Allosaurus is a genus of large carnosaurian theropod dinosaur that lived 155 to 145 million years ago during the Late Jurassic period.

The name is derived from the Greek words allos ("different") and sauros ("lizard")--an allusion to its unique (at the time of its discovery) concave vertebrae.

There is dramatic evidence for Allosaurus attacks on Stegosaurus, including an Allosaurus tail vertebra with a partially healed puncture wound that fits a Stegosaurus tail spike, and a Stegosaurus neck plate with a U-shaped wound that correlates well with an Allosaurus snout.

The skull had a pair of horns above and in front of the eyes. The horns were probably covered in a keratin sheath and may have had a variety of possible functions, including acting as sunshades for the eyes, being used for display, or being used in combat against other members of the same species.

It had three fingers per hand, tipped with large, strongly curved and pointed claws to help it rip prey apart.

Allosaurus was a massive carnivore. The largest specimens may have rivaled T. rex in size, growing up to 39 feet long and 16.5 feet tall.

A recent discovery shows that Allosaurus and many other carnivorous dinosaurs had a V-shaped wishbone. In flying birds, the wishbone is a spring that aids in flight by pushing the shoulder apart during the down stroke of the wings. In Allosaurus the wishbone is a rather delicate structure that spans the chest, although its purpose is unknown.

With its muscular legs, Allosaurus was built for speed. Estimates show it could run up to 21 miles per hour (33 km/h), which was faster than other dinosaurs of its time, including the Sauropods and Stegosaurs it hunted.

The structure of the inner ear was like that of a crocodilian, so Allosaurus probably could have heard lower frequencies best, and would have had trouble with subtle sounds.

It was known for over half of the 20th century as Antrodemus, but a study of the remains from the Cleveland-Lloyd Dinosaur Quarry brought the name Allosaurus back to prominence during the 1970s and established it as one of the best-known dinosaurs.