Oligocene

The Oligocene is a geologic epoch of the Paleogene period and extends from about 33.9 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 ὀλίγος (oligos, few) and καινός (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 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-looking ecosystems of the Miocene."[1] The Oligocene change in ecosystems is 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 major extinction event, a faunal replacement of European with Asian fauna except for the endemic rodent and marsupial families called the Grande Coupure. The Oligocene-Miocene boundary is not set at an easily identified worldwide event but rather at regional boundaries between the warmer late Oligocene [26,23 Ma] and the relatively cooler Miocene.

Subdivisions
Oligocene faunal stages from youngest to oldest are:
 * Chattian or Late Oligocene 	(28.4 ± 0.1 – 23.03 mya)
 * Rupelian or Early Oligocene 	(33.9 ± 0.1 – 28.4 ± 0.1 mya)