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Oligocene

2007 Schools Wikipedia Selection. Related subjects: Geology and geophysics

   Mesohippus.
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   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.

   See also: PaleoMap Project: Oligocene

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.
   Retrieved from " http://en.wikipedia.org/wiki/Oligocene"
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