The Oligocene epoch is a geologic period that 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 (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 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:

Climate

Paleogeography

During this period, the continents continued to drift toward their present positions. Antarctica continued to become more isolated and finally developed a permanent ice cap.(Haines)

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, isolating the remnants of Tethys Sea. 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 since the faunas of the two regions are very similar. During sometime in the Oligocene, South America was finally detached from Antarctica and drifted north towards North America. It also allowed the Antarctic Circumpolar Current to flow, rapidly cooling the continent.

Flora

Angiosperms continued their expansion throughout the world; tropical and sub-tropical forests were replaced by temperate deciduous woodlands. Open plains and deserts became more common. Grasses expanded from the water-bank habitat in the Eocene and moved out into open tracts; however even at the end of the period it was not quite common enough for modern savanna.(Haines)

In North America, subtropical species dominated with cashews and lychee trees present, and temperate trees such as roses, beech and pine were common. The legumes of the pea and bean family spread, and sedges, bulrushes and ferns continued their ascent.

Fauna

Hyaenodon.

Mesohippus.

Reptiles were abundant in the Oligocene. Choristodera, a group of semi-aquatic, crocodile-like, diapsid (archosauromorph?) reptiles originated in the Jurassic, possibly as far back as Late Triassic. Early in the Oligocene, the Choristodera became extinct, possibly due to climate changes. Snakes and lizards did diversify to a degree.

Mammals included:Brontotherium, Indricotherium, Entelodont, Hyaenodon, Mesohippus. Elephant-like forms, Proboscidea, were present.

The Oligocene oceans resembled today's fauna, such as the bivalves. The baleen and toothed cetaceans (whales) just appeared, and their ancestors, the Archaeoceti cetaceans remained relatively common but their numbers were falling as Oligocene progressed because of climate changes and competition with today's modern cetaceans and the Charcharinid sharks, which also appeared in this epoch. Pinnipeds probably appeared near the end of the epoch from a bear-like or otter-like ancestor.

Oceans

Oceans continued to cool, particularly around Antarctica.

Impact Events

Recorded extraterrestrial impacts:

• Nunavut, Canada (23 Ma, crater 24 km diameter,)

Supervolcanic explosions

La Garita Caldera (28 thru 26 million years ago, VEI=9.1)^[6]

See also

• List of fossil sites (with link directory)
• Turgai Sea

References

• Ogg, Jim; June, 2004, Overview of Global Boundary Stratotype Sections and Points (GSSP's) http://www.stratigraphy.org/gssp.htm Accessed April 30, 2006.

External links

• BBC Changing Worlds: Oligocene
• Paleos: Oligocene
• UCMP Berkeley Oligocene Page
• Prehistoric Pictures, in the Public Domain
• Oligocene Leaf Fossils
• Olicgocene Fish Fossils
• PaleoMap Project
• PaleoMap Project: Oligocene

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