The evolution of pelvic aspiration in archosaurs

Abstract

Movements of the pelvic girdle have recently been found to contribute to inspiratory airflow in both crocodilians and birds. Although the mechanisms are quite different in birds and crocodilians, participation of the pelvic girdle in the production of inspiration is rare among vertebrates. This raises the possibility that the pelvic musculoskeletal system may have played a role in the ventilation of basal archosaurs. Judging from 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 abdominal cavity. From this basal mechanism, several archosaur lineages appear to have evolved specialized gastralia, pelvic kinesis, and/or pelvic mobility. Kinetic pubes appear to have evolved independently in at least two clades of Crocodylomorpha. This convergence suggests that a diaphragmatic muscle may be basal for Crocodylomorpha. The pelvis of pterosaurs was long, open ventrally, and had prepubic elements that resembled the pubic bones of Recent crocodilians. These characters suggest convergence on the pelvic aspiratory systems of both birds and crocodilians. The derived configuration of the pubis, ischium and gastralia of non-avian theropods appears to have enhanced the basal gastral breathing mechanism. Changes in structure of the pelvic musculoskeletal system that were present in both dromaeosaurs and basal birds may have set the stage for a gradual reduction in the importance of gastral breathing and for the evolution of the pelvic aspiration system of Recent birds. Lastly, the structure of the pelvis of some ornithischians appears to have been permissive of pubic and ischial kinesis. Large platelike prepubic processes evolved three times in Ornithischia. These plates are suggested to have been instrumental in an active expansion of the lateral abdominal wall to produce inspiratory flow. Thus, many of the unique features found in the pelvic girdles of various archosaur groups may be related to the function of lung ventilation rather than to locomotion.