CORVALLIS, Ore. - Paleontologists at Oregon State University announced today in the journal Science more evidence that the theropod, or meat-eating dinosaurs, were cold-blooded - and that the first birds probably did not, as had long been believed, evolve from known dinosaurs.
The conclusions are largely based on examination of lung structure and ventilation in modern mammals, birds, and reptiles, which were used as models to understand lung structure and ventilation in dinosaurs and prehistoric birds.
"Our analysis suggests that it was a physiological impossibility for the lungs of birds to have evolved from the lungs of the theropod dinosaurs, as has been the conventional wisdom for decades," said John Ruben, an OSU professor of zoology and expert on dinosaur and avian evolution.
"For this to have happened, the lungs of such dinosaurs could not have functioned in any normal way," Ruben said. "Such an animal probably wouldn't have been active enough to even catch its own food, and there isn't much evolutionary or survival value to that."
This research clearly suggests birds did not come from known dinosaurs, Ruben said, but does not provide solid conclusions about where they did come from. It's possible some pre-dinosaurian reptile or early dinosaur that pre-dated the known dinosaurs may have been the ancestors of birds, he said.
In recent years, Ruben and OSU graduate students Nicholas Geist and Terry Jones have done studies on dinosaur evolution and metabolism that provided some of the first concrete evidence about the physiology of these ancient reptiles.
Among earlier findings - which were published previously in the journal Science - CAT scans of dinosaur nasal structure clearly suggested that dinosaurs were not warm-blooded, since they lacked the nasal "turbinates" that warm-blooded animals use to prevent excess heat and water loss while breathing.
In the latest study, the research looked at lungs.
Living reptiles have "septate" lungs that are like a pair of flabby, compartmentalized air-sacs. The reptilian lung is capable of supporting only relatively low rates of oxygen consumption.
Warm-blooded mammals and birds, however, need about 10 times more oxygen than cold-blooded reptiles, and have independently evolved two different types of "high performance" lung structures. Mammals have efficient "alveolar" lungs with millions of air sacs clustered like tiny grapes.
The lungs of birds are greatly modified septate lungs that allow for high rates of gas exchange and high activity levels. A very specialized, "hinged" rib structure and large sternum are required to ventilate the lungs of modern birds.
But no dinosaur or early bird had this peculiar rib and sternum arrangement, the scientists say.
This tells them that dinosaurs could not have had modern, bird-like lungs capable of maintaining the activity typical of warm-blooded animals. And, if their lungs were not like modern birds, dinosaur lungs would have to have been more conventionally reptilian, probably similar to crocodile lungs.
The evidence that the theropod dinosaurs possessed crocodile-like lungs is reinforced by skeletal analysis suggestive of a liver-diaphragm lung ventilation mechanism in these dinosaurs. Crocodiles have a non-muscular diaphragm, powered by muscles which attach to the liver and the pubic bones of the hip. It pulls the large liver backwards to inflate the lungs, and is associated with a distinctive hip structure.
"The theropod dinosaurs, which were the supposed ancestors of the birds, included such prominent species as the Tyrannosaurus rex and Velociraptor," Jones said. "In them we see a marked similarity between the hip structure of theropod dinosaurs and crocodiles."
This, along with the discovery of new evidence of a non-muscular diaphragm in two specimens of Sinosauropteryx, the recently discovered theropod from China, leads to the conclusion that theropod dinosaurs, like crocodilians, used a liver-diaphragm mechanism to ventilate their lungs.
The skeletal structure of the earliest birds indicates they also possessed a simple reptile-like lung, consistent with their being cold-blooded. But unlike the theropod dinosaurs, they lacked a diaphragm mechanism with which to fill their lungs.
For years now, Ruben said, paleontologists have worked with the first known true bird, Archaeopteryx, and tried to move its pelvic bones around to fit the necessary structure for it to have evolved from theropod dinosaurs.
It didn't fit, he said, because it lacked a diaphragm and may not have evolved from them after all.
These findings are also consistent with the conclusions of recent research done by scientists at the University of North Carolina in Chapel Hill, reported last month in Science, which also concluded birds could not have been closely related to dinosaurs. That research was based on evidence that birds lacked the embryonic thumb found in dinosaurs.