LUNG STRUCTURE

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The debate about dinosaur lung structure and its application to the question of thermoregulation stems from an analysis of Sinosauropteryx published by Ruben, Jones, Geist and Hillenius (the group also responsible for papers on respiratory turbinates and hatchling bone maturation critical of the endothermic dinosaur postulate).
They begin their arguement with a detailed morphological comparison of lung structure in reptiles and birds, suggesting that the simpler, bellows-like reptilian/crocodilian lung is probably incapable of supporting the increased metabolic rates necessary for active endotherms . While birds have a similar septate lung, modifications leading to a series of connected air sacs throughout the thorax and abdomen and a uni-directional airflow increase oxygen transport efficiency sufficiently to allow a very high metabolic rate . Ruben et al also maintain that the processes by which the lungs are powered differ significantly in reptiles and birds.
They then turn their attention to dinosaurs, observing that although these probably had the same septate lungs as reptiles and birds, they did not have the necessary skeletal and muscular mechanisms necessary to provide a bird-like, high efficiency air circulation and oxygen-extraction system. In particular, they focus on photographs of the first Sinosauropteryx specimen, claiming that it shows a clear division of the thoracic and abdominal cavities by a crocodile-like vertical partition. Further arguements on the shape and function of the pelvis of primitive birds such as Archaeopteryx and dinosaurs leads them to conclude not only that with reptilian style lungs dinosaurs could not have been endotherms , but also that it makes it much less likely that birds are descended from dinosaurs.
This paper has come under considerable criticism, as yet unpublished, by various experts including Greg Paul, Guy Leahy and, reputedly, Luis Chiappe. In particular they point out that the crocodilian system requires a mobile pelvis , and that the pelves of theropods are simply not able to substitute. Look out for papers in Science refuting many of the Ruben claims, particularly the presence of a vertical partition in dinosaurs and the Ruben analysis of pelvic structure and other skeletal modifications and air- sacs reputedly absent in dinosaurs.

A second, related paper by the same group was subsequently published in Science, this time dealing with the juvenile theropod Scipionyx. This small dinosaur from Italy is superbly preserved, to the extent that remains of internal organs are evident. Ruben et al interpret these internal organ structures as supporting their idea of an hepatic-piston respiratory system in theropods, and suggest that such a lung system would have enabled dinosaurs to rapidly increase oxygen intake to maintain high activity levels for extended periods without an endothermic basal metabolism; a sort of turbo-charged dinosaur. Again, their interpretation and conclusions have received considerable criticism.

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