Wing osteology, myology, and function of Rhea americana (Aves, Rheidae)

The Greater Rhea (Rhea americana, Rheidae) is a flightless paleognath with a wide geographical distribution in South America. The morphology of its shoulder girdle and wings are different from those of flying birds and some characteristics are similar to basal birds and paravian theropods. We present a detailed osteological, myological, and functional study of the shoulder and the wing of the Greater Rhea. Particular features of the anatomy of the pectoral girdle and wing of Rhea include the lack of triosseal canal, reduced origin area of the mm. pectoralis p. thoracica and supracoracoideus and the lack of a propatagium. The wing muscle mass is markedly reduced, reaching only 0.89% of total body mass (BM). Forelimb muscles mass values are low compared to those of flying birds and are congruent with the non-use of wings for active locomotion movements. R. americana does not flap the wings dorso-ventral as typical for flying birds, but predominantly in cranio-caudal direction, following a craniolateral to caudomedial abduction-adduction arc. When the wings are fully abducted, they are inverted L-shaped, with the inner surface caudally faced, and when the wings are folded against the body, they do not perform the complete automatic wing folding nor the circumduction of the manus, a movement performed by extant volant birds. This study complements our knowledge of the axial musculature of the flightless paleognaths and highlights the use of the Greater Rhea as a model, which may help understand the evolution of Palaeognathae, as well as the origin of flapping flight among paravian theropods.

Automated summary

This study presents a detailed analysis of the osteology, myology, and function of the shoulder and wing of the flightless bird Greater Rhea. The anatomy of the pectoral girdle and wing of the species is different from flying birds, but shares similarities with basal birds and paravian theropods. The study reveals unique features of the Greater Rhea's wing, including reduced muscle mass and different wing movement patterns compared to flying birds. This research contributes to our understanding of the musculature of flightless birds and provides insight into the evolution of Paleognathae and the origin of flapping flight in paravian theropods.