The Eocene Epoch is a Geologic Period that extends from about 36 million to 58 million years before the present. As with other older geologic periods, the rock beds that define the start and end are well identified, but the exact dates of the start and end of the period are slightly uncertain. The name Eocene refers to the dawn of modern ('new') mammalian faunas that appeared during the epoch. The Eocene follows the Paleocene Epoch and is followed by the Oligocene Epoch. The Eocene is the second epoch of the Tertiary Era.
The start of the Eocene is marked by the emergence of the first modern mammals. The End is set at a major extinction event that may be related to the impact of large extraterrestrial objects in Siberia and/or near Chesapeake Bay.
The Eocene is usually broken into Lower and Upper subdivisions. The Faunal stages from youngest to oldest are:
Ypresian/(Lower Claiborne) Lutetian/(Lower Claiborne) Bartonian/Auversian (Upper Claiborne) Priabonian/Jackson (Upper Claiborne)
Continents continued to drift toward their present positions. Climates remained warm although slow global cooling that eventual led to the Pleistocene glaciations started around end of the Eocene. Mountain building in Western North America started in the late Eocene. There appears to have been a land bridge between North America and Europe as the faunas of the two regions are very similar. Important Eocene land faunas are found in Western North America, Europe, Patagonia, Egypt and SouthEast Asia. Marine faunas are best known from South Asia and the SouthEast United States.
During the Eocene plants and marine faunas became quite modern. The first Charcharinid sharks appeared as did early marine mammals. On land, early forms of many modern mammalian orders appeared including ungulates, bats, probiscidians, prtimates, and rodents. Older forms declined. Many modern orders of birds first appear in the Eocene.