Critical innovations in the assembly of the modern flight apparatus in Early Cretaceous birds

A pivotal innovation in the evolution of powered flight in dinosaurs was the triosseal canal-a specialized passage formed by the scapula, coracoid, and furcula that guides the wing-elevation tendon. However, the origins of this structure remained obscure. Here, we applied integrated histological analysis and micro-computed tomography (CT) scanning of a new enantiornithine specimen and the basal ornithuromorph Archaeorhynchus. Our results indicate that the triosseal canal evolved first through paedomorphosis of the coracoscapular joint into a synchondrosis in ornithothoracines and subsequent acquisition of the acrocoracoclavicular joint in ornithuromorphs. The complete lack of a connection between the furcula and coracoid represents one of the crucial skeletal disparities between enantiornithines and ornithuromorphs. We propose that the closure of the triosseal canal in ornithuromorphs markedly improved tendon stability, facilitating a greater range of wing motion and more efficient flight compared to enantiornithines, serving as one of the critical functional triggers responsible for their ecological diversification.