Birds have a number of extra air sacs in their skeletons that supply their lungs with air and enhance their ability to exchange gases. Patrick M. O'Connor of the Ohio University College of Osteopathic Medicine and Leon P. A. M. Claessens of Harvard University analyzed a 67-million-year-old fossil of Majungatholus atopus, a primitive theropod that grew to several meters in length. Comparing the remains to data collected on more than 200 living birds, they found that the creature possessed a surprisingly avian anatomy. The dinosaur's vertebrae, in particular, exhibit adaptations like those seen in extant sarus cranes. "The pulmonary system of meat-eating dinosaurs such as T. rex in fact shares many structural similarities with that of modern birds, which, from an engineering point of view, may possess the most efficient respiratory system of any living vertebrate inhabiting the land or the sky," Claessens remarks.
The results, published in today's issue of the journal Nature, indicate that the system that birds use for breathing developed before birds themselves evolved. This respiratory adaptation, the authors note, is consistent with the hypothesis that predatory dinosaurs had elevated metabolic rates.