DEINONYCHUS (die-NON-ick-us)

Period: Early Cretaceous

Order, Suborder, Family: Saurischia, Theropoda, Dromaeosauridae

Location: North America (United States)

Size: 8-10 feet (2.5-3 meters)

At least three nearly complete skeletons of this fierce, small theropod were discovered in southern Montana in 1964. Deinonychus is the best-known member of the family Dromaeosauridae.

John Ostrom studied this dinosaur. Before his research, theropods were divided into two major groups: Carnosauria and Coelurosauria. He showed that Deinonychus had features of both groups. This helped convince other paleontologists that this division was incorrect. Also, several features of Deinonychus are also found in birds. Several paleontologists now think that Deinonychus and other members of its family, the Dromaeosauridae, are more closely related to birds than are any known dinosaurs.

Though Deinonychus was small compared to Tyrannosaurus or Allosaurus, it was an agile theropod capable of deadly attacks. Deinonychus had a large head with sharp teeth that pointed back. Its teeth were well suited to biting off pieces of flesh and keeping smaller prey from escaping its jaws. Its arms were long, and like most theropods, it had three fingers on each hand. Each of its fingers ended in a well-developed claw. The fingers could move, which means Deinonychus used its hands to attack and eat its prey.

The thigh bone (femur) of Deinonychus was shorter than the shin bones (tibia and fibula), which means the animal was a fast runner. Like other theropods and birds, the back foot was tridactyl, meaning that it walked on only three toes (the three middle toes). But Deinonychus had something completely different from other dinosaurs-a claw on the second toe that was very large, sharply pointed, and strongly curved. It is this "terrible claw" that the genus is named after. Similar claws are found only on other members of the family Dromaeosauridae, which includes Dromaeosaurus, Velociraptor, and Hulsanpes (a Mongolian specimen known only from fragmentary material). Studies of the sharp clawed second toe show that the claw was not used while walking but was carried off the ground in a raised position. Walking on the toe would have worn down the sharp tip.

Another interesting feature of this genus and the family Dromaeosauridae are thin rods of bone along the sides of the tail vertebrae (bones of the spine). These rods stiffen the back of the tail while allowing some flexibility. The part of the tail nearest the body does not have these stiffening rods so it can still move. This allowed the tail to be used as a balancing device. Just as the tight rope walker shifts his balance stick from side to side to keep his balance, Deinonychus could swing its tail from side to side and up and down.

Deinonychus may have hunted in packs; the remains of several animals were found with the plant-eating Tenontosaurus. During an attack, Deinonychus may have grasped its prey with its jaws and hands while kicking the victim's underbelly with the large, knifelike claw. If Deinonychus did hunt in packs, several animals may have brought down prey much larger than themselves.