A.ajax (Marsh,1877) >> A. laticollis (Marsh,1879) Atlantosaurus immanis (Marsh,1878) Apatosaurus  > A.excelsus (Marsh,1879) >> Brontosaurus excelsus (Marsh,1879) >>> Brontosaurus amplus (Marsh,1881) Elosaurus parvus (Peterson & Gilmore,1902) Apatosaurus  > A.louisae (Holland,1915) = Atlantosaurus louisae (Holland,1915 - Steel,1970) = Brontosaurus louisae (Holland,1915 - Bakker, 1986)

The anatomy of Apatosaurus (Marsh,1883; Riggs,1903; Gilmore,1936) is completely except for the mandible. Overall, it is similar to that of Diplodocus and Barosaurus, except that the bones, particularly those of the neck and forelimb, are much more robust, the caudal vertebrae are shorter and less specialized, and the typical forked diplodocid chevrons less developed.