The Eocene–Oligocene extinction event, also called the Eocene-Oligocene transition or Grande Coupure, is the transition between the end of the Eocene and the beginning of the Oligocene, an extinction event and faunal turnover occurring between 33.9 and 33.4 million years ago 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 ungulates, the "condylarths".
This was a time of major climatic change, especially cooling, not clearly caused by any single major impact or volcanic event. Extended volcanic activity is one possible cause. Another speculation points to several large meteorite impacts near this time, including those of the Chesapeake Bay crater 40 km (25 mi) and the Popigai crater 100 km (62 mi) of central Siberia, which scattered debris perhaps as far as Europe. New dating of the Popigai meteor strengthens its association with the extinction.
A leading model of climate cooling at this time predicts a decrease in atmospheric carbon dioxide, which slowly declined in the middle 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 occurred around 33.55 million years ago, marking the beginning of permanent ice sheet coverage on Antarctica.
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