A history of land mammals in the western hemisphere

by William Berryman Scott

At a very early period the order became divided into two main branches, one of which includes all the forms so far considered, and the other the very strange †Dinotherium. The †dinotheres entered Europe together with the †mastodons in the lower Miocene and continued into the Pliocene without much change and then died out, leaving no descendants. They never invaded North America, probably because they were of more or less aquatic habit, like the hippopotamuses, and therefore less likely to find suitable conditions in the narrow and unstable land-bridges which connected the Old World with the New, than were animals of purely terrestrial habitat. The †dinotheres were of huge size, equalling the larger elephants in this respect and closely resembling them in the skeleton of the body and limbs. As usual in this order, the generic peculiarities were to be found in the teeth and skull. There were no superior tusks, all the upper incisors and canines being lost, but there was a pair of large lower tusks, which were directed downward, with a strong backward curvature. The dental formula then was: i 0/1, c 0/0, p 2/2, m 3/3, × 2 = 22. The grinding teeth were relatively quite small and had, except the first molar, two transverse crests, giving a pattern singularly like that seen in the tapirs. The skull was remarkably long, low and flat, and no doubt these animals had a proboscis of some sort. That the †dinotheres were derived from the same ancestral stock as the †mastodons and elephants is perfectly obvious and is not questioned by any one, but it is not yet possible to trace the connection.

The proboscideans were late immigrants into South America, being known there only in the Pleistocene and late Pliocene times, and only the †mastodons entered the southern continent, where they gave rise to several peculiar local species in Argentina, Bolivia, Chili and Brazil; one of these (†Mastodon andium) had a deposit of cement on the crowns of the grinding teeth. Why the elephants, which extended to the northern border of the Neotropical region, should have failed to reach South America and maintain themselves there, is but one of many similar questions to which no assured answer can be given.

The evolution of the Proboscidea was, in a certain sense, very similar to that of the †oreodont family ([p. 381]) among the Artiodactyla, in that the developmental changes affected chiefly the dentition and the skull, the skeleton of the body and limbs having very early acquired a character which was afterward but little modified. Were the skull and teeth of the lower Miocene †Gomphotherium not known, we should hardly hesitate to refer the skeleton to the genus Elephas, and even in the Oligocene †Palæomastodon all the bones of the skeleton, other than the skull, were characteristically and unmistakably proboscidean. On the other hand, the transformations of the teeth and skull were very profound and far-reaching, very much more so than those which took place in the †oreodonts.

Fig. 229.—Evolution of the Proboscidea: on the right, a series of skulls; on the left, last lower molar. (After Lull, modified by Sinclair.) N.B. †Tetrabelodon should read †Gomphotherium.

In the dentition we may consider separately the development of the tusks and of the grinding teeth. The first step in the known series, as exemplified by †Mœritherium, was the enlargement of the second incisor in each jaw to form a tusk which, though actually quite long, was very small when judged by the proboscidean standard. The upper tusk was directed vertically downward and the lower one was procumbent, pointing almost directly forward; the third incisor and the canine were small and in the lower jaw already lost. In the next known stage, †Palæomastodon, all of the anterior teeth, except the tusks, had been suppressed; the upper tusks were longer and more curved and of an oval cross-section; they extended less directly downward and more forward, while the enamel was restricted to the outer side of the tusk; the lower tusks were more fully procumbent than in the preceding genus. The third stage, that of the lower Miocene †Gomphotherium, showed the upper tusks greatly elongated and directed more forward than downward, while the lower tusks were but little larger than before. From the middle Miocene two phyla may be distinguished by the tusks alone; in one, which was not destined to long life, the lower pair increased greatly both in length and in diameter, while in the other series they rapidly diminished and eventually disappeared. Even in the Pleistocene, however, the American †Mastodon had remnants of these tusks in the males. In the later †mastodons, the †stegodonts, and true elephants, the upper tusks, which alone remained, lost the enamel bands and attained enormous proportions, differing in the various genera and species in the extent and direction of curvature. An aberrant mode of tusk development was to be seen in the †dinotheres, in which the upper pair was suppressed and the lower pair enlarged and so curved that the points were directed backward.

The grinding teeth underwent much more radical and striking changes. At first (†Mœritherium) they were small, very low-crowned and of simple pig-like or quadritubercular pattern, making two interrupted cross-crests; all were in use simultaneously and the succession of milk-teeth and premolars was by vertical replacement, as in normal mammals generally. In †Palæomastodon there were three pairs of tubercles on the molars and in †Gomphotherium these coalesced into ridges, but in all the †mastodons there was more or less distinctness of the conical tubercles. In one or more phyla the three-ridged plan persisted for a long time, one such phylum terminating in the Pleistocene †Mastodon americanus. In the other series the number of ridges increased, first to four, then to five, six and more (†Stegodon); the crowns of the teeth became much larger and higher, and the ridges, as their number increased, became much thinner, and the valleys between them were filled with cement, and finally, in the true elephants, with their fully hypsodont, many-crested teeth, were thickly covered all over with cement. The vertical succession of milk-teeth and premolars was retained in †Gomphotherium, at least in some species, but the large molars, which could not find room to be exposed while the premolars were in place, came in successively from behind. This horizontal mode of succession is the only one to be seen in the true elephants, in which but one tooth, or parts of two, on each side of each jaw are in simultaneous use and the premolars have entirely disappeared, but the milk-teeth are retained.

The changes in the skull, which amounted to a reconstruction, were very largely conditioned by the great increase in the length and consequent weight of the tusks, in the size of the grinding teeth and the development of the proboscis. In the earliest known type (†Mœritherium) the skull had little about it that would, at first sight, suggest proboscidean affinities; it was long and narrow, with sagittal crest and occiput of normal type, very long cranial and very short facial region. The nasal opening was directed forward and the nasal canal was relatively long and horizontal in direction, but the nasal bones were already much shortened, indicating that the proboscis was probably in an incipient stage. The symphysis of the lower jaw was procumbent and somewhat elongated, but to only a comparatively slight degree.