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Oligocene
The Oligocene (symbol O ) is a geologic epoch of the Paleogene Period and extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that define the epoch are well identified but the exact dates of the start and end of the epoch are slightly uncertain. The name Oligocene comes from the Greek (''oligos'', few) and (''kainos'', new),〔(【引用サイトリンク】url=http://www.etymonline.com/index.php?term=Oligocene&allowed_in_frame=0 )〕 and refers to the sparsity of extant forms of molluscs. The Oligocene is preceded by the Eocene Epoch and is followed by the Miocene Epoch. The Oligocene is the third and final epoch of the Paleogene Period.
The Oligocene is often considered an important time of transition, a link between the archaic world of the tropical Eocene and the more modern ecosystems of the Miocene.〔Haines, Tim; ''Walking with Beasts: A Prehistoric Safari,'' (New York: Dorling Kindersley Publishing, Inc., 1999)〕 Major changes during the Oligocene included a global expansion of grasslands, and a regression of tropical broad leaf forests to the equatorial belt.
The start of the Oligocene is marked by a notable extinction event called the Grande Coupure; it featured the replacement of European fauna with Asian fauna, except for the endemic rodent and marsupial families. By contrast, the Oligocene–Miocene boundary is not set at an easily identified worldwide event but rather at regional boundaries between the warmer late Oligocene and the relatively cooler Miocene.
== Subdivisions ==
Oligocene faunal stages from youngest to oldest are:
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