| Birds and crocodylians are the only living representatives of the Archosauria, a once-diverse group that includes dinosaurs, pterosaurs, and other fossil forms. Archosaurs are highly divergent in terms of respiratory system specializations, and present an opportunity to examine the evolution of respiratory design and function.; In chapter 1, in vivo visceral and skeletal kinematics were examined during respiration in the crocodylian Alligator mississippiensis through cineradiography, intrapulmonary pressure recordings, and pneumotachometry. The crocodylian breathing mechanism consists of a primitive costostemal aspiration pump and a derived pelvovisceral (diaphragmatic) system. Tripartite ribs facilitate thoracic movement, allowing substantial excursions of the body wall. An abrupt shift in the position of the parapophyses between thoracic vertebrae two and three results in a greater increase in transverse dimensions of the posterior thorax.; In chapter 2, respiratory kinematics were examined through cineradiography in several basal birds, guinea fowl (Numida meleagris), emu (Dromaius novaehollandiae) and tinamou (Nothoprocta perdicaria). The avian skeletal aspiration pump has fewer degrees of freedom than the crocodylian thorax. Sternal movement is elliptical, and expansion of the body wall is greatest in the ventrocaudal region.; In chapter 3, theropod gastralia, dermal abdominal ossifications, are examined. Theropod medial gastralia imbricate with two contralateral elements in the ventral midline. Theropod gastralia may have served as an accessory component of the aspiration pump, and were appropriately positioned to ventilate abdominal air sacs.; In chapter 4, pelvic aspiration in crocodylians and birds is examined. Only in crocodylians is a pelvic bone, the pubis, involved in aspiration breathing. Fossils indicate that mobile pubic bones are a crocodyliform adaptation.; In chapter 5, the identity and function of the pterosaur prepubis is examined. The pterosaur prepubis probably represents the ossified distal pubic cartilage, and likely functioned in aspiration breathing analogous to the crocodylian pubis.; In chapter 6, skeletal indicators for the evolution of archosaur breathing mechanisms are discussed. Increased pulmonary complexity appears to be associated with increased levels of constraint on thoracic mobility. Many archosaur groups have evolved accessory breathing mechanisms. Cranial and caudal air sacs systems and the basic prerequisites for an avian-style aspiration pump evolved in nonavian theropod dinosaurs. |
