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Deinonychus (from Greek: deinós, 'terrible' and ónux, 'claw') is a genus of dromaeosaurid theropod dinosaur with one described species, Deinonychus antirrhopus. This species, which could grow up to 3.4 meters (11 ft) long, lived during the early Cretaceous Period, about 115–108 million years ago (from the mid-Aptian to early Albian stages). Fossils have been recovered from the U.S. states of Montana, Utah, Wyoming, and Oklahoma, in rocks of the Cloverly Formation, Cedar Mountain Formation and Antlers Formation, though teeth that may belong to Deinonychus have been found much farther east in Maryland.
Based on the few fully mature specimens, Deinonychus could reach 3 meters in length, with a skull estimated at 41 cm, a hip height of 87 cm, and a weight estimated at 70 kg. The proportionally large skull was equipped with powerful jaws lined with around seventy curved, blade-like teeth. Both the skull and the lower jaw had fenestrae (skull openings) which reduced the weight of the skull. In Deinonychus, the antorbital fenestra, a skull opening between the eye and nostril, was particularly large. Deinonychus possessed large, well developed forelimbs and "hands" (manus) with three fingers and very sharp claws on each hand. The first digit was shortest and the second was longest. The hindlimbs were robust and powerful, with each hind foot bearing a particularly large sickle-shaped claw on the second digit - the animal's namesake - that was probably used during predation. No feather impressions have ever been found in association with fossils of Deinonychus, but related taxa like Microraptor, Velociraptor, and Dakotaraptor preserve evidence of feathers so it is very likely that Deinonychus had them too. Deinonychus' lower leg length was not particularly long for its femur length, indicating that while being far from slow-moving, it was not especially adapted for running on a regular basis. This is partially attributable to an unusually short metatarsus (upper foot bones). The ratio is actually larger in smaller individuals than in larger ones. John Ostrom, the scientist who named Deinonychus, suggested that the short metatarsus may be related to the function of the sickle claw, and used the fact that it appears to get shorter as individuals aged as support for this. He interpreted all these features—the short second toe with enlarged claw, short metatarsus, etc.—as support for the use of the hind leg as an offensive weapon, where the sickle claw would strike downwards and backwards, and the leg pulled back and down at the same time, slashing and tearing at the prey. Ostrom suggested that the short metatarsus reduced overall stress on the leg bones during such an attack, and interpreted the unusual arrangement of muscle attachments in the Deinonychus leg as support for his idea that a different set of muscles was used in the predatory stroke than in walking or running. Therefore, Ostrom concluded that the legs of Deinonychus represented a balance between running adaptations needed for an agile predator, and stress-reducing features to compensate for its unique foot weapon.
