**Dienoycus** is a genus of dinosaur. Dienocus is a dromaeosaurid theropod dinosaur. There are one known species in this genus: *Deinonychus antirrhopus*.

Appearance
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This dinosaur could be up to 3.4 meters (11 ft) long. It lived during the early Cretaceous Period, about 115–108 million years ago (from the mid-Aptian to early Albian stages). Fossils have been found in the U.S. states of Montana, Utah, Wyoming, and Oklahoma, in rocks of the Cloverly Formation, Cedar Mountain Formation, and Antlers Formation. It had a large claw on its foot.

History of paleontology
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Paleontologist John Ostrom's study of *Deinonychus* in the late 1960s changed the way scientists thought about dinosaurs. It caused the "dinosaur renaissance" and made scientists argue whether dinosaurs were warm-blooded or cold-blooded. Before this, most people thought dinosaurs were big, slow, and looked and acted like modern reptiles. Ostrom told other scientists that *Dienocus* had a small body, sleek, horizontal posture, the same kind of spine that flightless birds have, and the large claws on its feet. All this, said Ostrom, meant *Deinonychus* was probably very fast and caught its food like a predator.

Etymology
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The name "Deinonychus" means "terrible claw."

"Terrible claw" refers to the unusually large, sickle-shaped talon on the second toe of each hindfoot. 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 millimeters (4.7 in) long. The species name *antirrhopus* means "counterbalance", 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. Still, 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*.

Discovery and naming
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Fossilized remains of *Deinonychus* have been recovered from the Cloverly Formation of Montana and Wyoming and in the roughly contemporary Antlers Formation of Oklahoma in North America. The Cloverly formation has been dated to the late Aptian through early Albian stages of the early Cretaceous, about 115 to 108 Ma. Additionally, teeth found in the Arundel Clay Facies (mid-Aptian), of the Potomac Formation on the Atlantic Coastal Plain of Maryland may be assigned to the genus. The first remains were uncovered in 1931 in southern Montana near the town of Billings. The team leader, paleontologist Barnum Brown, was primarily concerned with excavating and preparing the remains of the ornithopod dinosaur *Tenontosaurus*, but in his field report from the dig site to the American Museum of Natural History, he reported the discovery of a small carnivorous dinosaur close to a *Tenontosaurus* skeleton, "but encased in lime difficult to prepare." He informally called the animal "Daptosaurus agilis" and made preparations for describing it and having the skeleton, specimen AMNH 3015, put on display but never finished this work. Brown brought back from the Cloverly Formation the skeleton of a smaller theropod with seemingly oversized teeth that he informally named "Megadontosaurus". John Ostrom, reviewing this material decades later, realized that the teeth came from *Deinonychus*, but the skeleton came from a completely different animal. He named this skeleton *Microvenator*.