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.

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.

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.

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.

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.

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.

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.

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.