hoofs fit them for walking on firm ground and they speedily are made helpless in quicksand or bog. Did we know nothing of their mode of life, we might confidently infer from their teeth that the horses were grazers, feeding principally upon grass. A long-legged, grazing animal must needs have a neck of sufficient length to enable the mouth to reach the ground easily, unless a long proboscis is developed; and so we shall find in the history of the horses that the elongation of the head and neck kept pace with the lengthening of the legs and feet.

Though it can hardly be doubted that the horses passed through most of their development in North America, yet the immediate ancestry of all the existing species must be sought in the Old World, none of the many Pleistocene species of the western hemisphere having left any descendants. In North America all of the known Pleistocene forms belonged to the genus _Equus_, but the True Horse, _E. caballus_, was not among them. The more abundant and important of these species have been sufficiently described in Chapter VII (p. 199); it need only be recalled that there were ten or more distinct forms, ranging in size from the great _E. †giganteus_ of Texas to the minute _E. †tau_ of Mexico, while the plains and forests were the feeding grounds of moderate-sized species, about 14 hands high.

In the latest Pliocene, and no doubt earlier, species of the modern genus _Equus_ had already come into existence; and in association with these, at least in Florida, were the last survivors of the three-toed horses which were so characteristic of the early Pliocene and the Miocene. However, little is known about those earliest recorded American species of _Equus_, for the material so far obtained is very fragmentary. In the absence of any richly fossiliferous beds of the upper Pliocene generally, there is a painfully felt hiatus in the genealogy of the horses; and it is impossible to say, from present knowledge, whether all of the many species of horses which inhabited North America in the Pleistocene were autochthonous, derived from a purely American ancestry, or how large a proportion of them were migrants from the Old World, coming in when so many of the Pleistocene immigrants of other groups arrived. It is even possible, though not in the least likely, that all of the native American stocks became extinct in the upper Pliocene and that the Pleistocene species were all immigrants from the eastern hemisphere, or the slightly modified descendants of such immigrants; but, on the other hand, it is altogether probable that some of these numerous species were intruders. Unfortunately we are in no position yet to distinguish the native from the foreign stocks.

In the middle Pliocene, which also has preserved but a meagre and scanty record of its mammalian life, we again meet with horses in relative abundance, but of a far more primitive type. They are still incompletely known, but it is clear that they belonged to three parallel series, or phyla, of three-toed grazing horses, with teeth which, though high-crowned, had not attained to the extreme degree of hypsodontism seen in the species of _Equus_ and had a somewhat less complex pattern of the grinding surface, though distinctly foreshadowing the modern degree of complication. One of the genera (_†Pliohippus_) was not improbably the ancestor of a very peculiar horse (_†Hippidion_) of the South American Pleistocene. These middle Pliocene genera were much smaller animals than the Pleistocene horses, aside from the pygmy species of the latter, of light and more deer-like proportions, and with three functional toes or digits. The median digit (3d of the original five) was much the largest and carried most of the weight, on hard ground practically all of it; the lateral digits (2d and 4th) which in existing horses are represented by the rudimentary metapodials, or “splints,” though much more slender than the median digit, yet had the complete number of parts and each carried a small hoof. Mere “dew-claws” as these lateral toes were, they may have been of service in helping to support the weight in mud or snow. In all parts of the skeleton there are little details which show that these species of the middle Pliocene were not so advanced and differentiated as are their modern successors, but it would be unprofitable to enumerate these details, which are of interest only to the anatomist.

In the lower Pliocene the horses were very much more numerous and varied than in the middle portion of the epoch. The same three genera of grazing animals, represented by less advanced and modernized species, are found; and, in addition, there was an interesting survival (_†Merychippus_) from the middle Miocene of an intermediate type, together with several species of browsing horses (_†Parahippus_ and _†Hypohippus_). In these browsing forms the teeth were all low-crowned and early formed their roots, and the crowns were either without cement or with merely a thin film of it in the depressions of the grinding surface. The pattern of the grinding surface is so very much simpler than in the high-crowned, prismatic teeth of the grazers that it requires close analysis to detect the fundamental identity of plan. Such teeth imply that their possessors must have fed habitually upon a softer and less abrasive diet than grass, probably the leaves and soft shoots of trees and bushes and other succulent vegetable substances, very much in the fashion of existing deer, and must therefore have been chiefly inhabitants of the woods and groves and thickets along streams, as the grazing species were of the plains and open spaces. “This assemblage of the progressive and conservative types of horses was certainly one of the most distinctive features of Lower Pliocene time in North America” (Osborn).

[Illustration: FIG. 150.—Three-toed, grazing horse (_†Neohipparion whitneyi_) of the upper Miocene. Restored from skeletons in the American Museum of Natural History.]

In the upper Miocene very much the same conditions prevailed and, for the most part, the same genera of horses, with different and somewhat less advanced species, were found as in the lower Pliocene, so that no particular account of them is needed. In the middle Miocene, however, there was a change, the typically grazing horses being very rare or absent and those with intermediate forms of teeth taking their place. Evidently, it was about this time that the horses with more plastic organization and capable of readjustment to radically different conditions began to take to the grazing habit, while other phyla, less capable of advance, retained the ancient, low-crowned type of grinding teeth and, after persisting, as we have seen, into the lower Pliocene, became extinct before the middle of that epoch. It is of great interest to observe that in the genus (_†Merychippus_) intermediate between the browsing and grazing types, the milk-teeth retained the older and more primitive character of low crowns without covering of cement, while the permanent grinders had much higher, cement-covered and complex crowns. In the lower Miocene, the variety of horses was much diminished and all had the low-crowned, cement-free, browsing type of teeth. Reversing the statement, we see that in the middle and still more in the upper Miocene the primitive and more or less distinctly homogeneous phylum branched out into several series, like a tree, some of the branches continuing and further subdividing through the Pliocene and Pleistocene, while others, less progressive and less adaptable, underwent but little change and had died out before the middle Pliocene.

[Illustration: FIG. 151.—Skeleton of _†Neohipparion whitneyi_, American Museum.]

The Oligocene horses deserve more particular attention, for they were almost the half-way stage of development in the long backward ascent to the earliest known members of the family in the lower Eocene. We may pass over the John Day horses (_†Miohippus_), which were somewhat larger than those of the White River, but otherwise very like them, merely noting the presence of a slightly different genus (_†Anchitherium_) which was the probable ancestor of _†Hypohippus_ and the other non-progressive types of the Miocene and Pliocene. The genus (_†Mesohippus_) which characterizes the White River, or lower Oligocene, was a group of species of different sizes, becoming smaller as we go back in time, the commonest one being considerably smaller than a sheep and differing more or less in all its parts from the horses of the upper Miocene and all subsequent formations. The teeth were very low-crowned and fitted only for the mastication of soft vegetable tissue; but it is of particular interest to observe the beginnings of the “mark” in the upper incisors in the form of a low enamel-ridge arising behind the cutting edge of the tooth; the lower incisors still had the simple chisel-like crowns of the more ancient genera; all the premolars, except the first, had already acquired the molar-pattern.

[Illustration: FIG. 152.—The small, browsing, three-toed, short-necked horse (_†Mesohippus bairdi_) of the middle White River. Restored from a skeleton in the American Museum.]

The skull resembled that of a very small modern horse, but with many differences of detail, the most obvious of which is the shallowness of the jaws, for depth was not needed to carry the very low-crowned teeth, and, for the same reason, the ascending ramus of the lower jaw was short. The face was relatively short and the eye-socket, which was incompletely surrounded by bone, was directly above the hindmost teeth; the cranium was proportionately large and capacious and the brain, as is shown by the cast, was richly convoluted. The neck was relatively far shorter than in the Miocene genera, the ball-and-socket joints between its successive vertebræ were less elaborated and the odontoid process of the axis was in the first stage of assuming the spout-like form, being semicylindrical, with convex lower and flat upper surface. The trunk was proportionately long and the back sloped forward, owing to the greater length of the hind legs. The limbs and feet were elongate and very slender, but the fore-arm bones are only partially coössified, and the ulna, though greatly attenuated, was still complete. The same is true of the bones of the lower leg; the shaft of the fibula was hardly more than a thread of bone, but its full length was preserved. In the fore foot there were three functional digits, the median one enlarged and supporting most of the weight, but its hoof was much thinner and flatter than in the corresponding digit in the Miocene and subsequent genera; the lateral digits touched the ground and were not entirely functionless and, in addition, there was a small splint, the rudiment of the fifth digit. The hind foot was three-toed, without splint.

The little Uinta horse (_†Epihippus_) is still very incompletely known, but gives us one point at least of greater primitiveness than the White River genus in that only the last two premolars had taken on the molar-pattern, the forward two being smaller and simpler. The known species of the Uinta genus was very much smaller than any of the White River forms and even smaller than some of those of the preceding Bridger formation; but it should be remembered that the Uinta has been but partially explored and much remains to be learned regarding its fauna.

The Bridger horses are fortunately much better known. There are several species of the genus _†Orohippus_, which form a connected and progressive series; and, though much smaller than the smallest and oldest of the White River forms, they were somewhat larger than the known representative of the Uinta, _†Epihippus_, but distinctly more primitive in all other respects. The incisors were simple cutting teeth, with no trace of even an incipient “mark,” and only one premolar in each jaw, the hindmost one, had taken on the molar-pattern. The orbit was farther forward in the skull and less enclosed behind than in _†Mesohippus_, the cranium narrower and less capacious; the neck was even shorter and the odontoid process of the axis still retained the primitive peg-like form. The limbs and feet were conspicuously shorter in proportion than those of the White River genus; the ulna and fibula were stouter and less reduced and entirely separate from the radius and tibia respectively. The front foot had four functional toes; the fifth digit, which in _†Mesohippus_ had been reduced to a splint, was completely developed in the Bridger horses, but the hind foot was three-toed.

[Illustration: FIG. 153.—The “Dawn Horse” (_†Eohippus_) of the lower Eocene. Restored from a skeleton in the American Museum.]

