When paleontologist Richard Owen dug up a dinosaur in 1842, he thought it looked like a reptile—a "terrible lizard" with scales, slow-moving legs, and cold blood. But many dinosaurs are now known to have been fast, powerful, and energetic. And since the 1960s, scientists have argued over whether the cold-blooded physiology of a lumbering reptile could have powered something nimble and speedy like a Velociraptor. Now, scientists using a technique once reserved for climatologists have found that big four-legged dinos had body temperatures similar to those of mammals—evidence that they either were warm-blooded or were better at conserving body heat than modern reptiles are.
Evolutionary biologist Robert Eagle of the California Institute of Technology in Pasadena heard about a technique from Caltech colleagues, who were using it to reconstruct ancient climates. By analyzing minerals in old rocks, geochemists can determine the relative amounts of different chemical isotopes: atoms of the same chemical element that vary slightly in mass. Different isotopes tend to form depending on whether a chemical reaction took place at high or low temperatures.
The method, called "clumped-isotope thermometry," focuses on a reaction involving the bond between carbon and oxygen. The lower the temperature at which a mineral forms, the more the rare isotopes carbon-13 and oxygen-18 tend to bond, or "clump," together. By studying CO2 trapped in minerals, geochemist John Eiler of Caltech and other researchers were trying to determine how warm Earth had been when they formed.
Trained as a paleontologist, Eagle realized that he could apply the technique to bioapatite, the mineral that makes up the enamel of teeth. "It's like a thermometer," he says. The results might help add data to resolve the debate over whether dinosaurs were cold-blooded (ectothermic) or warm-blooded (endothermic), which he calls "the most long-standing debate in paleobiology." To try to resolve it, his group analyzed 11 fossilized teeth from the long-necked, 30-ton Brachiosaurus and the 50-ton Camarasaurus, which had been dug up in Oklahoma, Wyoming, and Tanzania. The 150-million-year-old enamel, they found, formed between 36° and 38°C, the same temperature as the mammalian body. Dinosaurs, it seems, were warm after all.
But this doesn't settle the debate. To be truly endothermic, dinosaurs would have to have constantly maintained the same high body temperature. Big endothermic animals, however, have a problem with heat: their huge bodies retain too much of it. This phenomenon, called gigantothermy, causes them to overheat. Modern behemoths such as elephants are always "right on the edge" of overheating, says Frank Paladino, a physiologist at Indiana University-Purdue University Fort Wayne in Indiana, who studies giant animals such as pandas and leatherback turtles and was not involved in the research. Previous modeling studies, he says, have found that if the biggest dinosaurs were endothermic in the same way mammals are, their body temperature would have reached 40° to 45°C—far too hot for most life.
So if dinosaurs were endothermic, they must have had some sort of refrigerant system. In their paper published online today in Science, the authors speculate that the dinosaurs might have released excess heat through their long necks and tails or through air sacs under their skin, or used some unknown mechanism to cool off.
Another possibility is that the body temperatures of giant dinosaurs fluctuated with their surroundings, as those of modern reptiles do. In that case, Paladino points out, they would have been ectothermic without being cold-blooded. "Obviously, dinosaurs were 'warm-blooded'; they lived in a warm climate," he says. The more important question, he says, is whether their cells produced energy at a high or a low rate, and that can't be determined by looking at isotopes in tooth enamel. In the hot climate of the Jurassic, he notes, there would be no environmental pressure for dinosaurs to evolve cellular mechanisms for producing a lot of heat. But as small bipedal dinosaurs began to evolve into birds, they would have needed more energy to be able to flap their wings; endothermy may have evolved at this point.
Eagle agrees. All the discovery shows is that dinosaurs weren't cold like modern reptiles are, he says. He plans to address the endothermy question by looking at teeth from pygmy relatives of the giant dinosaurs and from juveniles of the same species. If their enamel also formed at high temperatures, he guesses, it might be more evidence that the species was endothermic and not just retaining heat because of its size. "But I'm open-minded about which it is," he says. He also plans to use the technique to look at other species and reconstruct evolutionary trees to determine when endothermy evolved. "It's mind-boggling the amount we could do," he says.
This item has been corrected. Frank Paladino's institution was originally incorrect; it is Indiana University-Purdue University Fort Wayne.