The
Miocene (symbol M
I) is a
geological epoch of the
Neogene Period and extends from about (Ma). The Miocene was named by
Sir Charles Lyell. Its name comes from the Greek words ('''', “less”) and ('''', “new”) and means "less recent" because it has 18% fewer modern sea
invertebrates than the
Pliocene. The Miocene follows the
Oligocene Epoch and is followed by the
Pliocene Epoch. The Miocene is the first epoch of the
Neogene Period.
The earth went from the Oligocene Epoch through the Miocene and into the Pliocene as it cooled into a series of Ice Ages. The Miocene boundaries are not marked by a single distinct global event but consist rather of regional boundaries between the warmer Oligocene and the cooler Pliocene.
The plants and animals of the Miocene were fairly modern. Mammals and birds were well-established. Whales, seals, and kelp spread. At the end of this epoch, the Himalayas started to rise.
Subdivisions
The Miocene
faunal stages from youngest to oldest are typically named according to the
International Commission on Stratigraphy:
{|
| Messinian
| (7.246–5.332 Ma)
|-
| Tortonian
| (11.608–7.246 Ma)
|-
| Serravallian
| (13.65–11.608 Ma)
|-
| Langhian
| (15.97–13.65 Ma)
|-
| Burdigalian
| (20.43–15.97 Ma)
|-
| Aquitanian
| (23.03–20.43 Ma)
|}
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.
Regionally, other systems are used. These ages often extend across the ICS epoch boundary into the Pliocene and Oligocene:
Australia
Australian Miocene ages are very finely divided in the early Middle Miocene, while most of the rest of the Miocene had a rather constant fauna as far as is known:
{|
|
Mitchellian
| (10.5–5 Ma); extends into the Early Pliocene
|-
|
Bairnsdalian
| (15–10.5 Ma)
|-
|
Balcombian
| (15.5–15 Ma)
|-
|
Batesfordian
| (16.5–15.5 Ma)
|-
|
Longfordian
| (27.5–16.5 Ma); includes much of the Late Oligocene
|}
California
Californian sites provide a sequence distinct from the main North American one:
{|
|
Delmontian
| (7.5 –2.9 Ma); includes much of the Pliocene
|-
|
Mohnian
| (13.5–7.5 Ma)
|-
|
Luisian
| (15.5–13.5 Ma)
|-
|
Relizian
| (16.5–15.5 Ma)
|-
|
Saucesian
| (22–16.5 Ma)
|-
|
Zemorrian
| (33.5–22 Ma); includes nearly all the Oligocene
|}
Japan
Japanese Miocene ages only start in the mid-Burdigalian; the ICS ages are used in much of the Early Miocene:
{|
|
Yuian
| (9.5–3.6 Ma); includes the Early Pliocene
|-
|
Fujian
| (11.1–9.5 Ma)
|-
|
Kaburan
| (13.5–11.1 Ma)
|-
|
Tozawan
| (15.97–13.5 Ma)
|-
|
Haranoyan
| (18.2–15.97 Ma)
|}
New Zealand
In
New Zealand, the following ages are recognized:
{|
|
Kapitean
| (6–4.8 Ma); extends into the Early Pliocene
|-
|
Tongaporutuan
| (10–6 Ma)
|-
|
Waiauan
| (11.5–10 Ma)
|-
|
Lillburnian
| (15–11.5 Ma)
|-
|
Cliffdenian
| (16.5–15 Ma)
|-
|
Altonian
| (17.5–16.5 Ma)
|-
|
Awamoan
| (20–17.5 Ma)
|-
|
Hutchinsonian
| (21–20 Ma)
|-
|
Otaian
| (23.03–21 Ma)
|}
North America
In most of North America, faunal stages are defined according to the land mammal fauna (North American Land Mammal Ages or
NALMAs):
{|
| Hemphillian
| (9–4.75 Ma); includes much of the Early Pliocene
|-
| Clarendonian
| (11.8–9 Ma)
|-
| Barstovian
| (15.5–11.8 Ma)
|-
| Hemingfordian
| (19–15.5 Ma)
|-
| Arikareean
| (30.5–19 Ma); includes much of the Oligocene
|}
South America
In
South America, a system similar to the North American one is used; its periods are correspondingly called
SALMAs (South American Land Mammal Ages):
{|
|
Huayquerian
| (9–5.4 Ma); the
Montehermosan barely extends into the Pliocene
|-
|
Chasicoan
| (10–9 Ma)
|-
|
Mayoian
| (12–10 Ma)
|-
|
Laventan
| (13.8–12 Ma)
|-
|
Colloncurian
| (15.5–12 Ma)
|-
|
Friasian
| (16.3–15.5 Ma)
|-
|
Santacrucian
| (17.5–16.3 Ma)
|-
|
Colhuehuapian
| (21–17.5 Ma)
|-
|
Deseadan
| (29–21 Ma); includes much of the Oligocene
|}
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, Europe, and east Asia. 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 dramatic new 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 Ma. The 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 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.
