tĕrˈəsôrˌ [Gr.,=winged lizard], extinct flying reptile (commonly called pterodactyl [Gr.,=wing finger]) of the order Pterosauria, common in the late Triassic and Cretaceous periods, from approximately 228 to 65 million years ago. At least 60 genera of pterosaurs have been found, sizes ranging from that of a sparrow to that of the huge Quetzalcoatlus, the largest fossils of which have a wingspread of more than 40 ft (12 m). The earlier species (e.g., Rhamporhynchus) had fully toothed jaws and long tails, but in the later forms (e.g., Pteronodon) the tail was a stump, teeth were lacking, and the jaws were modified into a beak. The skulls of different species differ markedly, presumably as a result of adaptation to capturing different kinds of prey.

The flying apparatus of pterosaurs comprised a membranous wing stretched between the fourth finger of the hand and the side of the body. The fifth finger was degenerate, and the first three were free of the wing. The "pteroid" bone, unique to the pterosaur group, attached to the wrist and pointed toward the shoulder, also helped support the wing, and the wings were strengthened by numerous connective tissue fibers. There is no fossil evidence of feathers. Most researchers now believe that pterosaurs were adapted for active flight, not just gliding as was earlier believed. Their bones are large but hollow, and they possessed a keeled breastbone for the attachment of flying muscles. Debate continues regarding how pterosaurs moved when on the ground and how maneuverable they were in the air. Pterosaurs, unlike the flying dinosaur Archaeopteryx, were not ancestral to the birds but represented a wholly separate line of development. Like dinosaurs, pterosaurs were affected by the mass extinction at the end of the Cretaceous period.

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