Apatosaurus previously known as Brontosaurus, is a genus of sauropod dinosaurs that lived about 140 million years ago, during the Jurassic Period. They were some of the largest land animals that ever existed, about 4.5 metres (15 feet) tall at the hips, with a length of up to 21 m (70 feet) and a mass up to 35 metric tonnes (40 tons). Their name means 'deceptive lizard', so-named because the chevron bones were like those of Mosasaurus (Greek apatelos or apatelios meaning 'deceptive' and sauros meaning 'lizard').