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Miocene

2007 Schools Wikipedia Selection. Related subjects: Geology and geophysics

   The Miocene epoch is a period of time that extends from about 23 to 5.3
   million years before the present. As with other older geologic periods,
   the rock beds that define the start and end are well identified, but
   the exact dates of the start and end of the period are uncertain. The
   Miocene was named by Sir Charles Lyell. Its name comes from the Greek
   words meion (less) and ceno (new) and means "less recent" because it
   has 18% (less than the Pliocene) of modern sea invertebrates. The
   Miocene follows the Oligocene Epoch and is followed by the Pliocene
   Epoch. The Miocene is the first epoch of the Neogene period.

   The Miocene boundaries are not set at an easily identified worldwide
   event but rather at regional boundaries between the warmer Oligocene
   and the cooler Pliocene.

Miocene subdivisions

   The Miocene faunal stages from youngest to oldest are typically named
   according to the International Commission on Stratigraphy:
   Messinian    (7.246 – 5.332 mya)
   Tortonian    (11.608 – 7.246 mya)
   Serravallian (13.65 – 11.608 mya)
   Langhian     (15.97 – 13.65 mya)
   Burdigalian  (20.43 – 15.97 mya)
   Aquitanian   (23.03 – 20.43 mya)

   These subdivisions within the Miocene are defined by the relative
   abundance of different species of calcareous nanofossils (calcite
   platelets shed by brown single-celled algae) and foraminifera
   (single-celled protists with diagnostic shells). Two subdivisions each
   form the Early, Middle and Late Miocene.

   In most of North America, faunal stages are defined according to the
   land mammal fauna ( NALMA). They overlap the borders of the Miocene and
   Oligocene/Pliocene:
   Hemphillian   (9 – 4.75 mya); includes much of the Early Pliocene
   Clarendonian  (11.8 – 9 mya)
   Barstovian    (15.5 – 11.8 mya)
   Hemingfordian (19 – 15.5 mya)
   Arikareean    (30.5 – 19 mya); includes much of the Oligocene

   Californian sites, which are derived from the former Farallon Plate,
   provide another sequence which also overlaps with the epoch
   boundaries :
   Delmontian (7.5 – 2.9 mya); includes much of the Pliocene
   Mohnian    (13.5 – 7.5 mya)
   Luisian    (15.5 – 13.5 mya)
   Relizian   (16.5 – 15.5 mya)
   Saucesian  (22 – 16.5 mya)
   Zemorrian  (33.5 – 22 mya); includes nearly all the Oligocene

   Yet other systems are used to describe the Miocene stratigraphy of
   Japan, Australia and New Zealand.

Miocene climate

   Climates remained moderately warm although the slow global cooling that
   eventually led to the Pleistocene glaciations continued.

   Although a long-term cooling trend was well underway, there is evidence
   for a warm period during the Miocene when the global climate rivalled
   that of the Oligocene. The Miocene warming began 21 million years ago
   and continued until 14 million years ago, when global temperatures took
   a sharp drop. By 8 million years ago, temperatures dropped sharply once
   again, and the Antarctic ice sheet was already approaching its
   present-day size and thickness. Greenland may have begun to have large
   glaciers as early as 7 to 8 million years ago, although the climate for
   the most part remained warm enough to support forests there well into
   the Pliocene.

Miocene paleogeography

   Continents continued to drift toward their present positions. Of the
   modern geologic features, only the land bridge between South America
   and North America was absent.

   Mountain building took place in Western North America and Europe. Both
   continental and marine Miocene deposits are common worldwide with
   marine outcrops common near modern shorelines. Well studied continental
   exposures occur in the American Great Plains and in Argentina. India
   continued to collide with Asia, creating more mountain ranges. Tethys
   Seaway continued to shrink.

Miocene flora

   Grasslands underwent a major expansion as forests fell victim to a
   generally cooler and drier climate overall. Grasses also diversified
   greatly into a number of species and also caused a major increase in
   the biodiversity of large herbivores and grazers, including ruminants
   (of which modern cattle and deer belong).

Miocene fauna

   Both marine and continental fauna were fairly modern, although marine
   mammals were less numerous. Only in isolated South America and
   Australia did widely divergent fauna exist.

Mammals

   These were also modern, with recognizable wolves, raccoons, horses,
   beaver, deer, camels, and whales.

Birds

   Recognizable crows, ducks, auks, grouses and owls appear in the
   Miocene. By the epoch's end, all or almost all modern families are
   believed to have been present; the few post-Miocene bird fossils which
   cannot be placed in the evolutionary tree with full confidence are
   simply too badly preserved instead of too equivocal in character.
   Marine birds reached their highest diversity ever in the course of this
   epoch.

Sea life

   Brown algae plants, called kelp, proliferate, supporting new species of
   sea life, including otters, fish and various invertebrates. The
   cetaceans diversified and some modern genera have already appeared,
   such as the sperm whales. The pinnipeds, who appeared near the end of
   the Oligocene, became more and more aquatic.

Miocene oceans

   The oceans continue to cool as the poles were transformed into
   glaciers.

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