The appearance of the living animal, aside from the character of the hair, colour-pattern, etc., may be closely inferred from the skeleton. It was a much smaller and more graceful animal than its contemporary and relative †Diadiaphorus, as light and agile as a gazelle. The head had some resemblance to that of a small horse, but the neck was much shorter than in the horses; the body also was shorter than in the latter, and the proportions of the trunk and limbs were quite as in the smaller antelopes. But these likenesses to horses and antelopes were, it must again be emphasized, superficial; the fundamental characteristics of structure were more primitive than in the most ancient known artiodactyls and perissodactyls.
Fig. 253.—Left pes of †Thoatherium. Princeton University Museum. Letters as in [Fig. 250].
With the aid of the fragmentary material which alone represents the †proterotheres in the formations preceding and following the Santa Cruz in time, it is not practicable to trace the development of the various phyla in a satisfactory manner. Two of the Santa Cruz genera, †Diadiaphorus and †Proterotherium, continued into the lower Pliocene (Paraná), and two additional ones have been named, but little is known about them. The latest known member of the family so far discovered is a genus (†Epitherium) from the upper Pliocene of Monte Hermoso, a tridactyl form like †Diadiaphorus. It is a noteworthy fact that the most advanced and specialized genus of the entire family ended with the Santa Cruz, while the less differentiated types survived till a considerably later period. Possibly, it was the incoming of the highly efficient Carnivora from North America that led to the extermination of the last †proterotheres.
Turning backward from the Santa Cruz, the family may be traced without any question to the Deseado stage of the Oligocene, though nothing but teeth has yet been obtained, while in the Eocene it would appear to have become merged in the same group of small, †Condylarthra-like animals with quadritubercular molars, as those which are regarded as the probable ancestors of the †macrauchenids. However likely this conclusion may seem to be, its confirmation must await the discovery of much more complete specimens than are now available.
Order †Astrapotheria. †Astrapotheres
In the Santa Cruz another group of peculiar South American ungulates, the †Astrapotheria, made its last recorded appearance. Though not at all uncommon in that formation, no complete or even partial skeleton has yet been found, but merely the skull and a few bones of the limbs and feet. For this reason there is much doubt as to the systematic position and relationships of these animals, which were among the most curious of the many strange mammals which made up the Santa Cruz fauna. They were mentioned in connection with the †Amblypoda ([p. 456]) as possible representatives of that order in South America, but, as will be seen later, this is an improbable conclusion, and the group appears to have been indigenous in the southern continent, in which, at all events, it had a very long history. It has not been found in any formation later than the Santa Cruz, unless the Friasian fauna, which contains it, should be removed from that stage, of which it apparently forms the latest division.
I. †Astrapotheriidæ.
†Astrapotherium, Santa Cruz and Patagonian. †Astrapothericulus, Patagonian. †Parastrapotherium, Deseado. †Astraponotus, Astraponotus Beds. †Albertogaudrya, Casa Mayor.
II. †Trigonostylopidæ.