Variation of the coracoscapular joint (1 Viewer)

[Fred Ruhe]

Gerald Mayr, 2021

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

Zoomorphology (advance online publication)
doi: https://doi.org/10.1007/s00435-021-00528-2
https://link.springer.com/article/10.1007/s00435-021-00528-2

Free pdf:
https://link.springer.com/content/pdf/10.1007/s00435-021-00528-2.pdf

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.

Enjoy,

Fred

 

[Fred Ruhe]

Fig. 1 a Wing and pectoral girdle bones of a pigeon (Columba sp.),
with the dotted area indicating the surfaces of the coracoid and sternum occupied by musculus supracoracoideus (after George and
Berger 1966: Fig. III.1). b, c Coracoscapular joint of b the Common
Woodpigeon, Columba palumbus (Columbidae; left side, craniolateral
view), and c the Ring-necked Pheasant, Phasianus colchicus
(Phasianidae; right side, caudodorsal view). d‒f Omal extremity of
the left coracoid (dorsal view) of d the Early Cretaceous Yixianornis
grabaui with a deeply cup-shaped cotyla scapularis (IVPP V 1363;
right coracoid, mirrored; surrounding matrix digitally removed),
e Sagittarius serpentarius (Accipitriformes, Sagittariidae) with a
deeply cup-shaped cotyla scapularis, and f Rhynchotus rufescens
(Tinamiformes, Tinamidae) with a flat facies articularis scapularis.
The arrows in a‒c indicate the course of the tendon of the supracoracoideus muscle, which passes through the triosseal canal formed by the processus acrocoracoideus of the coracoid, the extremitas omalis of the furcula, and the acromion of the scapula. acr acromion of scapula, car carina sterni, cdp crista deltopectoralis of humerus, csc cotyla scapularis, csj coracoscapular joint, exo extremitas omalis of furcula, fah facies articularis humeralis of scapula, fas facies articularis scapularis, fur furcula, lco left coracoid, lhu left humerus, lsc
left scapula, pac processus acrocoracoideus of coracoid, ppc processus
procoracoideus of coracoid, rco right coracoid, ste sternum, sup
musculus supracoracoideus, tbd tuberculum dorsale of humerus. Not
to scale

Fred