An examination of forearm bone mobility inAlligator mississippiensis(Daudin, 1802) andStruthio camelusLinnaeus, 1758 reveals thatArchaeopteryxand dromaeosaurs shared an adaptation for gliding and/or flapping

Abstract

ABSTRACT In response to increased limb bone loads many tetrapod clades have converged upon similar adaptations to reinforce the elbow joint by reducing independent movements of the forearm bones. However prior studies have not examined how these changes occurred phylogenetically or functionally, such as during the transition from prehensile forelimbs in dinosaurs to gliding/flapping flight in bird wings. Here, a functional analysis of forearm bone mobility in extant archosaurs shows that crossing and uncrossing of the radius and ulna can be forced in alligators via a passive gliding mechanism recently described in lacertilians, while birds are adapted to inhibit this motion. A comparison of these findings with a sample of extinct quadrupedal archosaur forearms strongly suggests that, due to the highly conserved morphology of tetrapod forearms in general, the lacertilian mechanism broadly describes the plesiomorphic mechanism via which tetrapod forearm bones passively cross in response to locomotor-induced...