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

Mesozoic Era: (248-65 mya)

Triassic | Jurassic | Cretaceous

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Jurassic Period (206-144 mya)

The Pangaea supercontinent begins to drift apart. Notably, present-day Greenland and North America split from Europe and Africa, and the Rocky Mountain and Sierra Nevada ranges form. Sea levels rise to flood much of the continents, allowing extensive shallow-water habitats to establish. In the Jurassic's warm climate, tropical marine species, like various forms of phytoplankton and algae, prosper. Sponges, corals, bivalves, ammonoids, and belemnites also flourish. Among marine vertebrates, modern sharks share the seas with plentiful bony fishes.

On land, insects grow in number and kind, as do dinosaurs, which range in size from small iguanodonts to massive brachiosaurs. Abundant and lush vegetation supports the herbivores. Cycads and conifers are the dominant tall trees, while ferns feature among the smaller plants. The herbivores, in turn, support the carnivores. Some smaller, carnivorous theropod dinosaurs display feathers. Their descendants, the birds, soon join the pterosaurs in the skies. Mammals continue to evolve, but their heyday will not come until the demise of dinosaurs at the end of the Cretaceous.

200 mya: Pangaea starts to break apart

Accumulated heat from deep within Earth causes the Pangaea supercontinent to break apart. Rift valleys appear where continents separate. Over the next 160 million years, fragments reposition themselves as today's continents.

150 mya: Birds

Most authorities are now convinced that birds evolved directly from the small, bipedal theropod dinosaurs. Early birds share structural features with these dinosaurs, most notably the same type of pelvis, a long bony tail, hollow bones, clawed fingers, and a tooth-filled jaw. Modern bird features such as a breastbone and beak have not yet evolved. The fossil species Archaeopteryx, first discovered in Germany in the 1860s, is the oldest-known example of a bird. The shape of its feathers suggests that it flew. A recent discovery in northeast China provides support to the theory that theropods evolved feathers for warmth before birds used them in flight.

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Exaptation (150 mya)

It is commonly supposed that birds evolved feathers to facilitate flying. In fact, these anatomical features may have evolved for an entirely different reason.

Fossil discoveries suggest that feathers first evolved on theropod dinosaurs. The two-legged dromaeosaur, for example, a small, wingless meat-eater whose fossil was recently found in China, was coated with them. Scientists think feathers most likely served to insulate these dinosaurs' bodies. This provides further support to those who suggest that some lineages of dinosaurs (at least the ones from which warm-blooded birds evolved) may have been warm-blooded themselves.

As was the case with the limbs of early land vertebrates, feathers evolved for a reason other than that for which they were later used. This concept is known as exaptation, or, less formally, "pre-adaptation."

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-> Go to the Cretaceous Period

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

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