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Intro | Precambrian Eon | Paleozoic Era | Mesozoic Era | Cenozoic Era

Cenozoic Era: (248 mya-present)

Paleocene | Eocene | Oligocene | Miocene | Pliocene | Pleistocene | Holocene

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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.

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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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.

Miocene extinction

Date:

9 mya

Intensity:

3

Affected:

Many woodland plant-eating herbivores go extinct

Hypotheses:

Global cooling, changes in vegetation

Summary:

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.

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.

-> Go to the Pliocene Epoch

Intro | Precambrian Eon | Paleozoic Era | Mesozoic Era | Cenozoic Era

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