Short of breath. Ruben (left) says dino lungs were inefficient.
Ann Gibbons
Science 1997; 278: 1229-1230.
When John Ruben first laid eyes on a high-quality photo of the so-called "feathered" dinosaur from China last year, he was stunned. It wasn't the featherlike structures that riveted his attention--he dismissed them as collagen fibers (see sidebar)--but the theropod dinosaur's innards, which were outlined in the slab of stone. "My eyes popped out," recalls Ruben, a respiratory physiology expert at Oregon State University in Corvallis. "I realized that here was the first evidence in the soft tissue that theropods had the same kind of compartmentalization of lungs, liver, and intestines that you would find in a crocodile"--and not in a bird.
Short of breath. Ruben (left) says dino lungs were inefficient.
OREGON STATE UNIVERSITY
To prove that notion, Ruben and his graduate students sectioned crocodiles and other reptiles and found that their lung structures resembled the images of several flattened fossil dinosaurs from China. On page 1267, Ruben uses this lung evidence to argue not only that dinosaurs were incapable of the high rates of gas exchange needed for warm-bloodedness, but also that their bellowslike lungs could not have evolved into the high-performance lungs of modern birds. Thus, he challenges two of the reigning hypotheses concerning dinosaurs: that they were warm-blooded and that they gave rise to birds.
Coming hot on the heels of another controversial paper that concludes that digits in bird wings could not have developed from dinosaur forelimbs (Science, 24 October, p. 666), Ruben's report is part of a "one-two punch to the dinosaur origins of birds hypothesis," says paleontologist James Farlow of Indiana University-Purdue University in Fort Wayne. But while many dinosaur experts say they welcome Ruben's novel approach, few are willing to embrace his conclusions so far. "This is exactly the kind of research we need," says Lawrence Witmer, an evolutionary biologist at Ohio University College of Osteopathic Medicine in Athens. And it's definitely weakening the case for warm-blooded dinosaurs. But many researchers, including Farlow and Witmer, think there's persuasive evidence that birds are descendants of dinosaurs. Says Farlow: "[This] is like a breath of fresh air, but it's going to ruffle a lot of feathers."
To test whether dinosaurs were really endotherms--warm-blooded animals able to generate their own heat--Ruben and graduate students Terry Jones and Nick Geist have sought to identify the signatures of endothermy, such as a scroll-like structure in the nose, in the bones of living animals. They have argued that dinosaurs lack such structures (Science, 30 August 1996, p. 1204). But what they really needed was improbable--a look at a dinosaur's lungs to see if they were efficient enough to power a warm-blooded animal.
The improbable happened last year, however, when Ruben saw photos of several specimens of Sinosauropteryx, a small, meat-eating dinosaur from the 120-million-year-old Yixian formation in northeastern China. The fine silt from an ancient lake preserved the animals' soft structures, including a clear "silhouette of the lungs" of one dinosaur, says paleontologist Larry Martin of the University of Kansas, Lawrence, who has seen the fossils.
When Ruben looked at the photos, it was "immediately apparent" to him that the dinosaur's lungs were arranged in a way that closely matched that of crocodiles. The theropods had two major cavities--the thoracic cavity containing the lungs, liver, and heart; and the abdominal cavity containing intestines and other organs. These were completely separated from each other by the diaphragm, as is the case in crocodiles. Birds have no such separation.
In living crocodilians, the function of this separation is to provide an airtight seal between the cavities. Then, when the diaphragmatic muscles contract, they pull back the liver and create negative pressure in the thoracic cavity, allowing air to fill the bellows-type lungs. Birds don't need such a separation between the cavities, because air in their lungs moves one way through millions of tiny air passages, drawn by the expansion and contraction of air sacs throughout their bodies.
