Marsh's original classification scheme, most notably sinking P. longiceps as a synonym of P. ingens. Recognizing variation During the early 1990s, S. Christopher Bennett also published several major papers reviewing the anatomy, taxonomy and life history of Pteranodon. Fragmentary fossils assigned to Pteranodon have also been discovered in Skåne, Sweden. Description Pteranodon species are extremely well represented in the fossil record, allowing for detailed descriptions of their anatomy and analysis of their life history. Over 1,000 specimens have been identified, though less than half are complete enough to give researchers good anatomical information. Still, this is more fossils material than is known for any other pterosaur, and it includes both male and female specimens of various age groups and possibly species. Adult Pteranodon specimens from the two major species can be divided into two distinct size classes. The smaller class of specimens have small, rounded head crests and very wide pelvic canals, even wider than those of the much larger size class. The size of the pelvic canal probably allowed the laying of eggs, indicating that these smaller adults are females. The larger size class, representing male individuals, have narrow hips and very large crests, which were probably for display. Adult male Pteranodon were among the largest pterosaurs, and were the largest flying animals known until the late 20th century, when the giant azhdarchid pterosaurs were discovered. The wingspan of an average adult male Pteranodon was . Adult females were much smaller, averaging in wingspan. A large specimen of Pteranodon longiceps, USNM 50130, is estimated to have a wingspan of , body length of and body mass of . An even larger specimen had a wingspan of . While most specimens are found crushed, enough fossils exist to put together a detailed description of the animal. Methods used to estimate the mass of large male Pteranodon specimens (those with wingspans of about 7 meters) have been notoriously unreliable, producing a wide range of estimates. In a review of pterosaur size estimates published in 2010, researchers Mark Witton and Mike Habib argued that the largest estimate of 93 kg is much too high and an upper limit of 20 to 35 kg is more realistic. Witton and Habib considered the methods used by researchers who obtained smaller mass estimates equally flawed. Most have been produced by scaling modern animals such as bats and birds up to Pteranodon size, despite the fact