Allosaurus is a genus of carnivorous theropod dinosaur that lived 155 to 145 million years ago during the late Jurassic period. The name “Allosaurus” means “different lizard” alluding to its unique concave vertebrae. It is derived from the Greek ἄλλος and σαῦρος

(sauros) (“lizard / generic reptile”). The first fossil remains that could definitively be ascribed to this genus were described in 1877 by palaeontologist Othniel Charles Marsh. As one of the first well-known theropod dinosaurs, it has long attracted attention outside of paleontological circles.

Allosaurus was a large bidepal(meaning walking on 2 legs) Its skull was large and equipped with dozens of sharp, serrated teeth. It averaged 9.5 metres (31 ft) in length, though fragmentary remains suggest it could have reached over 12 m (39 ft). Relative to the large and powerful hindlimbs, its three-fingered forelimbs were small, and the body was balanced by a long and heavily muscled tail. It is classified as an allosaurid (included in theropods), a type of carnosaurian theropod dinosaur. The genus has a complicated taxonomy, and includes 3 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. It was known for over half of the 20th century as Antrodemus, but a 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 most abundant large predator in the Morrison Formation, Allosaurus was at the top of the food chain, probably preying on contemporaneous large herbivorous dinosaurs, and perhaps other predators. Potential prey included ornithopodsstegosaurids, and sauropods.