July 15, 2005 — Dinosaurs had a respiratory system similar to modern birds, according to a study which adds new evidence of an evolutionary link between these creatures and birds.

Writing in the journal Nature, lead author Patrick O'Connor reports that the peculiar pulmonary system of birds has an older history than previously thought.

"What was once formally considered unique to birds was present in some form in the ancestors of birds," said O'Connor, an assistant professor of biomedical sciences at Ohio University's College of Osteopathic Medicine.

Indeed, fossils finds suggest that a group of predatory dinosaurs such as Tyrannosaurus rex and velociraptor did not feature a crocodile-like pulmonary system as previously thought, but a much bigger, more complex mode of breathing with air sacs similar to that of today's birds.

The respiratory system of birds relies on nine flexible air sacs — tucked away in hollows of bones — that act like bellows to move air through the lungs.

The system keeps the volume of air in the lungs constant, enhances the ability to exchange gases, supplies oxygen for the high energy required for flight and allows for the high metabolic rates found in birds.

Analysis of the fossilized remains of the neotheropod Majungatholus atopus, a 67-million-year-old carnivorous dinosaur O'Connor discovered in Madagascar in 1996, pointed to "evidence for cervical and abdominal air-sac systems," said the researchers.

O'Connor, along with and Harvard University's Leon Claessens, compared the air sac structures of Majungatholus atopus with those of 234 modern birds, and looked at how the air system affected the skeleton around the neck, chest and hips.

It emerged that dinosaurs had a surprisingly avian anatomy, with many similarities to the vertebral column of birds.

"Our study indicates that basal neotheropods possessed the anatomical potential for flow-through ventilation of the pulmonary system," O'Connor and Claessens concluded.

Their research matches recent studies which suggest that dinosaurs possessed several features, such as bird-like postures and even feathers, once thought solely to characterize modern birds.

"These studies have linked anatomical, physiological or behavioral inferences with an increased metabolic potential, suggesting that if not bird-like in metabolism, theropods were at least 'more similar' to birds than to reptiles," said the researchers.

A pulmonary machinery for enhanced gas exchange in dinosaurs would have produced a quick metabolism — necessary for a predatory lifestyle — pushing dinosaurs closer to being warm-blooded creatures.

"This doesn't mean they maintained this warm-bloodedness all the time. They probably fell somewhere between cold- and warm-blooded," O' Connor said.

According to Paul Barrett, a palaeontologist at the Natural History Museum in London, the research adds to evidence that counters theories which dispute a link between dinosaurs and birds.

"This work is another nail in the coffin for that competing theory," Barrett told Nature.