Passing over, for lack of space, the transitional forms of the Wind River, we come finally to the most ancient known horses, the Wasatch species comprised in the genus _†Eohippus_, the “Dawn Horse,” as its name signifies; these were little creatures ranging in size from a cat to a small fox. Despite an unmistakably equine look in the skeletons of these diminutive animals, it is only the long intermediate series of species and genera, together forming a closely linked chain, which we have traced back from the Pleistocene to the lower Eocene, that leads us to regard _†Eohippus_ as the ancestral type of the horses. Were only the two ends of the chain known, he would be a daring speculator who should venture to connect them. In these little Wasatch horses we have the evidence of a still more ancient form with five fully developed toes in each foot, since the front foot had four functional digits and indication of a splint, and splints, as the whole history of the long series teaches, always are found to be functional digits in the ancestor; the hind foot had three toes and perhaps two splints. This preceding form is hardly to be looked for in America or Europe; it will be found, if ever, in the region whence the great migration came. In all other respects, as well, _†Eohippus_ was what we should expect the forerunner of the Wind River and Bridger horses to be. The premolars were all smaller and simpler than the molars and the latter in the upper jaw are particularly interesting, for they had no crests and ridges of enamel, but four principal conical cusps, arranged in two transverse pairs, and between the cusps of each pair was a tiny cuspule no bigger than the head of a pin. These cuspules were the first step in the formation of the transverse crests, which were destined to assume such importance in the subsequent members of the series. The neck was very short, the body long, with curved or arched back, the limbs and feet short, and the hind limb much longer than the fore, making the relative proportions of the various parts of the skeleton very different from what they afterwards became.

Reviewing this marvellous history of steady and long-continued change, beginning with the most ancient genus, _†Eohippus_, the following modifications may be noted:

(1) There was a nearly constant, if somewhat fluctuating, increase in size, leading by slow gradations from the diminutive horses of the lower Eocene to the great animals of the Pleistocene.

(2) The molar teeth, originally made up of conical cusps, changed to a highly complex pattern of crests and ridges, and the premolars, one by one, assumed the size and pattern of the molars; the low-crowned, rooted and cement-free teeth, fitted only for browsing, became very high-crowned, prismatic and cement-covered, admirably adapted to grazing. Beginning in the upper incisors of the White River _†Mesohippus_, the “mark” became established as an enamel-lined pit, growing in depth as the teeth increased their length.

(3) The face grew relatively longer, the eye-socket being shifted behind the teeth and becoming completely encircled in bone, and the jaws were greatly increased in depth to accommodate the very long teeth.

(4) The short neck was greatly elongated and the individual vertebræ modified so as to give flexibility with no loss of strength. The primitive peg-like odontoid process of the axis became first semicylindrical and then spout-shaped.

(5) The arched back was straightened and the neural spines, especially of the anterior dorsals, elongated.

(6) The limbs grew relatively much longer; the bones of the fore-arm and lower leg were fused together, the one on the inner side (radius and tibia) enlarging to carry the entire weight and the external one (ulna and fibula) becoming more or less atrophied.

[Illustration: FIG. 154.—Series of horse skulls in ascending geological order. _A._, _†Eohippus_, lower Eocene (after Cope). _B._, _†Mesohippus_, lower and middle Oligocene. _C._, _†Protohippus_, upper Miocene (after Cope). _D._, _Equus_.]

[Illustration: FIG. 155.—Right manus and left pes of _Equus_.]

[Illustration: FIG. 156.—Right manus and left pes of _†Protohippus_.]

(7) The feet were much elongated and the median (3d) digit of each gradually enlarged until it carried the whole weight, at the same time modifying the shape of the hoof so as to fit it to be the sole support of the body. The other toes gradually dwindled and became functionless, though often retained as splints. The first digit (pollex and hallux) was first lost, then the fifth, then the second and fourth were reduced to dew-claws and finally to splints. Thus the pentadactyl horses of the lower Eocene were transformed into the monodactyl species of the Pliocene and Pleistocene.

In South America the story of the horses was a brief one, for they were among the immigrants from the north and did not reach the southern continent till the Pliocene, probably late in that epoch, for none of the three-toed genera have been found in South America. So far as known, these southern equines were small and medium sized animals, with large heads, relatively short feet and somewhat ass-like proportions. There were two well-defined groups of these animals: (1) species of the genus _Equus_, which thus, at one time or another, inhabited every one of the continents, Australia excepted; (2) three genera peculiar to South America and developed there from northern ancestors, probably _†Pliohippus_. Two of these genera (_†Hippidion_ and _†Onohippidium_) displayed curious modifications of the nasal bones, which were extremely slender and attached to the skull only at their hinder ends, instead of being, as is normally the case, supported for nearly their whole length by lateral articulation with other bones. What can have been the significance and function of these excessively slender, splint-like nasals, it is difficult to conjecture. The third genus (_†Hyperhippidium_) was a small mountain-horse, with extremely short feet, which were well adapted to climbing.

[Illustration: FIG. 157.—Right manus and left pes of _†Mesohippus_.]

[Illustration: FIG. 158.—Right manus and pes of _†Eohippus_.]

This is the merest outline sketch of a most wonderful series of gradual and progressive modifications, a sketch that might readily be expanded into a volume, were all the details filled in. While each set of organs, teeth, skull, neck, body, limbs and feet, might appear to advance independently of the others, in reality there was no such independence, for at every stage of the progression all the parts must have been so coördinated into a harmonious whole, that the animal could thrive and hold its own in the stress of competition. Could we but discover all the facts of environment, on the one hand, and organization, on the other, we should doubtless learn that the little _†Eohippus_ was as exquisitely fitted to its place in the Wasatch world, as are the horses, asses and zebras of the present day to theirs. It was the response to changing needs, whether of food, climate, disease or competition, that was the main factor of development.

[Illustration: FIG. 159.—Skeleton of a Pampean horse (_†Hippidion neogæum_). National Museum, Buenos Aires. For restoration, see Fig. 119, p. 214. Note the splint-like nasal bones attached only at the hinder end.]

2. _†Titanotheriidæ. †Titanotheres_

This family, all of whose members vanished from the earth ages ago, was a comparatively short-lived group and nearly the whole of its recorded history was enacted in North America; only a few belated stragglers reached the eastern hemisphere, though the family may, nevertheless, have originated there.

[Illustration: FIG. 160.—White River †titanothere (_†Titanotherium robustum_) males fighting. Restored from a skeleton in the American Museum of Natural History.]

In the lowest of the three substages of the White River Oligocene the most conspicuous and abundant fossils are the †titanotheres, the latest members of which were huge animals of almost elephantine proportions. They belonged to four parallel, or rather slightly divergent, phyla, differing in the development of the horns, in the shape of the head and in the relative length and massiveness of the limbs. The teeth were all low-crowned, or brachyodont, the canines much too small to have been of any service as weapons and the incisors had curious little, button-shaped crowns, which can have had little or no functional importance, since they show hardly any wear, even in old animals. With such front teeth, a prehensile lip and long tongue would seem to have been necessary for gathering and taking in food.

[Illustration: FIG. 161.—Second upper molar, left side, of _†Titanotherium_. _A._, masticating surface; _B._, outer side of crown.]

The †titanotheres were one of two perissodactyl families in which the premolars never became so large and complex as the molars. The upper molars had a longitudinal outer wall, composed of two deeply concave cusps, and two internal conical cusps, but no transverse ridges; the lower molars were composed of two crescents, one behind the other, a pattern which was very widely distributed among the early and primitive artiodactyls and perissodactyls.

The so-called “horns” were not strictly such, but a pair of bony protuberances from the front of the skull and, from their shape, could hardly have been sheathed in horn. The long, immensely broad and massive head resembled that of some fantastic rhinoceros, as did also the body and limbs. The brain was quite absurdly small, the cavity for it, lost in the huge skull, would hardly contain the fist of an ordinary man; these great beasts must have been incredibly dull and stupid, surpassing even the modern rhinoceroses in this respect. As is generally true in mammals which have horns, antlers, or similar weapons borne upon the skull, or very large tusks, the bones of the brain-case were made enormously thick and yet lightened by an intricate system of communicating cavities or “sinuses,” separated by many bony braces and supports connecting the inner and outer denser layers, which form the surfaces of the bones. In this way the skull is made strong enough without any proportionate increase of weight to endure the severe shock of impact, when the horns or tusks are made use of. The principle is the same as the engineer employs in designing a steel truss-bridge. The upper profile of the head was deeply concave, just as it is in those rhinoceroses which are armed with nasal horns.

[Illustration: FIG. 162.—Skull of _†Titanotherium elatum_. American Museum.]

The neck was of moderate length and the body, as indicated by the long, arched ribs and the greatly expanded hip-bones, was extremely bulky and massive. The spines of the anterior dorsal vertebræ were excessively long, forming a great hump at the withers. The limbs and feet were columnar, like those of an elephant; the feet were supported on a great pad, while the hoofs were mere excrescences on the periphery of the foot. The bones of the fore-arm were entirely separate and the ulna was very stout; in the lower leg also the bones were not coössified, but the fibula was but moderately heavy. This is a sharp contrast to the arrangement found in the horses and in those hoofed animals generally which are swift runners and have slender, elongate limbs and feet, such as deer, antelopes, camels, etc. Heavy, slow-moving animals, like elephants, tapirs, rhinoceroses, etc., almost always have separate fore-arm and leg-bones and generally a heavy ulna. The number of digits was four in the front foot and three in the hind. The genera differed in the proportions of limbs and feet, one having them longer and less ponderous than another, and, no doubt, the former was of swifter gait.

