Allosaurus (æləˈsɔ rəs) is a genus of carnivorous theropod dinosaur that lived 155 to 150 million years ago during the late Jurassic period (Kimmeridgian to early Tithonian). The
name "Allosaurus" means "different
lizard" alluding to its
unique concave vertebrae (at the time of its discovery). It is derived from the Greek ἄλλος/allos ("different, other") and σαῦρος/sauros ("
lizard / generic reptile"). The first fossil remains that could definitively be ascribed to this genus were described in 1877 by paleontologist Othniel Charles
Marsh. As one of the first well-known theropod dinosaurs, it has long attracted attention outside of paleontological circles. Indeed, it has been a top feature in several films and documentaries about prehistoric life.
As the most abundant large predatory dinosaur 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. 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. Due to having a much weaker
bite force and a much wider
gape than most other large carnivorous dinosaurs, it may have attacked large prey by ambush, using its upper jaw
like a hatchet.