A secondary endochondral ossification centre in the furcula of extant birds and its significance for the evolution of the neornithine acrocoracoclavicular joint

The processus acrocoracoideus claviculae on the furcular epicleidium of some extant birds has been hypothesised to have developed independently from the main body of furcula, potentially representing a secondary ossification centre within avian furcula. However, the developmental pattern of the processus acrocoracoideus claviculae remains unclear, impeding our understanding of its homology and evolution. Here, we analysed the morphology, articulation and developmental pattern of the avian furcular epicleidium through X-ray computed tomography scanning and histochemical staining in several bird exemplars. The results show that the processus acrocoracoideus claviculae is a secondary endochondral ossification in the furcula, distinct from the intramembranously ossified main furcula and separated by connective tissue. The processus acrocoracoideus claviculae fuses with the furcula during postnatal development and forms the articular surface of the acrocoracoclavicular joint. The developmental pattern suggests that the left and right processus acrocoracoideus claviculae may be formed by the fusion of a sesamoid with the respective furcular ramus and evolved convergently among different neornithine lineages. This unique structure of furcula not only increases the morphological diversity of the acrocoracoclavicular joint in neornithine birds but also lays the foundation for their complex and diverse flight styles.