The Eocene, like the Palaeocene before it, had a climate much warmer than today. At the start of the Eocene the Palaeocene–Eocene Thermal Maximum was reached. This lasted for 100,000 years, and caused a large extinction event. The land was heavily forested, with temperate forests into arctic and antarctic regions, and the many herbivorous mammals were browsers, not grazers.
The end of the Eocene was the beginning of the Oligocene (33.9 million years ago). Many plant and animal species went extinct. This was the Eocene–Oligocene extinction event. The extinction event was probably caused by meteorite strikes in Siberia and Chesapeake Bay.