Functional morphology of the reptilian and avian respiratory systems and its implications for theropod dinosaurs

Based on indirect osteological evidence, modern analogies, functional constraints and general phylogenetic information, it is possible to narrow down the possibilities for the structure of the respiratory system in theropod dinosaurs. The lungs were probably large by mammalian standards and in all but the most advanced coelurosaurs lacked true avian air sacs. Instead they possessed cranial, ventral and caudal sac-like lung regions that graded dorsally and medially into regions of highly specialized, faveolar parenchyma. The lungs were multichambered and supported a crosscurrent gas exchange model. The evolution of this lung type into the avian lung-air sac system is feasible and would be precipitated by the formation of thoracic air sacs, which anatomically separated regions with gas storage and propulsion function (air sacs) from the exchanger (lung). This process could have taken place in the presence of a crocodylian-like diaphragmaticus muscle but did not require it. Unidirectional air flow, which is typical of the avian palaeopulmo, would have been a late development. Possible osteological indicators of the origin of true air sacs would include broadening of the ventral pelvic apparatus, elimination of the pubic bridge and the gastralia, and development of the ventral (sternal) ribs.