Fig. 244.—Santa Cruz †macrauchenid (†Theosodon garrettorum) and predaceous marsupial (†Borhyæna tuberata). Restored by C. Knight from skeletons in the museum of Princeton University.
Next in the ascending series, to use the genealogist’s term, came the genus †Theosodon of the Santa Cruz, of which almost all the skeletal parts are known and thus make possible a full comparison with †Macrauchenia, which assuredly was its direct descendant. In view of the great lapse of time involved, the differences between the two genera were less than might have been expected, though the more ancient animal was in all respects the more primitive. †Theosodon was, in the first place, considerably smaller, not much exceeding a llama in size; the teeth had lower crowns than even those of †Scalibrinitherium and the incisors were arranged in line with the grinding teeth, not in a transverse row, but curving inward slightly, so that those of the opposite sides nearly met in front. The incisors, canine and first premolar were simple, sharply pointed, conical teeth, which gave an almost reptilian expression to the anterior part of the skull. The upper molars were on the same fundamental plan as those of †Macrauchenia, but in a less advanced stage of development, the transverse crests being incomplete and the internal cusps had a certain degree of separateness from the crests and from each other. It is evident that the upper molars were derived from the quadritubercular type. The lower molars had the vertical pillar in the concavity of the posterior crescent very prominently developed.
The resemblance of the skull to that of †Macrauchenia is obvious at the first glance, but it was less specialized and departed less from the ordinary ungulate type. The cranium was longer and the face shorter, the orbit, which was incompletely closed behind, extending over the second molar. There was a sagittal crest, the length of which differed much in the various species; the nasal bones were already very short, though decidedly longer than in the subsequent genus †Scalibrinitherium, and the anterior nasal opening was extended forward as a long, narrow slit, because the maxillaries did not come into contact with each other in the superior median line, and the premaxillaries touched each other, but were not coössified. The nasal canal, though very short, was horizontal, not vertical. The skulls of the three genera thus displayed three successive stages in the backward shifting of the orbit and of the anterior nasal opening, in the shortening of the nasal bones and in the formation of a solid rostrum by the fusion of the upper jaw-bones. No doubt also the living animals exhibited a corresponding gradation in the development of the proboscis.
Fig. 245.—Development of the skull in the †Macrauchenidæ, side views. A, †Theosodon, Santa Cruz. B, †Scalibrinitherium, Paraná. (After Ameghino.) C, †Macrauchenia, Pampean. (After Burmeister.) n., nasal bones.
Fig. 246.—Development of the skull in the †Macrauchenidæ. A, †Theosodon. B, †Scalibrinitherium. (After Ameghino.) C, †Macrauchenia. (After Burmeister.)