The Oligocene period was one of unparallelled growth in mammalian evolution. Nowhere is evidence of this more common than in the badland country of the Dakotas and southern Nebraska, where in places, the main component of the rocks are the bones and skulls of animals such as the camels, pig-like animals, giant rhinoceri and miniature horses that inhabited North America during this period. Specimens found in the White River Beds of this region include the sheep-like oreodont Merycoidodon, the pug-like Bothriodon, the rhinoceras-like Uintatherium, Brontotherium and Hyracodon.