Deinonychus

***Deinonychus*** (meaning "terrible 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.

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 dinosaurs were warm-blooded or cold-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 millimetres (4.7 in) long. The species name *antirrhopus* means "counter balance", which refers to 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 bend to the sides with a high degree of flexibility. In both the Cloverly and Antlers formations, *Deinonychus* remains have been found closely associated with those of the ornithopod *Tenontosaurus*. Teeth discovered associated with *Tenontosaurus* specimens imply they were hunted, or at least scavenged upon, by *Deinonychus*.

## Description

Based on the few fully mature specimens, *Deinonychus* could reach 3.4 meters (11 ft) in length, with a skull length of 410 millimeters (16 in), a hip height of 0.87 meters (2.9 ft) and a weight of 73 kg (161 lb), though there is a higher estimate of 100 kg (220 lb). The skull was equipped with powerful jaws lined with around seventy 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.

*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 skin impressions 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. 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.

## *Deinonychus* in The Amazing Dinosaur Adventure

*Deinonychus* from *The Time of The Great Giving*