The Oligocene is a geologic epoch of the Paleogene period and extends from about 33.9 million to 23 million years before the present. 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.(1)Tim Haines (1999) Walking with Beasts: A Prehistoric Safari Major changes during the Oligocene included a global expansion of grasslands, and a regression of tropical broad leaf forests to the equatorial belt.
During this period, the continents continued to drift toward their present positions. 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. Marine fossils from the Oligocene are rare in North America. Sometime during 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 Antarctic continent. Antarctica became more isolated and finally developed a permanent ice cap.
Flowering plants continued their expansion throughout the world as tropical and sub-tropical forests were replaced by temperate deciduous forests. Open plains and deserts became more common and grasses expanded from their water-bank habitat in the Eocene moving out into open tracts. However, even at the end of the period grass was not as common as in modern savannas. In North America, subtropical species dominated the ecology with cashews and lychee trees present, along with temperate trees such as beeches and pines.
More open landscapes allowed animals to grow to larger sizes than they had earlier in the Paleogene. Marine animals became fairly modern, as did land animals on the northern continents. This was probably more as a result of older forms dying out than as a result of more modern forms evolving. Many groups, such as horses, entelodonts, rhinoceroses, oreodonts, and camels, became more able to run during this time, adapting to the plains that were spreading as the Eocene rainforests receded. The first cats originated in Asia during the late Oligocene and spread to Europe.(2)Maryann Mott (January 11, 2006) “Cats Climb New family Tree” National Geographic News South America was isolated from the other continents and evolved a quite distinct animals during the Oligocene. The South American continent became home to strange animals such as pyrotheres and astrapotheres, as well as litopterns and notoungulates. Sebecosuchian crocodiles, terror birds, and carnivorous marsupials, like the borhyaenids remained the dominant predators.
The Oligocene saw the beginning of modern ocean circulation, with tectonic shifts causing the opening and closing of ocean gateways. Cooling of the oceans had already commenced by the Eocene-Oligocene boundary,(3)Mitchell Lyle et al. (2008) “Pacific Ocean and Cenozoic evolution of climate” Reviews of Geophysics, Volume 46, Number 2, Pages 1–47 and they continued to cool as the Oligocene progressed. The formation of permanent Antarctic ice sheets during the early Oligocene and possible glacial activity in the Arctic may have influenced this oceanic cooling.
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|1.||⇑||Tim Haines (1999) Walking with Beasts: A Prehistoric Safari|
|2.||⇑||Maryann Mott (January 11, 2006) “Cats Climb New family Tree” National Geographic News|
|3.||⇑||Mitchell Lyle et al. (2008) “Pacific Ocean and Cenozoic evolution of climate” Reviews of Geophysics, Volume 46, Number 2, Pages 1–47|