A history of land mammals in the western hemisphere

by William Berryman Scott

The neck was of moderate length, sufficiently long to enable the animal to reach the ground with the lips, a necessity in the absence of a proboscis. The body was very long and, as is shown by the length and curvature of the ribs and the great breadth of the hip-bones, extremely bulky. The limbs were very massive, and the long bones had lost the marrow-cavities, being filled with spongy bone, as in the elephants, †titanotheres and most other very heavy mammals. The bones of the fore-arm and lower leg were separate. The hip- and thigh-bones and shin-bones were remarkably elephantine in character and, if found isolated, might readily be referred to some unknown proboscidean, but the bones of the fore limb were quite different from those of the elephants. The feet likewise had a very proboscidean appearance, notwithstanding important and significant deviations in structure; they had the same shortness and massiveness and a similar reduction in the size of the hoofs, and the presence of all five digits added to the resemblance. Undoubtedly, the feet had the same columnar shape and arrangement of elastic pads. The living animal must have had an appearance quite similar to that of a rather small elephant, not exceeding six or seven feet in height at the shoulders and therefore not surpassing the largest modern rhinoceroses, the broad-lipped species of Africa (Opsiceros simus). Of course, the head must be excepted from the comparison, as that was totally unlike the head of any existing creature; with its long and narrow shape, its fantastic protuberances and its lack of a proboscis, it had no suggestion of likeness to any proboscidean. Whether the great body was naked, or clothed with hair, it is of course impossible to determine with confidence, but, all things considered, it seems unlikely that the hair should have been completely lost in any terrestrial mammal at so early a period. As we have seen in the preceding chapters, hairy elephants and rhinoceroses continued into and through the Pleistocene, not only in the cold regions of the north, as is shown by the hair of the American †Mastodon. In the tropics conditions were different, and in that uniformly warm climate the loss of hair by the very large mammals probably took place long before the Pleistocene. At all events, it is a significant fact that no hairless land mammals are now known in any region which has severe winters. It is true that the middle Eocene climate over most of North America was warm-temperate or subtropical, and the †uintatheres may, in consequence, have been hairless, but there is no evidence of this.

Fig. 232.—Skull of †Elachoceras parvum (lower jaw restored). Princeton University Museum.

Within the limits of the †uintathere family, considerable modification and change may be traced, which, as in the case of the Proboscidea, principally affected the skull and the general stature. It is hardly worth while to deal separately with the two or more phyla which may be distinguished, for the differences between them are relatively unimportant. In the uppermost part of the Bridger stage almost the latest representatives of the family are found and the genus (†Eobasileus) was of the largest size. These animals had remarkably long and narrow heads and very large, shovel-shaped nasal protuberances; in the males the upper canine tusks were very long and curved back nearly in a semicircle. In the middle portion of the stage the species of †Uintatherium were somewhat smaller and had shorter, wider and higher heads, the tusks, though well developed, were not quite so long, nor so strongly recurved; in some species they were nearly straight, with “hastate” or spear-head point. In the same horizon is found a third genus (†Elachoceras) which was probably a survival persisting from the lower Bridger, in which none of these animals and little of anything else has yet been found. †Elachoceras was hardly half as large as the common species of †Uintatherium and its skull might be described as a preliminary sketch for that of the latter; the nasal horns were extremely small, or, more probably, entirely absent; the median pair were mere low knobs, hardly an inch in height, and the posterior pair were simply thickenings of the crest which enclosed the top of the cranium on three sides, scarcely rising above it. This crest itself was much less prominent than in †Uintatherium and the basin-like top of the skull, in consequence, very much shallower. The upper incisors and the first premolar had already been lost and the upper canine enlarged into a sabre-like tusk, which, however, was relatively smaller than in the succeeding genera. The grinding teeth were quite the same as in the latter. Unfortunately, the skull of †Elachoceras is the only part of the animal which is known, but, so far as that is concerned, it is precisely what we should expect the forerunner of †Uintatherium to be; an ancestor made to order could hardly be more diagrammatic. It might, of course, be objected that no such relation as that of ancestor and descendant could obtain between these two genera, because they were contemporaries, but the case is like that of the ancestral elephants described in the preceding chapter. †Mœritherium and †Palæomastodon are found together in the Egyptian Oligocene, the former surviving for a considerable time after it had given rise to the latter, and in the upper Eocene only †Mœritherium occurs. Many similar instances might be given, just as grandfathers often live long with their grandchildren.