At a certain level in the White River beds the †titanotheres abruptly cease, disappearing with what seems like startling suddenness. In all probability, however, the extinction was more gradual and its apparent abruptness was due, partly at least, to the break in the deposition of the beds, which is very obvious. Such a break, or “unconformity,” as geologists call it, almost always implies an unrecorded lapse of time, which may have been very long. However it came about, gradually or suddenly, the extinction of these great animals is difficult to explain; no Carnivora of the time could have been formidable enemies and they had no rivals in their own walk of life. Their stupidity may have been a factor, but it seems more likely that the onset of some new infectious disease, perhaps imported by incoming migrants from the eastern hemisphere, gave the _coup de grace_. In the lower substage, beneath the unconformity, where the remains of †titanotheres are so abundant, successive changes may be observed. The species with great “horns,” rounded, flattened or triangular, are confined to the upper levels; in the middle section other species, somewhat smaller and with shorter “horns,” are found, while in the bottom levels the animals are much smaller and have still smaller “horns.”

The Uinta †titanotheres were much more numerous and varied than those of the White River; in the upper part of these beds are found two genera (_†Diplacodon_ and _†Protitanotherium_) which already had quite prominent bony protuberances on the nose; their canines were large enough to be of value as weapons and the incisors were well developed and functional. Evidently, there was a change here in the manner of feeding, the front teeth were used for cropping and browsing, a function which in the White River members of the family must have been largely taken over by the lips and tongue, while the growth of the horn-like protuberances on the skull rendered the canines superfluous as weapons. This latter change is one which recurs frequently in different phyla of the hoofed animals, in which the earlier and more primitive members had canine tusks, and the later, more advanced representatives developed horns, the tusks diminishing as the horns increased. While this rule is a general one, it is not entirely without exceptions.

In the lower Uinta and in the Bridger the †titanotheres were extremely abundant and numerically they are the commonest of all fossils in those beds; no less than five series or phyla may be distinguished, three of them being added in the upper Bridger. The differences between the phyla, however, principally concern the forms of the teeth and the shape of the skull; in some the head was short and broad, in others long and narrow, and in others again of medium proportions; some had broad and extremely low-crowned grinding teeth, which in others were higher and more erect. But these are matters of minor detail, useful as they are in pointing the way to a proper arrangement of the various species; in essentials, the forms all agreed and constituted several series of closely allied genera. Comparing these Bridger animals with the great †titanotheres of the lower White River, the first and most obvious difference that strikes the observer is the very much smaller size of the more ancient types. With some variation in this respect, hardly any of the Bridger species exceeded a modern tapir in stature and very much resembled one in proportions. The canine teeth were tusks as large as those of a bear and must have been very effective weapons; the molar-pattern was identical with that found in the great Oligocene beasts, but the premolars were simpler and relatively smaller.

[Illustration: FIG. 163.—†Titanothere (_†Mesatirhinus superior_) with long, narrow head; Bridger stage. Restored from a skeleton in the American Museum.]

[Illustration: FIG. 164.—Second upper molar, right side of a Bridger †titanothere (_†Palæosyops_).]

The skull had a straight upper profile, though in several of the phyla small bony protuberances were developed over the eyes, and must clearly be regarded as incipient stages of the “horns” which were subsequently to become so long and prominent. Instead of being broad on top as it was in the White River genera, the cranium carried a high ridge of bone, the sagittal crest, which served for the attachment on each side of the great temporal muscle, one of the most important of the muscles of mastication. The trunk was less massive and the limbs were lighter than in the Oligocene genera, but the number of digits was the same, four in the front foot and three in the hind, and the hoofs were much better developed, serving actually to carry the weight and not being mere excrescences upon the periphery of a pad. Aside from the proboscis, which lends such a characteristic appearance to the existing tapirs, the †titanotheres of the Bridger must have looked much like tapirs, and in early days, when the mutual relationships had not been satisfactorily determined, they were frequently described as “tapiroid.” The term is unobjectionable in so far as it is understood that a merely superficial likeness is implied, not any real relationship other than that which unites all the perissodactyl families.

As noted above, the phyla of the †titanotheres were much more numerous in the later than in the earlier part of the Bridger stage, when they were reduced to two. In the still older Wind River stage these two united into one. The Wind River animals (_†Eotitanops_) were similar, but much smaller, and occurred in incomparably less variety and abundance. Indeed, one of the most striking differences between the Wind River and the Bridger faunas consists in the great increase and diversification of the †titanotheres in the latter. There was, it is true, a second phylum of the family in the Wind River, represented by the genus _†Lambdotherium_, but this was a short-lived series, which left no descendants in the Bridger or subsequent formations. These were the smallest known members of the family and were light, slender-limbed animals, a very notable difference from the others.

With the Wind River the history of the †titanotheres breaks off short, and from present information, can be carried no farther back. Possibly, there was a Wasatch ancestor, which only awaits discovery, but it seems more likely that these earliest known genera were belated immigrants from the same as yet unknown region, whence came the modernized and progressive elements of the Wasatch fauna. Except for its obscure beginning, the family was pre-eminently characteristic of North America, and only two representatives of it have been found outside of that continent, one in Hungary and one in Bulgaria. No doubt others will yet be found in Asia.

Both in its resemblances and its differences, as compared with the far longer and more complex story of the horses, the history of the †titanotheres has instructive bearings upon evolutionary theory.

(1) Starting with two phyla, one of which speedily died out, the other ramified into four or five, which continued until the disastrous end, pursuing a nearly parallel course of development.

(2) There was a great increase in size and especially in bulk and massiveness from species no bigger than a sheep in the Wind River stage to those which rivalled small elephants in the lower White River.

(3) The teeth underwent comparatively little change; the incisors dwindled and lost functional importance and the canines were reduced, horn-like growths taking their place as weapons; the premolars grew larger and more complicated, but never attained the full size and complexity of the molars, as they did in other perissodactyl families.

(4) Horn-like, bony protuberances appeared first as small humps and knobs over the eyes and steadily enlarged, at the same time shifting their position forward, until they finally attained great size and were on the nose.

(5) The skull was modified so as to support these weapons and endure the shock of impact when they were put to use, (_a_) by making the upper profile strongly concave from before backward; (_b_) by greatly widening the top of the cranium, where in the older and more primitive genera the high and thin sagittal crest was placed; (_c_) by immensely increasing the thickness of the cranial bones and at the same time hollowing them by means of an intricate system of cavities; in this way sufficient strength was secured without undue increase in weight.

[Illustration: FIG. 165.—Series of heads of †titanotheres in ascending geological order. _A._, _†Palæosyops_, lower Bridger. _B._, _†Manteoceras_, upper Bridger. _C._, _†Diplacodon_, Uinta. _D._, _†Titanotherium_, extreme development of horns, White River. From models in the American Museum and Princeton University.]

(6) The growth of the brain did not keep pace with the increase in the size and weight of the body and head, and this deficiency may have been a factor in determining the early extinction of the family.

(7) To support the huge head, stout ligaments and powerful muscles were needed in the neck and trunk and these in turn required large bony, surfaces for their attachment. To meet this need, the spines of the anterior trunk-vertebræ were very much lengthened, so as to form a hump at the shoulders, and this elongation of the spines went on in proportion to the growing weight of the head.

(8) The limb-bones increased in thickness until they became extremely massive, to carry the immense weight of the body, and they eventually lost the marrow-cavities, which were filled up with spongy bone, a great gain in strength. As is generally, though not universally, true of the large and heavy mammals, there was no coössification between the limb-bones and no great increase in their proportionate length. The thigh-bone, or femur, lost the cylindrical shape of the shaft, becoming flattened and very broad, and acquiring something of the appearance of the same bone in the elephants.

[Illustration: FIG. 166.—Right manus of †titanotheres. _A._, _†Titanotherium_, White River (after Marsh). _B._, _†Palæosyops_, Bridger, Princeton University Museum.]

(9) There was no loss or coössification of elements in wrist (carpus) or ankle (tarsus) and _no reduction of digits_ within the limits of the family. In the latest, largest and most specialized genera, as well as in the earliest, smallest and most primitive, there were four toes in the front foot and three in the hind. We have the most cogent reasons for assuming that all mammals were derived from ancestors which had five toes in each foot, neither more nor less. If this be true, then the most ancient known †titanotheres, which were small and light, had already suffered the loss of the first digit in the fore foot and of the first and fifth digits in the hind foot, but there reduction ceased. With the growing body-weight, long, narrow and slender feet would have been a detriment, whereas in swift-running animals, like horses and deer, long and very slender feet are a great advantage. The contrast is both striking and instructive, showing the importance of a short, broad, polydactyl and pillar-like foot to very large and heavy mammals, all of which have feet of this character.

(10) The hoofs, as shown by the terminal bones (ungual phalanges) which formed their bony cores, were reduced in size until they became mere nail-like excrescences around the border of the massive foot.

3, 4. _Tapiridæ and †Lophiodontidæ. Tapirs and †Lophiodonts_

The history of the tapir family is not at all satisfactorily known, partly because tapirs are comparatively rare as fossils in all of the Tertiary formations, and still more for the reason that the specimens so far collected are so fragmentary, not a single half-complete skeleton among them. Had these animals actually been as rare in North America as the fossils would seem to indicate, they could not possibly have maintained themselves for so long a time, throughout nearly the whole of the Tertiary and Quaternary periods. For some reason, probably because they have always been forest-haunting animals, their habits must have kept them in places remote from the areas where the accumulation of sediments was in progress, and thus only occasional stragglers were buried and preserved.

The rarity and incompleteness of the material render it impossible to give any such full account of the tapirs as is practicable for the horses and †titanotheres, but the circumstance is less unfortunate in the case of the tapirs than in that of many other families. This is because these creatures have been so conservative and unprogressive, that they have undergone comparatively little change since their earliest recorded appearance. They have been aptly termed “living fossils” and seem like belated survivors from some older world, out of place in the modern order of things. Attention has already been directed (p. 137) to the remarkable geographical distribution of the tapirs at the present time; Central and South America, southeastern Asia and the adjoining islands.

[Illustration: FIG. 167.—American Tapir (_Tapirus terrestris_). By permission of W. S. Berridge, London.]

