Dinosaur Majungatholus atopus had air sacs
Patrick O'Connor of Ohio University and Leon Claessens of Harvard University, compared air sacs in the neck and chest vertebrae of a 67 million-year-old carnivorous therapod dinosaur fossil, Majungatholus atopus, with those in more than 200 living birds. They reported last week in Nature that M. atopus' structures, while not identical, were very similar to birds. This, together with bird-like soft tissue preserved in a Tyrannosaurus rex, is more evidence that birds share a common ancestor with therapod dinosaurs.
The avian respiratory system
Birds have the most efficient respiratory system of any living vertebrate. Its unique flow-through design has flexible air sacs in hollow bones that continually move fresh air through the lungs, keeping the volume of air in them constant. This is radically unlike that of the dead-end bellows-type lungs of mammals or living reptiles. This unique avian respiratory system supplies high levels of oxygen, which in turn allows for the high metabolic rate necessary to sustain the energy requirements of flight. Therefore O'Connor and Claessens concluded that therapod dinosaurs had a respiratory system with the potential to support higher rates of metabolism than other reptiles, although not as high as in birds.
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1. Kardong, K.V., 2002, "Vertebrates: Comparative Anatomy, Function, Evolution," , McGraw Hill: Boston MA, Third Edition, pp.419-428.