| Paleogene period|
| Paleocene epoch|
| Eocene epoch|
| Oligocene epoch|
56 to 34 million years ago
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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.
The name Eocene refers to the dawn of modern ('new') mammalian fauna that appeared during the epoch.
As with other older geologic periods, the strata that define the start and end are well identified, but their exact dates are slightly uncertain.
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)
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 7°C at high latitudes) which lasted less than 100,000 years . 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.
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.
The oldest known fossils of most of the modern orders of mammals appear in a brief period during the Early Eocene. At the beginning of the Eocene several new mammal groups arrived in North America, modern mammals, like artiodactyls, perissodactyls and primates, animals with features like long, thin legs, feet and hands capable of grasping, and advanced teeth adapted for chewing. Dwarf forms reigned. All these new orders of mammals were small, under 10 kg: based on tooth size, Eocene mammals were only 60 per cent the size of the primitive Paleocene mammals that had preceded them, and they were also smaller than the mammals that followed them. Why mammals from hot climates should be smaller than from cool climates may have something to do with heat conservation in larger mammals.
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, proboscideans, primates, and rodents. Older primitive forms of mammals declined in variety and importance. Important Eocene land faunas are found in Western North America, Europe, Patagonia, Egypt and South-East Asia. Marine fauna are best known from South Asia and the South-East United States.
The first aquatic mammals, whales and sea cows appeared in the oceans. The whales belonged to an extinct lineage called archeocetes, and quickly grew to huge size: Zeuglodon (more correctly Basilosaurus, a confusing name because this was a mammal not a reptile) attained 20 to 25 meters in length.
The Eocene saw the appearance of modern birds such as eagles, pelicans, quail, and vultures, as well as the great flightless Diatrymiformes, 2 meters or more in height, with a huge hooked beak that clearly indicated carnivorous habits. Such giant flightless birds, curiously reminiscent of their ancestors, the great theropod dinosaurs of the bygone Mesozoic, may have been able to develop because carnivorous mammals remained primitive and not very efficient.
Africa was separated by ocean on all sides, which allowed the development of a unique fauna in isolation from Europe, Asia and North America. There evolved not only the ancestors of elephants but also the hyrax, the monkey, and strange extinct forms such as the rhinoceros-like embrithotheres.
The transition from the Eocene to the Oligocene is marked by an extinction event. Although minor compared to the ones at the end of the Permian of the Cretaceous it did result in considerable changes in e.g. the mammalian fauna of Europe. There it is also known as the Grande Coupure, the "great hiatus". Elsewhere it has received different names. Its cause(s) are not quite clear. There were considerable climatic changes underway that resulted in general cooling, but there is also evidence of impacts from space, both in the US (Chesapeake Bay) and in Siberia.
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