Lung Structure and Ventilation in Theropod Dinosaurs and Early Birds

Abstract

Reptiles and birds possess septate lungs rather than the alveolar-style lungs of mammals. The morphology of the unmodified, bellowslike septate lung restricts the maximum rates of respiratory gas exchange. Among taxa possessing septate lungs, only the modified avian flow-through lung is capable of the oxygen–carbon dioxide exchange rates that are typical of active endotherms. Paleontological and neontological evidence indicates that theropod dinosaurs possessed unmodified, bellowslike septate lungs that were ventilated with a crocodilelike hepatic-piston diaphragm. The earliest birds ( Archaeopteryx and enantiornithines) also possessed unmodified septate lungs but lacked a hepatic-piston diaphragm mechanism. These data are consistent with an ectothermic status for theropod dinosaurs and early birds.

Axial and appendicular pneumaticity inArchaeopteryx

“…It has previously been claimed that Archaeopteryx possessed simple, bellows-like lungs (Ruben et al 1997(Ruben et al , 1999. Theropod dinosaurs were also thought to have possessed such bellows-like lungs and, in addition, a diaphragmatic separation of the body cavity into an anterior pleuropericardial cavity and a posterior peritoneal cavity, as in extant crocodilians and mammals (Ruben et al 1997(Ruben et al , 1999.…”

“…Rather than rely on osteological evidence, Ruben et al (1997Ruben et al ( , 1999 based their conclusions on more dubious interpretations of soft-tissue preservation in two theropods and did not discuss the great volume of contradictory osteological evidence. In one case (Ruben et al 1999), the alleged identi¢cation of a crocodile-like, hepatic diaphragm in the basal coelurosaurian theropod Sinosauropteryx was not even based on a study of the fossil, but merely on examinations of photographs.…”

“…Theropod dinosaurs were also thought to have possessed such bellows-like lungs and, in addition, a diaphragmatic separation of the body cavity into an anterior pleuropericardial cavity and a posterior peritoneal cavity, as in extant crocodilians and mammals (Ruben et al 1997(Ruben et al , 1999. Based on the great deal of osteological evidence for bone pneumatization in theropods and sauropods (e.g.…”

Archosaurian respiration and the pelvic girdle aspiration breathing of crocodyliforms

“…Pelvic girdle-assisted lung ventilation has been hypothesized to be most important in resting birds in which the sternum is relatively fixed ( Baumel et al 1990). Large dorsoventral excursions of the postsacral vertebral column have been inferred on the basis of an extreme dorsal curvature of the tail that is known from desiccated skeletal specimens (Ruben et al 1997(Ruben et al , 2003, but have not been demonstrated through direct observation.…”

“…Based on the reported involvement of the pelvis in breathing in birds (Baumel et al 1990;Ruben et al 1997Ruben et al , 2003 and crocodylians (Rathke 1866;, pelvic aspiration has been hypothesized to be a primitive breathing mechanism for the Archosauria , despite the fact that the proposed pelvic breathing mechanisms of birds and crocodylians are very different. However, respiratory pelvic girdle movement has never been demonstrated in either taxon by direct quantitative evidence, but has been inferred from anatomy, electromyography or observed body wall movement (Rathke 1866;Baumel et al 1990;Ruben et al 1997Ruben et al , 2003.…”