The tapirs are all of moderate size, going back to very small forms at the beginning of their history and never at any period developing into large animals. The only striking and unusual feature about any of the existing members of the family is the long proboscis, a flexible, dependent snout, and, were they all extinct and nothing known of them but the skull, this proboscis could have been confidently predicated of them from the great shortening of the nasal bones. Small tusks, not showing when the mouth is closed, are formed in an exceptional way by the enlarged external upper incisor and the lower canine, the upper canine being much reduced and without function. The grinding teeth have very low crowns, premolars (except the first) and molars are all alike and of a very simple pattern, which has been independently repeated in several different orders of herbivorous mammals; in both upper and lower teeth, there are two elevated, straight, transverse crests.

[Illustration: FIG. 168.—Skull of American Tapir, right side.]

Except for the modification of the skull which is conditioned by the development of the proboscis, the skeleton might belong to any one of several Eocene or Oligocene families, and it is this generalized, indifferent character which has led to the dubbing of many early perissodactyls as “tapiroids.” The limbs are short and moderately heavy, the bones of the fore-arm and lower leg all separate and the number of toes is four in the front foot and three in the hind. The toes end in well-formed separate hoofs, but behind them is a pad, which carries most of the weight. The body is covered with smooth, short hair, which in the American species is of a uniform dark brown, but in the Asiatic species the head, neck and limbs are black and the body is white. In both, however, the young have longitudinal, light-coloured stripes and spots on a dark ground (_see_ Fig. 6, p. 47) indicating what the colour-pattern of the ancestral forms must have been. As might be inferred with certainty from the low-crowned teeth, the tapirs are browsing, not grazing, animals, feeding upon leaves and shoots and other soft vegetable tissues. They are shy and solitary in habit and live usually in thick forests and near water, which they frequently enter, both for bathing and as a place of refuge when pursued. Under modern conditions, the only perissodactyls of the western hemisphere are the tapirs of the Neotropical region, North America proper, which for ages was the principal home of the order, not having a single representative now.

In the Pleistocene, tapirs were apparently more abundant than in any of the Tertiary epochs, but this was probably due to the fact that the Pleistocene of the forested regions is far more fully recorded than is any Tertiary stage. One species, which was hardly distinguishable from the Recent Central American form, was common in the forested region east of the Mississippi and in California, and a second species (_Tapirus †haysii_) was larger and heavier than the other. Except in Texas, none have been found in the Great Plains area, nor are they likely to be, for that region, then as now, appears to have been devoid of forests. No doubt, these Pleistocene species had substantially the same habits as the existing ones, but they were adapted to a colder climate and a different vegetation, for, except the Pinchaque Tapir (_T. roulini_) of the high Andes, all the modern species are tropical in distribution.

Concerning the Pliocene and Miocene tapirs, but meagre information has been obtained. Enough material has been gathered by the collectors to demonstrate the continuous presence of the family in North America throughout those epochs, but the broken and fragmentary specimens are insufficient to show what the structural changes were. It should be remembered, however, that it is only in the region of the Great Plains and the Great Basin of Nevada that any considerable quantity of Miocene and Pliocene mammals have been found, and in those regions tapirs probably never were common. If the Peace Creek formation of Florida is properly classified as latest Pliocene, then at that time the American tapirs were essentially what they are to-day, for the Florida species is hardly separable from the modern _T. terrestris_.

Not till we reach the lower Oligocene, or White River beds, do we get material which permits the making of definite statements regarding the course of developmental changes. The White River genus, _†Protapirus_, which is also found in the middle Oligocene of Europe, was a much smaller animal than any of the known Pleistocene or Recent species, barely more than half the size, in fact. The teeth show that the small tusks were canines, both above and below, and that the curious substitution of the external upper incisor for the canine had not yet taken place. The grinding teeth were identical in pattern with those of the existing genus, but not all the premolars had yet acquired the form and size of the molars. In the skull the nasal bones had begun to shorten, but the change had not yet made much progress, and the proboscis must have been in merely an incipient stage of development. What little is known of the skeleton other than the skull was like that of the modern genus, but the bones were much smaller and proportionately lighter.

[Illustration: FIG. 169.—Skull of White River tapir (_†Protapirus validus_), left side. Princeton University Museum. N.B. This figure is much less reduced than Fig. 168.]

The Eocene tapirs are still very imperfectly known; all that can be said of them is that they become successively smaller as they are traced backward in time, and that in them the premolar teeth were all smaller and simpler than the molars. The Wasatch genus (_†Systemodon_) is the most ancient member of the series yet discovered. Dating from the Eocene immigration, the tapirs are to be regarded as a North American family, for there is here a complete continuity from the lower Eocene to the Pleistocene, while in Europe they first appeared, probably by migration from North America, in the middle Oligocene.

[Illustration: FIG. 170.—Head of the White River tapir (_†Protapirus validus_). Restored from a skull in the museum of Princeton University.]

In South America the history of the tapirs is even shorter and less eventful than that of the horses; the latter, as we have seen, reached the southern continent in the Pliocene and there gave rise to a number of peculiar and characteristic genera, but the tapirs have been found only in the Pleistocene of Argentina and Brazil and only the modern genus is represented.

Wofully broken and incomplete as the developmental history of the tapirs still is, the fragments are nevertheless sufficient to show a mode of evolution differing in certain important respects from that followed by the horses or †titanotheres. Certain features are common to all three groups, such as the increase in size and in proportionate stoutness from stage to stage and the gradual enlargement and complication of the premolar teeth. On the other hand, the tapirs have been very conservative, and they underwent far less radical changes than did either of the other families. Aside from the proboscis and the modifications of the skull which the development of that organ necessitated, these animals remain to-day very nearly what they were in Oligocene times. This, then, is an example of development practically restricted to a few organs, while all the other parts of the structure changed but little.

[Illustration: FIG. 171.—Upper teeth, left side, of tapirs, showing comparative sizes. _A_, _†Protapirus validus_, White River Oligocene. _B_, _Tapirus terrestris_, modern. _i3_, external incisor. _c_, canine. _m1_, first molar.]

The extinct †lophiodonts, like the tapirs, of which they would seem to have been near relatives, are known only from incomplete material, and comparatively little has been learned regarding their history. While they were abundant and varied in Europe, during the Eocene epoch, they never were a striking or prominent element among the mammals of North America, where they persisted one stage later, and they did not reach South America. In North America they are found from the Wasatch to the White River.

The White River genus (_†Colodon_), which is fairly well known, might almost be described as combining the characters of horses and tapirs; but such an expression is not to be interpreted as meaning that this genus is in any sense a connecting link or transition between the two families, but merely that in certain important respects its course of development ran parallel with that followed by the horses. The teeth were very tapir-like, especially those of the lower jaw, which, indeed, are hardly distinguishable from those of a tapir, and the premolars had the molar-pattern. The limbs were very light and slender and the feet long and narrow; the fore foot retained a small fifth digit; the feet, especially the hinder one, had a resemblance to those of the contemporary horses (_†Mesohippus_), though the median digit was not so much enlarged, nor the lateral ones so far reduced. It is highly probable that, had this family persisted till the Pleistocene, instead of dying out in the lower Oligocene, it would have eventually terminated in monodactyl forms.

The †lophiodonts of the Eocene are represented by very fragmentary material; so far as that material goes, it does not show much change from the White River genus, except that the premolar teeth were smaller and simpler, the limbs and feet retaining the same characteristics of length and slenderness. The Wasatch genus (_†Heptodon_) had a similar lightness of limb and narrowness of feet, these characters thus appearing at the very beginning of the family history, so far as their North American career is concerned.

5. _Rhinocerotidæ. True Rhinoceroses_

The history of the great group of rhinoceroses and rhinoceros-like animals is a very long and complicated one, inferior in its completeness only to that of the horses. The complexity of the story arises from the large number of phyla into which the families are divisible, and, despite the great wealth of material and the admirable preservation of much of it, it is extremely difficult to find a clew through the mazes of this labyrinthine genealogy. From the standpoint of the existing geographical distribution of animals, few mammals could seem more foreign and exotic to North American life than do the rhinoceroses, and yet for a very long time that continent was one of the chief areas of their development, so far, at least, as that development can be followed. It is even probable, though not clearly demonstrable, that the family originated here and subsequently spread to the Old World, but not to South America, where no member of it has ever been found. The later history of the rhinoceroses ran its course in the Old World entirely, and the highest specializations within the family are to be found there; in North America these animals are not known to have persisted beyond the lower Pliocene, and if they did survive, it was only as a few stragglers in out of the way places.

The modern rhinoceroses are restricted to Africa, southern Asia and some of the larger Malay islands, Borneo, Sumatra and Java, and within these wide geographical limits are to be found the terminal representatives of at least three separate and quite distinct phyla, the African, Indian and Sumatran genera respectively (_Opsiceros_, _Rhinoceros_, _Dicerorhinus_). It will be advisable to begin the study of this peculiarly interesting family with a brief examination of its modern members, even though none of these are found in the western hemisphere.

[Illustration: FIG. 172.—Skull of the Javan Rhinoceros (_R. sondaicus_). Note the single upper incisor, and the rough surface on the nasal bones for the attachment of the single horn.]

All the existing rhinoceroses are large and massive animals, ranging from four feet to six feet six inches in height at the shoulder, and all have solid dermal horns, except in most females of the Javan species[6] (_R. sondaicus_). The Indian and Javan species have a single horn on the nose, while those of Africa and Sumatra have, in addition to the nasal horn, a second one on the forehead. The horns, thus, do not form a transverse pair, but are placed in the median line of the head, one behind the other; it should also be noted that these horns are solid, dermal structures, made up of agglutinated fibres or hairs and not having a bony core formed by outgrowths of the skull, as do the horns of most ruminants, such as oxen, sheep and antelopes, which are therefore called “hollow-horned” (Cavicornia). The skull, however, betrays the presence of horns by the extremely rough areas which serve for their attachment and thus the presence or absence of these weapons may be readily determined in the case of an extinct species of which only the skeleton remains. The skin is very thick and coarse, typically “pachydermatous,” and is quite naked in most of the species; but in the Sumatran form there is a sparse coat of hair, which is quite thick in the young animal. In the Indian _Rhinoceros unicornis_ the enormously thick skin has conspicuous and regularly arranged folds, which make the creature look as though encased in armour; the ears and tail are tufted with hair. In the African and Sumatran genera the folds are obscurely marked and not definitely arranged, giving the body a smoother appearance. All the existing species, except one, are browsers and feed upon leaves and twigs, and they frequent forests and marshes where their food is abundant. Not that these and other browsing animals do not occasionally eat grass, but it is not their principal diet. The exception noted is the largest of all the living species, the Broad-Lipped Rhinoceros (erroneously called “White”) of Africa, _Opsiceros simus_, which is strictly a grazing animal and therefore frequents more open country than the other African species, _O. bicornis_.

