Comparison of forelimb function between Deinonychus and Bambiraptor(Theropoda: Dromaeosauridae)

Abstract

Fossils and casts of forelimb bones of the dromaeosaurids Deinonychus antirrhopus and Bambiraptor feinbergi were manually manipulated to determine range of motion and to test functional hypotheses. Shoulder motion in Bambiraptor resembles that found by a previous study on Deinonychus. The humerus can be retracted and elevated to subhorizontal positions and protracted somewhat beyond the vertical. In both taxa, the elbow can be strongly flexed but cannot be fully extended. Supination and pronation cannot occur by movement of the radius, which is immobile relative to the ulna. The palms therefore face medially except during wrist extension, which causes obligatory supination. The fingers of Deinonychus remain spread during flexion. In contrast, torsion of the distal articular surface of metacarpal I and the long axis of phalanx III-3 cause the first and third digits of Bambiraptor to approach each other during flexion, the first known instance of opposable fingers in a dinosaur. The morphology and range of motion in the forelimbs of Deinonychus and Bambiraptor enable two-handed prehension with the wrist flexed, one-handed clutching of objects to the chest, use of the hand as a hook, arm-swinging or -raising displays, and use of the forelimbs to maintain balance. Feathered wings, if present, precluded manual apprehension of objects on the ground, two-handed clutching of objects to the chest, and use of digit II to probe crevices. The forelimbs could not be used to dig. Opposability of the fingers of Bambiraptor enabled one-handed prehension, whereas Deinonychus required both hands to hold objects.