Osteological and Soft-Tissue Evidence for Pneumatization in the Cervical Column of the Ostrich (Struthio camelus) and Observations on the Vertebral Columns of Non-Volant, Semi-Volant and Semi-Aquatic Birds.

Naomi E Apostolaki,Paul M Barrett,Emily J Rayfield,Andrew A Farke

doi:10.1371/journal.pone.0143834

Naomi E Apostolaki, Paul M Barrett + Show 2 more

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https://doi.org/10.1371/journal.pone.0143834

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Journal: PloS one	Publication Date: Dec 9, 2015
Citations: 12	License type: CC BY 4.0

Affiliation: Natural History Museum, University of Bristol

Abstract

Postcranial skeletal pneumaticity (PSP) is a condition most notably found in birds, but that is also present in other saurischian dinosaurs and pterosaurs. In birds, skeletal pneumatization occurs where bones are penetrated by pneumatic diverticula, membranous extensions that originate from air sacs that serve in the ventilation of the lung. Key questions that remain to be addressed include further characterizing (1) the skeletal features that can be used to infer the presence/absence and extent of PSP in birds and non-avian dinosaurs, and (2) the association between vertebral laminae and specific components of the avian respiratory system. Previous work has used vertebral features such as pneumatic foramina, fossae, and laminae to identify/infer the presence of air sacs and diverticula, and to discuss the range of possible functions of such features. Here, we tabulate pneumatic features in the vertebral column of 11 avian taxa, including the flightless ratites and selected members of semi-volant and semi-aquatic Neornithes. We investigate the associations of these osteological features with each other and, in the case of Struthio camelus, with the specific presence of pneumatic diverticula. We find that the mere presence of vertebral laminae does not indicate the presence of skeletal pneumaticity, since laminae are not always associated with pneumatic foramina or fossae. Nevertheless, laminae are more strongly developed when adjacent to foramina or fossae. In addition, membranous air sac extensions and adjacent musculature share the same attachment points on the vertebrae, rendering the use of such features for reconstructing respiratory soft tissue features ambiguous. Finally, pneumatic diverticula attach to the margins of laminae, foramina, and/or fossae prior to their intraosseous course. Similarities in PSP distribution among the examined taxa are concordant with their phylogenetic interrelationships. The possible functions of PSP are discussed in brief, based upon variation in the extent of PSP between taxa with differing ecologies.

Highlights

The avian respiratory system is composed of two immobile, dorsally fixed lungs, which are the sites of gas exchange, and nine large air sacs extending from the lungs via diverticula, which act as mechanical bellows to ventilate the lungs but that have very limited potential (5%) for gas exchange [1,2,3,4]
The ostrich neck is composed of 17 vertebrae, but the axis and atlas were missing from our specimen (BRSMG Ag1174.1–15), leaving 15 cervicals (CV3–17) in total
Having observed vertebral laminae and their close association with other pneumatic features and cervical air sac diverticula we argue that the laminae, they may represent only ambiguous evidence for the presence of a heterogeneous respiratory system, serve a combined function in many cases, acting as the attachment points for both muscles and pneumatic diverticula, agreeing with observations made by previous studies (e.g., [4,13,14])

Summary

Introduction

The avian respiratory systemThe avian respiratory system is composed of two immobile, dorsally fixed lungs, which are the sites of gas exchange, and (variably) nine large air sacs extending from the lungs via diverticula, which act as mechanical bellows to ventilate the lungs but that have very limited potential (5%) for gas exchange [1,2,3,4]. Lung volume remains constant during breathing, whereas the air sacs contract and expand during ventilation and occupy a substantial proportion of the body cavity [3,5,6]. The lung is functionally and anatomically subdivided into paleopulmo and neopulmo portions. Paleopulmonic airflow is unidirectional during the course of the respiratory cycle [3,5,7]. Most ratites have poorly developed neopulmonic [8] but well-developed palaeopulmonic parabronchi, emus have minimally developed neopulmo portions in their lungs [5,7]. All penguins have strictly paleopulmonic lungs; i.e., they have no neopulmonic parabronchi [2,3,4,7]. Grebes, and loons have both neopulmonic and paleopulmonic lungs [3,7]

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