Miocene

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The Miocene Epoch is a period of time that extends from about 23.03 to 5.332 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 μείων (meioon, less) and καινός (kainos, 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.

1 Subdivisions
2 Climate
3 Paleogeography
4 Flora and fauna
5 Oceans
6 See also
7 References
8 External links

[edit] 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.

[edit] 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 of 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 - the Middle Miocene Climate Transition (MMCT). 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.

[edit] 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, although South America was approaching the western subduction zone in the Pacific Ocean, causing both the rise of the Andes and a southward extension of the Meso-American peninsula.

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. The Tethys Seaway continued to shrink and then disappeared as Africa collided with Eurasia in the Turkish-Arabian region between 19 and 12 mya. Subsequent uplift of mountains in the western Mediterranean region and a global fall in sea levels combined to cause a temporary drying up of the Mediterranean Sea (known as the Messinian salinity crisis) near the end of the Miocene.

The global trend was one towards increasing aridity caused primarily by global cooling reducing the ability of the atmosphere to absorb moisture. Uplift of East Africa in the Late Miocene was partly responsible for the shrinking of tropical rain forests in that region, and Australia got drier as it entered a zone of low rainfall in the Late Miocene.

[edit] Flora and fauna

Grasslands underwent a major expansion as forests fell victim to a generally cooler and drier climate overall. Grasses also diversified greatly into different species and also caused a major increase in the biodiversity of large herbivores and grazers, including ruminants.

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

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.

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 appeared, such as the sperm whales. The pinnipeds, which appeared near the end of the Oligocene, became more aquatic.

Perhaps most important were the 100 or so species of apes that lived during this time. They occupied much of the Old World and ranged in size, diet, and anatomy. Due to scanty fossil evidence it is unclear which ape or apes contributed to the modern hominoid clade, but molecular evidence indicates this ape lived from between 15 to 12 million years ago.