The coracoscapular joint of neornithine birds—extensive homoplasy in a widely neglected articular surface of the avian pectoral girdle and its possible functional correlates

Abstract

A survey is given of the morphological variation of the coracoscapular joint of neornithine birds. In Mesozoic stem group representatives, the coracoid exhibits a deeply concave cotyla scapularis, which articulates with a globose tuberculum coracoideum of the scapula. This morphology is likely to be functionally related to the development of a powerful supracoracoideus muscle and the formation of a triosseal canal as a pulley for the tendon of this muscle. In neornithine birds, the coracoid articulates with the scapula either via a concave cotyla or a flat facies articularis, with the latter largely restricting movements of the coracoid to the paramedian plane. Ancestral state reconstruction suggests that a cotyla scapularis is plesiomorphic for Neornithes and that a flat facies articularis scapularis evolved at least 13 times independently within the clade. For several lineages, the transition to a flat facies articularis scapularis can be traced in the fossil record, and the replacement of a cup-shaped cotyla by a flat articular facet seems to have been due to various functional demands. Often, a flat facies articularis scapularis is associated with reduced shafts of the furcula. A weakly developed furcula enables transverse movements of the coracoid and therefore enables a restriction of the mobility of the coracoscapular joint to the paramedian plane. In taxa with a large crop, a flat facies articularis scapularis is likely to be associated with a reorganization of the pectoral musculature, whereas in procellariiform birds, the transition from a cotyla to a facies articularis appears to have been correlated with the capacity for sustained soaring without wing flapping.

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“…Conversely, modern birds have been considered to display the opposite condition, with the main scapular articular surface on the coracoid being concave and that on the scapula convex. This purported discrepancy is the source of the clade name Enantiornithes, meaning ‘opposite birds.’ As mentioned above, however, a convex scapular articular surface is also seen on the coracoids of some non-avialan theropods, such as Sinovenator , and in some crown birds that are secondarily evolved ( Mayr, 2021 ), though the convexity is less prominent in these taxa than in late-diverging enantiornithine birds. Furthermore, the scapular articular surface on the coracoid is shallowly concave in some enantiornithines, such as pengornithids (e.g., IVPP V 18687 and V 18632).…”

“…Therefore, the triosseal canal is not necessarily formed by all three pectoral elements and is not necessarily a fully enclosed bony passage. Also, the procoracoid process is absent in certain volant crown birds that possess a triosseal canal, including Pavo muticus ( Figure 4 ) and Colius striatus ( Mayr, 2021 ) , as well as in the Late Cretaceous galliform-like genus Palintropus ( Longrich, 2009 ). Accordingly, the procoracoid process cannot be considered an essential constituent of the triosseal canal.…”

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“…7). As in crown group Strigiformes, the coracoid furthermore exhibits a shallow facies articularis scapularis, whereas there is a concave cotyla scapularis in Primoptynx (a cotyla scapularis is plesiomorphic for neornithine birds; Mayr 2021b). On the other hand, Primoptynx and crown group Strigiformes share a rounded omal extremity of the coracoid with a long (in the sterno‐omal direction) facies articularis clavicularis, whereas the omal extremity is hook‐shaped in Ypresiglaux .…”

Transformation of the pectoral girdle in pennaraptorans: critical steps in the formation of the modern avian shoulder joint

“…7 ) ( Zhou & Zhang, 2001 ; Clarke, 2002 ; You et al, 2006 ; Wang et al, 2020b ). This morphology is often described as a “ball and socket” joint ( Turner, Makovicky & Norell, 2012 ), but this interpretation is debated ( Mayr, 2021 ). Furthermore, a flat articular surface for the scapula on the coracoid has evolved in crown birds at least 13 times ( Mayr, 2021 ).…”

“…There have been many transformations in the morphology of the pectoral girdle that occurred during the evolution from non-avialan maniraptoran theropod dinosaurs (hereafter simply maniraptorans or theropods) to birds (Avialae) as summarized by previous studies , such as the elongation of the coracoid body from a quadrangular (often described as trapezoidal) to strut-like morphology, the separation of the scapula and coracoid from a fused (or connected by long suture prior to fusion) scapulocoracoid to separate bones articulating through a ball and socket joint, the change in the angle demarcated by the scapulocoracoid from obtuse to acute, the orientation of the glenoid fossa from caudoventral to dorsolateral, the elongation of the acrocoracoid process (homologous to “coracoid tubercle” or “biceps tubercle” of theropods) and the rotation of the coracoid body from laterally facing to craniocaudally then ventrally facing ( Turner, Makovicky & Norell, 2012 ; Lü et al, 2016 ; Wang, Stidham & Zhou, 2018 ; Novas et al, 2021b ). The osseous triosseal canal is an important feature of extant birds that was absent in non-avialan theropods, forming a pulley-like passage that guides the motion of the main muscle responsible for the upstroke, the M. supracoracoideus ( Mayr, 2021 ; Wang et al, 2022b ). This canal is formed by the acromion process of the scapula, the acrocoracoid process of the coracoid and the epicleideal process of the furcula, mainly through the coracoscapular joint, the scapuloclavicular joint and the acrocoracoclavicular joint ( Baumel et al, 1993 ; Ando & Fukata, 2018 ).…”