Birds' flow-through lung system has plenty of surface area and is especially efficient at exchanging oxygen for carbon dioxide. (Mammals have yet another system that allows efficient gas exchange.) The bellowslike reptilian lung, however, provides much less area for gas exchange, and reptiles cannot absorb oxygen at the high rates needed to sustain intense activity. Ruben also showed that theropods and crocodiles share a distinct hip structure, linked to muscles that help bring air into the bellowslike lungs. All in all, says Ruben, it's "pretty solid evidence that theropods could not have had a modern, high-performance avian-style lung ... and were stuck with an unmodified, bellowslike lung." Says Martin: "Support for the hot-blooded dinosaur hypothesis now has the rigidity of a marshmallow." The evolutionary implications are even more far-reaching. Ruben argues that a transition from a crocodilian to a bird lung would be impossible, because the transitional animal would have a life-threatening hernia or hole in its diaphragm. "There may well be a relationship between dinosaurs and birds, but it's not the linear relationship you see in museum displays," he says.
Ruben's analysis is "another nail in the coffin of the warm-blooded dinosaur theory," says paleontologist Peter Dodson of the University of Pennsylvania, Philadelphia. But many other researchers say his case is not airtight. They point out that Ruben relied on photos showing a lung outline that is little more than "smudges on rock," says Witmer. What's more, Ruben's inferences are based on a flattened, two-dimensional fossil. "You would expect some deformation when the organs squish out," says Witmer, who suggests, only half-jokingly, that Ruben flatten his alligators with a steamroller for comparison. And the evolutionary transition from the actual theropod lung, rather than the modern crocodilian analog, might be easier.
Indeed, even if Ruben's analysis of lung structure holds up, it would have to be weighed against "a mountain" of other evidence supporting the dinosaurian origin of birds, says Farlow. Still, he finds Ruben's findings of a crocodilian-type lung for theropods "compelling." Fortunately, the Yixian formation is so rich in fossils that more specimens of Sinosauropteryx are likely to turn up. And if the same lung structure appears in enough fossils, Ruben's case will gather considerable weight.
Ann Gibbons
Exactly 1 year ago, paleontologists were abuzz about photos of a so-called "feathered dinosaur," which were passed around the halls at the annual meeting of the Society of Vertebrate Paleontology (Science, 1 November 1996, p. 720). The Sinosauropteryx specimen from the Yixian Formation in China made the front page of The New York Times, and was viewed by some as confirming the dinosaurian origins of birds. But at this year's vertebrate paleontology meeting in Chicago late last month, the verdict was a bit different: The structures are not modern feathers, say the roughly half-dozen Western paleontologists who have seen the specimens.
Feathered friend? Collagen fibers in a sea snake's tail resemble
feathers.
OREGON STATE UNIVERSITY
The stiff, bristlelike fibers that outline the fossils lack the detailed organization seen in modern feathers, says Alan Brush, an ornithologist at the University of Connecticut, Storrs, who specializes in feather structure. Brush was part of a "dream team" sent to China this spring by The Academy of Sciences in Philadelphia to view the fossils.
But just what the structures are--and whether they link birds and dinosaurs--is still under debate. Noting that the outline of the dinosaur skin is hard to discern in the fossilized stone, another dream team member, paleontologist Larry Martin of Kansas University, Lawrence, thinks the structures are frayed collagenous fibers beneath the skin--and so have nothing to do with birds. Zoologist John Ruben of Oregon State University in Corvallis dissected a sea snake's tail to show that such fibers can indeed look feathery (see photo). Others, including Brush and Philip Currie, a paleontologist at the Royal Tyrrell Museum of Palaeontology in Drumheller, Canada, describe the bristlelike fibers as "protofeathers"--fibers that may be hollow and made of the same kind of keratin as feathers.
Meanwhile, Ji Qiang, director of the Chinese Geology Museum in
Beijing, insists that the fibers are "obvious primitive feathers." But a paper
in press at Nature by another group of Chinese researchers doesn't make that
claim, says Currie. Measuring the width of the fibers under a scanning electron microscope
or testing whether they're made of collagen or keratin could resolve the debate. Some of
these tests are under way, Currie adds.