Though some scientists have proposed that predatory dinosaurs had lungs similar to crocodiles and other reptiles, a new study published in this week's issue of the journal Nature suggests the ancient beasts boasted a much bigger, more complex system of air sacs similar to that in today's birds. The finding is one of several studies in recent years to paint a new, more avian-like portrait of meat-eaters such as T. rex: The creatures may have had feathers, incubated their eggs, grown quickly and perhaps even breathed like birds.
"What was once formally considered unique to birds was present in some form in the ancestors of birds," said Patrick O'Connor, an assistant professor of biomedical sciences at Ohio University's College of Osteopathic Medicine and lead author on the study, which was funded in part by the National Science Foundation.
O'Connor and collaborator Leon Claessens of Harvard University visited museums in New York, Berkeley, Chicago, Pittsburgh, Washington, D.C., Berlin and London to examine the bones of ancient beasts, and also studied a 67-million-year-old dinosaur, Majungatholus atopus, that O'Connor had discovered in Madagascar as a graduate student in 1996. They compared the dinosaur skeletons with those of modern birds to draw comparisons of how the soft tissues in the dinosaurs may have been structured.
Birds long have fascinated biologists because of their unusual pulmonary system. Pulmonary air sacs prompt air to pass through the lungs twice during ventilation. This system also creates holes in the skeleton of birds, which has led to a popular notion that birds have "air in their bones," O'Connor said.
The new study, which examined how the air syst em invades the skeleton in areas such as the neck, chest and hips, finds similarities between the vertebral column of dinosaurs and birds that point to a common soft tissue system as the culprit. Though probably not identical to living birds, "it's nothing like the crocodile system as we know it," O'Connor said.
"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 sky," said Claessens, who received a Ph.D. from Harvard in organismic and evolutionary biology last month and will join the faculty at the College of the Holy Cross in Worcester, Mass., this fall.
In birds, this special anatomical configuration increases the gas exchange potential within the lungs, boosting metabolism and creating warm-bloodedness. The researchers are quick to point out, however, that the new study doesn't clearly peg predatory dinosaurs as habitually warm-blooded animals. The creatures probably had a more complex strategy, falling somewhere between what scientists define as cold- and warm-blooded. It appears that these animals had the pulmonary machinery for enhanced gas exchange, O'Connor explained, which would have pushed them closer to being warm-blooded creatures.
Previous research that pointed to a more crocodilian-like pulmonary system was based on a study of two dinosaur skeletons encased in rock. O'Connor and Claessens have expanded on that research by studying a broader collection of dinosaur skeletal remains, and are the first to integrate both anatomical and functional studies of modern birds as models of how the ancient creatures' air sacs were structured.
The scientists are part of a reinvigorated movement of researchers who are examining dinosaur bones and comparing them with modern animals to learn more about the anatomy of th ese extinct beasts.