Oligocene Epoch (33.9 - 23 MYA)
An extract from Rudolph F. Zallinger’s mural “The Age of Mammals,” showing reconstructions of Oligocene mammals. From left to right these are the creodont Hyaenodon; the saber toothed nimravid Hoplophoneus; the horse Mesohippus; Subhyracodon, a hornless rhino; Protapirus, an early tapir; the primitive camel Poebrotherium; and the anthracotherid artiodactyl Bothriodon. © Yale University
The Oligocene is an important transition from the Eocene, which in many respects was very like the world inhabited by the last of the dinosaurs, to the more recognizably modern world of the Miocene. The beginning of the Oligocene is marked by a sudden decrease in earth's temperature. The Antarctic continent had drifted to its current position at the south pole and a permanent ice cap had started to form. Open plains and deserts became more common and grasslands began to spread. A vast inland sea that had once separated Europe and Asia dried up and increased ease of movement of animals meant that the faunas of the two continents became very similar.
The Oligocene represents a low-point in the overall diversity of mammalian species, partly due to continued global cooling and reduced endemism. Some perissodactyls, such as brontotheres, went extinct although rhinos, tapirs, and horses all survived through the Oligocene. The hornless rhino Indricotherium, from the Oligocene of Central Asia, was the largest land mammal ever. However, the Oligocene is also the period when artiodactyls, including oreodonts, camels, and the early ancestors of deer and cattle, became far more common than perissodactyls. The White River Badlands of Wyoming, Nebraska, and South Dakota, which are famous for their late Eocene mammal fossils, also contain the best fossil record of Oligocene mammals in North America.