The functional morphology of the forelimb of Deinonychus antirrhopus and its importance for the origin of avian flight

The discovery of Deinonychus antirrhopus, brought about a renaissance in dinosaur studies, especially because it led to the revival of the theory that birds are descended from dinosaurs. Because of Deinonychus's close phylogenetic position to flying dinosaurs, its morphology is of particular interest for the study of the functional transition that occurred as use of the forelimbs changed from predation to flight.; The bones of Deinonychus were carefully examined to determine if they possessed those features necessary to complete an avian upstroke-downstroke cycle. The kinematics of the forelimb skeleton of Deinonychus was reconstructed to accurately test the range of motion of the forelimb. This biomechanical model shows a range of motion consistent with that of the avian flight stroke. Further, Deinonychus exhibits the skeletal components of the avian automatic flexion and extension system. Deinonychus had an avian-like triangular radiale that slides in the trochlea carpalis of the semilunate carpal. This motion was driven by the radius sliding parallel to the ulna as it rode over the radial condyle of the humerus.; The musculature of Deinonychus was then reconstructed by comparison to the extant phylogenetic bracket of living dinosaurs (avians) and alligators to determine what muscular components of the avian flight system were present. Based on this phylogenetic bracketing, muscle homologues were drawn between alligators and avians and new muscle names were proposed to reflect these homologues. This bracket shows that Deinonychus possessed an avian style ligament system in the wrist and the wrist crossing musculature that is necessary for the coordinated, automatic flexion and extension of the wrist with the elbow. Further Deinonychus possessed the osteological correlates for insertions of the m. extensor metacarpi radialis on mtc I which is necessary for automatic extension of the hand. It also had an insertion scar on the second metacarpal for the m. extensor carpi ulnaris which drives automatic flexion. These combined data show that Deinonychus possessed both the skeletal and muscular components of the avian flight system.; The presence in Deinonychus of so many components of the avian flight system indicates that much of the avian flight system evolved prior to flight and was later exapted for flight in avians. Predatory behavior is a logical precursor to avian flight behavior. The same stroke which allowed a terrestrial bipedal animal to catch its prey with its “hands” may have enabled its possessor to run right off the ground.