A history of land mammals in the western hemisphere

by William Berryman Scott

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.

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.

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.