Deinonychus — dromaeosaurid theropod of the Early Cretaceous

As the largest known specimens show, Deinonychus could reach a length of 3.4 meters. An individual of this size possessed a skull length of 41 centimeters, a hip height of 87 centimeters, and a mass of about 73 kilograms. The skull was equipped with powerful jaws bearing about 70 curved, blade-like teeth. All the teeth were serrated on the front as well as the back, the denticles on the back of the teeth being almost twice as large as those on the front. John Ostrom's first skull reconstruction, still based on very fragmentary remains, shows a relatively compact, broad skull similar to that of Allosaurus. However, more recent skull finds of Deinonychus and closely related genera with good three-dimensional preservation show that the palate was more curved than John Ostrom had suspected, leading to the reconstruction of a much narrower and longer snout. At the same time, the paired zygomatic bone (jugal) widened the skull in the eye region, allowing good stereoscopic vision. The skull of Deinonychus differed from that of Velociraptor by, among other things, the deeper, more robust mandibles and by the more robust skullcap, which was similar to that of Dromaeosaurus. Furthermore, the nasal bone was not flattened like that of Velociraptor. Both the skull and the mandible showed cranial windows that reduced the weight of the skull. In Deinonychus, the antorbital window located in front of the eyes was particularly large.

As in all dromaeosaurids, the forelimbs were very long, reaching 70 percent of the length of the hind legs in Deinonychus. The large hand accounted for 40 percent of the total length of the forelimbs. The first of the three fingers was the shortest, while the second was the longest. Each hind leg bore a crescent-shaped, unusually large claw on the second toe. In the animal's life, the claws were larger than they have survived fossil, as dinosaurs possessed a covering of horn over their claws, as do archosaurs living today. John Ostrom compared a crescent claw from Deinonychus (specimen number YPM 5205) to claws of present-day crocodiles and birds and concluded that the claw measured over 120 millimeters in length in the living animal. Both anatomical evidence and evidence from fossil footprints show that only the third and fourth toes touched the ground during locomotion, while the second toe bearing the sickle claw was held above the ground during locomotion. The ischium was shorter in relation to the pubis than in other dromaeosaurids. The caudal vertebrae were equipped with a series of ossified tendons and greatly elongated vertebral processes - the prezygapophyses on the top and chevron bones on the underside of the vertebrae. John Ostrom suggested that this device made the tail a stiff counterweight. However, a fossil of the closely related Velociraptor mongoliensis (IGM 100/986) has a tail in the anatomical composite that was bent horizontally in an S-shape. This suggests considerable lateral mobility of the tail.

Although no feathers have yet been discovered in association with Deinonychus fossils, evidence of feathers in a variety of other dromaeosaurids suggests that all members of the group including Deinonychus were feathered (principle of phylogenetic clasping). Thus, the feathered dromaeosaurid genus Microraptor is both geologically older and phylogenetically more primitive than Deinonychus. Several Microraptor fossils show contour feathers on the arms, legs, and tail that match those of modern birds. An ulna of Velociraptor, closely related to Deinonychus, shows quill knobs, suggesting long arm feathers in Velociraptor.

Deinonychus is one of the best known dromaeosaurids and a close relative of the smaller Velociraptor, which comes from younger strata of the Upper Cretaceous of northern China and Mongolia. Two other Upper Cretaceous genera, Tsaagan from Mongolia and the little-known Saurornitholestes from North America, may also have been close relatives of Deinonychus.

Velociraptor and Deinonychus together form the group Velociraptorinae, which was established by Rinchen Barsbold in 1983 and originally included Velociraptor as the only genus. The long-nosed Velociraptorinae probably used more of their claws and less of their skulls to overpower prey, unlike other dromaeosaurids such as the Dromaeosaurinae, which had stockier skulls. In 1995, Philip Currie attributed Deinonychus as well as a number of other dromaeosaurids to the Velociraptorinae, which gained general acceptance.

However, a more recent study by Nicholas Longrich and Philip Currie in 2009 classifies Deinonychus outside the Velociraptorinae and posits a group called Eudromaeosauria to include Deinonychus, the Velociraptorinae, and the Dromaeosaurinae. Together with the troodontids, the dromaeosaurids form the Deinonychosauria, which many researchers believe is the sister group to birds and thus the dinosaur group most closely related to birds.