In the early Oligocene Epoch (about 36 million years ago), Paleomastodon was found in North Africa. This animal was the ancestor of the other elephant-like animals that would appear in Africa. There is a gap in the fossil record before the next proboscidean fossil is found, in the early Miocene Epoch of Africa. By this time, two distinct groups of mastodons had appeared. Both groups would spread through Africa and into Europe during the Miocene Period. Then into North America via the Bering Strait. Some even crossed Central America and moved into South America.
The group of mastodons that we are probably most familiar with are the zygodont mastodons. Zygodont refers to their teeth. In zygodont mastodons, such as the familiar American Mastodon (Mammut americanus), they have lost the lower tusks that we see in Paleomastodon, and their lower jaw resembles that of modern elephants.
The teeth of the American Mastodon are different, however, than those of the Mammoth and modern elephants. Instead of the many "labyrinths" that we see in Mammoth teeth, Mastodons have "cones" or "lophs". These structures allowed them to eat a diet of herbs, shrubs, and tree bark, unlike the Mammoths, whose teeth were shaped for grinding of grasses. Mammoths are referred to as "grazers" for their habit of eating grass, while Mastodons are called "browsers" for eating a more varied and tougher diet.
The American Mastodon reached a height of about three meters (10 feet) at the shoulder, which was a bit smaller than our modern African elephants, they were also quite a bit smaller than the Mammoths that lived at the same time. The Mastodon had a coat of shaggy brown hair that protected it, and allowed it to live in the tundra conditions.
The Stegodons (not to be confused with Stegosaurus, the dinosaur!) were another type of zygodont that lived in India during the Pleistocene (1.6 million years ago to 10, 000 years ago). They had long, spirally curved upper tusks and vestigial lower tusks. (A vestigial structure is one that once was a fully operational body part, but has now lost much of its usefulness, and now exists in a greatly reduced form. An example is the human appendix; in our distant ancestors, it was much larger and was used for digesting a high fiber diet, but has now become much smaller, almost disappearing, and is not actually used in digestion.) The Stegodons also had ridged teeth, more like what we see in the Mammoths and modern elephants. This similarity in the teeth, led many paleontologists to believe that modern elephants were descended from the Stegodons, but we have since established that they were actually distant cousins who had many similar traits.
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