Biologists peck at bird-dinosaur link.

In the paleontological paternity suit of the century, researchers are battling over the widely held idea that dinosaurs sired birds. A small but vocal group of naysayers; has scored points recently with two separate studies arguing that the lungs and hands of birds were starkly different from those of theropod dinosaurs--the small bipedal carnivores that supposedly evolved feathers and ultimately flight.

"Our work strongly suggests that dinosaurs may not have been the direct ancestor of birds," says John A. Ruben, a physiologist at Oregon State University in Corvallis. In the Nov. 14 Science, Ruben and his colleagues compared the lungs of living birds and crocodiles with evidence from dinosaur fossils.

Crocodiles, like mammals, draw air into their lungs with the help of a diaphragm, a movable tissue that divides the chest cavity from the abdomen. In crocodiles, muscles from the pubic bone in the pelvis attach to the liver, which connects to the diaphragm, creating a pistonlike system for pulling the diaphragm toward the animal's tail. When these muscles contract, the movement of the diaphragm enlarges the chest cavity and pulls air into the lungs.

Birds have a different lung system, one that doesn't rely on a diaphragm to alter the pressure in the chest cavity. Instead, muscles connected to the ribs draw air into a network of sacs located in the abdomen--an extremely efficient system for supplying the oxygen needed during flight.

Ruben's team contends that the pubic bones of theropod fossils closely resemble those of crocodiles, providing evidence that dinosaurs had a piston-driven diaphragm.

They gleaned additional clues about the breathing mechanism of dinosaurs from a Chinese dinosaur called Sinosauropteryx (SN: 5/3/97, p. 271). One fossil of this animal preserves evidence of internal organs that normally don't show up in dinosaur remains. These soft-tissue impressions in the chest cavity of Sinosauropteryx match the shape and placement of the diaphragm in crocodiles, offering further hints that dinosaurs breathed like crocodiles, says Ruben.

He and his colleagues argue that birds could not have evolved from animals equipped with diaphragms because the two breathing systems work in incompatible ways.

That conclusion supports the findings of another study, published in the Oct. 24 Science, which focuses on the hand bones of birds and dinosaurs. All land-dwelling vertebrates--from swallows to snakes--descended from ancestors that had five digits on each limb. Most theropod dinosaurs lost two of the digits on their hands during evolution, a pattern broadly similar to the three digits in the wings of birds.

Ann C. Burke and Alan Feduccia of the University of North Carolina at Chapel Hill contend, however, that the resemblance between bird wings and dinosaur hands is superficial. The researchers studied the embryos of birds, turtles, and alligators to decipher how the digits of the hands grow.

They observed that the fourth digit develops first in all modern vertebrate embryos that have five digits. This finding enabled them to number the developing digits in bird embryos. They determined that birds lack the first and fifth digits of the hand. Dinosaurs, though, lacked the fourth and fifth digits.

"The implication is that it's virtually impossible to imagine how dinosaurs could have given rise to birds," says Feduccia.

Most paleontologists dismiss the impact of the new findings and argue that the evidence linking birds to dinosaurs is growing (SN: 8/23/97, p. 120).

Kevin Padian of the University of California, Berkeley criticizes the hand study. Burke and Feduccia's interpretation of bird hands rests on rules of development derived from modern animals, and Padian says that such assumptions cannot explain the pattern seen in dinosaurs. "The [hand study] neglects the fact that theropods clearly violate all the laws that they regard as inviolable. It's complete nonsense."

Paleontologist Lawrence M. Witmer of Ohio University in Athens calls the lung study intriguing, but he questions the conclusions. Theropod dinosaurs walked on two feet and had an immobile pubic bone, whereas crocodiles walk on four feet and have a pubic bone that shifts during movement. "The pelvic structures of crocodiles and dinosaurs were quite different. I'm not sure how to interpret the similarities that we see."

Witmer also says that the Sinosauropteryx specimen is squashed, making it difficult to interpret the tissue preserved in its chest cavity.