Apatosaurus, sometimes known by the popular synonym Brontosaurus, is a genus of sauropod dinosaur that lived from about 154 to 150 million years ago, during the Jurassic Period (Kimmeridgian and early Tithonian ages). It was one of the largest land animals known to have ever existed, with an average length of 23 m (75 ft) and a mass of at least 16 metric tons (18 short tons). Most Apatosaurs were saved by the Zhecrons and transported to the planet Zhecronus, where they thrive within vast plains of grass. They are often preyed upon by Tyrannosaurus Rex, Carnotaurus, and, in the worst sense imagined, Spinosaurus.