classification. Eaton found that most of the differences in bone shapes could be easily explained by the pressures of fossilization, and concluded that no Pteranodon skeletons had any significant differences from each other besides their size. Therefore, Eaton was left to decide his classification scheme based on differences in the skulls alone, which he assigned to species just as Marsh did, by their size. In the end, Eaton recognized only three valid species: P. occidentalis, P. ingens, and P. longiceps. The discovery of specimens with upright crests, classified by Harksen in 1966 as the new species Pteranodon sternbergi, complicated the situation even further, prompting another revision of the genus by Miller in 1972. Because it was impossible to determine crest shape for all of the species based on headless skeletons, Miller concluded that all Pteranodon species except the two based on skulls (P. longiceps and P. sternbergi) must be considered nomena dubia and abandoned. The skull Eaton thought belonged to P. ingens was placed in the new species Pteranodon marshi, and the skull Eaton assigned to P. occidentalis was re-named Pteranodon eatoni. Miller also recognized another species based on a skull with a crest similar to that of P. sternbergi; Miller named this Pteranodon walkeri. To help bring order to this tangle of names, Miller created three categories or "subgenera" for them. P. marshi and P. longiceps were placed in the subgenus Longicepia, though this was later changed to simply Pteranodon due to the rules of priority. P. sternbergi and P. walkeri, the upright-crested species, were given the subgenus Sternbergia, which was later changed to Geosternbergia because Sternbergia was already in use ("preoccupied"). Finally, Miller named the subgenus Occidentalia for P. eatoni, the skull formerly associated with P. occidentalis. Miller further expanded the concept of Pteranodon to include Nyctosaurus as a fourth subgenus. Miller considered these to be an evolutionary progression, with the primitive Nyctosaurus, at the time thought to be crestless, giving rise to Occidentalia (with a small crest), which in turn gave rise to Pteranodon with its long backwards crest, finally leading to Geosternbergia with its large, upright crest. However, Miller made several mistakes in his study concerning which specimens Marsh had assigned to which species, and most scientists disregarded his work on the subject in their later research, though Wellnhofer (1978) followed Miller's species list. and Schoch (1984) somewhat oddly published another revision that essentially returned to