The Oligocene is the last geological epoch in the Palaeogene. It lasted for about 11 million years, from 34 million years ago to 23 million years ago. It was preceded by the Eocene and followed by the Miocene. During the Oligocene, the world continued to get cooler from its peak in the mid-Eocene. The world was still largely forested, but grasslands began to spread. Ice formed in Antarctica. The period is seen as one of transition, both in climate and in the biota.
- Haines, Tim 1999. Walking with beasts: a prehistoric safari. New York: Dorling Kindersley.