Deinonychus

Deinonychus (meaning 'terrible claw') is a genus of carnivorous dromaeosaurid dinosaur. There is one described species, Deinonychus antirrhopus. This 3.4 meter (11 ft) long dinosaur 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, Wyoming, and Oklahoma, in rocks of the Cloverly Formation and Antlers Formation, though teeth that may belong to Deinonychus have been found much farther east in Maryland.

Paleontologist John Ostrom's study of Deinonychus in the late 1960s revolutionized the way scientists thought about dinosaurs, leading to the "Dinosaur renaissance" and 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 the small body, sleek, horizontal, posture, ratite-like spine, and - especially - the enlarged raptorial claws on the feet, which suggested an active, agile predator.

"Terrible claw" refers to the unusually large, sickle-shaped talon on the second toe of each hind foot. The fossil YPM 5205 preserves a large, strongly curved ungual. In life, archosaurs have a horny sheath over this bone which extends the length. Ostrom looked at crocodile and bird claws and reconstructed the claw for YPM 5205 as over 120 mm long. The species name antirrhopus means “counter balance”, which refers to John Ostrom's idea about the function of the tail. As in other dromaeosaurids, the tail vertebrae have a series of ossified tendons and super-elongated bone processes. These features seemed to make the tail into a stiff counterbalance, but a fossil of the very closely related Velociraptor mongoliensis (IGM 100/986) has an articulated tail skeleton that is curved laterally in a long S–shape. This suggests that, in life, the tail could swish to the sides with a high degree of flexibility.

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.

Description
Based on the largest known specimens, Deinonychus could reach 3.4 meters (11.1 ft), with a maximum skull length of 410 mm (16.4 in), a hip height of 0.87 meters (2.85 ft), a maximum weight of 73 kilograms (161 lb). Its skull was equipped with powerful jaws lined with around sixty curved, blade-like teeth. Studies of the skull have progressed a great deal over the decades. Ostrom reconstructed the partial, imperfectly preserved, skulls that he had as triangular, broad, and fairly similar to Allosaurus. Additional Deinonychus skull material and closely related species found with good three-dimensional preservation show that the palate was more vaulted than Ostrom thought, making the snout far narrower, while the jugals flared broadly, giving greater stereoscopic vision. The skull of Deinonychus was different from that of Velociraptor, however, in that it had a more robust skull roof like that of Dromaeosaurus, and did not have the depressed nasals of Velociraptor. 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.

Like all dromaeosaurs, Deinonychus possessed large hands (manus) with three claws on each forelimb. The first digit was shortest and the second was longest. Each hind foot bore a sickle-shaped claw on the second digit, which was probably used during predation.

No feathers have ever been found in association with fossils of Deinonychus. Nonetheless, the evidence suggests that the Dromaeosauridae, including Deinonychus, had feathers. The genus Microraptor is both older geologically and more primitive phylogenetically than Deinonychus, and within the same family. Multiple fossils of Microraptor preserve pennaceous, vaned feathers like those of modern birds on the arms, legs, and tail, along with covert and contour feathers. Velociraptor is geologically younger than Deinonychus, but even more closely related (within the subfamily velociraptorinae, see Classification, below). A specimen of Velociraptor has been found with quill knobs on the ulna. Quill knobs are where the follicular ligaments attached, and are a direct indicator of feathers of modern aspect.