Abstract
The Archosauria are a highly successful group of vertebrates, and their evolution is marked by the appearance of diverse respiratory and metabolic strategies. This review examines respiratory function in living and fossil archosaurs, focusing on the anatomy and biomechanics of the respiratory system, and their physiological consequences. The first archosaurs shared a heterogeneously partitioned parabronchial lung with unidirectional air flow; from this common ancestral lung morphology, we trace the diverging respiratory designs of bird- and crocodilian-line archosaurs. We review the latest evidence of osteological correlates for lung structure and the presence and distribution of accessory air sacs, with a focus on the evolution of the avian lung-air sac system and the functional separation of gas exchange and ventilation. In addition, we discuss the evolution of ventilation mechanics across archosaurs, citing new biomechanical data from extant taxa and how this informs our reconstructions of fossils. This improved understanding of respiratory form and function should help to reconstruct key physiological parameters in fossil taxa. We highlight key events in archosaur evolution where respiratory physiology likely played a major role, such as their radiation at a time of relative hypoxia following the Permo-Triassic mass extinction, and their evolution of elevated metabolic rates.This article is part of the theme issue ‘Vertebrate palaeophysiology’.
Highlights
The Archosauria are a highly successful group of tetrapod vertebrates, represented today by birds and crocodylians
In addition to their evolutionary and ecological success, archosaurs are marked by the appearance of diverse respiratory and metabolic strategies, providing an excellent opportunity to study the functional evolution of the respiratory system [4]
Other models have since been developed that better approximate the avian respiratory system e.g. by accounting for cross-current gas exchange [106], but given the uncertainty on the gas exchange mechanism in crocodilians, it is unclear which model is most appropriate for fossil archosaurs
Summary
The Archosauria (ruling reptiles) are a highly successful group of tetrapod vertebrates, represented today by birds and crocodylians. Qualitative and quantitative comparisons of the ribs and vertebrae in both non-avian dinosaurs and the non-dinosaurian dinosauriform Silesaurus found that they possessed a furrowed thoracic ceiling, with forked ribs which would have incised the lung’s dorsal surface and rendered dorsal components of the lung immobile [36,53] (figure 2), similar to modern birds The lungs in these taxa would have had a great deal of structural support, and so would have fulfilled the functional prerequisites for the increased subdivision of the parabronchi and thinning of the blood–gas barrier seen in the lungs of extant birds [8]. This hypothesis remains to be tested, and does not rule out an alternative extra-costal mode of ventilation which expanded the lungs and air sacs in the dorsoventral plane to avoid affecting stability
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