Pteranodon walkeri YPM 2525 (Bennett 1991,1993, 2001) was derived from a sister to P. walkeri KUVP 221 and represents the last of this lineage. Normally such scrappy remains would go unnoticed. Here they clear up a widely-held misconception.

KUVP 221 is only known from a drawer full of assorted bones. The mandible length and width identify it as a sister to P. walkeri, which is only known from a skull. In KUVP 221 the scapula and coracoid were unfused. Bennett (1991, 1993, 2001) considered KUVP 221 an immature specimen, but actually adults in this lineage were all smaller than preceding taxa, such as the giant Jenkins specimen. In other pterosaurs lineages following a size reduction the scapula and coracoid became unfused even in adults. It is the same here.

Bennett (1991, 1993, 2001) and others considered pterosaurs to be archosaurs with archosaur-like growth patterns. But that is false. Pterosaurs were lizards with lizard-like growth patterns. In living lepidosaurs, Maisano (2002) reported, “no terminal fusion universally coincides with the achievement of either sexually or skeletally mature size and that some squamates continue to grow long after the fusion of various elements.” Thus pterosaurs, as squamates, could have fused bones early in ontogeny. They could also have never fused certain bones before they died of old age. Under that squamate paradigm, “immature” bone texture could have been retained in phylogenetically smaller adults. This is a key fact that has been previously overlooked or ignored.

Size reduction in prior taxa enabled the survival of tiny pterosaur lines while larger forms became extinct. Derived Pteranodon taxa became smaller but were not able to become small enough to survive the extinction that terminated the Cretaceous period.

The family tree of Pteranodon is here. The family tree of the Ornithocephalia and Germanodactylia is here. The expanded family tree of the Pterosauria is here.