A history of land mammals in the western hemisphere

by William Berryman Scott

Fig. 237.—†Nesodon imbricatus, Santa Cruz stage. Restored by C. Knight from a skeleton in the museum of Princeton University.

A few years ago Dr. Ameghino announced the very surprising discovery that, instead of having merely the normal arrangement of two dentitions, the milk and the permanent, †Nesodon developed three successive dentitions, one preceding the milk-series, and therefore called pre-lacteal. In certain other mammals traces of a pre-lacteal series had already been found, in the shape of tooth-germs, which never attain full development or even cut the gum; and quite recently Dr. Ameghino has shown that in the tapir at least one functional pre-lacteal premolar is formed. The significance of this fully developed pre-lacteal dentition in †Nesodon is not yet clear, though it seems reasonable to suppose that it was the almost uniquely late retention of a primitive character.

The skull was closely similar to that of †Toxodon, on a smaller scale, but there were several minor differences, which were, in part, conditioned by the larger and much more completely hypsodont teeth of the Pampean genus, as well as by its generally increased size and bulk. In †Nesodon the sagittal and occipital crests were much more prominent and the former was much longer, while the thickening of the cranial bones was in only an incipient stage. The nasal bones were considerably longer. The jaws were lower and shallower, in correlation with the less perfectly hypsodont teeth, and in the lower jaw the chin was much more erect and rounded. The entire head of this curious Santa Cruz animal had something remarkably rodent-like in its appearance, though it is quite inadmissible to suppose that the likeness was due to relationship.

The skeleton was far smaller and lighter and otherwise differently proportioned from that of †Toxodon, but there was, nevertheless, a close agreement between the two genera. The neck was of moderate length and thickness, the body long and heavy, but with no such relative bulk as in the Pampean genus. The hump at the shoulders, as indicated by the spines of the anterior dorsal vertebræ, though already well defined, was less prominent. The shoulder-blade (scapula) was relatively broader than in †Toxodon, its spine had a distinct acromion and two very long and conspicuous processes given off backward from the spine, only one of which, and that a mere vestige, is indicated in †Toxodon. The hip-bones were almost parallel with the backbone and were not nearly so broad or so everted as in the latter, a difference which is amply accounted for by the great discrepancy in girth.

Fig. 238.—Left pes of †Toxodon. La Plata Museum. Cal., calcaneum. As., astragalus. N., navicular. Cn. 1 and 2, coössified internal and middle cuneiforms. Cn. 3, external cuneiform. Cb., cuboid.

The limbs were of nearly equal length and there was no such shortening of the fore-arm or elongation of the thigh as in †Toxodon, and so the descent of the backbone forward, which gave such grotesqueness to the skeleton of the latter, was far less pronounced. The limb-bones were rather slender, in size and proportions not unlike those of a tapir, but in structure very like the very much larger and more massive ones of †Toxodon. The bones of the fore-arm were separate, but those of the lower leg were coössified in the same exceptional manner as in the Pampean genus, that is, the upper ends, but not the lower, were fused together. The thigh-bone was not flattened, but had the normal cylindrical shaft and a conspicuous third trochanter. The feet, in which the digits were already reduced to three, were extremely small in comparison with the size of the animal; in structure, they were almost identical with those of †Toxodon, but were far narrower and more slender. The heel-bone (calcaneum) articulated with the other bones of the tarsus in a normal manner. The digits were well separated and the hoof-bones quite strongly developed, indicating that the hoofs were functional, supporting most of the weight. In short, the difference in the external appearance of the feet between the two genera was much the same as between the tapirs and rhinoceroses.