The Eocene epoch, 55.8 to 33.9 million years ago, was a time when mammals diversified and occupied most niches. The climate was mild, with tropical conditions extending up to 45 degrees from the equator. Antarctica was still connected to South America, and much of Europe, the Middle East, and Southeast Asia were inundated. The Eocene was a little before the Age of Herbs, and most of the planet was covered in forests rather than grasslands. Many modern mammalian orders originated during this period.
The Eocene epoch, 55.8 to 33.9 million years ago, began about 10 million years after the extinction of the dinosaurs, when mammals were diversifying and had already risen to prominence, occupying most of the niches available. As was the case for much of Earth’s history, the climate during the Eocene was relatively mild, with tropical conditions extending up to 45 degrees from the equator and a temperate climate extending to the poles. During the Eocene, the climate at the poles would have been comparable to that of the Pacific Northwest.
The continental layout during the Eocene was similar to today, except Antarctica was still connected to South America and much of Europe, the Middle East, and Southeast Asia were inundated. This is due to the near absence of continental ice sheets at the poles, meaning that all of the ice was in the form of water, filling the oceans. Because Antarctica was still connected to South America, there was no freezing circumpolar current. Antarctica was temperate, and the worldwide circulation of ocean currents has given the global climate a degree of homogeneity not seen since.
The Eocene was a little before the Age of Herbs, meaning that most of the planet was covered in forests rather than grasslands. Arboreal mammals were most common, and nearly all animals were small: few exceeded 10 kg (22 lb). On average, they were 60% smaller than the Paleocene animals that preceded them, and even smaller than the large animals that would have evolved shortly after the epoch ended. This small size is thought to have helped the animals cope better with the heat.
Representatives of many modern mammalian orders are thought to have originated during the Eocene, including bats, trunks (elephants and relatives), primates (although these may have evolved much earlier), rodents, and many groups of marsupials. “Modern” plant and animal life evolved during this period, meaning it would have had many early ecosystems that people today would have considered normal rather than distinctly ancient, such as the cycad/dinosaur ecosystems of the Mesozoic.
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