Apatosaurus opponent of Marsh and his taxa, labeled the "Apatosaurus" mount of the American Museum of Natural History "Brontosaurus". Because of this decision the name "Brontosaurus" was commonly used outside of scientific literature for what Riggs considered "Apatosaurus", and the museum's popularity meant that "Brontosaurus" became one of the best known dinosaurs, even though it was invalid throughout nearly all of the 20th and early 21st centuries. It was not until 1909 that an "Apatosaurus" skull was found during the first expedition, led by Earl Douglass, to what would become known as the Carnegie Quarry at Dinosaur National Monument. The skull
Apatosaurus of another "Apatosaurus" individual originally described as "Atlantosaurus immanis"; as a consequence, some elements cannot be ascribed to either specimen with confidence. Marsh distinguished the new genus "Apatosaurus" from "Atlantosaurus" on the basis of the number of sacral vertebrae, with "Apatosaurus" possessing three and "Atlantosaurus" four. Two years later, Marsh announced the discovery of a larger and more complete specimen at Como Bluff, Wyoming. He gave this specimen a new name based on the conventions of his age and the relatively sparse fossil record available at that time. It was later recognised that the features he had used to distinguish