Apatosaurus is a type of dinosaur that lived during the Late Jurassic period. It is also known as Brontosaurus, a name that was previously thought to be a separate species. Other synonyms for Apatosaurus include Elosaurus, Atlantosaurus, and Brontosaurus excelsus. Apatosaurus belongs to the family Diplodocidae, which includes other long-necked sauropods such as Diplodocus and Barosaurus. These dinosaurs were herbivorous and were believed to have reached lengths of up to 75 feet. Despite being one of the largest land animals to have ever existed, Apatosaurus had a relatively small brain size, suggesting that it may not have been particularly intelligent.