Life
Flora
Grasslands underwent a major expansion; forests fell victim to a generally cooler and drier climate overall. Grasses also diversified greatly,
co-evolving with large herbivores and grazers, including
ruminants. Between 7 and 6 million years ago, there occurred a sudden expansion of grasses which were able to assimilate
carbon dioxide more efficiently but were also richer in
silica, causing a worldwide extinction of large herbivores. The expansion of grasslands and
radiations among terrestrial herbivores such as horses can be linked to fluctuations in CO
2.
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.
In the Early Miocene, several Oligocene groups were still diverse, including
nimravids,
entelodonts, and three-toed horses. Like in the previous Oligocene epoch,
oreodonts were still diverse, only to disappear in the earliest
Pliocene. During the later Miocene mammals were more modern, with recognizable
dogs,
raccoons,
horses,
beaver,
deer,
camels, and
whales, along with now extinct groups like
borophagine dogs,
gomphotheres,
three-toed horses, and semi-aquatic and hornless rhinos like ''
Teleoceras'' and ''
Aphelops''. Islands began to form between South and North America in the Late Miocene, allowing ground sloths like ''
Thinobadistes'' to
island-hop to North America.
Unequivocally recognizable dabbling ducks, plovers, typical owls, cockatoos and crows appear during the Miocene. By the epoch's end, all or almost all modern bird 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.
Approximately 100 species of apes lived during this time. They ranged over much of the Old World and varied widely in size, diet, and anatomy. Due to scanty fossil evidence it is unclear which ape or apes contributed to the modern hominid clade, but molecular evidence indicates this ape lived from between 15 to 12 million years ago.
In the oceans, Brown algae, called kelp, proliferated, supporting new species of sea life, including otters, fish and various invertebrates.
Cetaceans attained their greatest diversity during the Miocene, with over 20 recognized genera in comparison to only six living genera. This diversification correlates with emergence of gigantic macro-predators such as megatoothed sharks and raptorial sperm whales. Prominent examples are ''C. megalodon'' and ''L. melvillei''.
Crocodilians also showed signs of diversification during Miocene. The largest form among them was a gigantic caiman ''Purussaurus'' which inhabited South America. Another gigantic form was a false gharial ''Rhamphosuchus'', which inhabited modern age India. A strange form ''Mourasuchus'' also thrived alongside ''Purussaurus''. This species developed a specialized filter-feeding mechanism, and it likely preyed upon small fauna despite of its gigantic size.
The pinnipeds, which appeared near the end of the Oligocene, became more aquatic. Prominent genus was Allodesmus.
Oceans
There is evidence from oxygen isotopes at
Deep Sea Drilling Program sites that ice began to build up in Antarctica about 36 Ma during the
Eocene. Further marked decreases in temperature during the
Middle Miocene at 15 Ma probably reflect increased ice growth in Antarctica. It can therefore be assumed that East Antarctica had some glaciers during the early to mid Miocene (23–15 Ma). Oceans cooled partly due the formation of the
Antarctic Circumpolar Current, and about 15 million years ago the ice cap in the southern hemisphere started to grow to its present form. The Greenland ice cap developed later, in the
Middle Pliocene time, about 3 million years ago.
Middle Miocene disruption
See also
Geologic Time Scale
List of fossil sites
:Category:Miocene animals
Lystromycter
Footnotes
==References==
(1998): The Cenozoic. Retrieved 2008-SEP-20.
(2008): The impact of Miocene atmospheric carbon dioxide fluctuations on climate and the evolution of terrestrial ecosystems. ''PNAS'' 105(2): 449–53. (HTML abstract) Supporting tables
(2005): GeoWhen Database. Retrieved 2011-03-08.
(1999): ''Earth system history''. Freeman, New York. ISBN 0716728826
Further reading
(1993): ''Biogeography. An ecological and evolutionary approach'' (5th ed.). Blackwell Scientific Publications, Cambridge. ISBN 0632029676
(2004):
Overview of Global Boundary Stratotype Sections and Points (GSSP's). Retrieved 2006-04-30.
External links
PBS Deep Time: Miocene
UCMP Berkeley Miocene Epoch Page
Miocene Microfossils: 200+ images of Miocene Foraminifera
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