Allosaurus is an extinct genus of large 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. The first fossil remains that could definitively be ascribed to this genus were described in 1877 by Othniel Charles Marsh. The genus has a very complicated taxonomy and includes at least three valid species, the best known of which is A. fragilis. From Wikipedia
