Integration of ventilation and locomotion in archosaurs

Movements of the pelvis have recently been found to contribute to ventilation in both crocodilians and birds. Alligators have a kinetic pelvis in which the ischiopubic and ischiotruncus muscles rotate the pubic bones ventrally to increase abdominal volume and thereby facilitate inspiration. In birds, the entire pelvis rocks on the axial skeleton to produce ventilation. Although the mechanisms of pelvic aspiration are very different in crocodilians and birds, it is unusual among vertebrates for the pelvic musculoskeletal system to play an active role in inspiration. This observation raises the possibility that the pelvic musculoskeletal system may have played an important role in the ventilation of basal archosaurs. Based on the mechanism of pelvic aspiration in crocodilians and the structure of gastralia in basal archosaurs, we suggest that an ischiotruncus muscle pulled the medial aspect of the gastralia caudally, and thereby helped to produce inspiration by increasing the volume of the cuirassal basket. The proposed mechanism of cuirassal breathing in non-avian theropods leads us to suggest that the phase relationship of the ventilatory and locomotor cycles in running theropods was the opposite of that observed in running birds. Furthermore, we suggest that the ventilatory cycle of flying pterosaurs was entrained to the locomotor cycle with a phase relationship that was the opposite of that observed during flight in modern birds.