J.A. Ruben, T.D. Jones, N.R. Geist, and W.J. Hillenius.  1997.  Science 278:1267-1270.

Abstract: Reptiles and birds possess septate lungs, rather than the alveolar-style lungs typical of mammals. Morphological constraints restrict capacity for maximal rates of respiratory gas exchange in the unmodified, bellows-like septate lung. Among those taxa possessing septate lungs, only the highly modified avian "flow-through" lung is capable of O₂-CO₂ exchange rates typical of active endotherms. New paleontological and neontological evidence indicates that theropod dinosaurs possessed unmodified, bellows-like septate lungs that were ventilated with a crocodile-like "hepatic-piston" diaphragm. Most early birds (e.g., Archaeopteryx, enantiornithines) also possessed relatively unmodified septate lungs but probably lacked a hepatic-piston diaphragm mechanism. These data are consistent with an ectothermic status for theropod dinosaurs and early birds.