Apatosaurus is one of the best known dinosaurs. It was up to 65 feet long and weighed nearly 30 tons, though recent analysis suggests it may have been even bigger. Apatosaurus may have been big, but it's brain was small. In a battle against a large predator like Allosaurus or Ceratosaurus, Apatosaurus probably relied on instinct. For years it was thought that Apatosaurus and Brontosaurus were the same animal. However, it's now understood that they were in fact seperate. These dinosaurs lived 150 million years ago in North America.