1. Department of Ecology and Evolutionary Biology, Brown University, Providence, Rhode Island 02912, USA
2. Department of Organismic and Evolutionary Biology and Museum of Comparative Zoology, Harvard University, Cambridge, Massachusetts 02138, USA

Correspondence to: David B. Baier¹ Correspondence and requests for materials should be addressed to D.B.B. (Email: David_Baier@brown.edu).

Abstract

Despite recent advances in aerodynamic^{1, 2}, neuromuscular^{3, 4, 5} and kinematic^{6, 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 debated^{9, 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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