The shape, structure, function, and evolution of the pterosaurian uropatagium

The wing membrane of pterosaurs is the earliest innovation in the evolution of vertebrate powered flight and led to pterosaurs dominating the Mesozoic skies. Most studies of pterosaur flight focus on the main wing surface, the brachiopatagium, whereas the small but aerodynamically potentially important tail membrane, the uropatagium, is comparatively understudied. This review presents the current knowledge of the shape, extent, structure and function of the uropatagium that lies between the legs in pterosaurs, based on the available fossil evidence. Both direct evidence from soft tissue preservation and indirect evidence from osteological and ichnological data, suggest variation in the uropatagium among taxa, although evolutionary trends can still be outlined. While early branching pterosaurs had a proportionally large, confluent uropatagium integrated to varying extent with the tail and attached to the elongate fifth toe, pterodactyloid pterosaurs had a greatly reduced and split uropatagium spanning a much smaller triangular area between the knee joint and the base of the tail. The uropatagium was an important component of flight in early pterosaurs and its modification and reduction in derived forms is part of a major transition with the origins of the pterodactyloids and their changing flight and walking apparatus.