The pteroid bone is a rod-like element found only in pterosaurs, the flying reptiles of the Mesozoic. It articulated at the wrist, and supported a membranous forewing in front of the inner part of the wing spar. The function of this bone, particularly its orientation, has been much debated. It is widely believed that it pointed towards the body, and that the forewing was relatively narrow. An alternative hypothesis states that it was directed forwards during flight, resulting in a much broader forewing that acted as a leading edge flap. We tested scale models in a wind tunnel to determine the aerodynamic consequences of these conflicting hypotheses, and found that performance is greatly improved if the pteroid is directed forwards: the lift: drag ratios are superior and the maximum lift is exceptionally high in comparison with conventional aerofoils. This high lift capability may have enabled even the largest pterosaurs to take off and land without difficulty.

This study reports on the three-dimensional spatial arrangement and movements of the skeleton of Anhanguera santanae (Pterodactyloidea: Ornithocheiridae), determined using exceptionally well-preserved uncrushed fossil material, and a rigid-body method for analysing the joints of extinct animals. The geometric results of this analysis suggest that the ornithocheirids were inherently unstable in pitch and yaw. As a result, pitch control would probably have been brought about by direct adjustment of the angle of attack of the wing, by raising or lowering the trailing edge from the root using the legs if, as is indicated in soft-tissue specimens of a number of unrelated pterosaur species, the legs were attached to the main wing membrane, or by using long-axis rotations at the shoulder or wrist to raise and lower the trailing edge from the wingtip. An analysis of the three-dimensional morphology of the wrist lends support to the idea that the pteroid -a long, slender wrist bone unique to pterosaurs that supported a membranous forewing -was directed forwards in flight, not towards the body. As a result, the forewing could have fulfilled the function of an air-brake and high-lift device, and may also have had an important role in pitch, yaw, and roll control. The joint analysis is consistent with a semi-erect quadrupedal model of terrestrial locomotion in the ornithocheirids.

SUMMARY Pterosaur wings bore a striking resemblance to sails, having a bony spar at the leading edge, formed by the forelimb and one enormously elongated digit,and an elastic wing membrane. Such simple wings would be expected to have performed badly due to excessive deformation, membrane flutter and poor control characteristics. Here I discuss how certain anatomical features,specifically a forewing membrane in the inner part of the wing and a system of fibres embedded in the distal part, may have countered these shortcomings. The forewing, supported by the unique pteroid bone, would have reduced the wings'geometric twist, and has been shown in wind tunnel tests to improve membrane stability at low angles of attack and dramatically increase the maximum lift coefficient at high angles of attack. The function of the fibres is poorly understood, but it is suggested that they improved membrane stability and optimised twist nearer the wingtips.