Over 30 million years ago, the Oligocene epoch saw the start of the global cooling that would eventually shift the Earth's climate to one where glaciers were present and ice ages were possible. Worldwide, this was the time when grasslands began to expand and forests - especially tropical ones - shrank correspondingly. Animals evolved to fit the new, open landscape and many fast-running prey and predator species arose as a result.
Began: 33.7 million years ago
Ended: 23.8 million years ago
Reconstruction of the Earth in the Oligocene epoch, 33.7 million - 23.8 million years ago. Credit: Dr Ron Blakey, NAU Geology.
During this period the following extinction level events are thought to have occurred.