Fig. 226.—The American †Mastodon (†Mastodon americanus), Pleistocene. Restored from a skeleton in the museum of Princeton University.
The fourth of the Pleistocene proboscideans of North America was a member of a different and much more ancient genus, †Mastodon, which in the Old World became extinct before the end of the Pliocene. The American †Mastodon (†M. americanus) was thus a belated survival of an ancient type, seemingly out of place even in the strange Pleistocene world, which had so many bizarre creatures. The distinguishing characteristic of the genus was in the simple, low-crowned and comparatively small grinding teeth, which had three or four prominent transverse ridges, covered with heavy enamel, and, usually, with no cement on the crowns. As these teeth were so much smaller than those of the elephants, as many as three on each side of each jaw might be in simultaneous use. In this species there was no vertical succession of teeth, but in some of the Tertiary †mastodons such succession has been observed. The long tusks were directed nearly straight forward and were almost parallel, with but slight curvature, the convexity downward. In the males there was a short single tusk or, less commonly, a pair of such tusks, in the lower jaw, which were probably not visible externally; these were the vanishing remnants of an earlier stage of development, when the †mastodons had a fully developed pair of lower tusks, nearly as large as the superior pair.
Fig. 227.—Last lower molar of the American †Mastodon.
The skull, while essentially proboscidean, was yet much lower and flatter and less dome-like than in the elephants; the thickening of the cranial bones was less extreme. The remainder of the skeleton differed so little from that of the elephants as to require no description. In size, this species about equalled the †Mammoth, the larger individuals measuring nine feet six inches at the shoulder. Remains have been found which prove that the American †Mastodon had a covering of long, coarse hair, and that it fed upon the leaves, shoots and small branches of trees, especially of conifers. There is much reason to believe that the species outlived the elephants in this continent and persisted until after the establishment here of the American Indian, and it may well have been human agency which finally extinguished the dwindling race. The range of the species nearly coincided with that of the †Columbian Elephant, but did not extend so far into Mexico, and in the central part of the continent reached much farther north, even into Alaska.
In the Pliocene of Texas, Nebraska and Idaho lived the American representatives of a genus (†Stegodon) which was a connecting link between the elephants and the †mastodons, and which was especially characteristic of the Pliocene of India. The tusks, which were confined to the upper jaw, had lost their enamel and the last molar, above and below, had five or six enamel ridges, but the crowns, which in the Asiatic species were buried in cement, had but a small amount of this material. Several species of †Mastodon occur in the same beds, but only isolated teeth have been found.
The †mastodons, in a broad sense of the term, have been divided into several genera and subgenera in accordance with different schemes; the simplest perhaps is to group into a second genus those species which had fully developed lower tusks. This four-tusked genus has received several names, of which †Tetrabelodon is most commonly used in this country, but the term †Gomphotherium is much older and, according to the law of priority, must therefore be employed. The lower Pliocene species of †Gomphotherium had a pair of large lower tusks, of cylindrical shape, and both upper and lower tusks had longitudinal bands of enamel, and in order to support the weight of these great tusks the symphyseal, or chin, region of the lower jaw was greatly elongated; the molars had four cross-crests.
