The findings are just the latest in a series of studies to indicate that some dinosaurs had avian features.
Forget giant lizards. When it comes to the breathing apparatus of dinosaurs, they were more like giant sparrows, according to a new study comparing dino skeletons with those of modern birds.
Writing in the journal Nature, researchers from Ohio and Harvard universities report that rather than reptilian lungs, dinosaurs sported a much bigger and more complex system of air sacs similar to that found in today's 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 at Ohio's College of Osteopathic Medicine and lead author of the study.
O'Connor and co-author Leon Claessens, an evolutionary biologist, visited museums across the United States and in Berlin and London to examine dinosaur bones, as well as those of a 67 million-year-old dinosaur called Majungatholus atopus -- a Tyrannosaurus rex cousin -- that O'Connor discovered in Madagascar in 1996.
Birds' lungs are unique in the animal world because they have an opening at each end, so that new air comes in one way and used air goes out the other. Nine air sacs, tucked away in hollows of bones, function like bellows to hold the air at either end, and the whole system works together to allow birds to put fresh air through their lungs when they're breathing in and out.
This makes the bird breathing system more efficient at supplying oxygen for the high energy required for flight and allows birds to travel easily in thin air.
The new study looked closely at how the air system affects the skeleton around the neck, chest and hips, and found similarities between the vertebral column of dinosaurs and birds that point to a similar type of breathing system for the dinos.
While the system probably wasn't exactly the same in dinosaurs as in today's birds, "it's nothing like the crocodile system as we know it," O'Connor said.
The exact function of the skeletal modifications is not understood, but many scientists think the gaps in the bones evolved to lighten bone structure, allowing dinosaurs to walk upright and made it easier for birds to fly.
"The pulmonary system of meat-eating dinosaurs such as T. rex in fact shares many structural similarities with that of modern birds," added Claessens.
O'Connor said the existence of an air-sac system with flow through ventilation of the lungs suggests that this type of dinosaur, at least, could have maintained a stable and quite high metabolism, putting it much closer to a warm-blooded existence.
The findings are just the latest of a series of studies in recent years that indicate at least some branches of the dinosaur family had avian features, including feathers, eggs that they incubated in nests and rapid growth after hatching.