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A pterosaur ("winged lizard") was a flying reptile of the order Pterosauria. They existed from the late Triassic to the Cretaceous periods (from 228 to 65 million years ago). The earlier species had long, fully-toothed jaws and long tails, while later forms had a stump for a tail, no teeth and a jaw more like a beak than the elongated jaw of the earlier species. Pterosaurs are classified within an overarching group (Archosaurs) that also includes crocodiles, dinosaurs (including birds) and thecodonts.

Pterosaurs were first discovered in 1784 by the Italian naturalist Cosimo Collini. He initally belived that pterosaurs were aquatic animals, not flyers. In the 19th century Baron Georges Cuvier proposed that pterosaurs flew.

At least 60 genera of pterosaurs have been found, ranging from the size of a small bird to wingspans in excess of 40 feet. Since the first pterosaur fossil was discovered in the late Jurassic Solnhofen limestone in 1784, twenty-nine kinds of pterosaurs have been found in those deposits alone. Most paleontologists now believe that pterosaurs were adapted for active flight, not just gliding as was earlier believed.

Most pterosaur fossils did not preserve well. Their bones were hollow, and when sediments piled on top of them, the bones were flatened. The best preserved fossils have come from Aripe, Brazil. For some reason, when the bones were deposited, the sediments encapsualted the bones, rather then crushing them. This created the dimensional fossils for palentologists to study. The Aripe find was discovered in 1974.

Pterosaur wings were thin membranes of skin, strengthened by closely spaced fibers and similar to the wings of bats, attached to the extremely long fourth finger of each arm and extending along the sides of the body. There is no fossil evidence of feathers, but pterosaurs were unique among reptiles in that at least some of them were covered with hair, similar but not homologous to mammalian hair. Although in some cases fibers in the wing membrane have been mistaken for hair, some fossils such as those of Sordes pilosus ( the "hairy demon") do show the unmistakeable imprints of hair on the head and body, not unlike modern-day bats. The presence of fur (and the demands of flight) imply that pterosaurs were warm-blooded ('endothermic').

Their bones were hollow and had openings at each end. Unlike typical reptiles, pterosaurs had a keeled breastbone that was developed for the attachment of flight muscles and a brain that was more developed than comparable dinosaurs of similar sizes.

A jumble of pterosaur bones found in the Atacama desert in Chile yielded a proportionally large number of juvenile individuals. This would indicate that, like some modern-day shorebirds, pterosaurs roosted in rookeries, and that the young passed through a nesting stage while their parents cared for them until they were ready to fly on their own.

It is believed that competition with early bird species may have resulted in the extinction of the pterosaurs. By the end of the Cretaceous, only large species of pterosaurs survived. The smaller versions were extinct, and replaced by birds. After the impact of the famous Chicxulub bolide that ended the Cretaceous period, the larger animals all died, including the pterosaurs. Birds, being smaller creatures, survived.

Examples of pterosaurs include

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