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

ABSTRACT

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.

KEYWORDS:

• Processus acrocoracoideus claviculae
• sesamoid
• neomorph
• furcular development

Acknowledgments

We thank Zhang Limin for facilitating lab access, Li Dongsheng and Geng Binghe for facilitating access to the collection room and Ying Pengfei for assistance with the CT scanning. We also appreciate the helpful discussion of the results with Alida M. Bailleul, Tom Stidham and Wang Shiying. Special thanks to Yinmai O’Connor for her help in editing this paper.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Author’s Contribution

Wu Qian, Li Zhiheng, and Zhou Zhonghe designed the project. Wu Qian collected and analysed the data. Wu Qian, Li Zhiheng and Zhou Zhonghe wrote the paper.

Additional information

Funding

This work was supported by the National Natural Science Foundation of China under grants 42302012 and 42288201.