Apatosaurus considered a junior synonym of "Apatosaurus"; its only species was reclassified as "A.excelsus" in 1903. A 2015 study concluded that "Brontosaurus" is a valid genus of sauropod distinct from "Apatosaurus", but not all paleontologists agree with this division. As it existed in North America during the late Jurassic, "Apatosaurus" would have lived alongside dinosaurs such as "Allosaurus", "Camarasaurus", "Diplodocus", and "Stegosaurus". "Apatosaurus" was a large, long-necked, quadrupedal animal with a long, whip-like tail. Its forelimbs were slightly shorter than its hindlimbs. Most size estimates are based on specimen CM3018, the type specimen of "A.louisae". In 1936 this was measured to
Apatosaurus names; he grouped "Apatosaurus" within Opisthocoelia. Sauropoda is still used as the group name. In 2011, John Whitlock published a study that placed "Apatosaurus" a more basal diplodocid, sometimes less basal than "Supersaurus". Cladogram of the Diplodocidae after Tschopp, Mateus, and Benson (2015). It was believed throughout the 19th and early 20th centuries that sauropods like "Apatosaurus" were too massive to support their own weight on dry land. It was theorized that they lived partly submerged in water, perhaps in swamps. More recent findings do not support this; sauropods are now thought to have been fully terrestrial animals. A study