At left, a detail of a painting by Jay Matternes shows typical animals of this forested, humid environment. At right, the Badlands National Park in South Dakota, contains many Oligocene fossils.

During the Oligocene, the even-toed ungulates (artiodactyls) became more diverse, with more species than the perissodactyls. Pig-like oreodonts were common artiodactyls in the Oligocene. Their fossils are quite numerous in parts of the Great Plains.