Though their habitat spanned a large territory, mastodons were most common in the ice age spruce forests of the eastern United States, as well as in warmer lowland environments. Their remains have been found as far as 300 kilometers offshore of the northeastern United States, in areas that were dry land during the low sea level stand of the last ice age. Mastodon fossils have been found in South America; on the Olympic Peninsula of Washington state, USA (Manis Mastodon Site), in Kentucky (particularly noteworthy are early finds in what is now Big Bone Lick State Park); the Kimmswick Bone Bed in Missouri; in Stewiacke, Nova Scotia, Canada; in Richland County, Wisconsin; La Grange, Texas; Southern Louisiana; and north of Fort Wayne, Indiana, USA.
While mastodons were furry like woolly mammoths and similar in height at roughly three meters at the shoulder, the resemblance was superficial. They differed from mammoths primarily in the blunt, conical, nipple-like projections on the crowns of their molars, which were more suited to chewing leaves than the high-crowned teeth mammoths used for grazing; the name mastodon (or mastodont) means "nipple teeth" and is also an obsolete name for their genus. Their skulls were larger and flatter than those of mammoths, while their skeleton was stockier and more robust. Mastodons also seem to have lacked the undercoat characteristic of mammoths.
The tusks of the mastodon sometimes exceeded five meters in length and were nearly horizontal, in contrast with the more curved mammoth tusks. Young males had vestigial lower tusks that were lost in adulthood. However, it has been proven that female mastodons had lower pairs of tusks. The tusks were probably used to break branches and twigs, although some evidence suggests males may have used them in mating challenges; one tusk is often shorter than the other, suggesting that, like humans and modern elephants, mastodons may have had laterality. Examination of fossilized tusks revealed a series of regularly spaced shallow pits on the underside of the tusks. Microscopic examination showed damage to the dentin under the pits. It is theorized that the damage was caused when the males were fighting over mating rights. The curved shape of the tusks would have forced them downward with each blow, causing damage to the newly forming ivory at the base of the tusk. The regularity of the damage in the growth patterns of the tusks indicates that this was an annual occurrence, probably occurring during the spring and early summer.
Another influencing factor to their eventual extinction in America during the late Pleistocene may have been the presence of Paleo-Indians, who entered the American continent in relatively large numbers 13,000 years ago. Their hunting caused a gradual attrition to the mastodon and mammoth populations, significant enough that over time the mastodons were hunted to extinction.
In September 2007, Mark Holley, an underwater archeologist with the Grand Traverse Bay Underwater Preserve Council who teaches at Northwestern Michigan College in Traverse City, Michigan, said that they might have discovered a boulder (3.5 to high x long) with a prehistoric carving in the Grand Traverse Bay of Lake Michigan. The granite rock has markings that resemble a mastodon with a spear in its side. Confirmation that the markings are an ancient petroglyph will require more evidence.
Current excavations are going on annually at the Hiscock site in Byron, New York, for mastodon and related paleo-Indian artifacts. The site was discovered in 1959 by the Hiscock family while digging a pond with a backhoe; they found a large tusk and stopped digging. The Buffalo Museum of Science has organized the dig since 1983. It has been called one of the richest sites available for mastodon-related artifacts. The site sits on swampland that was covered by Lake Tonawanda, which was a glacier runoff lake formed over 10,000 years ago. It has been confirmed that mastodons would flock there to eat the sodium-rich clay during one of the last great droughts of the paleolithic.
In august 2008, miners in Romania have unearthed the skeleton of a 2.5 million-year-old mastodon, believed to be one of the best preserved in Europe. 90% of the skeleton's bones were intact, with damage to the skull and tusks.