The American Mastodon was among the largest land animals that lived during the Pleistocene period (Ice Age / ~2 million - 11,500 years ago). During the Pleistocene period, its population was distributed throughout North America from coast to coast, beginning in Canada, and extending South past Florida, then into Central Mexico.
Emerging in North Africa ~ 35 million years ago during the late Eocene period, Mastodons (Greek for "Nipple Tooth") migrated into Eurasia during the late Oligocene period, (~ 25 million years ago). They then entered North America ~20 million years ago by means of the Bering Isthmus which was once a land mass connection between North America and Asia. However, the oldest actual specimen of "American Mastodon" (Mammut americanum) fossil discovered is determined to be ~ 3.5 million years old.
Mastodon's are from the family order of Mammutidae as opposed to the Woolly Mammoth, which is from the family order of Elephantidae. Many people confuse Mastodons with Mammoths, though there are several distinct differences. Asides from the fact that Mastodons emerged much earlier than Mammoths, Mastodons were shorter, more compact in stature and had a more muscular looking torso. Mastodon's tusks were not as long and were less curved than Mammoth's. This helped them maneuver within their forest environment and aided them in being able to snap branches and twigs from trees. With their cone shaped teeth which were coated with several enamel layers they easily crushed and chewed that material along with shrubs and other vegetation for meals. By contrast, the Mammoth's teeth looked more similarly to the smooth bottom of a tennis shoe, as its diet consisted primarily of prairie grasses. Also, Mastodon's skull was more elongated and it had a shorter trunk than Mammoth.
Similarly to Mammoth "and today's modern elephant", Mastodon went through several (6) sets of teeth. Its last set of teeth emerging when the animal was around 45 years of age and lasted until it's death. (~ 70 years old). It's last set of teeth eventually wore down to the point that the animal could no longer chew enough meals to sustain itself, thereby hastening the animal's death through starvation.
American Mastodons died out ~10,000 years ago. Although there is some evidence of man's contribution, the more widely accepted theory of its extinction is placed upon the dramatic climatic fluctuations during the Pleistocene period, which severely affected the land's vegetation and the Mastodons ability to find enough nutrition to survive upon.