| Paleogene period|
| Paleocene epoch|
| Eocene epoch|
| Oligocene epoch|
33.9 to 23.0 million years ago
The Oligocene epoch is a geologic period of time that extends from about 34 million to 23 million years before the present. As with other older geologic periods, the rock beds that define the period are well identified, but the exact dates of the start and end of the period are slightly uncertain. The name Oligocene comes from the Greek oligos (few) and kainos (new) and 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 Oligocene is the third and final epoch of the Palaeogene period.
The start of the Oligocene 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:
Chattian (28.4 ± 0.1 – 23.03 MYA)
Rupelian (33.9 ± 0.1 – 28.4 ± 0.1 MYA)
Climates remained warm, although the slow global cooling that eventually led to the Pleistocene glaciations started around the end of the epoch.
During this period, the continents continued to drift toward their present positions.
Mountain building in western North America continued, and the Alps started to rise in Europe as the African plate continued to push north into the Eurasian plate. A brief marine incursion marks the early Oligocene 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.
Angiosperms continued their expansion throughout the world; tropical and sub-tropical forests were replaced by temperate deciduous woodlands. Grasses expanded from the water-bank habitat in the Eocene, and moved out into open tracts.
In North America, subtropical species dominated with cashews and lychee trees present, and temperate trees such as roses, beech and pine common. The legumes of the pea and bean family spread, and sedges, bulrushes and ferns continued their ascent.
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. This was probably more as a result of older forms dying out than as a result of more modern forms evolving.
South America was apparently isolated from the other continents and evolved a quite distinct fauna during the Oligocene.
Oceans continued to cool, particularly around Antarctica.
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