Mastodons were very distantly related to mammoths and modern elephants. You have to go back tens of millions of years, though, to find a common ancestor. Mastodons and mammoths both lived during the Pleistocene between about 2.6 million and 11,500 years ago and their fossilized remains look similar. One way to tell them apart is by their teeth, which are very different. Full-grown mastodons also are smaller than mammoths. Mastodons probably overlapped with humans in Kansas.