The evolution of pelvic aspiration in archosaurs

Movements of the pelvic girdle have recently been found to contribute to in spiratory airflow in both crocodilians and birds. Although the mechanisms a re quite different in birds and crocodilians, participation of the pelvic g irdle in the production of inspiration is rare among vertebrates. This rais es 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 aspec t of the gastralia caudally, and thereby helped to produce inspiration by i ncreasing the volume of the abdominal cavity. From this basal mechanism, se veral archosaur lineages appear to have evolved specialized gastralia, pelv ic kinesis, and/or pelvic mobility. Kinetic pubes appear to have evolved in dependently in at least two clades of Crocodylomorpha. This convergence sug gests that a diaphragmatic muscle may be basal for Crocodylomorpha. The pel vis 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-avia n theropods arrears to have enhanced the basal gastral breathing mechanism. Changes in structure of the pelvic musculoskeletal system that were presen t 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 p elvis of some ornithischians appears to have been permissive of pubic and i schial kinesis. Large platelike prepubic processes evolved three times in O rnithischia. These plates are suggested to have been instrumental in an act ive expansion of the lateral abdominal wall to produce inspiratory flow. Th us, many of the unique features found in the pelvic girdles of various arch osaur groups may be related to the function of lung ventilation rather than to locomotion.