Oligocene

Mesohippus.

The Oligocene epoch is a geologic period 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 ceno (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 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."(Haines)

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.

Paleogene period
Paleocene epoch	Eocene epoch	Oligocene epoch
Danian \| Selandian Thanetian	Ypresian \| Lutetian Bartonian \| Priabonian	Rupelian \| Chattian

Oligocene Subdivisions

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)

Oligocene Climate

Climates remained warm, although the slow global cooling that eventualty led to the Pleistocene glaciations started around the end of the epoch.

Oligocene 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. 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. During sometime in the Oligocene, South America was finally separated from Antarctica, and drifted north, towards North America. It also allowed the Circumpolar currents to flow around Antarctica, rapidly cooling the continent.

Oligocene 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 common. The legumes of the pea and bean family spread, and sedges, bulrushes and ferns continued their ascent.

Oligocene Fauna

Important Oligocene land faunas are found on all continents except Australia. Even more open landscapes allowed animals to grow to larger sizes than they had earlier in the Paleogene.(Haines) 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.

Mammals

Brontotherium, Indricotherium, Enteledont, Hyaenodon, Mesohippus

Reptiles

Reptiles were abundant in the Oligocene. Choristodera, a group of semi-aquatic, crocodile-like, diapsid (archosauromorph?) reptiles. They 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 in a degree.

Birds

Eurotrochilus

Sea Life

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 due to climate changes and competition with today's modern cetaceans and the Charcharinid sharks, who also appeared in this epoch. Pinnipeds probably appeared near the end of the epoch from a bear-like or otter-like ancestor.

Oligocene Oceans

Oceans continued to cool, particularly around Antarctica.

References
ISBN links support NWE through referral fees

Haines, Tim; Walking with Beasts: A Prehistoric Safari, (New York: Dorling Kindersley Publishing, Inc., 1999)
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

Credits

New World Encyclopedia writers and editors rewrote and completed the Wikipedia article in accordance with New World Encyclopedia standards. This article abides by terms of the Creative Commons CC-by-sa 3.0 License (CC-by-sa), which may be used and disseminated with proper attribution. Credit is due under the terms of this license that can reference both the New World Encyclopedia contributors and the selfless volunteer contributors of the Wikimedia Foundation. To cite this article click here for a list of acceptable citing formats.The history of earlier contributions by wikipedians is accessible to researchers here:

Oligocene ^history

The history of this article since it was imported to New World Encyclopedia:

History of "Oligocene"

Note: Some restrictions may apply to use of individual images which are separately licensed.