Allosaurus lived around 150 million years ago in the late Jurassic Period. The name Allosaurus means ‘different lizard’. Allosaurus was first described by Othniel Marsh in 1877 and become one of the first well-known dinosaurs. Marsh also named and gave the first description of the Apatosaurus (Brontosaurus) in that same year. Most Allosaurus fossils have been found in the Morrison Formation, a distinctive band of sedimentary rock found in the western United States. Allosaurus had a large skull and walked on two legs. Its body and head were balanced by a long and heavy tail. Like the Tyrannosaurs rex, the Allosaurus had short arms. It had three fingers that were each tipped with sharp, curved claws. Research suggests that the average length of an Allosaurus was around 8.5 meters (28 feet) and that it weighed around 2.3 tons. Allosaurus was a carnivore (meat eater) and featured big, sharp teeth that had edges like saws.