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Deinonychus (meaning 'terrible claw') was a genus of carnivorous dromaeosaurid dinosaur. This 3-4 metre (10-13 ft) long dinosaur lived during the early Cretaceous Period (Aptian - Albian stages, 121 to 98.9 million years ago). Fossils of the only named species (D. antirrhopus) have been recovered from Montana, Wyoming and Oklahoma, though teeth that may belong to Deinonychus have been found much farther east in Maryland.

Its name refers to the unusually large, sickle-shaped talon on the second toe of each hind foot, which was probably held retracted while the dinosaur walked on the third and fourth toes. It was commonly thought that Deinonychus would kick with the sickle claw to slash at its prey but recent tests on reconstructions of similar Velociraptor talons suggest that the claw was used to stab, not slash. As in other dromaeosaurids, the tail was stiffened by a series of elongated bones and bone processes. This might have given Deinonychus greater balance and turning ability. In both the Cloverly and Antlers Formation, Deinonychus remains have been found closely associated with those of the ornithopod Tenontosaurus. Teeth discovered associated with Tenontosaurus specimens imply it was hunted or at least scavenged upon by Deinonychus.

Paleontologist John Ostrom's study of Deinonychus in the late 1960s revolutionized the way scientists thought about dinosaurs, igniting the debate on whether or not dinosaurs were warm-blooded. Before this, the popular conception of dinosaurs had been one of plodding, reptilian giants. Ostrom noted lightweight bones and stiffened tendons which revealed an active, agile predator. (see more...)