Alternative classifications Due to the subtle variations between specimens of pteranodontid from the Niobrara Formation, most researchers have assigned all of them to the single genus Pteranodon, in at least two species (P. longiceps and P. sternbergi) distinguished mainly by the shape of the crest. However, the classification of these two forms has varied from researcher to researcher. In 1972, Halsey Wilkinson Miller published a paper arguing that the various forms of Pteranodon were different enough to be placed in distinct subgenera. He named these Pteranodon (Occidentalia) occidentalis (for the now-disused species P. occidentalis) and Pteranodon (Sternbergia) sternbergi. However, the name Sternbergia was preoccupied, and in 1978 Miller re-named the species Pteranodon (Geosternbergia) sternbergi, and named a third subgenus/species combination for P. longiceps, as Pteranodon (Longicepia) longiceps. Most prominent pterosaur researchers of the late 20th century however, including S. Christopher Bennett and Peter Wellnhofer, did not adopt these subgeneric names, and continued to place all pteranodont species into the single genus Pteranodon. In 2010, pterosaur researcher Alexander Kellner revisited H.W. Miller's classification. Kellner followed Miller's opinion that the differences between the Pteranodon species were great enough to place them into different genera. He placed P. sternbergi into the genus named by Miller, Geosternbergia, along with the Pierre Shale skull specimen which Bennett had previously considered to be a large male P. longiceps. Kellner argued that this specimen's crest, though incompletely preserved, was most similar to Geosternbergia. Because the specimen was millions of years younger than any known Geosternbergia, he assigned it to the new species Geosternbergia maysei. Numerous other pteranodont specimens are known from the same formation and time period, and Kellner suggested they may belong to the same species as G. maysei, but because they lack skulls, he could not confidently identify them. Disused species A number of additional species of Pteranodon have been named since the 1870s, although most now are considered to be junior synonyms of two or three valid species. The best-supported is the type species, P. longiceps, based on the well-preserved specimen including the first-known skull found by S. W. Williston. This individual had a wingspan of . Other valid species include the possibly larger P. sternbergi, with a wingspan originally estimated at . P. oweni (P. occidentalis), P. velox, P. umbrosus, P. harpyia, and P. comptus are considered to be nomina dubia by Bennett (1994) and others who question their validity. All probably are