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
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.