separate Pteranodon from other pterosaurs was its lack of teeth, Williston concluded that "Ornithostoma" must be considered the senior synonym of Pteranodon. However, in 1901, Pleininger pointed out that "Ornithostoma" had never been scientifically described or even assigned a species name until Williston's work, and therefore had been a nomen nudum and could not beat out Pteranodon for naming priority. Williston accepted this conclusion and went back to calling the genus Pteranodon. However, both Williston and Pleininger were incorrect, because unnoticed by both of them was the fact that, in 1891, Seeley himself had finally described and properly named Ornithostoma, assigning it to the species O. sedgwicki. In the 2010s, more research on the identity of Ornithostoma showed that it was probably not Pteranodon or even a close relative, but may in fact have been an azhdarchoid, a different type of toothless pterosaur. Revising species Williston was also the first scientist to critically evaluate all of the pteranodon species classified by Cope and Marsh. He agreed with most of Marsh's classification, with a few exceptions. First, he did not believe that P. ingens and P. umbrosus could be considered synonyms, which even Cope had come to believe. He considered both P. velox and P. longiceps to be dubious; the first was based on non-diagnostic fragments, and the second, though known from a complete skull, probably belonged to one of the other, previously-named species. In 1903, Williston revisited the question of Pteranodon classification, and revised his earlier conclusion that there were seven species down to just three. He considered both P. comptus and P. nanus to be specimens of Nyctosaurus, and divided the others into small (P. velox), medium (P. occidentalis), and large species (P. ingens), based primarily on the shape of their upper arm bones. He thought P. longiceps, the only one known from a skull, could be a synonym of either P. velox or P. occidentalis, based on its size. In 1910, Eaton became the first scientist to publish a more detailed description of the entire Pteranodon skeleton, as it was known at the time. He used his findings to revise the classification of the genus once again based on a better understanding of the differences in pteranodont anatomy. Eaton conducted experiments using clay models of bones to help determine the effects of crushing and flattening on the shapes of the arm bones Williston had used in his own