Comments on the Morphology of Basal Paravian Shoulder Girdle: New Data Based on Unenlagiid Theropods and Paleognath Birds

Abstract

In 1976 John Ostrom published an enlightening paper about the anatomical transformations in the shoulder girdle and forelimb elements along the origin of birds. Most of his ideas were based on comparing Archaeopteryx lithographica with the extant New World vulture Cathartes aura. Ostrom offered innovative ideas about range of movements and function of wing elements in the basal bird Archaeopteryx. Further, he explored anatomical transformations that may have occurred at early stages of the evolution of flight and established several hypothetical steps toward the acquisition of flapping flight in modern birds. Since then, however, our understanding of paravian diversity and anatomy has increased dramatically. Based on novel information derived from recent experiments, and currently available anatomical evidence of basal paravians, the present paper aims to review some important topics on pectoral girdle anatomy related to flight origins. Further, a brief analysis of pectoral girdle osteology and myology of the extant paleognath Rhea americana is also included with the aim to test whether Ostrom’s ideas still remain valid under this new context, based on available phylogenetic and anatomical frameworks.

Unusual pectoral apparatus in a predatory dinosaur resolves avian wishbone homology

“…The origin of the peculiar biology of birds has been investigated at several levels and from different functional and phylogenetic perspectives [1][2][3][4][5][6][7][8][9][10] . The evolution of the active flight of birds has been subjected to hierarchically nested analyses, which have reconstructed the sequence of adaptive regimes that shaped the avian bauplan [1][2][3]8,9 , have focused on the origin of locomotor modularity 4 , and have discussed the emergence of the anatomical novelties involved in powered flight [5][6][7]10 . In birds, the furcula (or wishbone) is a peculiar unpaired dermal element of the pectoral apparatus that articulates to each of the scapulocoracoids, working as a strut between the shoulders 11 .…”

Digital restoration of the pectoral girdles of two Early Cretaceous birds and implications for early-flight evolution

“…In most euornithines (e.g., Figure 4D–F ), this process is proportionally longer and extends much further beyond the glenoid than in non-euornithine birds. In non-avialan theropods and Archaeopteryx , by contrast, the acrocoracoid process (frequently described as coracoid tubercle or biceps tubercle) is located cranioventral to the glenoid fossa ( Mayr et al, 2005 ; Novas et al, 2021 ). The acrocoracoid process of Sapeornis forms a shelf-like structure projecting dorsally, cranially, and laterally from the lateralmost part of the coracoid, a condition not known in other birds.…”

“…In Rhea , the Lig. acrocoracoacromiale bridges the tendon of M. supracoracoideus, contributing to a what is functionally a ‘triosseal canal’ ( Novas et al, 2021 ). In certain other birds, a closed bony canal is formed by the coracoid and scapula only, with no contribution from the furcula, or even formed by the coracoid alone via an ossified bridge connecting the acrocoracoid and procoracoid processes (e.g., Upupa epops and Columba livia ) ( Baumel and Witmer, 1993 ).…”

“…In most euornithine specimens (e.g., Yixianornis and Gansus ) ( Clarke et al, 2006 ; Wang et al, 2016c ), the acrocoracoid process is medially hooked and a prominent procoracoid process is present, features that suggest the existence of a typical, fully enclosed bony triosseal canal formed by the scapula, coracoid, and furcula. Many previous studies have denied the presence of a triosseal canal in non-euornithine birds because of the lack of a long medially hooked acrocoracoid process and a procoracoid process ( Novas et al, 2021 ; Wang and Zhou, 2017a ). Although a small and pointed procoracoid process has been reported in Protopteryx ( Zhang and Zhou, 2000 ; Chiappe et al, 2020 ), this cannot be confirmed in the provided figures and preservation of the omal region is poor.…”

“…In species with an extremely small scapular wing, the supracoracoid foramen is either absent or located in the sternal wing. Figure 1 illustrates the key morphological features of the pectoral girdle in different theropod groups and the terms used in this article, though we admit that a complete evolutionary picture of the theropod pectoral girdle has yet to be presented and there are different opinions on what terms should be used for certain structures ( Xu, 2002 ; Novas et al, 2021 ).…”