During the Eocene Period, Megacerops reigned as the largest mammal in North America, standing around seven feet high and boasting a pair of horns. They are one of the best represented mammals in the North American fossil record, possibly inspiring the Lakota people's legend of great thunder beasts.

During the paleontological digs of the nineteenth century, more and more Megacerops fossils were discovered, upending our understanding of the size and adaptations the mammal family is capable of.

This specimen is a showcase chunk of a bone from a Megacerops recovered on private land in the Chadron Formation of South Dakota. It dates to the Late Eocene, over 34 million years ago.

Other showcase pieces and smaller display case items from Megacerops are all available below!

Megacerops, formerly known as Brontotherium, is a genus of perissodactyl mammals, related to other odd-toed ungulates like rhinos and zebras. It was one of the largest members of the Brontotheriidae family, standing over seven feet high and weighing over 8,000 pounds. To complement its massive size, Megacerops sported a pair of large blunt horns, believed to be used for wrestling rival mates.

These giant animals lived in North America during the Late Eocene Period, going extinct around 33.9 million years ago. They were the last living Brontotheres on the planet. Their impressive skeletons have left quite a mark on the fossil record, being one of the best-represented mammal genera.

Fossils from the wider Brontotheriidae family can be found across North America and Asia, but the skulls from the badlands of the United States command particular attention. Most common in this area is Megacerops coloradensis, also known as titanothere AKA the “thunder beast.” This name comes from Lakota oral tradition and their fossils may have been the basis for legends of giants associated with thunderstorms.

In life, Megacerops were intimidating in size but not so much in diet, subsisting on fruits and leaves while traveling together in packs for protection from predators. Wear on their teeth suggests that Megacerops had a browser diet, looking for soft food on forest floors.

Just how Megacerops and the other members of the Brontotheriidae family died out remains a matter of speculation, but environmental stresses are the most likely cause. As the Eocene Period gave way to the Oligocene, the lush forests of North America turned cool and dry, and the vegetation became unsuitable for Megacerops’s eating habits.

Since the North American source was the final population of Brontotheres to die off, they were also the first to be discovered, as these remains were closest to the Earth’s surface. During the fossil boom of the mid-nineteenth century, Megacerops fossils helped scientists to understand the lost world of the Eocene, with all its titanic mammals and heavy greenery.