Brontops belongs to a family of extinct mammals called brontotheres -- meaning "thunder beasts" -- which are evolutionary cousins of horses and rhinoceroses. Thirty-five million years ago, Brontops roamed the warm woodlands of what we now call the Badlands of South Dakota. The shape of its teeth, particularly the low crowns, indicate that this animal most likely subsisted on a diet of soft leaves. Teeth like these would be quickly worn away by tough grasses.

Rising above the nose of Brontops' skull is a most unusual pair of horns. When the animal was alive, these were covered with tough skin, and probably helped the animals recognize other brontotheres; they may also have been used for courtship displays and as weapons.

One of this specimen's ribs has a lump of bone, representing a fracture that healed during the animal's lifetime. This could have been an injury sustained in a fight with another Brontops.

There are no forests in the Badlands now. At the end of the epoch we call the Eocene, about 34 million years ago, the climate in North America became cooler and drier and the Thunder Beasts disappeared.