A history of land mammals in the western hemisphere

by William Berryman Scott

Fig. 241.—Skull of Adinotherium, top-view, showing the rugosity on the forehead for the small frontal horn.—Princeton University Museum.

Another imperfectly known family, that of the †Notohippidæ, occurred in the Patagonian stage, but was most abundant in the Deseado, where several genera of it have been found. These animals had mostly hypsodont teeth, forming roots in old age, and the teeth were in closed series, but there was no tusk-like enlargement of the incisors. In the later genera, those of the Patagonian stage (†Notohippus, †Argyrohippus), the crowns of the grinding teeth had a thick covering of cement, and those of the lower jaw had some resemblance, though not at all a close one, to the teeth of horses. The skull also had a certain suggestion of likeness to the horses and Dr. Ameghino was persuaded that these animals were ancestors of the horses. The family went back to the Astraponotus stage, but can be traced no farther.

Suborder †Typotheria. †Typotheres

This suborder was composed of much smaller animals than the †Toxodonta and contained no large forms; some, indeed, were exceedingly small, no larger than rabbits. It was much the most diversified of the suborders, as is made evident by the table of families and genera. Two of these families, the †Typotheriidæ and the †Hegetotheriidæ, continued into the older Pleistocene. Of the former there was the genus first named and described, †Typotherium, which has given its name to the family and suborder, and the species of which were much the largest of the entire group, almost equalling a large pig in size. At the first glance this genus might easily be mistaken for a large rodent, and indeed it has actually been referred to that order, but the resemblance was a purely superficial one and involved no relationship.

In †Typotherium the teeth were considerably reduced in number, the formula being: i 1/2, c 0/0, p 2/1, m 3/3, × 2 = 24. The first incisor in each jaw was a broad, scalpriform, persistently growing tooth, which much resembled the corresponding tooth in the rodents, but was not, as it is in the latter, worn to a sharp chisel-edge by attrition, but was abruptly truncated. There was a second similar, but much smaller, tooth in the lower jaw; the other incisors and all the canines had been lost and the premolars reduced to two in the upper and one in the lower jaw. The molars were large, persistently growing and thoroughly hypsodont; in pattern they were very similar to those of †Toxodon. The skull without the lower jaw was low and the cranial portion broad and flattened, but retaining a long sagittal crest. The eye-sockets were nearly, but not quite, closed behind by the very long and slender postorbital processes of the frontal bones. In front of the eyes the face was suddenly constricted into a long, narrow rostrum, and it is this shape of the skull which, together with the persistently growing, scalpriform incisors, gave such a rodent-like appearance to the head. The auditory region had the same remarkable structure as in the †Toxodonta. The lower jaw had a short horizontal portion and very high vertical portion, which gave the head great vertical depth.

The skeleton, so far as it is known, was decidedly more primitive than that of the contemporary †Toxodon, as is shown by the presence of collar-bones (clavicles) and by the larger number of digits, five in the front foot and four in the hind. The hoof-bones, or ungual phalanges, were narrow, pointed and nail-like, though in the hind foot they were broader and more hoof-like.

Little can be done as yet in tracing back the history of this family, the Santa Cruz beds having yielded no member of it. In the Deseado stage, the genus †Eutrachytherus differed surprisingly little from †Typotherium, in view of the long hiatus in time between them. The Deseado genus already had thoroughly hypsodont and rootless teeth, and the molar pattern was quite the same as in †Typotherium, but the teeth were much more numerous, the formula being: i 3/2, c 1/1, p 4/4, m 3/3, × 2 = 42. Nothing is known of the skeleton. The family arose probably from one of the Eocene families (†Archæopithecidæ or †Acœlodidæ) with low-crowned teeth, but the connection cannot be made out. Presumably, the development of this family ran its chief course in some part of South America far to the north of the fossil-beds of Patagonia.

The second family which was represented in Pampean times was that of the †Hegetotheriidæ, and the sole genus of it which survived so late was †Pachyrukhos, a little creature no larger than a rabbit. The genus went back without any noteworthy change to the Santa Cruz stage of the Miocene, from which complete skeletons have been obtained. The dental formula was nearly as in †Typotherium: i 1/2, c 0/0, p 3/3, m 3/3, × 2 = 30, and the enlarged, rootless and scalpriform incisors were similar. The grinding teeth were thoroughly hypsodont and had a thin coating of cement; the molar-pattern was fundamentally like that of †Nesodon, in simpler form, but can be seen only in freshly erupted and unworn teeth.

The skull was very rodent-like in appearance, its flat top and narrow, tapering facial region, and the gnawing incisors adding much to the resemblance. The very large eye-sockets and the enormously developed auditory region suggest nocturnal habits, and, no doubt, the timid, defenceless little creatures hid themselves by day, perhaps in burrows. The enlargement of the accessory auditory chambers, which all of the †Toxodontia possessed, reached its maximum in †Pachyrukhos, and the chambers formed great, inflated protuberances at the postero-external angles of the skull. The neck was short, the body long and the tail very short, much like that of a rabbit. Collar-bones were present, as they probably were in all of the other members of the suborder †Typotheria, though this has not been definitely ascertained in all cases. The limbs were relatively long, especially the hind legs, and very slender; the bones of the fore-arm were separate, but those of the lower leg were coössified at both ends. The feet, which had four digits each, were of unequal size, the posterior pair being much longer than the anterior, and the hoofs were long, slender and pointed, almost claw-like. The entire skeleton suggests a leaping gait and its proportions and general appearance were remarkably like those of a rabbit-skeleton. In the restoration ([Fig. 300, p. 639]) Mr. Knight has followed these indications and drawn an animal which might readily be mistaken for a curious, short-eared rabbit; and there is every justification for doing this, though the character of the fur and the form of the ears are, of course, merely conjectural. Perhaps the ears are too small.