A history of land mammals in the western hemisphere

by William Berryman Scott

Fig. 235.—Skull of †Toxodon, Pampean formation, the upper molars much broken. La Plata Museum.

The material from the lower Pliocene adds nothing to our knowledge of this suborder, but in the Santa Cruz time of Patagonia, which was Miocene, it was very abundantly represented and preponderatingly by the genus †Nesodon, which was the first discovered member of the marvellous Santa Cruz fauna, named nearly 70 years ago by Sir Richard Owen. It also chanced that Owen’s specimen was the imperfect lower jaw of a young animal with the milk-teeth, which were mistaken for the permanent dentition, and when the latter was found long afterwards, it was naturally supposed to belong to a different animal and received a different generic name. Nor was this all; the changes which took place in the appearance and relative size of the permanent teeth within the life-time of the individual were so remarkable, that the successive stages of development were by several investigators supposed to be distinct genera and species and named accordingly. In this way nearly 30 different names have, at one time or another, been assigned to the common species, †N. imbricatus; and it was not until the late Dr. Ameghino had brought together a complete series of skulls and jaws illustrating these changes, and showing the gradual transition from one to the other, that the confusion could be cleared up.

Fig. 236.—Skull of Santa Cruz †toxodont, †Nesodon; same scale of reduction as [Fig. 235].

There was a long hiatus in time between †Toxodon and †Nesodon and so great was the structural difference between them, that there is much doubt whether the latter was directly ancestral to the former; in any event, †Nesodon so nearly represents what the desired ancestor must have been, as to serve for all practical purposes of the study.

All the species of this Santa Cruz genus were much smaller animals than the species of †Toxodon, †N. imbricatus being no longer than a tapir, with considerably shorter legs, and of much slighter and more slender build than †Toxodon, though every tooth and every bone proclaims its relationship to the latter.

In †Nesodon the dental formula was unreduced; i 3/3, c 1/1, p 4/4, m 3/3, × 2 = 44, though several of the teeth were much reduced in size, so as to have lost their functional importance, and frequently individuals are found in which one or more of these insignificant teeth are lacking. The first upper incisor was a broad, chisel-shaped tooth, which continued to grow for a period, then formed its root, and growth ceased; the second incisor was a pointed, triangular tusk, which grew throughout life, becoming longer with advancing age; while the third, which was lost in †Toxodon, was small and unimportant. In the lower jaw the first and second incisors were chisel-like and had a limited growth; being rather narrow, they both bit against the broad first upper incisor; the third incisor was a persistently growing tusk, not so large as the upper one, against the posterior face of which it impinged and was obliquely truncated by wear, so that its length was limited, while the upper tusk continued to elongate and was made narrower and sharper by wear. All the lower incisors were far less procumbent than in †Toxodon, and were directed obliquely upward and forward. The remarkable changes of appearance which, as mentioned above, took place within the life-time of the individual, were largely due to the differential growth of the incisors. The milk-incisors were all nearly alike and formed no tusks; when the permanent incisors were first protruded, the first upper and the first and second lower were large and the tusks were not visible, and, when the latter did appear, they were for some time smaller than the other incisors. These, however, formed roots and ceased to grow, actually becoming smaller with advancing age, for the crowns narrowed to the roots and, the more they were worn down, the smaller they became. The tusks, on the other hand, grew throughout life and became larger as the other incisors were reduced by wear, and thus the whole appearance of the anterior part of the jaw was totally changed.

This mode of forming the tusks by the enlargement of the second upper and third lower incisor is an unusual one, though it was repeated in another South American ungulate order, the †Litopterna, and nearly so in the Proboscidea, in which both upper and lower tusks were the second of the three original incisors.

In both jaws, the canines of †Nesodon were insignificant and sometimes absent. The premolars, which were smaller and simpler than the molars, had quite high crowns, but early ceased to grow and formed long roots. The molars were truly hypsodont and formed no roots till late in life; they were constructed on the same plan as those of †Toxodon, but were decidedly more complex, the upper ones having several spurs and crests given off inward from the external wall, in addition to the two principal transverse crests, and they had a certain superficial likeness to the teeth of a rhinoceros. As in †Toxodon, these upper molars were curved inward, so as almost to meet those of the opposite side above the palate. The lower molars had the same bicrescentic plan as in †Toxodon, but were more complicated, and in the concavity of the hinder crescent was a vertical pillar, which was well-nigh universal among the indigenous South American ungulates.

If †Nesodon was really the ancestor of †Toxodon, then the development of the grinding teeth must have been a process of completing the hypsodontism, until the teeth grew persistently, never forming roots, and, at the same time, of simplifying the pattern. This is contrary to the usual course of evolution, in which the pattern grew more complex in the successive stages; but such steadily increasing complexity was not invariable, and several instances of undoubted simplification are known among mammals, though not yet in other ungulates. Only the recovery of the intermediate genera will enable us to determine whether †Nesodon was the actual ancestor of †Toxodon, or whether it was merely one of a short-lived branch from the main stem, in which the teeth had acquired an unusual degree of complexity.