There are considerable differences in proportions and general appearance among the various species, but they all have short necks, very long and massive bodies, short and heavy limbs and short, columnar feet, which look much like those of elephants, but have only three toes each. In all but two of the living species the upper lip is prehensile and characteristically pointed and can be used to pick up very small objects, like the “finger” on an elephant’s trunk; in the Sumatran species (_Dicerorhinus sumatrensis_) the lip, though pointed, is horny and inflexible, while in the African _O. simus_ it is broad and straight-edged.

The teeth of the modern rhinoceroses are extremely characteristic and may always be recognized at a glance. In the African genus (_Opsiceros_) there are no front teeth, all the incisors and canines being lost; the other genera have on each side a single large and trenchant upper incisor, in shape like a broad, obliquely edged chisel, which shears against a still larger elongate and tusk-like lower incisor, that is procumbent and points directly forward. The Indian Rhinoceros (_R. unicornis_) is said to use its tusks as weapons in very much the same fashion as the Wild Boar. Between the large lower tusks there is a pair of very small incisors, which can have little or no functional value; the third lower incisor has been suppressed, as have also the canines of both jaws. The dental formula then is: _i_ 1/2 or 0/0, _c_ 0/0, _p_ 4/4, _m_ 3/3, × 2 = 28 or 34 (see p. 93). The premolars, except the first, though somewhat smaller than the molars, have essentially the same pattern. The upper molars have moderately high crowns, yet they are purely brachyodont, except in the grazing, broad-lipped African species (_O. simus_), in which they may fairly be called hypsodont. The external wall of the tooth is broad and nearly smooth, not divided into cusps, as it is in the horses and tapirs, and the two transverse crests, which in the tapirs are directly transverse, are very oblique. In all the existing species additional complications are given by the short spurs, which project inward from the outer wall or from the transverse crests. The lower molars are formed each of two crescents, one behind the other, but their arms or horns are angulate, not curved as they are in other perissodactyls which have crescentic lower teeth.

The upper surface of the skull is very concave in the antero-posterior direction and very broad over the cranium, where there is no sagittal crest. The nasal bones are immensely thick and strongly arched, with the convexity upward; both this arching of the nasals and the fore-and-aft concavity of the skull are devices for giving a strong and solid attachment to the great nasal horn, for the attachment of which these bones have an extremely rough surface, and in the two-horned species, a second roughened area on the forehead marks the place of attachment of the frontal horn. The bones of the cranium are very thick, but lightened by the many chambers which traverse them. The articulation of the lower jaw with the skull is in some respects unique among mammals; the postglenoid process is a long spike, which fits inside of a bony lump (the _postcotyloid process_) behind the condyle of the lower jaw, and the posterior margin of the latter is greatly thickened. The neck is short and stout, the trunk very long, broad and deep, the long and strongly arched ribs and the widely expanded hip-bones providing space for the great mass of viscera. The bones of the limbs are short and very massive; the humerus has a very prominent deltoid ridge and the femur an unusually large third trochanter; the bones of the fore-arm and lower leg are separate, as in the massive ungulates generally. The foot-bones are likewise extremely short and heavy, and the number of digits is three in each foot. Each of the five or more existing species has its skeletal peculiarities, every portion of the bony structure showing characteristic features; but these are only minor modifications of the general plan and may be neglected in any comprehensive account of the living representatives of the family.

[Illustration: FIG. 173.—Left manus of Indian Rhinoceros (_R. unicornis_).]

In order to find any American members of this family, it is necessary to go back to the lower Pliocene, where a great abundance of them is encountered, representing, according to Osborn’s view, four or five phyla; and just as in the case of the horses of the same formation, they were an assemblage curiously made up of progressive and old-fashioned, conservative genera,—some were persistent native stocks, others the descendants of immigrants from the Old World, which reached America in the middle Miocene. There was great variety of form, size and proportions among these animals, North America at that time having a larger number of genera and species than Africa and Asia combined have now. Some were quite small, some large, though none equalled the larger modern species. Some of the genera had relatively long legs, but in one genus, _†Teleoceras_ (Fig. 125, p. 230), an Old World type, they were most grotesquely short, the belly almost touching the ground, as in a hippopotamus. Most of these rhinoceroses were hornless, but _†Teleoceras_ had a small horn on the very tip of the nose. In consequence of the lack of horns, the nasal bones were thin and weak, in marked contrast to the massive, convex nasals of the modern species, and, for the same reason, the upper profile of the skull was nearly straight. Except for minor details, the dentition was in very nearly the modern stage of development; there was a single trenchant upper incisor on each side, a procumbent lower tusk and between the tusks a pair of small incisors; the other incisors and the canines were already lost. One genus (_†Peraceras_) had lost all the upper front teeth. The grinding teeth had the same character as in the existing species, but were somewhat simpler, owing to less development of the accessory spurs. In the more progressive types the teeth were rather high-crowned, though in none were they actually hypsodont; while the persistent ancient genera had teeth with much lower crowns.

Aside from the differences in the skull, which are obviously to be correlated with the absence or very small size of the horn, the skeleton in these Pliocene genera differed but little from the type common to the existing rhinoceroses, and in all the species the feet were three-toed. In short, the dentition and skeleton, except the skull, had already attained to substantially the modern conditions. While the Old World at that time had both horned and hornless rhinoceroses in abundance, none of the genera with large and fully developed horns ever migrated to the western hemisphere. This is the more remarkable in that the great †Woolly Rhinoceros (_Opsiceros †antiquitatis_) of the Pleistocene, which had two very large horns, inhabited Siberia with the †Mammoth (_Elephas †primigenius_). The latter extended its range through Alaska and the northern United States, but the rhinoceros, for some unknown reason, did not accompany it in its eastward wanderings.

The rhinoceroses of the upper Miocene did not differ sufficiently from those of the lower Pliocene to call for particular attention. Needless to say, there were differences between the species of the two epochs, but in such a sketch as this only the broader and more obvious changes can be taken into account. Even in the middle Miocene the only feature which calls for notice was the first appearance in North America of the Old World genus _†Teleoceras_, which became so abundant in the upper Miocene and lower Pliocene. The middle Miocene species (_†T. medicornutus_) would seem to have been descended from _†T. aurelianensis_ of the lower Miocene of France; the two species agreed not only in having a small horn on the tip of the nose, but also in the presence of a still smaller one on the forehead.

In the lower Miocene but two phyla of rhinoceroses have been found, both of which were the comparatively little changed descendants of Oligocene ancestors; and there was thus a notable difference from the rhinoceroses of the middle Miocene and subsequent stages, which were decidedly more modern in character. One of these phyla was constituted by those rhinoceroses (_†Diceratherium_, Fig. 129, p. 239) which had a transversely placed pair of horns on the nose, not one behind the other, as in all of the subsequent two-horned species, of which North America had but the one middle Miocene form (_†T. medicornutus_) mentioned above. The lower Miocene species of _†Diceratherium_ was a very small animal, and smaller than any member of the family from later formations. The †diceratheres originated in North America, and the stages of their development may be clearly made out; they also migrated to the eastern hemisphere and have been found in France, though it is possible that the genus was not truly monophyletic and arose independently in both hemispheres.

The second phylum is that of the hornless forms (_†Cænopus_) which were so abundantly represented in the Oligocene and persisted with little change into the Pliocene.

In the upper Oligocene, or John Day, the †diceratheres are the only rhinoceroses certainly yet obtained, and of these there were several species, large and small. The hornless forms may have been present in Oregon, but this has not been clearly demonstrated. That they continued to exist somewhere during that stage is hardly open to question, for they reappeared in the lower Miocene.

From the White River, or lower Oligocene, many well-preserved rhinoceroses, including complete skeletons, have been gathered in the various collections and display very interesting differences in the three substages of the White River beds. In the uppermost substage is found the apparent beginning of the †dicerathere phylum, though it may be traced back to the middle substage; the nasal bones had become much thickened so as to serve as a support for the horns, and these are indicated by a small, but very rough, area on the outer side of each nasal. Comparing this White River species with those of the upper Oligocene and lower Miocene, two differences may be observed: in the later species the horn-supports were well defined bony knobs or prominences, and these knobs were close to the anterior ends of the nasals; while in the White River animal the places for the attachment of the horns were mere roughened areas, and these were well behind the tips of the nasals. This is not an infrequent sort of change, that horns should shift their position forward or that the portion of the nasals in front of the horns should be shortened. Parallel changes occurred among the †titanotheres.

In the middle White River all the rhinoceroses were hornless, but the same two phyla may be distinguished; the actual starting point of the †diceratheres had no indication of the nasal horns, but may be identified as such by their close resemblance in other respects to the species of the upper substage in which the incipient horns appeared. Much commoner were the members of the typical hornless line (see Fig. 135, p. 256), which, though true and unmistakable rhinoceroses, were yet far removed in many details of structure from the progressive genera of the middle and upper Miocene. There are several species in this phylum, which constitute a series of diminishing size almost in proportion to their increasing antiquity. The dentition was already thoroughly and characteristically rhinoceros-like, but a more primitive feature was the presence of a second upper incisor, a small tooth placed behind the trenchant one, making the incisor formula 2/2; the third incisor and the canines of both jaws were already lost. The assumption of the molar-pattern by the premolars varied much in degree of completeness in the different species; the upper molars, while having all the essentials of the rhinocerotic plan of structure, had a much less complex appearance than in the Recent genera, because of the absence of the accessory spurs; and all the grinding teeth were very low-crowned, in strong contrast to the high-crowned (yet not properly hypsodont) teeth of the middle Miocene and subsequent genera.

