Article: An unusual flight mechanism in the Pterosauria

The motion at the shoulder joint of the Cretaceous pterodactyloid pterosaur Santanadactylus brasilensis is investigated. Shoulder movement is important because it determines the orientation and amplitude of the wing stroke during flapping flight. In previously studied pterosaurs, the principal shoulder motion is in the transverse vertical plane with, in at least one species, a degree of rotation about an axis along the length of the wing. However, in Santanadactylus brasilensis, evidence from direct articulation of the bones and from morphological analogues indicates that shoulder movement was restricted to rotation of the humerus about its long axis. The nature of the flight stroke produced by this motion depends upon the position of the humerus relative to the body. At maximum extension, humeral rotation was converted into rotation of the wing about the axis joining shoulder and wing tip, while if the humerus was held as close as possible to the body humeral rotation would give rise to a small amount of vertical wing movement, as well as rotation about the axis joining the shoulder to wing tip. The former wing motion might be used in braking, while the latter wing motion could have produced useful aerodynamic force for maintaining horizontal flight. Before this unusual shoulder motion could have evolved, flapping flight must have reduced in importance, and wing rotation may have arisen in response to pressures to minimize wing inertia.