Meeting Abstract
There are three main hypotheses about how large pterodactyloids may have launched themselves from the ground: (1) a running takeoff in bipedal posture, (2) a standing takeoff in bipedal posture, and (3) a standing takeoff in quadrupedal posture, powered almost entirely by the forelimbs. Manipulation of fossils at the UT Austin Vertebrate Paleontology Laboratory enabled us to test these hypotheses. (1) Thrust from the legs during running must exceed stalling velocity, but wingstroke amplitude, which with wingbeat frequency determines the thrust of the wings, is limited by shoulder height, body size, and wing length. Because wing length is positively correlated with body size (and wingbeat frequency is negatively correlated), large pterosaurs could not have achieved a stroke reaching 40° below the horizontal (considered minimally effective for thrust) without leaping. So a simple running takeoff is unlikely. (2) The proximal hindlimb segments of most large pterosaurs were nearly as long as the forelimbs, and each set of bones was three times the gleno-acetabular length. The erect parasagittal hindlimbs, proportionally longer relative to body length than those of herons and egrets, needed to effect a wing-assisted jump to bring the animal to approximately three hip heights above the ground to enable a wingstroke to reach 40° below the horizontal. This appears possible, so (2) is plausible. (3) When the humerus is laterally extended, rotation effecting retraction of the forelimb is prevented by a bony stop at the deltopectoral crest. To retract the forelimb for a quadrupedal launch, it must be supinated at least 135°, and thrust must be provided by a sudden extension of the elbow and wrist joints, for which no adequate musculature is known. Hypothesis (2) appears to be the only plausible one.
