A smudge in ancient siltstone has led a group of researchers to challenge the prevailing view that dinosaurs were warm-blooded and gave rise to birds. A report in tomorrow's Science, suggests that dinosaurs breathed with crocodilelike lungs that could not suck in enough oxygen to keep their bodies warm and could not have led to bird lungs.
To test whether dinosaurs were really endotherms--warm-blooded animals able to generate their own heat--a team led by John Ruben, a respiratory physiology expert at Oregon State University in Corvallis, has searched for hallmark signs of endothermy in living animals--such as a scroll-like bone in the nose--in the fossil record. But what they really needed seemed beyond reach: a look at a dinosaur's lungs to see if they had enough oxygen capacity to power a warm-blooded animal.
Then the improbable happened. Last year Ruben saw photos of several specimens of Sinosauropteryx, a small, meat-eating dinosaur from the 120-million-year-old Yixian formation in northeastern China. Ancient lake silt had preserved the animals' soft structures, including a clear silhouette of the lungs. The imprints, Ruben says, revealed a thoracic cavity with the lungs, a liver, and a heart, and an abdominal cavity with intestines and other organs. The two were completely separated by an airtight diaphragm--just like in crocodiles. When the diaphragm muscles in a crocodile contract, they lower the pressure in the thoracic cavity, allowing air to rush into the bellows-type lungs.
Ruben argues that dinosaurs could not have been warm-blooded because their lungs--which among other restrictions have a limited surface area compared to birds' lungs--could not absorb enough oxygen. Ruben maintains that the crocodilian lung could not have evolved into a bird lung, since the transitional animal would have a fatal hole in its diaphragm.
Dinosaur experts applaud the approach but few are willing to embrace Ruben's conclusions so far. They point out that the silt imprints may not reflect the true arrangement of organs: "You would expect some deformation when the organs squish out," says Lawrence Witmer, an evolutionary biologist at Ohio University College of Osteopathic Medicine in Athens. Indeed, even if Ruben's analysis of lung structure holds up, says paleontologist James Farlow of Indiana-Purdue University in Fort Wayne, Indiana, it must be weighed against "a mountain" of other evidence that favors dinosaurs giving rise to birds.