The species of †Nesodon, of which many have been named on very questionable grounds, differed but little in size and were of such variable and fluctuating character that a proper discrimination of them is exceedingly difficult. One of these species (†N. cornutus) gives indications of having possessed a small dermal horn on the forehead and was thus a possible ancestor of †Trigodon.
Fig. 239.—Left pes of †Nesodon, Princeton University Museum. Letters as in [Fig. 238] and scale of reduction the same.
A second phylum of the suborder was represented in the Santa Cruz stage by the genus †Adinotherium, the species of which, not equalling a sheep in size, were very much smaller animals than those of †Nesodon, but closely like them in other respects. The dentition, including the pre-lacteal series, and the skull were almost identical in the two genera, with the exception that a large proportion of the individuals of †Adinotherium had the small frontal horn, while others had no trace of it. While it is quite possible that the presence or absence of the horn, which was always inconspicuous, may have been a matter of specific distinction, a more probable explanation is that it was a sexual character, the males horned and the females hornless. Much the same thing is to be observed in the modern Javan Rhinoceros (R. sondaicus) in which the females have a very small horn, or none at all, and the males a large one.
In the skeleton also there were few differences, other than those of size, between †Adinotherium and †Nesodon; the former was not only smaller, but also lighter and more slender proportionately, and there was no hump at the shoulders, the spines of the dorsal and lumbar vertebræ all reaching the same level, so that the back must have been nearly straight in the living animal. From the more general and constant presence of the frontal horn, †Adinotherium was more probably the ancestor of the horned †Trigodon than was †Nesodon, but until the intermediate forms shall have been recovered, no definite decision can be made.
Fig. 240.—†Adinotherium ovinum, small, horned †toxodont of the Santa Cruz. Restored from a skeleton in the museum of Princeton University.—Note the minute horn on the forehead.
The same or very nearly the same genera of the family †Toxodontidæ lived in the Patagonian and Deseado stages, but there the record breaks off and can, for the present at least, be followed no farther. It remains to be determined whether the series originated in regions farther to the north, or whether the ancestral types will be found in Patagonia.
The other two families are still very incompletely known, but sufficiently to justify their inclusion in the present suborder. In the †Leontiniidæ, which are known only from the Deseado stage (†Leontinia), we have a curious variant of the †toxodont type. The tusks were decidedly smaller than in the Santa Cruz members of the preceding family, the grinding teeth with lower crowns and simpler structure. The skull was much like that of †Nesodon, but the anterior nasal opening was of quite a different shape, being carried much farther back on the sides, so that the nasal bones had a far longer portion which was freely projecting and unsupported; these bones were shorter and much thicker than in the Santa Cruz genera and, to all appearances, supported a small, median horn on their anterior ends. The feet, so far as they have been recovered, did not differ in any significant manner from those of the preceding family.
