Allosaurus (pronounced /ˌælɵˈsɔrəs/) was a genus of large theropod dinosaur that lived 155 to 145 million years ago, in the late Jurassic period (Kimmeridgian to Tithonian). The name Allosaurus means "different lizard" and is derived from the Greek αλλος/allos ("different, strange") and σαυρος/sauros ("lizard"). The first remains that can definitely be ascribed to this genus were described in 1877 by Othniel Charles Marsh. As one of the first well-known theropod dinosaurs, it has long attracted attention outside of paleontological circles, and has been a top feature in several films and documentaries.
Allosaurus was a large bipedal predator with a large skull, equipped with dozens of large, sharp teeth. It averaged 8.5 meters (28 ft) in length, though fragmentary remains suggest it could have reached over 12 meters (39 ft). Relative to the large and powerful hindlimbs, its three-fingered forelimbs were small, and the body was balanced by a long, heavy tail. It is classified as an allosaurid, a type of carnosaurian theropod dinosaur. The genus has a complicated taxonomy, and includes an uncertain number of valid species, the best known of which is A. fragilis. The bulk of Allosaurus remains have come from North America's Morrison Formation, with material also known from Portugal and possibly Tanzania. It was known for over half of the 20th century as Antrodemus, but study of the copious remains from the Cleveland Lloyd Dinosaur Quarry brought the name Allosaurus back to prominence, and established it as one of the best-known dinosaurs.
As the prominent large predator in the Morrison Formation, Allosaurus was at the top of the food chain, probably preying on contemporaneous large herbivorous dinosaurs and perhaps even other predators (e.g. Ceratosaurus). Potential prey included ornithopods, stegosaurids, and sauropods. Some paleontologists interpret Allosaurus as having had cooperative social behavior, and hunting in packs, while others believe individuals may have been aggressive toward each other, and that congregations of this genus are the result of lone individuals feeding on the same carcasses. It may have attacked large prey by ambush, using its upper jaw like a hatchet.