Despite recent advances in aerodynamic1,2, neuromuscular3,4,5 and kinematic6,7 aspects of avian flight and dozens of relevant fossil discoveries8, the origin of aerial locomotion and the transition from limbs to wings continue to be debated9,10. Interpreting this transition depends on understanding the mechanical interplay of forces in living birds, particularly at the shoulder where most wing motion takes place. Shoulder function depends on a balance of forces from muscles, ligaments and articular cartilages, as well as inertial, gravitational and aerodynamic loads on the wing11. Here we show that the force balance system of the shoulder evolved from a primarily muscular mechanism to one in which the acrocoracohumeral ligament has a critical role. Features of the shoulder of Mesozoic birds and closely related theropod dinosaurs indicate that the evolution of flight preceded the acquisition of the ligament-based force balance system and that some basal birds are intermediate in shoulder morphology.
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We are grateful to K. Dial for the pigeon specimen, K. Middleton for help with VTK, C. Sullivan and L. Claessens for sharing alligator video and specimens, Z. Zhou for access to fossils at IVPP, T. Roberts for methodological advice, and the Brown Morphology group. We thank Autodesk for Maya software support and the SHAPE lab at Brown University for access to 3D laser scanning equipment. Funding was provided by the National Science Foundation (S.M.G., SHAPE lab), a Bushnell Faculty Research Grant (S.M.G.), the Paleontological Society (D.B.B.) and Sigma Xi (D.B.B.).
Reprints and permissions information is available at www.nature.com/reprints. The authors declare no competing financial interests.
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Baier, D., Gatesy, S. & Jenkins, F. A critical ligamentous mechanism in the evolution of avian flight. Nature 445, 307–310 (2007). https://doi.org/10.1038/nature05435
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