Mid-oligocene extinction event in north american land mammals.

Interest in extraterrestrial causes for the apparent 26-to 32-million-year periodicity of mass extinctions has focused on the terminal Eocene event and older events, although there is now evidence of a mid-Oligocene event near the early/late Oligocene boundary, or about 32.4 million years ago. An abrupt (200,000 years or less) mid-Oligocene extinction event appears in the record of North American land mammals, which results in the selective disappearance of archaic members of the fauna and later diversification of other taxa. The selective nature of the extinctions suggests climatic and ecological causes rather than an extraterrestrial catastrophe. Increased mid-Oligocene glaciation, worldwide cooling, a major regression event, and abrupt changes in the flora are probably the immediate causes, and these may have resulted from changes in global oceanic circulation.