GEIST, N.R.; JONES, T.D.; HILLENIUS, W.J.; RUBEN, J.A.; Sonoma State Univ.; Stephen F. Austin State Univ.; College of Charleston; Oregon State Univ.: New Evidence for Diaphragm Breathing in Pterosaurs

With few exceptions, amniotes ventilate their lungs by ribcage expansion. Mammals and crocodilians supplement costally powered lung ventilation with active diaphragms, and birds couple thoracic expansion with dorso-ventral rotation of an extensive sternum to ventilate the lung air-sac system. Alternately, at least some pterosaurs seem to have possessed fixed, immobile ribcages and we have previously suggested that these taxa may have been obligate diaphragm breathers. Examination of articulated, three-dimensionally preserved specimens of several taxa of Early Cretaceous (Aptian-Albian) pterodactyloid pterosaurs confirm that thoracic immobility was widespread in these forms. These taxa exhibit extensive ankylosis of anterior dorsal vertebrae to form a rigid notarium. Thoracic rigidity was further increased by fusion of four or more anterior ribs into the notaria in mature specimens. The presence of extensive thoracic fusion in these forms, as in other large pterodactyloids, would have constrained, or entirely negated, costally powered thoracic expansion as the primary mechanism of lung ventilation. Further, the morphology of sternocoracoid and sternocostal joints in these specimens suggests that the sternum had only very limited mobility, which is inconsistent with bird-like ventilatory mechanisms and an avian type lung air sac system. These data support the hypothesis that obligate diaphragm breathing was widespread in pterosaurs.