The Titanothere is an extinct rhinoceros-like mammal that lived 35 million years ago during the Oligocene epoch. One specimen was found during the 1987 field program. Titanotheres were huge mammals related to the horse. It was one of the largest of the family Brontotheriidae, which first appeared in North America 54 million years ago and spread to Asia and eastern Europe. The last members of the family died out 22 million years ago.
Brontotherium stood 8 ft (2.4 m) high at the shoulder, was 15 ft (4.5 m) long, and weighed about 5 tons (4.5 tonne). It had weak teeth and fed on soft vegetation. Like many of the late brontotheres, it bore a horn-like growth on its snout. This feature consisted of bone covered by thick skin. The fossil jaw pictured is from a juvenile.
The Entelodont is an ancient mammal intermediate between pigs and hippos. It roamed the Great Plains of North America about 35 million years ago (Oligocene, Chadron Fm.). Despite the large canine teeth, this one probably mostly ate plants. This animal has no direct living relatives but its closest cousin is the Wart Hog of Africa. It was about the size of a small horse. The skull on display was discovered by Sierra College students during the 1987 summer field class to the Badlands of South Dakota.
The Unitatherium is an extinct, large, primitive-hoofed mammal found in North America in terrestrial fossil deposits of the middle and late Eocene Epoch (52 million to 36.6 million years ago). They had massive skeletons and large, powerful canines modified into sabers. Their molars were large and flattened for grinding vegetation. It was as large as a modern rhinoceros. It was replaced among the herbivores by the titanotheres.