Apatosaurus "Apatosaurus" may have been more solitary than other Morrison Formation dinosaurs. Fossils of the genus have only been found in the upper levels of the formation. Those of "Apatosaurus ajax" are known exclusively from the upper Brushy Basin Member, about 152–151 mya. "A.louisae" fossils are rare, known only from one site in the upper Brushy Basin Member; they date to the late Kimmeridgian stage, about 151mya. Additional "Apatosaurus" remains are known from similarly aged or slightly younger rocks, but they have not been identified as any particular species, and thus may instead belong to "Brontosaurus". The Morrison Formation records a
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