System Series Stage Age (Ma) Neogene Miocene Aquitanian younger Paleogene Oligocene Chattian 23.03–28.4 Rupelian 28.4–33.9 Eocene Priabonian 33.9–37.2 Bartonian 37.2–40.4 Lutetian 40.4–48.6 Ypresian 48.6–55.8 Paleocene Thanetian 55.8–58.7 Selandian 58.7–61.7 Danian 61.7–65.5 Cretaceous Upper Maastrichtian older Subdivision of the Paleogene Period according to the IUGS, as of July 2009.
The Oligocene (symbol OG) is a geologic epoch of the Paleogene Period and extends from about 34 million to 23 million years before the present (33.9±0.1 to 23.03±0.05 Ma). 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." Major changes during the Oligocene included 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 and the relatively cooler Miocene.
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)
The Paleogene Period general temperature decline is interrupted by an Oligocene 7M-year stepwise climate change. A deeper 8.2oC 0.4M-year temperature depression leads the 2oC 7M-year stepwise climate change 33.5Ma. The stepwise climate spanned 7M-years 25.5Ma through 32.5Ma as depicted in the PaleoTemps chart. The Oligocene climate change was a global  increase in ice volume and a 55m decrease in sea level (35.7-33.5Ma) with a closely related (25.5-32.5Ma) temperature depression. The 7M-year depression abruptly terminated within 1-2M-year of the La Garita Caldera volcanism event 28-26 Ma. A deep 400 k-year glaciating Oligocene Miocene boundary event is recorded at McMurdo Sound and King George Island.
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 the 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.
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 were present, and temperate trees such as roses,[clarification needed] beech and pine were common. The legumes of the pea and bean family spread, and sedges, bulrushes and ferns continued their ascent.
Important Oligocene land faunas are found on all continents at this time. Even more open landscapes allowed animals to grow to larger sizes than they had earlier in the Paleogene. 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. Many groups, such as horses, entelodonts, rhinoceroses, oreodonts, and camels, became more cursorial during this time, adapting to the plains that were spreading as the Eocene rainforests receded.
South America was isolated from the other continents and evolved a quite distinct fauna during the Oligocene, 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. Brontotheres died out in the Earliest Oligocene, and creodonts died out outside Africa and the Middle East at the end of the period. Multituberculates, an ancient lineage of primitive mammals, also went extinct in the Oligocene. The Oligocene was home to a wide variety of strange mammals. A good example of this would be in the White River Badlands of the United States, which were formerly a semi-arid prairie home to many different types of endemic mammals, including entelodonts like Archaeotherium, camels (such as Poebrotherium), running rhinos, three-toed horses (such as Mesohippus), nimravids, protoceratids, and early dogs like Hesperocyon. Oreodonts, an endemic American group, were very diverse during this time. In Asia during the Oligocene, a group of running rhinos gave rise to the indricotheres, like Indricotherium, which were the largest land mammals ever to walk the Earth.
The marine animals of Oligocene oceans resembled today's fauna, such as the bivalves. The fossil record of marine mammals is a little spotty during this time, and not as well known as the Eocene or Miocene, but some fossils have been found. The baleen and toothed cetaceans (whales) just appeared, and their ancestors, the archaeocete cetaceans began to decrease in diversity due to their lack of echolocation, which was very useful as the water became colder and cloudier. Other factors to their decline could include climate changes and competition with today's modern cetaceans and the carcharhinid sharks, which also appeared in this epoch. Early desmostylians, like Behemotops, are known from the Oligocene. Pinnipeds probably appeared near the end of the epoch from a bear-like or otter-like ancestor.
Oceans continued to cool, particularly around Antarctica.
Recorded extraterrestrial impacts:
- Nunavut, Canada (23 Ma, crater 24 km (15 mi) diameter,)
- ^ "Geologic Age Symbol Font (StratagemAge)". USGS. 99-430. http://pubs.usgs.gov/of/1999/of99-430/of99-430_sec38.pdf. Retrieved 2011-06-22.
- ^ Haines, Tim; Walking with Beasts: A Prehistoric Safari, (New York: Dorling Kindersley Publishing, Inc., 1999)
- ^ A.Zanazzi (et al.) 2007 'Large Temperature Drop across the Eocene Oligocene in central North America' Nature, Vol. 445, 8 February 2007
- ^ C.R.Riesselman (et al.) 2007 'High Resolution stable isotope and carbonate variability during the early Oligocene climate transition: Walvis Ridge (ODP Site 1263) USGS OF-2007-1047
- ^ Lorraine E. Lisiecki Nov 2004; A Pliocene-Pleistocene stack of 57 globally distributed benthic δ18O records Brown University, PALEOCEANOGRAPHY, VOL. 20
- ^ Kenneth G. Miller Jan-Feb 2006; Eocene–Oligocene global climate and sea-level changes St. Stephens Quarry, Alabama GSA Bulletin, Rutgers University, NJ 
- ^ Breining, Greg (2007). "Most-Super Volcanoes". Super Volcano: The Ticking Time Bomb Beneath Yellowstone National Park. St. Paul, MN: Voyageur Press. pp. 256 pg. ISBN 978-0-7603-2925-2.
- Ogg, Jim; June, 2004, Overview of Global Boundary Stratotype Sections and Points (GSSP's) http://www.stratigraphy.org/gssp.htm Accessed April 30, 2006.
- Paleos: Oligocene
- UCMP Berkeley Oligocene Page
- Prehistoric Pictures, in the Public Domain
- Oligocene Leaf Fossils
- Olicgocene Fish Fossils
- PaleoMap Project: Oligocene
- Oligocene Microfossils: 300+ images of Foraminifera
Paleogene Period Paleocene Epoch Eocene Epoch Oligocene Epoch Danian | Selandian
Ypresian | Lutetian
Bartonian | Priabonian
Rupelian | Chattian
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