A novel, and very interesting, interpretation in this remarkably comma-free paper:
Farmer, C.G. 2006. On the origin of avian air sacs; pp. 89-106 in Milsom, W.K., Powell, F.L., and Mitchell, G.S. (eds.), Frontiers in Comparative Physiology. II. Respiratory Rhythm, Pattern and Responses to Environmental Change. Respiratory Physiology & Neurobiology 154.
From a quadrupedal ancestry theropod dinosaurs evolved a bipedal posture.
Theropods evolved bipedal posture or simply inherited it from a ancestral saurischian dinosaur which in turn inherited from a ancestral dinosaur which in turn inherited from a ancestral dinosauriform organism?
The ptero hollow bones was lined with air-sacs?
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Roberto Takata