| Ammosaurus, Agrosaurus, Arstanosaurus, Aublysodon, Amygdalodon|
Atlantosaurus (meaning "Atlas lizard") is a dubious genus of sauropod dinosaur. It contains a single species, Atlantosaurus montanus, from the upper Morrison Formation of Colorado, United States. Atlantosaurus was the first sauropod to be described during the infamous 19th century Bone Wars, during which scientific methodology suffered in favor of pursuit of academic acclaim.
The type specimen, found by Arthur Lakes in the Morrison Formation of Colorado, United States, was described by Othniel Charles Marsh, a Professor of Paleontology at Yale University (then called Yale College), in 1877 as Titanosaurus montanus. Marsh soon learned that the name Titanosaurus had already been used earlier that year to describe a different sauropod, so he renamed it Atlantosaurus montanus. The skeletal remains discovered were initially distinguished by their immense size and by the pleurocoels (air-filled pockets) in the vertebrae. However, since the time of its discovery, these features have been found to be widespread among sauropods, making it nearly impossible to distinguish the two known vertebrae of Atlantosaurus from those of its relatives. Since it is unclear whether or not Atlantosaurus montanus actually represents a distinct species, it is considered a nomen dubium ("dubious name"), though some researchers have considered it a likely synonym of Apatosaurus ajax.