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First published online May 8, 2007
Journal of Experimental Biology 210, 1663-1671 (2007)
Published by The Company of Biologists 2007
doi: 10.1242/jeb.000307

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Sailing the skies: the improbable aeronautical success of the pterosaurs

Matthew T. Wilkinson

Department of Zoology, University of Cambridge, Downing Street, Cambridge CB2 3EJ, UK

View larger version (7K): [in this window] [in a new window]   Fig. 1. Skeletal reconstruction of the Cretaceous pterosaur Anhanguera santanae in dorsal view, showing the elongated wing-finger (wf) supporting the cheiropatagium (ch), the unique pteroid bone (pt) supporting the propatagium (pro) and the cruropatagium (cr) medial to the leg. Two possible reconstructions of the pteroid are shown, with corresponding outlines of the propatagium: a forward-pointing orientation (solid line), and a medial orientation (broken line). Scale bar, 200 mm. Additional abbreviations: dc, distal carpal; f, femur; h, humerus; mc, medial carpal; pc, proximal carpal; r, radius; t, tibiotarsus; u, ulna; wf, wing-finger; wm, wing-finger metacarpal.

View larger version (7K): [in this window] [in a new window]   Fig. 2. Theoretically predicted hysteresis behaviour of an inextensible sail profile of given slackness in inviscid flow, showing changes in sail shape and the lift coefficient as angle of attack is decreased from a high value (1) through ideal incidence (2) and beyond, when the sail becomes S-shaped (3) and finally pops through (broken line), adopting a wholly convex shape with negative camber (4). A subsequent increase in the angle of attack from this point causes the profile to become S-shaped again (5) before positive camber is restored. Profile chord lines are indicated in red.

View larger version (39K): [in this window] [in a new window]   Fig. 3. (A) Articular movement of the right pteroid as reconstructed by Wilkinson et al. (Wilkinson et al., 2006). From the forward-pointing orientation, flexion at first takes the form of pure depression, so that the pteroid is confined to the indicated vertical plane that lies perpendicular to the wing spar. Depression then gives way to adduction, and the pteroid swings towards the body, eventually coming to occupy a plane parallel to the wing spar at the limit of flexion (broken outline). (B) Dorsal (top) and anterior (bottom) views of a virtual model of Anhanguera, with an extended propatagium (indicated in dark blue) and forward-pointing pteroid (indicated in red). (C) As in B, but with the pteroid fully flexed and the propatagium furled. Abbreviations as in Fig. 1.

View larger version (17K): [in this window] [in a new window]   Fig. 4. (A) Lateral view of the right medial carpal in articulation with the distal carpal of the Cretaceous pterosaur Coloborhynchus robustus. Scale bar, 25 mm. (B) Traditionally recognised articular surfaces of the right carpal-pteroid joint of Coloborhynchus, the medial carpal in distal (anterior) view showing the fovea (fov), and the head of the pteroid in proximal view. Specimen details can be found in Wilkinson et al. (Wilkinson et al., 2006). Scale as in A. (C) Reconstruction of the right wrist of Coloborhynchus in dorsal view according to descriptions provided by Bennett (Bennett, 2001; Bennett, 2006), with a sesamoid bone (ses) within the fovea, and the pteroid articulating on the side of the medial carpal. Note that the medial carpal has been rotated about its long axis by 180° with respect to A and B. The postulated trajectory of the wing-finger metacarpal extensor tendon (ten), in which the sesamoid is embedded, is also shown. Scale bar, 50 mm. (D) Reconstruction of the right wrist in dorsal view according to Wilkinson et al. (Wilkinson et al., 2006), with the pteroid at maximum elevation, and an alternative reconstruction of the sesamoid, which is shown in close association with the carpal-pteroid joint, embedded within a putative pteroid extensor tendon (origin and insertion points unknown). Scale as in C. Abbreviations as in Fig. 1. Broken line indicates a continuation of the extent of the tendon.

View larger version (8K): [in this window] [in a new window]   Fig. 5. Lift coefficients of spar-and-membrane profile models, with (pro) and without a propatagium (no pro), measured in a wind tunnel at a Reynolds number of 1.2x105, an appropriate value for large pterosaurs (Wilkinson, 2002). Data re-plotted from Wilkinson et al. (Wilkinson et al., 2006). Measurements were nominally made at angles of attack from –2° to 20°, but below 2° the model without a propatagium luffed so severely that readings could not be taken. Above 18° the same model stalled, and again became too unstable for the lift to be measured. For the model with a propatagium, the best performance was obtained if propatagium deflection (pd), measured with respect to the chord line of the cheiropatagium, was increased from 30° to 50° as indicated.

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