[Illustration: FIG. 174.—Skull of †hornless rhinoceros (_†Cænopus tridactylus_); middle White River stage. (After Osborn.)]

[Illustration: FIG. 175.—Second upper molar, left side, of _†Cænopus_, showing the masticating surface.]

As already mentioned, there was much variation in size among the species, but none was as large as those of the Miocene and Pliocene genera, not to mention the enormous animals of the Pleistocene and Recent epochs in the Old World. The commoner species of the middle White River substage (_†Cænopus occidentalis_) was an animal nearly equalling in size the American Tapir (_T. terrestris_) and quite like that species in its proportions, the limbs being relatively longer and less heavy and the feet narrower than in the rhinoceroses of the subsequent geological epochs. The skull, being hornless, had thin, pointed and nearly flat nasal bones, an almost straight and horizontal upper profile, and a short and low, but distinct, sagittal crest; the cranial bones were quite thin, there being no extensive development of sinuses within them. The articulation of the lower jaw with the skull was only beginning to take on the characteristic peculiarities seen in the later genera, and the hinder margin of the lower jaw was not much thickened. Thus, many of the features which distinguish the skull in all Recent and Pleistocene and most Pliocene, and upper and middle Miocene rhinoceroses were entirely lacking in _†Cænopus_, yet no anatomist could doubt that the White River animal was a genuine rhinoceros.

The neck was short, but not very heavy, the trunk elongate, but not massive, the ribs not being inordinately long nor strongly arched, and the hip-bones so little expanded that they were tapiroid rather than rhinocerotic in appearance. The limb-bones were relatively much more slender than in any existing species, and, although every one of them was characteristically that of a rhinoceros, yet the comparative lightness of body and slenderness of limb gave to these bones a certain resemblance to those of tapirs. The feet, which were moderately elongate and rather narrow, were three-toed, as in all subsequent North American species and in all existing members of the family.

The most ancient and primitive representative of the true rhinoceroses so far discovered occurs in the lowest division of the White River beds and is of particular interest as throwing light upon the origin of the family. The genus (_†Trigonias_) differed from that (_†Cænopus_) which was so abundant in the middle White River substage in several highly significant particulars, though on a merely casual inspection one might easily be misled into thinking that the two animals were nearly identical, for _†Trigonias_ was an undoubted rhinoceros. Such an identification, however, would be a great mistake, for the differences, though not striking, are very important. In the upper jaw the first or anterior incisor had already assumed the characteristic trenchant, chisel-like shape, but two other incisors were present also, thus bringing the number up to the original three, common to all early perissodactyls; even more interesting is the presence of a small upper canine. The lower jaw likewise had three incisors on each side, the first and third small, the second enlarged and tusk-like, but the canine had already been suppressed, and thus the dental formula was: _i_ 3/3, _c_ 1/0, _p_ 4/4, _m_ 3/3, × 2 = 42, or 14 more than the formula of the existing African species. The premolars were smaller and less complex than the molars.

[Illustration: FIG. 176.—Skull of _†Trigonias osborni_, lower White River. (After Hatcher.)]

[Illustration: FIG. 177.—Anterior end of right upper jaw of _†Trigonias osborni_ (after Lucas). _c._, canine. _i 3_, external incisor. _i 2_, middle incisor. _i 1_, first incisor.]

From this ancient genus may readily be inferred the steps by which the peculiar characters of the anterior teeth in the true rhinoceroses were attained. The first stage was undoubtedly an animal in which, as in all other Eocene perissodactyls, there were three well-developed incisors on each side of both jaws, 12 in all, and moderately prominent canine tusks; all these teeth were erect. The second stage was the enlargement of the first upper and second lower incisors, the latter becoming less erect and beginning to assume the recumbent position; at the same time the other incisors and the canines were reduced in size and were so little used that they lost their functional importance. The third stage, in which the first and second lower incisors were horizontal and pointed directly forward, and the first upper and second lower teeth were still further enlarged, the non-functional teeth reduced in size and the lower canine suppressed, was realized in the genus _†Trigonias_. There were thus but two hypothetical stages between this lower White Region genus and the tapir-like forms of the middle Eocene, so far, at least, as the anterior teeth are concerned.

[Illustration: FIG. 178.—Anterior end of left upper jaw of _†Cænopus_, _A_, adult; _B_, immature animal (after Osborn). _I 1_, first incisor; _I 2_, second incisor; _C_, canine.]

The skeleton of _†Trigonias_ was, on the whole, very much like that of the succeeding genus, _†Cænopus_, of the middle substage of the White River, but with the important exception that the front foot had four digits instead of three. The pollex, or first of the original five, almost always the first to disappear, had been suppressed, the third or median digit was already the largest of the series, both in length and breadth; the second and fourth, somewhat shorter together made a symmetrical pair, while the fifth, though much the most slender of all, was still functional and had retained all of its parts. In the hind foot the digits had been reduced to three. This arrangement, four toes in the manus and three in the pes, is the same as is found in the existing tapirs and in the Eocene perissodactyls generally, with only two or three known exceptions. In the Oligocene, on the other hand, all the genera except the †titanotheres, tapirs, †lophiodonts and †amynodonts were tridactyl both before and behind.

[Illustration: FIG. 179.—Left manus of _†Trigonias osborni_. (After Hatcher.)]

With _†Trigonias_ the definitely known history of the true rhinoceroses breaks off abruptly, and it is possible that that genus was an immigrant, though it is perhaps more likely that its ancestors existed in the upper and middle Eocene (Uinta and Bridger stages) of North America. Some fragmentary specimens from the Uinta beds, too imperfect for any definitive identification, are an encouragement to hope that the forerunner and direct ancestor of _†Trigonias_ may yet be discovered in that formation. It is also quite possible that one of the larger species of the genus _†Hyrachyus_, so abundant in the Bridger and going back to the Wind River, may take its place in the same series.

6. _†Hyracodontidæ. †Cursorial and †Aquatic Rhinoceroses_

The luxuriant diversification of the rhinoceros-stem was not exhausted by the many phyla of what we have called the true rhinoceroses. Two other series, very distinctly marked and rather distantly connected with the first, are yet to be considered. These two series, the †hyracodonts (in the narrow sense) and the †amynodonts, ran courses which, in certain respects, were singularly alike; both were of North American origin and one, the †hyracodonts, was entirely confined to that continent, while the other sent out late migrants, which entered Europe, no doubt through Asia, and both ended their careers before the close of the White River time. Their history was thus a brief one when compared with that of the true rhinoceroses, three phyla of which persist to the present day, though their geographical range is greatly restricted in comparison with what it was in the Miocene and Pliocene, when they ranged over every continent except Australia and South America.

Just how to classify these three series of rhinoceroses and rhinoceros-like animals, so as most accurately to express their mutual relationships, is a question that has received several answers. One method suggested is to include them all in a single family and to make a subfamily for each of the three well-distinguished series; this is the arrangement which personally I should prefer. A second plan is to accord family rank to each of the three groups; while the most elaborate scheme, that of Professor Osborn, is as follows: for the rhinoceroses, in the broader sense, he makes two families, the Rhinocerotidæ and the †Hyracodontidæ, and divides the former into four subfamilies, which include all of the true rhinoceroses, living and extinct, of the Old and New Worlds, and the latter into two subfamilies, the †Hyracodontinæ and †Amynodontinæ. It is not a matter of very great moment as to which of these three schemes is followed, and I shall therefore adopt the one proposed by Professor Osborn, in order to avoid, so far as possible, the confusing effect of different methods of classification.

[Illustration: FIG. 180.—†Cursorial rhinoceros (_†Hyracodon nebrascensis_), White River stage. Restored from a skeleton in the Museum of Princeton University.]

As before mentioned, the subfamily of the †hyracodonts (†Hyracodontinæ) became extinct in White River times, during most of which it was represented by the single genus _†Hyracodon_, whence are derived the names for the family and subfamily. The series was purely North American, and no member of it has ever been found in any other continent. The species of _†Hyracodon_ were altogether different in appearance and proportions from the true rhinoceroses, being lightly built, slender, cursorial creatures, suggestive rather of horses than of rhinoceroses, to which they bore much the same relation as the slender-limbed, narrow-footed †lophiodonts did to the tapirs (_see_ p. 326); in size, they were somewhat taller and considerably heavier than a sheep.

The low-crowned grinding teeth had the unmistakable rhinoceros-pattern, and between them and the teeth of the contemporary _†Cænopus_ the difference was merely one of size, except for one small, but not insignificant feature. The last upper molar had not perfectly acquired the triangular form characteristic of all the true rhinoceroses, caused by the complete fusion of the outer wall with the posterior crest, but the wall projected a little behind the crest, as in perissodactyls generally. Premolars (except the first) and molars were alike in structure and of nearly the same size. While the grinding teeth were thus hardly to be distinguished from those of the true rhinoceroses, the anterior teeth, incisors and canines, were totally different; they were very small and had simple, pointed and slightly recurved crowns, and were all very much alike in size and form. Thus, there were in the front of the mouth eight small, hook-like teeth, above and below, which were obviously quite useless as weapons; and as the skull had no horn, the animal was defenceless, and must have depended entirely upon speed for its safety from the attacks of the larger and more powerful beasts of prey.

The skull was short, deep and thick, and the head must have been heavy and clumsy, quite out of keeping with the body and limbs. The neck was surprisingly long, longer indeed proportionately than in the contemporary genus of horses (_†Mesohippus_), but the neck-vertebræ were relatively stout and strong, as was required for the muscles to move and control the heavy head. The body was rather elongate, but not deep or massive, and the limbs were proportionately much longer than in any of the known rhinoceroses. The limb-bones, one and all, despite their length and slenderness, bore an unquestionable likeness to those of the true rhinoceroses. In this elongation of the limbs the fore-arm and thigh were the parts most affected, and the slenderness, though in notable contrast to the proportions both of the true rhinoceroses and the †amynodonts, was yet much less marked than in the middle Eocene representatives of the †hyracodonts themselves. The feet were long and narrow, approximating, though not actually attaining the proportions of the feet in the White River horses (_†Mesohippus_). There were three digits in each foot, and the median toe (third of the original five) was so much enlarged and the lateral toes (second and fourth) so reduced, though still functional, as strongly to suggest a monodactyl foot as the outcome of this course of development, had not the early extinction of the subfamily put an end to it. It is interesting to reflect that, had the †lophiodonts and †hyracodonts continued their existence to the present time and had persisted in advancing along their particular lines of specialization, we should, in all probability, have had monodactyl tapirs and rhinoceroses, as well as horses.

