Everything about The Miocene totally explained
The
Miocene Epoch is a
period of time that extends from about 23.03 to 5.33 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% (fewer 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.
As the earth cooled, it went from the Oligocene epoch through the Miocene and into the Pliocene. 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.
The plants and animals of the Miocene were fairly modern. Mammals and birds were well-established. Whales, seals, and kelp spread.
Subdivisions
The Miocene
faunal stages from youngest to oldest are typically named according to the
International Commission on Stratigraphy:
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 (North American Land Mammal Ages or
NALMAs). They overlap the borders of the Miocene and Oligocene/Pliocene:
Californian sites, which are derived from the former
Farallon Plate, provide another sequence which also overlaps with the epoch boundaries:
Yet other systems are used to describe the Miocene stratigraphy of
Japan,
Australia and
New Zealand.
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.
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.
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 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 bird
families are believed to have been present; the few post-Miocene bird fossils which can't 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, 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.
Approximately 100 species of
apes lived during this time. They occupied much of the
Old World and ranged in size, diet, and anatomy. Due to scanty fossil evidence it's 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.
In the oceans, modern
sharks appeared at this time including the huge
Megalodon.
Cetaceans, such as
dolphins,
whales, and
porpoises evolved. Their ancestors the
Archaeoceti, however, were becoming less common and eventually became extinct.
Oceans
East Antarctica had some glaciers during the early Miocene (23-15 million years ago). Oceans cooled partly due the formation of
ACC, 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
Pliocene time, about 3 million years ago.
Middle Miocene disruption
Footnotes
Further Information
Get more info on 'Miocene'.
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