Eocene

The Eocene epoch, lasting from 55.8 ± 0.2 to 33.9 ± 0.1 Ma (million years ago), is a major division of the geologic timescale and the second epoch of the Palaeogene period in the Cenozoic era. The Eocene spans the time from the end of the Paleocene epoch to the beginning of the Oligocene epoch. The start of the Eocene is marked by the emergence of the first modern mammals. The end is set at a major extinction event called Grande Coupure (the "Great Break" in continuity), which may be related to the impact of one or more large bolides in Siberia and in what is now Chesapeake Bay. As with other geologic periods, the strata that define the start and end of the epoch are well identified, though their exact dates are slightly uncertain.

The name Eocene comes from the Greek ἠώς (eos, dawn) and καινός (kainos, new) and refers to the "dawn" of modern ('new') mammalian fauna that appeared during the epoch.

Subdivisions

The Eocene epoch is usually broken into Early and Late, or - more usually - Early, Middle, and Late subdivisions. The corresponding rocks are referred to as Lower, Middle, and Upper Eocene. The Faunal stages from youngest to oldest are:

The Ypresian and occasionally the Lutetian constitute the Lower, the Priabonian and sometimes the Bartonian the Upper subsection; alternatively, the Lutetian and Bartonian are united as the Middle Eocene.

Climate

Marking the start of the Eocene, Earth heated up in one of the most rapid (in geologic terms) and extreme global warming events recorded in geologic history, called 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) that lasted less than 100,000 years . The Thermal Maximum provoked a sharp extinction event that distinguishes Eocene fauna from the ecosystems of the Paleocene.

The Eocene global climate was perhaps the most homogeneous of the Cenozoic; the temperature gradient from equator to pole was only half that of today's, and deep ocean currents were exceptionally warm. The polar regions were much warmer than today, perhaps as mild as the modern-day Pacific Northwest; temperate forests extended right to the poles, while rainy tropical climates extended as far north as 45°. The difference was greatest in the temperate latitudes; the climate in the tropics however, was probably similar to today's. The recent discovery of a giant snake (about the size of a school bus) in Columbia that may have lived during the Eocene suggests, on the contrary, that the tropics were much warmer than today, a conclusion in accord with numerical simulations of the climate during the Eocene.

Although the global climate remained comparatively warm throughout the rest of the Eocene it was this epoch that marked the start of a slow global cooling trend, possibly triggered by the Arctic Ocean Azolla event and the formation of the Antarctic circumpolar current following the final break up of Gondwana. This trend would eventually lead to the Pleistocene glaciations.

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