Evolution of Cuirassal Breathing

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.

Phylogeny of terapods showing distribution of gastralia and our hypothesis (denoted by question mark) for the origin of cuirassal breathing in archosaurs.

Illistration of the mechanism of ventilation in the American alligator In alligators, expiration is produced by caudal rotation of the ribs and constriction of the abdominal cavity, which produce a cranial translation of the viscera. Constriction of the abdomen results from activity of the transversus abdominis muscle and the rectus abdominis muscle, the latter of which rotates the pubes dorsally. Inspiration is produced by cranial rotation of the ribs and caudal translation of the viscera. The viscera are pulled caudally by constraction of the diaphragmaticus muscle. The ischiopubic muscle increases the volume of the posterior abdomen by rotating the pubes ventrally.	Illustration of the mechanism of ventilation in the pigeon During inspiration in pigeons the sternum rotates ventrally (ventrally pointing arrows) and activity of the longissimus dorsi muscle elevates the pelvic girdle on the dorsal vertebrae (dorsally pointing arrows). During expiration, activity of the suprapubic abdominal and infrapubic abdominal muscles depresses the pelvis and tail.
Illustration of the prooposed cuirassal breathing mechanism in the theropod Deionychus antirrhopus The orientation and size of the pubes indicate that an ischiotruncus muscle was not present in maniraptoid theropods. For cuirassal breathing to work, a caudotruncus muscle would have to have been present	Illustration of the prooposed cuirassal breathing mechanism in the early bird Sinornis santensis In early birds, inspiratory activity of the caudotruncus muscle would have required simultaneous activity of the epaxial muscles to stabilize the tail against ventral rotation. Note that if the cudotruncus muscle orriginated on the chevrons and pygostyle, as illustrated the digestive tract would have had to pass through the belly of the muscle to reach the vent. Alternatively, the caudotruncus muscle could have taken its origin fron the superficial myosepta of the tail, as it does in the American alligator.

Illistration of the mechanism of ventilation in the American alligator

In alligators, expiration is produced by caudal rotation of the ribs and constriction of the abdominal cavity, which produce a cranial translation of the viscera. Constriction of the abdomen results from activity of the transversus abdominis muscle and the rectus abdominis muscle, the latter of which rotates the pubes dorsally. Inspiration is produced by cranial rotation of the ribs and caudal translation of the viscera. The viscera are pulled caudally by constraction of the diaphragmaticus muscle. The ischiopubic muscle increases the volume of the posterior abdomen by rotating the pubes ventrally.

Illustration of the mechanism of ventilation in the pigeon

During inspiration in pigeons the sternum rotates ventrally (ventrally pointing arrows) and activity of the longissimus dorsi muscle elevates the pelvic girdle on the dorsal vertebrae (dorsally pointing arrows). During expiration, activity of the suprapubic abdominal and infrapubic abdominal muscles depresses the pelvis and tail.

Illustration of the prooposed cuirassal breathing mechanism in the theropod Deionychus antirrhopus

The orientation and size of the pubes indicate that an ischiotruncus muscle was not present in maniraptoid theropods. For cuirassal breathing to work, a caudotruncus muscle would have to have been present

Illustration of the prooposed cuirassal breathing mechanism in the early bird Sinornis santensis

In early birds, inspiratory activity of the caudotruncus muscle would have required simultaneous activity of the epaxial muscles to stabilize the tail against ventral rotation. Note that if the cudotruncus muscle orriginated on the chevrons and pygostyle, as illustrated the digestive tract would have had to pass through the belly of the muscle to reach the vent. Alternatively, the caudotruncus muscle could have taken its origin fron the superficial myosepta of the tail, as it does in the American alligator.