Dinosaur shifts metabolic gears.

The beautifully preserved but flattened innards of a fossil dinosaur suggest that the extinct beast may have breathed much like modern mammals. Paired with other, more characteristically reptilian traits, this finding suggests that dinosaurs' metabolism functioned unlike that of any living animal.

In the Jan. 22 Science, physiologist John A. Ruben of Oregon State University in Corvallis and his colleagues describe a fossil of Scipionyx samniticus, a small meat-eating theropod, found in Italy. They mapped the traces of internal organs and muscles by using ultraviolet light, which makes residual liver pigments fluoresce.

The pattern supports Ruben's earlier analysis of a Sinosauropteryx fossil (SN: 11/15/97, p. 310). In both, he says, the liver connected to a diaphragm that separated the lungs from the viscera. A muscle pulled the diaphragm back and sucked air into the lungs. Modern crocodiles have a similar structure, perhaps a remnant from more active ancestors, he says.

Most modern reptiles, cold-blooded creatures that maintain a low resting metabolic rate, breathe by expanding their rib cages. Mammals and birds use both rib-based and diaphragm-driven respiration. The diaphragm system provides extra oxygen for sustained, intense activity.

Ruben starts with the assumption that theropods were cold-blooded. While resting, their respiration and metabolism would have been as slow as a modern reptile's. When necessary, however, these dinosaurs and ancestral crocodiles could shift gears from rib-based respiration to diaphragm-driven overdrive. "They had the best of both worlds, metabolically," says Ruben.

The dual-metabolism theory, says paleontologist James O. Farlow of Indiana University-Purdue University at Fort Wayne, "accounts for some otherwise odd features about dinosaurs." They were cold-blooded, he says, yet fossil evidence suggests that theropods were as active as warm-blooded animals.

The squashed, two-dimensional remains could be interpreted differently, warns paleontologist Philip J. Currie of the Royal Tyrrell Museum of Palaeontology in Drumheller, Alberta. "I think this is pushing the bounds of what can be said on the basis of the evidence." The supposed liver in the Scipionyx fossil could be a smudge from the animal's stomach contents. Of the dual metabolism hypothesis, he says, "It's a nice thought experiment."