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 This epoch is part of the
Tertiary period and the
Paleogene subperiod.

The Eocene Epoch is a period of time,the second epoch of the Tertiary Era. The Eocene follows the Paleocene Epoch and is followed by the Oligocene Epoch. It extends from about 55.5 million to 38 million years before the present. 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 (a) large extraterrestrial object(s) in Siberia and in what is now Chesapeake Bay.

Table of contents
1 Eocene naming
2 Eocene dating
3 Eocene subdivisions
4 Eocene climate
5 Eocene paleogeography
6 Eocene fauna

Eocene naming

The name Eocene refers to the dawn of modern ('new') mammalian faunas that appeared during the epoch.

Eocene dating

As with other older geologic periods, the strata that define the start and end are well identified, but their exact dates are slightly uncertain.

Eocene subdivisions

The Eocene is usually broken into Lower and Upper subdivisions. The Faunal stages from youngest to oldest are:
  1. Ypresian/(Lower Claiborne)
  2. Lutetian/(Lower Claiborne)
  3. Bartonian/Auversian (Upper Claiborne)
  4. Priabonian/Jackson (Upper Claiborne)

Eocene climate

Marking the start of the Eocene, the planet heated up in one of the most rapid (in geologic terms) and extreme global warming events recorded in geologic history, currently being identified as the
Paleocene-Eocene Thermal Maximum or Initial Eocene Thermal Maximum (PETM or IETM). This was an episode of rapid and intense warming (up to 7C at high latitudes) which lasted less than 100,000 years [1]. The Thermal Maximum lasted some 200,000 years, and provoked a sharp extinction event that strongly distinguishes Eocene fauna from the ecosystems of the Paleocene.

Climates remained warm through the rest of the Eocene, although slow global cooling, which eventual led to the Pleistocene glaciations, started around end of the Eocene.

Eocene paleogeography

Continents continued to drift toward their present positions. 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.

Eocene fauna

The oldest known fossils of most of the modern orders of mammals appear in a brief period during the Early Eocene and all were small, under 10 kg. Both groups of modern ungulates (Artiodactyla and Perissodactyla) became prevalent mammals at this time, due to a major radiation between Europe and North America. Early forms of many other modern mammalian orders appeared, including ungulates, bats, probiscidians, primates, and rodents. Older primitive forms of mammals declined in variety and importance. Important Eocene land faunaa are found in Western North America, Europe, Patagonia, Egypt and South-East Asia. Marine faunas are best known from South Asia and the South-East United States.

During the Eocene plants and marine faunas became quite modern. The first Charcharinid sharks appeared as did early marine mammals. . Many modern orders of birds first appear in the Eocene.

See also: Geologic Time Scale