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

Fernando E Novas,Federico L Agnolín,Gastón E Lo Coco,Matías J Motta,Federico Brissón Egli,Sebastián Rozadilla

doi:10.3389/feart.2021.662167

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 comparingArchaeopteryx lithographicawith the extant New World vultureCathartes aura. Ostrom offered innovative ideas about range of movements and function of wing elements in the basal birdArchaeopteryx. 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 paleognathRhea americanais 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.

Highlights

The origin and early evolution of birds and flight are one of the most debated topics on evolutionary biology
We present here a summary description of the main muscle masses of the shoulder girdle of the Greater Rhea (Rhea americana), those muscles that originate on the coracoid and acrocoracoid, constituting the most important muscles for the movement of the wing
Inspired by Ostrom (1976) research, we evaluated some keyfeatures of pectoral girdle morphology that may have importance in the rise of bird flight

Summary

Introduction

The origin and early evolution of birds and flight are one of the most debated topics on evolutionary biology. In 1976, Ostrom published a detailed comparison between the early bird Archaeopteryx lithographica and the extant New World vulture Cathartes aura, entitled “Some hypothetical anatomical stages in the evolution of avian flight”. In this contribution he speculated about the anatomical transformations and evolutionary steps in the shoulder girdle and forelimb during avian evolution, taking Archaeopteryx as the “starting point” of this sequence of evolutionary changes. Ostrom offered ideas about range of movements and function of limb elements in Archaeopteryx He gave special importance to the modifications of the dinosaurian “biceps tubercle” (homologous with the modern avian acrocoracoidal process) and its direct relation with the course of one of the main wing elevators, the m.

Objectives

Methods

Results

Discussion

Conclusion