Eocene–Oligocene extinction event

The transition between the end of the Eocene and the beginning of the Oligocene is marked by large-scale extinction and floral and faunal turnover (although minor in comparison to the largest mass extinctions). Most of the affected organisms were marine or aquatic in nature. They included the last of the ancient cetaceans, the Archaeoceti.

This was a time of major climatic change, especially cooling, not obviously linked with any single major impact or any catastrophic volcanic event. One cause of the extinction event is speculated to be extended volcanic activity. Another speculation is that the extinctions are related to several large meteorite impacts that occurred about this time. One such event caused the Chesapeake Bay impact crater (40 km), and another at the Popigai crater (100 km) of central Siberia, scattering debris perhaps as far as Europe. New dating of the Popigai meteor suggests it may be a cause of the mass extinction.

A leading scientific theory on climate cooling at this time is decrease in atmospheric carbon dioxide, which slowly declined in the mid to late Eocene and possibly reached some threshold approximately 34 million years ago. This boundary is closely linked with the Oligocene Oi-1 event, an oxygen isotope excursion that marks the beginning of ice sheet coverage on Antarctica.

The Grande Coupure, or "great break" in continuity, with a major European turnover in mammalian fauna about 33.5 Ma, marks the end of the last phase of Eocene assemblages, the Priabonian, and the arrival in Europe of Asian species. The Grande Coupure is characterized by widespread extinctions and allopatric speciation in small isolated relict populations. It was given its name in 1910 by the Swiss palaeontologist Hans Georg Stehlin, to characterise the dramatic turnover of European mammalian fauna, which he placed at the Eocene-Oligocene boundary. A comparable turnover in Asian fauna has since been called the "Mongolian Remodelling".

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