Allosaurus (pronounce /ˌæləˈsɔrəs/) is a genus of large theropod dinosaur that lived 155 to 145 million years ago, in the late Jurassic period. 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 lead dinosaur in several films and documentaries.