[Illustration: FIG. 181.—Left manus of †cursorial rhinoceroses. _A_, _†Triplopus cubitalis_ (after Cope), upper Bridger. _B_, _†Hyracodon nebrascensis_, White River.]

As in the case of so many other mammalian series, the †hyracodonts of the but partially explored Uinta formation are still very imperfectly known. Almost all that can be positively stated about them is that they were smaller than their White River successors and that the assumption of the molar-pattern by the premolars was incomplete. In the upper Bridger beds also not very much is known regarding the then representatives of the series, (_†Triplopus_). So much is clear, however, that they were still smaller and lighter animals, that the limbs were very light, and that the number of digits in the fore foot had already been reduced to three, the only known Bridger perissodactyl of which this is true, all the others having four digits in the manus and three in the pes.

[Illustration: FIG. 182.—Primitive †cursorial rhinoceros (_†Hyrachyus eximius_), lower Bridger. Restored from a skeleton in the American Museum of Natural History.]

In the middle and lower Bridger, and even in the Wind River, occurs a genus (_†Hyrachyus_) which contained a large number of species, ranging in size from a full-grown modern tapir to creatures no larger than foxes. It is among these smaller species that the most ancient member of the †hyracodont line is to be sought, though it is not yet practicable to select any particular one. _†Hyrachyus_, indeed, may very possibly have contained among its many species the ancestors of all three lines of the rhinoceroses and rhinoceros-like animals, and thus formed the starting point from which they developed in diverging series. It is always a very significant fact when two or more groups approach one another the more closely, the farther back in time they are traced, because that can only be interpreted to mean that ultimately they converged into a common term, even though that common ancestor should elude discovery.

_†Hyrachyus_ may be described as a generalized, relatively undifferentiated perissodactyl, from which almost any other family of the order, except the horses and the †titanotheres, might have been derived. The incisors, present in undiminished number, were well developed and functional, but not large, and the canines were moderately enlarged, forming small tusks. The premolars were all smaller and less complex than the molars, which had a strong resemblance to those of the tapirs; in the lower jaw they were identical with the latter, but in the upper jaw there was more than a suggestion of likeness to the rhinoceroses. The skull was long, narrow and low, hornless, and with thin, slender nasals and straight, horizontal upper contour. The neck was short, the body very long and the limbs of medium length and weight; though relatively stouter than in _†Triplopus_ of the upper Bridger and Uinta beds, they cannot be called heavy. The feet were not especially elongate and rather slender; the manus had four toes and the pes three.

[Illustration: FIG. 183.—Skull of _†Hyrachyus_. (After Osborn.)]

A brief and short-lived branch of this stock existed in the Bridger stage, but was not, so far as is known, represented in any of the subsequent stages, and was made up of a single genus (_†Colonoceras_) which had a small pair of dermal horns upon the nasal bones. In other respects, it was like _†Hyrachyus_. It is surprising to find that the horned series should have so speedily died out, while the defenceless forms not only persisted, but actually became more defenceless through the reduction of the canine tusks. _À priori_, one would have expected the opposite result, but the key to the enigma is probably to be found in the more perfect adaptation of the surviving kinds to swift running.

The second subdivision (†Amynodontinæ) of this family contains a series of animals which developed in a very divergent fashion and went to quite the opposite extreme from the cursorial †hyracodonts, resembling the latter (aside from the fundamental characteristics common to all rhinoceroses, in the broadest sense of that term) only in the pattern of the molar teeth and in the absence of horns. The terminal member of the †amynodont series was a White River genus (_†Metamynodon_) of which the remains have been found almost exclusively in the consolidated and cemented sands filling the old river-channels of the middle substage of the White River beds. This fact, together with certain structural features of the skull and skeleton, leads at once to the suggestion that these animals were chiefly aquatic in their habits and somewhat like hippopotamuses in mode of life. _†Metamynodon_ was quite a large animal, the heaviest and most massive creature of its time, after the disappearance of the giant †titanotheres, but was low and short-legged.

The true rhinoceroses, save those which, like the existing African species, have lost all the front teeth, all agree in the peculiar differentiation of the incisors, which was fully described in the preceding section of this chapter. The †hyracodonts had a second scheme, the incisors and canines being all similar in shape, small, pointed and recurved, while still a third mode of development was exemplified by the †amynodonts, in which the canines became large and formidable tusks, a very notable difference from all other rhinoceroses whatever.

[Illustration: FIG. 184.—Supposedly †aquatic rhinoceros (_†Metamynodon planifrons_) of the White River. Restored from a skeleton in the American Museum.]

In _†Metamynodon_ the incisors were not enlarged, but were unreduced and functional; the upper canine was a short, heavy tusk, obliquely truncated by the abrasion of the lower tusk, which was very large. Another striking difference from all the other groups of rhinoceroses was the reduction of the premolar teeth, which, instead of equalling the molars in size, were much smaller and were diminished to three in the upper, two in the lower jaw. The molars were of the characteristic rhinoceros-pattern, but were very narrow, especially the inferior ones, in which the enamel did not surround the whole crown, as it normally does, but was lacking along vertical bands, where the dentine formed the surface. The skull was extremely peculiar and, with its very long and high sagittal crest and immensely expanded and heavy zygomatic arches, had a surprising likeness to the skull of some great beast of prey. The face was very much shortened and the skull depressed, so that the head was remarkably low, broad and flat, proportions which did not recur in any other group of rhinoceroses. The neck was short, the body very long and very massive, as is shown by the long and strongly arched ribs. The limbs were short and stout and the feet quite primitive in character, the front foot retaining four fully developed and functional digits. No other perissodactyls of the middle White River beds, except the †lophiodonts and tapirs, had more than three digits in the manus, and thus _†Metamyodon_ was a belated exception to the general rule.

The Uinta member of this series was _†Amynodon_, a similar but smaller and lighter animal. The canine tusks were of more moderate size and none of the premolars had been lost, but were considerably smaller than the molars, and the last two had assumed the molar-pattern. The face was not conspicuously shortened and the zygomatic arches of the skull were not so heavy or so widely expanded as in the White River genus, and the skeleton was less massive.

The genus _†Amynodon_ is also represented in the upper Bridger beds, but by a species different from that of the Uinta stage. This more ancient species was a smaller animal than its upper Eocene successor and had less enlarged canine tusks, but it already possessed the typical rhinoceros molar teeth, the only Bridger mammal of which this is true. Beyond this species the line, as at present understood, cannot be traced, though probably some species of _†Hyrachyus_, or an allied genus, will prove to be the ancestor sought; but the connecting link has not yet been brought to light.

The history of the rhinoceroses and rhinoceros-like animals, of which a very much simplified sketch has just been given, is a highly complex one, much more so than that of the horses, †titanotheres, or tapirs, and is less fully recorded, the earlier chapters of the story being still missing. However, in the progress of discovery these chapters will almost certainly be recovered, and it is already possible to draw close inferences as to what they will reveal. The complexity of the history is chiefly due to the fact that, as compared with the other perissodactyl groups, the rhinoceros stem ramified more widely and gave rise to more divergent and diversified forms. At one extreme, we find huge, massive, slow-moving types; and, at the other, light, slender, cursorial creatures, almost horse-like in appearance, with intermediate forms of moderate size. Some were long and others short legged, mostly adapted to terrestrial life, but some with aquatic habits. The three very different sorts of modification which the anterior teeth (incisors and canines) underwent in the three principal series may be taken as an illustration of this divergent development, and to these may be added a fourth, the complete suppression of all the incisors and canines above and below, as is exemplified by the modern African species.

Of the three rhinoceros groups, whatever rank be assigned them, family or subfamily, much the most prolific in divergent forms was that of the true rhinoceroses (Rhinocerotidæ) of which seven or more phyla have been distinguished, three of them surviving to the present time. Only in this series were horns frequently present, the brief experiment, as it might be called, of the Bridger genus _†Colonoceras_, being the only known instances of horns among the †hyracodonts, and the †amynodonts were all hornless. In making the comparison as to degree of ramification among the three series, it should be borne in mind that the true rhinoceroses were the only long-lived group, the other two dying out before or at the end of the White River stage. Within the series or family of the true rhinoceroses, there was no great divergence of type, and all the members were much alike, heavy and slow animals, but with very great variety in the details of structure. Take, for instance, the matter of horns; we find both hornless and horned genera, the former preceding the latter in time, but, so far as North America is concerned, continuing in association with them till the end. Among the horned genera, the horn may be single, double in a transverse pair (_†Diceratherium_) or arranged one behind the other in the median line of the head (_Dicerorhinus_, _Opsiceros_, etc.). The single horn may be on the nose or the forehead; if on the nose, it may be on the upper side of the nasal bones (_Rhinoceros_) or on the extreme tip and pointing obliquely forward (_†Teleoceras_). The single frontal horn was much less common, but in the extraordinary _†Elasmotherium_, of the European and Siberian Pleistocene, the horn was of gigantic size and the surface for its attachment an enormous, dome-like boss on the forehead.

All three of the series had their most ancient known representatives in North America, and it seems probable, though by no means certain, that they all originated here by divergence from a common stock, which was represented more or less closely by the genus _†Hyrachyus_ of the Bridger and Wind River stages of the Eocene. However that may be, true rhinoceroses flourished exceedingly in the Old World from the upper Oligocene to the Pleistocene, the events of the latter epoch restricting them to their present range. The significance of the American genera for the ancestry of the modern types can be found only in the most ancient forms, _†Trigonias_ and _†Cænopus_; the subsequent development which led up to the existing species of Asia and Africa went on entirely in the eastern hemisphere. The †hyracodont subfamily had no known representatives outside of North America, but the †amynodonts sent out emigrants, which appeared for a brief time in the Oligocene of Europe.

