Modern birds are the most diverse group of living tetrapods, and vital to their success is the evolution of an extremely efficient respiratory system, which supplies them with the oxygen necessary to cope with the energetic demands of other aspects of their âhigh performanceâ biology e.g. endothermy and powered flight. Ventilation of the respiratory system by the musculoskeletal system is the first step in the delivery of oxygen to respiring tissues, and is a major determinant of oxygen uptake rates. The mechanics of ventilation can therefore tell us a great deal about respiration, and hence metabolism and locomotion capabilities. Birds are part of the wider group Archosauria, and in order to more fully understand the functional evolution of the avian respiratory system, we must study both their living and fossil relatives. The overall objective of this thesis was to investigate the mechanics of ventilation in both living and extinct archosaurs, as well as examining broader form-function relationships between bone morphology and functional respiratory soft tissue anatomy. In order to fulfil this objective, I first measured in vivo rib kinematics during breathing in living archosaurs, a crocodilian (American alligator) and a bird (wild turkey). I used X-ray reconstruction of moving morphology (XROMM) to measure joint rotations in 3D and test how well bony morphology of the costovertebral joints between the rib and vertebrae can actually predict motion. The results show that in alligators, morphology could only predict general motion patterns, but in birds there was a tighter correlation between observed and predicted rib motions. Secondly, I performed a geometric morphometric study into vertebral shape and costovertebral joint orientation across birds, crocodilians, and dinosaurs. I showed that overall vertebral shape in dinosaurs is more similar birds than crocodilians, and the costovertebral joint in particular is definitely more bird-like. This serves as an osteological correlate not just for predicting motion, but also lung structure. Lastly, I integrated results of my previous chapters to reconstruct rib motion and ventilation mechanics in a non-avian dinosaur. I animated the ribs of Tyrannosaurus rex in order to reconstruct ventilation in this taxon, and estimated important physiological parameters e.g. tidal volume. These data broadly agreed with predictions from allometric scaling of respiratory variables, and suggest possible rates of oxygen uptake more than sufficient to meet the oxygen demands of previously estimated metabolic rates in T. rex. Taken together, the results presented in this thesis form a framework for inferring the evolution of ventilation and the respiratory system as a whole through the use of osteological correlates and ribcage functional morphology. This framework can then be used to test hypotheses on the physiology and metabolism of now extinct organisms.