THE SAILING PEFORMANCE OF THE CRESTED PTERODACTYLOID TAPEJARA FROM THE EARLY CRETACEOUS OF BRAZIL

The function of pterosaur wings for powered flight has been known for more than two centuries, but the sailing performance of their wings has never been explored. The long and narrow outer wing of pterosaur, the dactylopatagium, bore a striking resemblance to a sail, having a bony spar at the leading edge, formed by one hyper-elongated fourth finger, and an elastic wing, reinforced by the actinofibrils. Here we explore the aero-hydrodynamic function of the Early Cretaceous crested pterodactyloid Tapejara wellnhoferi, based on an excellent 3-D skeleton cast. We used ‘Griffolyn' to reconstruct the wing membrane, and a thin plastic sheet for the soft-membraned cranial crest. During the sailing mode, the gliding wingplan was rotated forward at the glenoid and wrist; the inner wing was partially folded and oriented above the water surface allowing the outer wing to be directed vertically without any change in chord. The upright wing planform appears to be a stable configuration without much muscular effort. Tapejara had an enormous membranous cranial sail stretching upward and backward from the sagittal crest like a thin stiff vertical blade. During surface riding, pterosaurs looked like a three-masted carvel. The cranial sail served as a jib and a front rudder to steer the animal, whereas two upright triangular wings projecting transverse to the body axis functioned as a pair of mainsails to catch the wind and supply the thrust. The hindlimbs were backwardly directed for surface paddling and stability and prevented the animal from sliding sidewise. Apparently pterosaurs were buoyant and could glide on the water surface in the downwind direction, controlled by the webbed feet. In cross-section, the submerged body would mimic a trimaran where the hindlimbs served as two lateral hulls and the rounded sternum as the median hull. Actinofibrils conferred stiffness to the main sail to hold its shape well and maintain flatter camber no matter how strong the wind blows. Trimming the mainsail differently by pulling and pushing the knees would allow the animal to sail at different angles to the wind. Both sets of sails could be swiveled slightly for increased maneuverability. We calculated the sailing performance of Tapejara during foraging small fish and crustaceans. In a moderate tailwind (5m/s), the animal could sail at somewhere between 2 and 4 m/s as a free ride.