Apatosaurus (IPA: /əˌpætəˈsɔrəs/), formerly known as Brontosaurus, is a genus of sauropo dinosaur that lived about 150 million years ago, during the Jurassic Period (Kimmeridgian and Tithonian ages). It was one of the largest land animals that ever existed, with an average length of 23 meters (75 ft) and a mass of at least 23 metric tons (25 short tons). The name Apatosaurus means 'deceptive lizard', so-given because the chevron bones were similar to those of a prehistoric marine lizard, Mosasaurus. The name Apatosaurus comes from the Greek ἀπατέλος or ἀπατέλιος meaning 'deceptive' and σαῦρος meaning 'lizard'.