In the varied history of the rhinoceroses, the principles of evolutionary change which may be deduced from the recorded development of the horses, tapirs and †titanotheres are found to be applicable.

(1) There was the same gradual increase in size from the earlier to the later geological stages. Not that all the phyla kept equal pace in this respect, and even within the same phylum it was the rule rather than the exception to find larger and smaller contemporary species.

(2) In all of the early forms, up to the middle Miocene, the teeth were low-crowned; after that time there was a decided increase in the height of the teeth, though only in _†Elasmotherium_ was the fully hypsodont, cement-covered crown attained. In the existing African Broad-Lipped Rhinoceros (_Opsiceros simus_), which is a grazing animal, the high, cement-covered teeth may also fairly be called hypsodont.

(3) In all of the lines, as in the other perissodactyl families, the premolars gradually took on the pattern of the molars; only in the †amynodonts were the premolars notably reduced in number and size.

(4) The three different modes of development of the anterior teeth, exemplified by the true rhinoceroses, the †hyracodonts and †amynodonts respectively, need not be recapitulated here. It is sufficient to call attention to the fact that the three kinds of modification diverged from a common starting-point such as may be seen in the middle Eocene perissodactyls generally, and that in each series the transformation was gradual.

(5) The modification of the skull followed several different courses, as designated by the major and minor subdivisions of families, subfamilies and phyla. The development of horns, whether single or double, in transverse or longitudinal pairs, was the most important single influence in transforming the skull, as determined by the mechanical adjustment necessary to make these weapons effective, but even in the hornless forms changes went on, and in all the phyla the skull departed more and more widely from the primitive Eocene type in each succeeding geological stage. The most aberrant form of skull was that of the hornless and presumably aquatic _†Metamynodon_, in which the greatly shortened face, high sagittal crest and extremely wide zygomatic arches were altogether exceptional.

(6) When the history of any horned phylum is at all complete, the development of the horns may be followed step by step from the marks which they left upon the skull. As a rule, the story was one of gradual enlargement, but, in one case at least, an incipient horn apparently failed to enlarge and was eventually lost.

(7) In the light, slender and cursorial †hyracodonts the mode of development resembled that of the horses, as appears in the elongation of the neck, limbs and feet, in the enlargement of the median toe and concomitant reduction of the lateral digits. Also, as in the horses, the elongation of the limbs began to be noteworthy while the body-weight was small and was consequently accompanied by great slenderness; as the body-weight increased, the limbs became stouter, to yield the necessary support.

(8) In the phyla composed of massive animals the principle of change agreed with that exemplified by the †titanotheres, increasing body-weight being the determining factor in both cases. When this increase began to be decided, the reduction of digits ceased at the point which had already been reached in any particular series, three in both manus and pes in the true rhinoceroses, four in the manus and three in the pes in the †amynodonts. Very heavy animals require broad, columnar feet to support them, and hence the similarity of appearance in such widely separated groups as elephants, rhinoceroses and hippopotamuses, not to mention several extinct orders and families. Among the larger and heavier rhinoceroses, as in those of the present time, there was great variety in the proportionate lengths of the limbs, body and feet.

In brief, the great complexity of the history of the rhinoceroses is due to the many divergent and parallel phyla into which these animals may be grouped. Broadly speaking, they may be subdivided into the slender, cursorial types and the heavy, slow-moving types, the former developing in a manner similar to that shown by the horses, while the latter were modified after the fashion of the †titanotheres. Obviously the load to be supported by the legs and feet was a very important factor in determining the character of evolutionary change.

II. †ANCYLOPODA. †CLAWED PERISSODACTYLS

The very extraordinary and aberrant animals which are referable to this suborder have been understood only since the year 1888, for, as was shown in an earlier chapter (p. 41) their scattered parts had been assigned to two different mammalian orders, the skull to the perissodactyls and the feet to the pangolins, or scaly anteaters (Pholidota) of the Old World, since it occurred to no one that the same animal could have such a skull and teeth in combination with such feet.

The history of the Ancylopoda is still very incomplete, only four genera, of the lower Pliocene, middle and lower Miocene, and the middle Eocene respectively, being at all adequately known, but even in this imperfect form the story is worth telling. The suborder was probably of American origin and its most ancient known member existed in the middle Eocene. Both in Europe and North America the group persisted into the lower Pliocene and it is believed, though not clearly demonstrated, that in eastern Asia it continued even into the Pleistocene. All the genera of the suborder may be included in a single family.

7. _†Chalicotheriidæ. †Chalicotheres_

The specimens which so far have been found in the American middle and upper Miocene and lower Pliocene are very fragmentary, consisting of little more than teeth, and give no information other than to demonstrate the presence of the family in North America during that period of time. On the other hand, the European genera of the middle Miocene and lower Pliocene are well known and may or may not have been closely similar to their American contemporaries, though they were undoubtedly larger. In these most peculiar and grotesque animals (_†Macrotherium_ and _†Chalicotherium_) the head was relatively small, the teeth were very low-crowned and adapted only to a diet of soft vegetable substances and the mode of feeding must have been that of browsing upon leaves and shoots of trees and bushes; the premolars had not acquired the molar-pattern, which was very exceptional for perissodactyls of so late a time, such a difference between the two classes of teeth being characteristic of the Eocene members of the order; the incisors and canines were reduced, but the formula is not definitely known.

The neck was of moderate length, the body very long, and the limbs were also elongate, especially the anterior pair, in consequence of which the back sloped downward from the shoulders to the rump; the two fore-arm bones were fused together, and these, with the thigh-bones, were the longest segments of the limbs. The special peculiarity of these animals was in the character of the feet, which had three toes, each armed with a huge claw, instead of terminating in a hoof, as it does in all normal perissodactyls. The external digit, which, in the absence of the fifth, was the fourth, was the largest of the series and apparently bore the most of the weight, a notable departure from the normal perissodactyl symmetry, in which the third or median toe is the largest. The hind feet were considerably smaller than the fore, but had similar claws.

Many suggestions have been offered as to the manner in which these great claws were employed. The teeth demonstrate that these animals could not have had predaceous habits, but must have been inoffensive plant-feeders. As no such herbivorous creatures are living now, it is impossible to reach a definitive solution of the problem, which is further complicated by the fact that in two other orders of hoofed mammals, Artiodactyla and †Toxodontia, a more or less similar transformation of hoofs into claws took place, and among the edentates the large, herbivorous †ground-sloths (†Gravigrada) had enormous claws. It is inadmissible to suppose that these great †chalicotheres could have been burrowers, or tree-climbers, or that they pursued and slaughtered prey of any kind, for, aside from the character of the teeth, such heavy and slow-moving beasts would have been utterly inefficient at work of that sort. No doubt, the claws were used, to some extent, as weapons of defence, as the existing South American Ant-Bear (_Myrmecophaga jubata_) uses his formidable claws; probably also some, if not all, of these clawed ungulates would employ the fore feet in digging for roots and tubers, as is done by the bears generally. Many years ago, the late Sir Richard Owen suggested with reference to the †ground-sloths that the principal use of the fore feet, other than that of locomotion, was to draw down within reach of the long tongue and prehensile upper lip the branches upon which they browsed. This explanation may perhaps be applicable to all of these aberrant and exceptional groups of hoofed animals.

[Illustration: FIG. 185.—Left manus of lower Miocene †chalicothere (_†Moropus_). (After Peterson.)]

In the lower Miocene (Arikaree stage) of North America well-nigh complete skeletons of a large †chalicothere (_†Moropus_, Fig. 130, p. 240) have been obtained, an animal which considerably exceeded a large horse in bulk and stature. In structure this genus had departed less widely from the normal perissodactyl type than the genera of the European Miocene and Pliocene above described and was in many respects more primitive. It could not, however, have been directly ancestral to the European forms, though indicating in a general sort of way what the ancestral type must have been. _†Moropus_ had a relatively small, slender and pointed head, a long neck, much longer than in the European genera, and long fore legs; the shorter hind legs gave the back a steep inclination from the shoulders to the rump. The proportions of the head, neck and limbs suggest those of a giraffe, in less exaggerated form, but the likeness is more marked in the skeleton than in the restoration and is at best a distant one. The feet were armed with the great claws characteristic of the suborder, but the fore foot, in addition to the three functional toes, had a long splint, representing the rudimentary fifth digit; of the first, or pollex, no trace remained. The perissodactyl plan of symmetry had not yet been lost, the third or median digit being the longest of the series. In the hind foot, which had only three toes, the departure from the perissodactyl arrangement had already begun, and the third and fourth digits (_i.e._ of the original five) were of nearly equal size, both in length and thickness, while the second was smaller.

The family is represented in the John Day, or upper Oligocene, by specimens which are sufficiently characteristic to prove that they are properly referable to this group. They have been assigned to the same genus as that of the lower Miocene, but whether the identification is justified remains to be determined.

In the lower White River beds of Canada is found a much smaller animal of this family, but the material is too fragmentary for generic identification. Something more is known of a genus (_†Schizotherium_) from the European Oligocene, likewise much smaller than the Miocene and Pliocene forms, which had four, or possibly even five, functional digits, in the manus, but it has not been ascertained whether the transformation of hoofs into claws had already taken place.

It is not yet practicable to determine the relationships of the European and American †chalicotheres to one another, because of the imperfect nature of most of the material.

The molar teeth of the †chalicotheres were suggestively like those of the †titanotheres, and, were the teeth alone to be taken into account, no one could hesitate to regard the two families as closely related.

The most ancient known member of the family is the genus _†Eomoropus_, from the Bridger Eocene, which will be described by Professor Osborn in a paper soon to appear. _†Eomoropus_ was much nearer to the normal perissodactyls than were the genera from the Oligocene and Miocene above described.