In the Wind River stage, or upper division of the lower Eocene, lived the still incompletely known †Bathyopsis, of which, however, sufficient material has been obtained to show that it was much less specialized than any of the Bridger genera. This genus comprised animals much smaller than its successor, †Elachoceras of the middle Eocene, being smaller than a tapir; it stood in much the same relation to †Elachoceras as the latter did to †Uintatherium. In the American Museum of Natural History is a highly interesting skull of †Bathyopsis, which will shortly be described by Professor Osborn. The premaxillaries have not been preserved, and it is therefore impossible to say whether the upper incisors had already been suppressed or not, and though the upper canine has not been found, there can be no reasonable doubt that it was a tusk. The lower canine had not yet gone over to the incisor series, but was a thin though large tusk. There was one more lower premolar, four in all, than †Uintatherium possessed, and all the premolars were somewhat smaller and simpler than the molars. The small skull had a broad and somewhat concave cranial roof, with slightly raised enclosing crest, and the horn-like protuberances of the posterior and median pairs were present in an incipient stage. Whether those of the nasal pair were also indicated is not known, but probably they were not. The lower jaw was of very peculiar shape; the flange of the inferior border was not so well defined as in †Uintatherium, but had no hinder margin and rose very gradually backward.

The series of genera in descending order, †Eobasileus, †Uintatherium, †Elachoceras and †Bathyopsis, immediately impresses the observer as being a natural phylogenetic series of successive ancestors and descendants. Unfortunately, only the skull is known in the two last named, but there is no ground for supposing that the discovery of the skeletons would require any alteration in the series as we now have it. No member of this series has yet been found in the Wasatch, but there can be no doubt that it was represented in that stage, for a recent expedition from the American Museum has collected teeth of a †Bathyopsis-like form in still older beds.

SUBORDER †PANTODONTA

During the older part of the lower Eocene the †uintatheres must have been a rare and unimportant element of the fauna, at least in those parts of the continent whose history we know. Their place was taken by another suborder, the †Pantodonta, which was not ancestral to them, but collaterally related and descended from a common ancestry. The largest and most dominating of Wasatch mammals was the genus †Coryphodon, which also occurred in the lower Eocene of Europe, and the species of which ranged in stature from a tapir to an ox, though of much heavier form than the latter. The latest surviving species lived in the Wind River stage as a contemporary of †Bathyopsis, but then the suborder gave way to the †uintatheres.

In †Coryphodon (see [Fig. 142, p. 279]) the number of teeth was unreduced, a fact which is recorded in the name of the suborder, the dental formula typical of all the primitive ungulates being applicable to the genus. This formula was: i 3/3, c 1/1, p 4/4, m 3/3, × 2 = 44. The upper incisors were rather small, but functional, and the canines of both jaws were formidable tusks, though not rivalling in size the great sabres of the †uintatheres; the premolars had a simpler structure than the molars, which resembled those of the †uintatheres in a general way, but not closely. The skull differed greatly from that of the †uintatheres in having no horn-like protuberances, and was relatively large and heavy, the cranium having a broad, flat roof and no sagittal crest, and the lower jaw had no descending flange from the inferior border; in every way this skull was more normal and less bizarre-looking. The neck was proportionately longer than in the †uintatheres, the body long and the tail of medium length; the trunk-vertebræ had surprisingly small and weak spines, perhaps an indication of aquatic habits. The limbs were quite short and very heavy, and the bones, in comparison with those of the †uintatheres, were less proboscidean and more perissodactyl in character. For example, the femur retained the third trochanter and the long bones had marrow-cavities. The feet, on the contrary, were very like those of the †uintatheres, being extremely short and five-toed and with reduced, nodular hoof-bones; even in the details of the wrist and ankle joints there was no important difference between the two groups.