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Miocene Epoch (24-5.3 mya)
Early in the Miocene, temperatures begin to rise. Extensive mountain
building in the Americas and Asia alters air circulation and weather patterns, contributing
to drier overall conditions. Antarctica's separation from South America during the Oligocene,
however, inhibits the mixing of warm, tropical waters with cool, polar waters. Antarctica
freezes, and global climate cools again. Marine mammals like seals and whales flourish as
deep, nutrient-rich waters are stirred up in the process.
Land bridges, which form as the oceans recede and as inland waters
dry out, encourage new waves of animal migration between continents. In the cooler,
drier conditions, grasslands begin to dominate the Americas and eastern Eurasia.
Grassland habitats offer plenty of food for evolving grazers, including deer and antelope.
Horses, which first appeared in the Eocene, grow considerably larger, and some now display
a single toe on each limb instead of three or four. Animals that do not adapt to the more
severe climate and coarser vegetation, such as giant camels, which feed on leafy trees
much like a giraffe, teeter on the verge of extinction.
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24 mya: Short-term warming and drying
The Miocene begins with a gradual, short-lived warming, and some
tropical forests expand. Drying accompanies this temperature change, and tough scrub
plants evolve as a new form of vegetation.
20 mya: Great mountain building
Where continental plates collide, Earth's crust is lifted. Over
about 10 million years, mountains form in North America (the Cascades), South America
(the Andes), and Asia (the Himalayas). These massive formations disrupt wind and weather
patterns, altering rainfall distribution. As newly exposed rock surfaces weather,
carbon dioxide is removed from the atmosphere. This might contribute to global cooling.
20 mya: Kelp forests
Communities of large brown algae, called kelp, support evolving
marine life, such as sea otters, as well as established groups of fishes and
invertebrates. Though kelp is a plant, it is not closely related to its land
counterparts. For one thing, its cells use different pigments for photosynthesis.
Kelp grows in cool, shallow waters, where it attaches to rocks and coral
or sometimes floats freely. Because marine plants do not preserve well over time,
scientists can date kelp only to the Miocene, when animals it supports are known
to appear, but it may exist in earlier periods.
20 mya: Inland seas dry out
The shifting continents, changing climate patterns, and formation
of a polar ice cap cause sea levels to drop and inland seas to shrink. Land routes
open between continents, most notably between Africa and Eurasia, and Eurasia and
North America.
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Read more
Crossing over (20 mya)
The great diversification of land mammals during the Miocene is
due in large part to the formation of land bridges. These routes, which emerge as
sea levels drop and inland seas dry out, connect continents previously separated by
water. They provide access to new habitats and enable migrating animals to greatly
extend their geographic ranges.
Routes between Africa, Eurasia, and North America are the primary
migratory paths. The once-great Tethys Ocean no longer divides Africa and Eurasia.
Elephants and apes are among the mammals that venture out of Africa and settle in
parts of Eurasia, while rabbits, pigs, saber-toothed cats, and modern rhinos move
in the opposite direction. To the north, a dry corridor, the Bering land bridge,
connects what are now Siberia and Alaska. Eventually, both elephants and rhinos make
their way to North America, perhaps crossing paths with horses on their way to
Eurasia.
At the close of the Miocene, North America and South America are
close enough for some species to cross the narrow dividing waters. Ground sloths,
which had evolved in isolation with other South American species, make their first
appearance in the north, while raccoons can be found in the south. The island
continent of Australia welcomes visitors from southeast Asia, like rodents, which
may travel along the Malaysian island chain to get there.
Surprisingly, the large numbers of "invading" species probably do
not force great numbers of native species to extinction. Animals that go extinct in
the Miocene more likely do so because they fail to adapt to changes in climate and
vegetation.
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18 mya: Grazing horses
Horses first appeared in the early Eocene as cat-sized herbivores,
feeding on leafy vegetation. As coarse grasses replace this woodland vegetation,
however, some horse species evolve larger jaws and deep-rooted teeth with protective
enamel. They also evolve larger guts, because grasses are relatively poor in nutrition
and must be eaten in higher quantities to compensate. Grazing horses are larger than
their browsing cousins, with longer legs and hooves that enable them to run faster
than those with padded feet. They quickly spread from North America to Europe and
Asia, and from there to Africa, where some species become today's zebras.
16 mya: Orangutan line separates from African ape/hominid line
Orangutans, or Asian apes, are modern, tree-dwelling primates,
specialized for four-limbed climbing. While numerous fossils reveal a great deal
about the skulls of their ancestors, scientists still know little about the body
anatomy, posture, and locomotion of the earliest Asian apes.
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Miocene extinction
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Date:
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9 mya
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Intensity:
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3
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Affected:
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Many woodland plant-eating
herbivores go extinct
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Hypotheses:
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Global cooling, changes
in vegetation
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Summary:
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Antarctica's deep freeze, the
spread of grasslands, and perhaps even the effects of mountain building probably
contribute to the extinction of several browsing mammal species. Animals adapted
for cooler conditions and coarse, grassy vegetation fare well. Many browsing
herbivores that favor fruits and soft, broad-leaved ferns and shrubby plants,
however, perish. Certain groups illustrate this apparent pattern. Grazing horses
flourish, while browsing horses go extinct. Of the early camels, hypsodont forms
-- those with long, heavily enameled teeth -- survive, while short-toothed
species disappear.
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6 mya: Chimpanzee and hominid lines separate
Chimpanzees are the most closely related primates to humans. Chimps
share about 98.8 percent of their DNA with humans, and, based on the slight differences
in their DNA sequence, scientists calculate that they split off on a separate evolutionary
line only about 6 mya.
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