Anatomical atlas and three-dimensional musculoskeletal model of the flight apparatus in the Southern Screamer (Anseriformes: Chauna torquata )

Abstract The three bird species within the family Anhimidae (screamers) are of great interest to the big picture of avian evolution due to both their phylogenetic position and their unusual combination of features. As screamers are early diverging members of Anseriformes (waterfowl), which itself is an early diverging clade within the extant bird radiation, their anatomy may help shed light on that of the common ancestor of both Anseriformes and crown-group birds as a whole. To reveal new information about the wing anatomy of anhimids, we generated a three-dimensional musculoskeletal model of the flight apparatus of the extant Southern Screamer (Chauna torquata) using diffusible iodine-based contrast-enhanced computed tomography (diceCT). We found that the wing muscle anatomy of Chauna reflects the soaring flight style employed by anhimids, as opposed to the rapid continuous wingbeats used by most other anseriforms. In Chauna, this manifests as a more distal insertion of the pectoralis muscle on the humerus to facilitate slower, stronger wingbeats, and a reduction in the length of antebrachial muscle origins and insertions that serves to reduce the distal inertia of the wing. Our three-dimensional dataset represents a typical musculoskeletal system of a bird with strong flight capabilities. This, combined with our observation that most flight-related muscles in Chauna leave recognizable osteological correlates associated with their attachments to the skeleton, means that this work can help form a basis for future comparative studies of the avian musculoskeletal system, with implications for reconstructing the flight apparatus of fossil birds.