The neck of †Theosodon was even longer proportionately than in †Macrauchenia and the transference of the canal for the vertebral artery from the transverse processes to the neural arch had already taken place, except in the first, sixth and seventh vertebræ, and was thus less complete than in the Pampean genus, in which all the vertebræ of the neck, save the seventh, had the canal in its exceptional position. The odontoid process of the axis was less modified than in the latter, being relatively longer and more conical. The body was rather short, and the spines of the trunk-vertebræ were proportionally higher and more prominent. No caudal vertebræ have been found, but, from the shape of the sacrum, it is evident that the tail was short.
The limbs were long, but more slender and less elongate than in †Macrauchenia, in which the growth of the neck did not keep pace with that of the limbs, the lengthening of the proboscis probably compensating for this. The shoulder-blade had two conspicuous metacromia, very much as in the contemporary †toxodont, †Nesodon, but shorter and more widely separated. The humerus was short and quite slender and the fore-arm bones, which were much longer, did not coössify. The femur had a more slender and rounded shaft than in †Macrauchenia and a much larger third trochanter; the leg-bones were also separate from each other. The tridactyl feet were so like those of the Pampean genus, that no particular account of them is necessary, and the proportions of the limb segments were similar in both genera, short upper arm and lower leg, very long fore-arm and thigh, and short feet.
Fig. 247. Left manus of †Theosodon. S., scaphoid. L., lunar. Py., pyramidal. Tm., trapezium. Td., trapezoid. M., magnum. U., unciform. V., rudimentary fifth metacarpal.
The appearance of the living animal, as shown in the restoration, was no doubt somewhat like that of †Macrauchenia, but less bizarre. That there must have been some sort of a proboscis or prehensile upper lip, is indicated by the greatly shortened nasal bones, but this may not have been longer than in the existing Moose or Saiga Antelope. The long neck, short body and tail and long limbs suggest an animal not unlike a Guanaco, but larger and heavier. The hair may or may not have had the woolly character given to it in the drawing; upon such a point there can be no certainty.
In the older formations preceding the Santa Cruz, the †macrauchenids are known only from fragmentary material, though something of their history may be made out even from these fragments. †Protheosodon, of the Deseado stage, was considerably smaller than the Santa Cruz genus and had more primitive upper molars, in that the internal cusps and intermediate cuspules were isolated and conical, not forming transverse crests. Still smaller were the several genera (†Lambdaconus, etc.) related to the †macrauchenids found in the Casa Mayor Eocene, which have been referred, perhaps correctly, to the †Condylarthra. In these the formation of the external wall of the almost bunodont upper molars was in progress, by the fore-and-aft extension and transverse thinning of the external cusps; the internal pair of cusps and the cuspules were separate and conical. With much confidence, it may be inferred that in these little animals the skull was normal, the nasal bones were long and that the feet were five-toed, but demonstration is lacking.
The second family of the †Litopterna, the †Proterotheriidæ, were remarkable for their many deceptive resemblances to the horses. Even though those who contend that the †Litopterna should be included in the Perissodactyla should prove to be in the right, there can be no doubt that the †proterotheres were not closely related to the horses, but formed a most striking illustration of the independent acquisition of similar characters through parallel or convergent development. The family was not represented in the Pleistocene, having died out before that epoch, and the latest known members of it lived in the upper Pliocene of Monte Hermoso. In the still older Paraná formation more numerous and varied forms occurred, but only from the Santa Cruz have materials been obtained of sufficient completeness to furnish a full account of the structure of these extraordinary animals. Not that this remarkable character was due to grotesque proportions; on the contrary, they looked far more like the ordinary ungulates of the northern hemisphere than did any of their South American contemporaries; it is precisely this resemblance that is so notable.
In Santa Cruz times the family was represented by a large number of species, which have been grouped in four or five genera, which differed sufficiently to require generic separation, yet were closely similar. In all of them the dental formula was: i 1/2, c 0/1, p 4/4, m 3/3, × 2 = 36. Except in one genus (†Thoatherium) a pair of small tusks was formed by the enlargement of the second upper and third lower incisors, as in the †toxodonts, but the first upper and lower and the third upper incisors, which were retained in the †toxodonts, were lost in this family, as was also the upper canine, and the lower canine was very small, of no functional use. The teeth were brachyodont and, except the small tusks, displayed no tendency at any time toward the acquisition of high crowns. The premolars were less complex than the molars, though the last one approximated the molar-pattern. The upper molars had two crescentic outer cusps, meeting in a vertical ridge and together forming the outer wall; the transverse crests were imperfect, especially the hinder one which was often merely the intermediate cuspule, and did not fuse with the external wall. The lower molars had the two crescents, one behind the other, which recur in the †macrauchenids, all the suborders of the †Toxodontia, except the †Pyrotheria, and other South American ungulates, but the pillar in the posterior crescent, which was so characteristic of the groups named, was reduced to very small proportions and sometimes suppressed altogether. It should be noted, however, that this was the loss of an element which was formerly present.
The skull had a long cranium and rather short face, with long, high sagittal crest. The neck was short, the odontoid process of the axis peg-shaped, and the canal for the vertebral artery was in its normal position. The body was rather short, like that of a deer or antelope; the number of trunk-vertebræ is not definitely known in any of the genera, but was very probably 19 or 20, and the tail must have been short.