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)

"thunder lizard" Named for its great size and powerful build ("one of the largest reptiles yet discovered"), similar in meaning to Marsh's earlier mammal name Brontotherium "thunder beast" (1873). Brontes was also the name of a giant in Greek mythology. Contrary to a common explanation for the name, Marsh did not indicate that his "thunder lizard" was supposed to make a sound like thunder when it walked. 

He recognized that his Apatosaurus and Brontosaurus were closely related taxa, but distinguished the two forms primarily based on the number of fused vertebrae in the sacrum of the type specimens (three in Apatosaurus ajax, five in Brontosaurus excelsus), a feature now known to reflect the age of individuals. Elmer Riggs could not find sufficient grounds for treating both as separate genera, and made the well-known name Brontosaurus a junior synonyn of Apatosaurus in 1903. 

Surprisingly, though, Riggs also thought that the type species itself could not be identified in an adult form: "Apatosaurus ajax is based upon a specimen too young to admit of specific determination"--a situation, which, arguably, could have been grounds for treating the name Apatosaurus as a nomen dubium and using Brontosaurus instead. 

Modern authorities consider Apatosaurus ajax diagnosable, however. The nomenclatural issues surrounding the name are unrelated to Marsh's mistaken choice of a Camarasaurus skull for his reconstruction of "Brontosaurus."