The Miocene is the first geological epoch of the Neogene period and extends from about
23.03 to 5.332 million years ago. The Miocene was named by Sir Charles Lyell. Its name comes
from the Greek words μείων and καινός 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 earth went from the Oligocene 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 apes arose and diversified during the Miocene epoch, becoming widespread in the
Old World. In fact, by the end of this epoch, the ancestors of humans had split away from
the ancestors of the chimpanzees to follow their own evolutionary path. As in the Oligocene
before it, grasslands continued to expand and forests to dwindle in extent. In the Miocene
seas, kelp forests made their first appearance and soon became one of Earth's most productive
ecosystems. The plants and animals of the Miocene were fairly modern. Mammals and birds
were well-established. Whales, seals, and kelp spread. The Miocene epoch is of particular
interest to geologists and palaeoclimatologists as major phases of Himalayan uplift had occurred
during the Miocene epoch affecting monsoonal patterns in Asia, which were interlinked with
glaciations in the northern hemisphere.
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 and foraminifera. 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. 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 North 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 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. 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. 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.
Life Life during the Miocene Epoch was mostly supported
by the two newly formed biomes, kelp forests and grasslands. This allows for more grazers,
such as horses, rhinoceroses,and hippos. Ninety five percent of modern plants existed by the
end of this epoch. Flora
The coevolution of gritty, fibrous, fire-tolerant grasses and long-legged gregarious ungulates
with high-crowned teeth, led to a major expansion of grass-grazer ecosystems, with roaming herds
of large, swift grazers pursued by predators across broad sweeps of open grasslands, displacing
desert, woodland, and browsers. The higher organic content and water retention of the
deeper and richer grassland soils, with long term burial of carbon in sediments, produced
a carbon and water vapor sink. This, combined with higher surface albedo and lower evapotranspiration
of grassland, contributed to a cooler, drier climate. C4 grasses, which are able to assimilate
carbon dioxide and water more efficiently than C3 grasses, expanded to become ecologically
significant near the end of the Miocene between 6 and 7 million years ago. The expansion of
grasslands and radiations among terrestrial herbivores correlates to fluctuations in CO2.
Cycads between 11.5 and 5 m.y.a. began to rediversify after previous declines in variety
due to climatic changes, and thus modern cycads are not a good model for a "living fossil".
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 easily recognizable dogs, bears, 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. The expansion of silica-rich C4 grasses led
to worldwide extinctions of herbivorous species without high-crowned teeth.
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,
rather than 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, ranging throughout Africa, Asia and Europe and varying 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 between
7 and 8 million years ago. The first hominins appeared in Africa at the very end of the
Miocene, including Sahelanthropus, Orrorin, and an early form of Ardipithecus.
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.
Other notable large sharks were C. chubutensis, Isurus hastalis, and Hemipristis serra.
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 its gigantic size.
The pinnipeds, which appeared near the end of the Oligocene, became more aquatic. Prominent
genus was Allodesmus. A ferocious walrus, Pelagiarctos may have preyed upon other species
of pinnipeds including Allodesmus. Furthermore, South American waters witnessed
the arrival of Megapiranha paranensis, which were considerably larger than modern age piranhas.
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. Oceans cooled partly due to 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
The "Middle Miocene disruption" refers to a wave of extinctions of terrestrial and aquatic
life forms that occurred following the Miocene Climatic Optimum, around 14.8 to 14.5 million
years ago, during the Langhian stage of the mid-Miocene. A major and permanent cooling
step occurred between 14.8 and 14.1 Ma, associated with increased production of cold Antarctic
deep waters and a major growth of the East Antarctic ice sheet. A Middle Miocene δ18O
increase, that is, a relative increase in the heavier isotope of oxygen, has been noted
in the Pacific, the Southern Ocean and the South Atlantic.
See also Astaracian
Turolian Vallesian
Geologic time scale List of fossil sites
Category:Miocene animals References
Further reading Cox, C. Barry & Moore, Peter D.: Biogeography.
An ecological and evolutionary approach. Blackwell Scientific Publications, Cambridge. ISBN 0-632-02967-6
Ogg, Jim: Overview of Global Boundary Stratotype Sections and Points. Retrieved 2006-04-30.
External links PBS Deep Time: Miocene
UCMP Berkeley Miocene Epoch Page Miocene Microfossils: 200+ images of Miocene
Foraminifera