Pectoral girdles of Sapeornis and Piscivorenantiornis (1 Viewer)

[albertonykus]

Wang, S., Y. Ma, Q. Wu, M. Wang, D. Hu, C. Sullivan, and X. Xu (2022)
Digital restoration of the pectoral girdles of two Early Cretaceous birds, and implications for early flight evolution
eLife 11: e76086
doi: 10.7554/eLife.76086

The morphology of the pectoral girdle, the skeletal structure connecting the wing to the body, is a key determinant of flight capability, but in some respects is poorly known among stem birds. Here, the pectoral girdles of the Early Cretaceous birds Sapeornis and Piscivorenantiornis are reconstructed for the first time based on computed tomography and three-dimensional visualization, revealing key morphological details that are important for our understanding of early flight evolution. Sapeornis exhibits a double articulation system (widely present in non-enantiornithine pennaraptoran theropods including crown birds) which involves, alongside the main scapula-coracoid joint, a small subsidiary joint, though variation exists with respect to the shape and size of the main and subsidiary articular contacts in non-enantiornithine pennaraptorans. This double articulation system contrasts with Piscivorenantiornis in which a spatially restricted scapula-coracoid joint formed by a single set of opposing articular surfaces, a feature also present in other members of Enantiornithines, a major clade of stem birds known only from the Cretaceous. The unique single articulation system may reflect correspondingly unique flight behavior in enantiornithine birds, but this hypothesis requires further investigation from a functional perspective. Our renderings indicate that both Sapeornis and Piscivorenantiornis had a partially closed triosseal canal (a passage for muscle tendon that plays a key role in raising the wing), and our study suggests that this type of triosseal canal occurred in all known non-euornithine birds except Archaeopteryx, representing a transitional stage in flight apparatus evolution before the appearance of a fully closed bony triosseal canal as in modern birds. Our study reveals additional lineage-specific variations in pectoral girdle anatomy, as well as significant modification of the pectoral girdle along the line to crown birds. These modifications produced diverse pectoral girdle morphologies among Mesozoic birds, which allowed a commensurate range of capability levels and styles to emerge during the early evolution of flight.

 

[Fred Ruhe]

Figure 1. Pectoral girdle bones of Sapeornis chaoyangensis PMOL-AB00015.
(A-D), left scapula in lateral, dorsal, medial (costal), and ventral views. (E-H), left
coracoid in cranial, caudal, lateral, and proximal views; (I-L), furcula in cranial,
caudal, lateral, and distal views. Abbreviations: ac, acromial crest; acr, acromion
process; acra, articular surface for the acromion of the scapula; af, acromial flange;
ap, acrocoracoid process; fos, fossa; gf, glenoid fossa; gro, groove of scapula; hp, hypocleidium; mcoa, main coracoidal articular surface; msca, main scapular articular
surface; scoa, subsidiary coracoidal articular surface;scp, scapular part of coracoid;
snf, supracoracoidal nerve foramen; ssca, subsidiary scapular articular surface; stp, sternal part of coracoid; tsc, tubercle of the scapula. The black arrows in J and L indicate the concave surface for the tendon of m. supracoracoideus.

Figure 2. Pectoral girdle bones of Piscivorenantiornis inusitatus IVPP V 22582.
(A-D), left scapula in lateral, dorsal, medial (costal), and ventral views. (E-H), right coracoid in cranial, caudal, lateral, and proximal views; (I-L), furcula in cranial, caudal, lateral, and distal views. Abbreviations: acr, acromion process; acra, articular surface for the acromion of the scapula; ap, acrocoracoid process; coa, coracoidal articular surface; cg, caudal groove; gf, glenoid fossa; hp, hypocleidium; ia, impression for the Lig. acrocoracohumerale; sca, scapular articular surface; scp, scapular part of coracoid; sms, sulcus m. supracoracoideus; snf, supracoracoidal nerve foramen; stp, sternal part of coracoid.

Figure 3. Comparison of scapula and coracoid morphology across various paravian taxa. Each panel shows articulated left scapula and coracoid in cranial view (on left) and opposing articular surfaces of left scapula and coracoid (on right, with cranial direction toward top of figure for both scapula and coracoid). (A), Sinovenator changii (mirrored); (B), Sapeornis chaoyangensis; (C), Piscivorenantiornis inusitatus; (D), Tyto alba; (E), Egretta garzetta; (F), Pavo muticus. Abbreviations: acr, acromion process; ap, acrocoracoid process; coa, coracoidal articular surface; ct, coracoid tubercle; gf, glenoid fossa; mcoa, main coracoidal articular surface; msca, main scapular articular surface; pp, procoracoid process; sca, scapular articular
surface; scoa, subsidiary coracoidal articular surface; scp, scapular part of coracoid; snf, supracoracoidal nerve foramen; ssca, subsidiary scapular articular surface.

Figure 4. Simplified phylogeny with hypothetical steps of pectoral girdle evolution. The pectoral girdles of Sapeornis chaoyangensis, Piscivorenantiornis inusitatus, and Pavo muticus (from top to bottom) are shown in cranial, dorsal, and left lateral views. Abbreviations: tc, triosseal canal. The grey line in the dorsal view of the Sapeornis reconstruction represents the coracoclavicular ligament that connects the coracoid and furcula. Phylogenetic framework following Wang et al., (2018).

Fred