Morphology of the forelimb of <i>Confuciusornis</i> and its implications for early flight evolution

Abstract The morphological transformation of a highly mobile, grasping forelimb into a flapping wing is a key transition in the evolution of bird flight. This evolutionary transformation is characterized by more canalized (restricted) elbow and wrist mobility and a diminished prehensile (grasping) ability of the digits, requiring a suite of osteological modifications. Here, we employ micro-computed tomographic (μCT) imaging to reconstruct the forelimbs of Confuciusornis based on a new specimen from the Lower Cretaceous Jehol Group in western Liaoning, China. Our rendering is the first three-dimensional reconstruction of the forelimb for this key Early Cretaceous bird, revealing critical osteological details that advance our understanding of early flight evolution. The reconstruction reveals previously unrecognized features including canalized structures in the elbow and wrist joints, a relatively smaller triangular pisiform, a hook-like projection on the alular metacarpal, and distinctive digital morphology. We provide detailed morphological descriptions of the elbow, wrist, and metacarpophalangeal and interphalangeal joints, which are essential for evaluating flight capabilities, forelimb folding mechanics, and manual grasping ability. Furthermore, our data support the presence of a cushion-like bone at the distal end of the alular metacarpal of Confuciusornis and demonstrate fusion of this cushion-like bone with the cranial condyle.