GEIST, N. R.*; JONES, T. D.; RUBEN, J. A. ; HILLENIUS, W. J.; FREY, E. D.: Lung Ventilation Mechanisms of Pterodactyloid Pterosaurs

Pteranodon and several other genera of large pterodactyloid pterosaurs (e.g. Ornithocheirus, Quetzalcoatalus) exhibit a suite of skeletal features that severely constrained costally powered expansion of the thoracic cavity. These include: a) fusion of up to six dorsal vertebrae to form a notarium; b) solid fusion of up to six of the anterior vertebral ribs into this notarium; and c) the presence of a large sternum with limited mobility of the costo-sternal and coraco-sternal joints. This distinctive morphology in known large pterosaurs indicates a reduced capacity for costally powered aspiration. Significantly, the presence of these features coupled with the absence of gastralia in many fossil pterosaurs would have minimized or eliminated curaissal expansion of the thorax and abdomen during the respiratory cycle. Consequently, larger pterosaurs would probably have required an accessory means of ventilating their lungs. Here we suggest many large pterosaurs likely possessed lungs ventilated primarily by a crocodilian-like hepatic piston mechanism. The presence of mobile prepubic bones in pterosaurs that are morphologically analogous to the pubic bones of extant crocodilians may provide additional evidence for the presence of diaphragmatic musculature facilitating a hepatic piston-type lung ventilation mechanism. The extensive pneumaticity of skeletal elements in these forms was likely facilitated by anteriorly located pulmonary or tracheal diverticulae.