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 which good skull material was known at the time. The mount construction was overseen by Adam Hermann, who failed to find "Apatosaurus" skulls. Hermann was forced to sculpt a stand-in skull by hand. Osborn said in a publication that the skull was "largely conjectural and based on that of "Morosaurus"" (now "Camarasaurus"). In 1903 Elmer Riggs published a study that described a well-preserved skeleton of a diplodocid from the Grand River Valley near Fruita, Colorado, Field Museum of Natural History specimen P25112. Riggs thought that the deposits were similar in age to those of the Como Bluff in Wyoming from