The Oligocene Epoch is a Geologic Period that extends from about 24 million to 36 million years before the present. As with other older geologic periods, the rock beds that define the start and end are well identified, but the exact dates of the start and end of the period are slightly uncertain. The name Oligocene refers to the sparsity of additional modern mammalian faunas after a burst of evolution during the Eocene. The Oligocene follows the Eocene Epoch and is followed by the Miocene Epoch. The Eocene is the third epoch of the Tertiary Era.
The start of the Eocene is marked by a major extinction event that may be related to the impact of large extraterrestrial object in Siberia and/or near Chesapeake Bay. The Oligocene Miocene boundary is not set at an easily identified worldwide event but rather at regional boundaries between the warmer Oligocene and the relatively cooler Miocene.
Oligocene faunal stages from youngest to oldest are:
Continents continued to drift toward their present positions. Climates remained warm although slow global cooling that eventual led to the Pleistocene glaciations started around end of the Eocene. Mountain building in Western North America continued and the Alps started to rise in Europe. A brief marine incursion marks the early Paleocene in Europe. Oligocene marine exposures are rare in North America. There appears to have been a land bridge in the early Oligocene between North America and Europe as the faunas of the two regions are very similar. Important Oligocene land faunas are found on all continents except Australia. Marine faunas became fairly modern as did terrestrial vertebrate faunas in the Northern continents as older forms died out. South America was apparently isolated from the other continents and evolved quite distinct faunas during the Oligocene.