Associated with †Pachyrukhos in the Santa Cruz stage was another genus of the family, †Hegetotherium, which, though it cannot possibly have been ancestral to the former, yet serves to indicate, in general terms, what the ancestor must have been. This is another example of the long-continued survival of the more primitive together with the more advanced and specialized form. †Hegetotherium persisted into the Pliocene, but is not known from the Pleistocene. In this genus one upper and two lower incisors were already enlarged, rootless and scalpriform, but none of the teeth had been lost; it is interesting to note, however, that the teeth which were lacking in †Pachyrukhos were all very small and ready to disappear. The Santa Cruz species of †Hegetotherium were considerably larger and more robust animals than those of †Pachyrukhos.
Both of these genera were preceded by very similar, almost identical forms in the Patagonian, Deseado and Astraponotus stages, but the family cannot be definitely traced farther back than the lower Oligocene, but it very probably arose from some one of the groups, with low-crowned teeth, of the Casa Mayor stage.
Fig. 242.—Santa Cruz †typothere (†Protypotherium australe) and armadillo (†Stegotherium tesselatum). Restored by C. Knight from skeletons in the museum of Princeton University.
The family †Interatheriidæ was, in most respects, more conservative and underwent less change than either of the preceding groups. A persistently primitive type was the genus †Protypotherium, which appeared for the last time in the Pliocene of Monte Hermoso, but was much more abundant and better preserved in the Santa Cruz. The animal was small and had the full complement of teeth, which were arranged in each jaw in a continuous series, and were fully hypsodont and rootless, except incisors and canine, which were rooted. None of the incisors was specially enlarged, but there was a gradual transition of increasing size and complexity from the incisors to the molars. A remarkable feature of this genus was the deeply cleft form of the lower incisors, giving them a fork-like shape, somewhat as in the modern Hyracoidea. The ulna and radius in the fore-arm and the tibia and fibula in the lower leg were separate, but the digits were already reduced to four in each foot. This was one of the few Santa Cruz ungulates which possessed a long and heavy tail. The limbs were relatively long and the feet were armed with such slender hoofs that they looked almost like claws. The restoration shows the animal to have had, like nearly all of the †Typotheria, a very rodent-like appearance, a likeness which may, perhaps, be unduly increased by the form given to the ears.
In the allied genus, †Interatherium, from which the family is named, the head was short, broad and deep, almost bullet-like; the first incisor was enlarged and chisel-shaped, and the other incisors and the canines were much reduced in size. It is an interesting fact, observed as yet only in this genus, but probably true also of all the smaller members of the suborder which had hypsodont teeth, that the milk-premolars were rooted and comparatively low-crowned, while their permanent successors were completely hypsodont and rootless. The limbs were considerably shorter than those of †Protypotherium and the tail long and thick, except for which, the general appearance of the skeleton suggests that of the modern “conies” or “klipdases” (Hyracoidea) of Africa and Syria, a suggestion which Mr. Knight has followed in the drawing ([Fig. 297, p. 636]).
This family was represented in the Deseado stage by a genus (†Plagiarthrus) in which the teeth developed roots in old age, but is not known from more ancient formations. Their probable ancestors of the Eocene were very small animals, with brachyodont teeth, the premolars smaller and of simpler pattern than the molars. The upper molars had a continuous external wall, with indication of separate cusps, and two transverse crests, as in the †Toxodonta, and the lower molars were composed of two incomplete crescents. The teeth were present in undiminished number and the anterior incisors were but little enlarged. Nothing is known of the skeleton.