Exploring elbow kinematics in the central bearded dragon (Pogona vitticeps) using XROMM: Implications for the role of forearm long-axis rotation in non-avian reptile posture and mobility

The functional morphology and kinematics of the elbow joint remain relatively understudied in squamates. Previous investigations of lizard elbow morphology and kinematics suggest long-axis rotation (LAR) of the radius and ulna during stance allows the manus to remain pronated during forelimb retraction. Using XROMM (X-ray Reconstruction of Moving Morphology), we explored the range of 3D movements and kinematics of the humerus, radius, and ulna in three adult male Central bearded dragons (Pogona vitticeps) during trackway walking. Our data indicate that the elbow joint of P. vitticeps experiences significant rotations in all three dimensions and that the radius and ulna adduct and rotate laterally on their long axes relative to the elbow joint and to one another during stance. These movements allow the distal ends of the radius and ulna to remain in a configuration necessary for manus pronation. These data support previous inferences that the radius and ulna of lizards move independently at the wrist joint. We suggest that independent LAR of the radius and ulna relative to the elbow joint and to one another may be an ancestral mechanism in lizards and perhaps more broadly across non-avian reptiles.