Hot under the collar over dinosaurs.

Two researchers have added a hot new twist to the simmering debate over dinosaur physiology. By analyzing oxygen stored within the bones of a Tyrannosaurus rex skeleton, they have found evidence suggesting that the king of all carnivores had a warm-blooded metabolism more like that of mammals than that of reptiles.

Their study, however, has received a cool reception from researchers who question the validity of applying this innovative technique to fossil samples.

Reese E. Barrick and William J. Showers of North Carolina State University in Raleigh studied an exceptionally well preserved T. rex skeleton from the late Cretaceous, a period that ended 65 million years ago. To gauge the body temperature of the animal during its life, they measured the ratio of two oxygen isotopes in bones from several different parts of the body. A high ratio of oxygen-18 to oxygen-16 indicates that the bones developed at relatively cool temperatures.

Barrick and Showers contend that isotopic tests can tell warm-blooded from cold-blooded metabolisms. Because of their high metabolisms, mammals and other endotherms show little temperature variation throughout the year, the researchers suggest. They also surmise that endotherms keep their extremities at almost the same temperature as their body core.

Given that supposed pattern, the isotopic signature of T rex bones suggests that the animal was endothermic. The dinosaur's limbs and tail averaged only 2[degrees]C cooler than its core, and its overall body temperature varied less than 4[degrees]C for different times in its life, they report in the July 8 SCIENCE.

But physiologist John Ruben of Oregon State University in Corvallis argues that Barrick and Showers have jumped to conclusions without studying enough about modern endotherms. Mammals, says Ruben, sometimes keep their limbs much cooler than their bodies.

Other investigators also question the study. Among the skeptics is Yehoshua Kolodny of the Hebrew University of Jerusalem, who applied the oxygen isotope technique to the study of fossils. At an international meeting last year in Oxford, England, Kolodny reported that his experiments with fossil dinosaurs, fish, mammals, and aquatic reptiles revealed that the fossilization process altered the oxygen isotope ratios, wiping out the original information.

Anusuya Chinsamy of the University of Pennsylvania in Philadelphia contends that Barrick and Showers should have tested whether the isotopic technique can discern a difference between fossil mammals and reptiles before reporting the T rex data. "They haven't done the basic research yet," she says.

Chinsamy has other reasons to doubt the findings. In her own preliminary studies of T rex bone, she has found growth rings, a characteristic of ectothermic animals (SN: 5/14/94, p.312). "The growth rings indicate that these animals were not endothermic," she says.