Anatomy, biomechanics, and evolution of the archosaur ribcage with implications for the evolution of ventilation
Creators: Wang, Yan-Yin
Publisher: name: University of Alberta Library
Year: 2023

<p>Extant birds and crocodylians are modern representatives of Archosauria, a group of amniote vertebrates with a long evolutionary history that may be traced back to the Triassic Period. On the paths to extant bird and crocodylians, early archosaurs diversified and occupied many niches available for large-bodied amniotes throughout the Mesozoic Era. As with other amniotes, respiration is one of the fundamental biological processes needed to sustain the life of archosaurs, which consists of multiple steps that range from ventilating air into and out of the respiratory organs, to gas exchange at the blood-air barrier in the lung, to cellular respiration. This contribution pertains to the trunk anatomy and ventilatory biomechanics in archosaurs. <br>
The cervicodorsal transition marks the start of the trunk, and is traditionally defined by the first connection between the vertebral column and the sternum via rib segments. However, most archosaurs had cartilaginous sterna and sternal ribs, which are not well documented in the fossil record, and no consensus exists on how to identify cervicodorsal transitions in archosaurs without preserved sterna. We survey 29 extant and 32 fossil archosaurs for anatomical features that appear to change across the cervicodorsal transition, and use this information to propose criteria that identify regions of the vertebral series where the first sternal connection is likely to be located. Supervised statistical models based on features of vertebrae are created using a combination of linear discriminant analysis, logistic regressions, NaïveBayes classifier, and RandomForest, but the models can only accurately identify the cervicodorsal transition in sampled extant archosaurs. <br>
Vertebral ribs of the trunk in extant birds carry bony prongs called uncinate processes, which have been hypothesised to enhance a bird’s ability to expand the trunk and draw ventilatory airflow into the respiratory organs. Tab-like cartilaginous uncinate processes are present in crocodylians. In the fossil record, uncinate processes occur as ossified prongs only in maniraptoriform dinosaurs, but occur as mineralised cartilage in several ornithischian dinosaurs and one notosuchian crocodyliform. We establish an osteological correlate, termed the uncinate scar, by removing uncinate processes from vertebral ribs of extant archosaurs, and use the correlate to infer the presence of cartilaginous uncinate processes in 19 fossil archosaurs. Using the augmented distribution of uncinate processes by inferences from uncinate scars, we further infer that uncinate processes, with their capacity to enhance ventilation, are likely a homologous feature shared by most dinosaurs and may even be plesiomorphic for Archosauria. Histological sections through dorsal vertebral ribs carrying uncinate processes or uncinate scars are made for two extant archosaurs and four fossil dinosaurs, and we consistently find bundles of coarse collagen fibres at uncinate scars, bolstering the conclusion that uncinate scars are attachment sites for uncinate processes.<br><br>
Trunk muscles of the common raven Corvus corax, the emu Dromaius novaehollandiae, and three crocodylians are dissected to document the attachment sites of trunk muscles, which are used as a basis to compare the trunk muscle configuration among archosaurs carrying and lacking uncinate processes. We find that m. appendicocsotalis, a muscle attached to uncinate processes in most birds, remains present in the emu Dromaius novaehollandiae, which lacks uncinate processes. <br>
We construct three kinematic models, from ribcages of the ostrich Struthio camelus and the spectacled caiman Caiman crocodilus. Using tidal volume documented in the literature, we estimate the plausible motions of the ribcage, which are comparable to observations from in vivo studies. To infer the contributions made to ventilation by various trunk muscles, we construct two three-dimensional kinetic models to estimate changes in their moment arms during ventilation. The results suggest that uncinate processes in palaeognath birds provide an enhanced ability to protract the vertebral ribs, while limiting their potential for abduction by altering the mechanical leverage and fibre orientation of m. appendicocostalis. By comparison, uncinate processes in crocodylians likely provide an enhanced ability to protract and abduct simultaneously. The contrasts in muscle function between palaeognath birds and crocodylians are likely related to the morphology and muscle configuration of the trunk.<br>
 Our anatomical and biomechanical studies, taken together, indicate that ventilation enhanced by the additional mechanical leverage provided by uncinate processes was likely widespread in, and may even have been plesiomorphic for, Archosauria.  </p>