Inferences regarding the biology of extinct taxa are often founded primarily on cladistically-based phylogenetic hypotheses. However, these inferences are frequently inconsistent with the morphology and physiology of modern animals. I suggest that reference to the biology of modern animals might be a more appropriate tool to aid in the reconstruction of the morphology and physiology of extinct animals. Here, this methodology is applied to: (1) reconstruction of the respiratory anatomy and physiology of some Paleozoic archosaurs (dinosaurs, early birds, pterosaurs, and early crocodilians), (2) understanding cursorial locomotion in birds and bipedal dinosaurs, and (3) a reinterpretation of an enigmatic archosaur, Longisquama. The conclusions drawn from these data raise questions regarding current thinking regarding relationships between birds and dinosaurs. / Graduation date: 2000