CREATION BY EVOLUTION
died out 15,000 to 20,000 years ago. They had attained a height of about ten feet, which is about the size of most of the larger living elephants. They were, however, shorter legged and more massive in build than the living elephant.
Though the jaw was shortened, the mastodons never developed complex grinding teeth, but had five rather simple grinders in each jaw. Apparently they remained browsers to the end of their days.
While the mastodons were developing their upper tusks and losing their lower ones, there arose a group of elephants which lost the upper tusks and retained the lower ones, though the lower jaw had shortened so much that it could no longer reach the ground. These animals were the dinotheres, which arose in early Miocene time from long-jawed mastodons and increased to a maximum size of but little less than that of the mastodons themselves. By late Miocene time they reached their height of development, only to die out in early Pliocene time. In the dinotheres the lower tusks were not simply retained but were enlarged and recurved. It is hard to guess the use to which such tusks could be put. The back teeth of the dinothere resemble those of the mastodon, and probably the animal had a proboscis, similar to that of the elephant, else it would have had no means of reaching the ground to drink.
In late Pliocene time, while the typical mastodons were browsing on leaves and twigs, some of the group began to feed on grass. As already suggested, grass carries considerable silica in its stems and leaves, so that animals which feed on it must have hard teeth. The first change taken to harden the teeth was to increase the number of cross ridges from three or four to six or eight, and then to increase the height of each ridge. Forms that attained this stage of development are known as stegodons. They were short-jawed and
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