Smithsonian National Museum of Natural History Geologic Time The Story of a Changing Earth
Presented by the Department of Paleobiology.
The Eocene
Eocene Overview
Terrestrial Life during the Eocene
Marine Life in the Eocene
Shifting Continents and Changing Climates
A fossil weevil from the Green River
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Dawn horse
Pseudocrypturus cereamaxis
Extinct whale
Snail shell
Fern leaf
Palm leaf
March fly

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Eocene 55.8–33.9 mya
Defining Characteristics:
  • • first appearances of many modern mammal orders
  • • maximum extent of warm climate and tropical vegetation
  • • Map of the Eocene World
Secondary Characteristics:
  • • evolution of marine mammals
  • • cooling climate

In 1833, Charles Lyell derived the name Eocene from the Greek words Eos (meaning “dawn”) and Kainos (meaning “recent”). At the time, the Paleocene had not yet been named, so the Eocene became the “dawn of the recent” (Cenozoic Era). Lyell chose this term because only about 3.5% of fossil mollusks from sediments of this age were recent species. During the Eocene, volcanoes were active in the Rocky Mountains as the uplift of this region was completed. The rising mountains were eroded into sediment that filled the adjacent basins, which (along with nearby large lakes) became important fossil sites in Wyoming and Colorado. Spectacular Eocene fossils come from lake deposits at Messel, in Germany, and from the Green River Formation in southwestern Wyoming.

A dramatic warming event occurred at the onset of the Eocene, probably due to the release of methane that had been trapped in sediments on the ocean floor. In fact, the first 5 million years of the Eocene were warmer than any other time in the Cenozoic. Polar-region fossils include warm-weather species of plants, alligators, turtles, and flying lemurs. However, after the middle of the Eocene the climate became cooler and drier, a trend that continued for the rest of the Cenozoic Era. Throughout the epoch, mammals continued their rapid post-Cretaceous diversification. Giant titanothere herbivores, the first whales and sea cows, numerous hoofed mammals, primates, and rodents populated the landscapes.


Eocene Overview | Terrestrial Life during the Eocene | Marine Life in the Eocene
Shifting Continents and Changing Climates

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