Image: Sinosauropteryx fossil
This fossilized Sinosauropteryx, found in China, shows evidence of a division within the body cavity. Some researchers say the evidence argues against a direct link between such dinosaurs and modern birds.

The case for a direct link between dinosaurs and modern birds suffered another blow when researchers said dinosaurs had more in common with crocodiles than birds in the respiration department.

The view that birds are the direct descendants of dinosaurs has become the prevailing wisdom in recent years, leading some researchers to claim that “birds are living dinosaurs.”

But a backlash has been building in some areas of the scientific community. In October, a study published in the journal Science contended that birds could not have descended from dinosaurs, based in part on a comparison of dinosaurs’ fossilized “fingers” and the corresponding digits seen in bird embryos.

Now comes another critical study, written by researchers from Oregon State University and the College of Charleston in South Carolina. These researchers compared the respiratory structures of modern birds, mammals and crocodiles with those seen in the fossils of early birds and a class of dinosaurs known as theropods. Their work appears in the Nov. 14 issue of Science.

Crocodiles, like mammals, have diaphragms that aid in respiration - although the piston-style diaphragms of crocodiles are structured differently from those of humans and other mammals. The diaphragm separates the body cavity; its back-and-forth movement helps suck fresh air into the lungs, then push the spent air out.

The dinosaur fossils appear to show a separation in the visceral cavity and key skeletal characteristics similar to those of crocodiles, indicating that they had crocodile-like diaphragms. Researchers looked at fossils from 65 million to 200 million years old, but key evidence was found in a Sinosauropteryx fossil from China, dating back about 125 million years. That fossil showed traces of a soft-tissue partition within the body cavity.

“To have both skeletal and soft-tissue evidence was pretty critical,” said Oregon State University zoologist John Ruben. “That’s about as good as you can do when you’re talking about extinct animals.”

In contrast, birds have no diaphragms separating the body cavity. Instead, they breathe through the heaving of their chests, aided by movements in the pelvis and the tail. They also have a “flow-through” lung to take up oxygen efficiently.

Fossils of an early bird called Archaeopteryx showed a skeletal structure consistent with the creature’s modern-day descendants, although its lung probably was not as efficient.

Ruben and his colleagues - Terry Jones and Nicholas Geist of Oregon State University, and W. Jaap Hillenius of the College of Charleston - say there are “fundamental problems” with the idea that diaphragmless birds evolved from dinosaurs with diaphragms. Any animal that started to make the evolutionary transition would experience a hernia, they say.

“Such a debilitating condition would have immediately compromised the entire pulmonary ventilatory apparatus and seems unlikely to have been of any selective advantage,” the researchers wrote.

Like most other scientists in the field, Ruben says there are clearly similarities between birds and dinosaurs that hint at a common ancestor.

“The bottom line is not that there’s not a relationship between birds and theropod dinosaurs,” Ruben said. “But certainly the evidence is beginning to point against this simplistic, linear relationship.”

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