In geologic time, the Oligocene Epoch occurs during the Tertiary Period (also sometimes divided or referred to in terms of a Paleogene Period and a Neogene Period) of the Cenozoic Era of the Phanerozoic Eon. The Oligocene Epoch is the third epoch in the Tertiary Period (in the alternative, the latest (most recent) epoch in the Paleogene Period).

The Oligocene Epoch lasts from approximately 34 million years ago (mya) to 23 mya.

The Oligocene Epoch is further subdivided into (from earliest to most recent) Rupelian (34 mya to 29 mya) and Chattian (29 mya to 23 mya) stages. The Oligocene Epoch was preceded by the Eocene Epoch and was followed by the Miocene Epoch.

Large impact craters dating to the end of the Eocene Epoch and the start of the start of the Oligocene Epoch are evident in Russia (Popigal crater) and in the Chesapeake Bay of the United States. Craters dating to the end of the Oligocene Epoch and start of the Miocene Epoch can be studied in Northwest Canada and in Logancha, Russia. Volcanic activity also increased during the Oligocene Epoch.

The Oligocene Epoch climate was warmer than the modern climate. Evidence of the start of a generalized cooling trend is, however, in accord with the rise of warm-blooded mammals as the dominant land species. The Oligocene Epoch continued to present the slow climatic changes that allowed continued development and diversification of mammals.

Notable finds in the fossil record that date to the Oligocene Epoch include Branisella monkeys. The first fossils of Australian marsupials date to Oligocene Epoch fossil beds. Roses and orchids appeared by the end of the Oligocene Epoch.

See Also

Archean; Cambrian Period; Cretaceous Period; Dating Methods; Devonian Period; Evolution, Evidence Of; Fossils and Fossilization; Historical Geology; Holocene Epoch; Jurassic Period; Mesozoic Era; Mississippian Period; Ordovician Period; Paleocene Epoch; Paleozoic Era; Pennsylvanian Period; Pleistocene Epoch; Pliocene Epoch; Precambrian; Proterozoic Era; Quaternary Period; Silurian Period; Supercontinents; Triassic Period