Telltale Dino Heart Hints at Warm Blood.

He could tell just by looking that this dame had a heart of stone.

When PI (paleontological investigator) Michael (Mike) Hammer stumbled across the big-eyed dinosaur that he calls Willo on a South Dakota ranch in 1993, he noticed her chest cavity right away. Preserved in the curve of the dinosaur's ribs was a rock that Hammer was convinced was a heart.

The professional fossil collector, who runs Hammer and Hammer Paleotek in Jacksonville, Ore., took the dinosaur in for a medical X-ray scan. The scan confirmed that the beast had a heart with a structure that could finally help dinosaurs beat the rap as cold-blooded killers.

The fossil heart is built more like bird and mammal hearts than like the hearts of reptiles, says Dale A. Russell, a paleontologist at North Carolina State University in Raleigh, who was part of the team that reported the finding in the April 21 SCIENCE.

Like those of birds and mammals, this heart probably had four chambers with a single aorta--a large artery that carries blood to the body, says Russell. Crocodiles, alligators, and other reptiles have two aortas. Hammer and his colleagues say they can clearly make out the aorta and two ventricles in the X rays.

Hearts with a single aorta tend to be more efficient at delivering oxygen to the body, says Michael K. Stoskopf, a North Carolina State veterinarian on the team. A more efficient heart could support a more active animal and may indicate that dinosaurs had high metabolic rates--a prerequisite for being warm-blooded, the authors say.

"This is finally evidence for our side," says paleontologist Jack Horner of the Museum of the Rockies at Montana State University in Bozeman, who has argued that dinosaurs were warm-blooded animals.

The organ is remarkably well preserved. "Really, it's a stunner," says Russell.

Still, the fossil has lost some components that would strengthen the team's case, says John A. Ruben of Oregon State University in Corvallis. Ruben described the liver and diaphragm of another dinosaur last year (SN: 2/20/99, p. 127). "It's too incomplete to say anything as definitive as the authors would like to," he says.

A pulmonary vessel--which carries blood to the lungs--is conspicuous in its absence and may be a clue that the dinosaur had a second aorta, Ruben says. "It's like the dog that didn't bark in Sherlock Holmes," he explains.

"It would have been nice to see [the pulmonary artery]," agrees Stoskopf, but he adds that the aorta is much heartier and more likely to survive in fossil form. The missing pulmonary vessel "doesn't negate what we did see," Stoskopf says.

If the authors have correctly interpreted the results, "it's a very cool discovery," says James O. Farlow of Indiana University-Purdue University in Fort Wayne. However, just because the dinosaur had a heart that could support a high metabolism doesn't mean the animal was warm-blooded, he says.

Willo is a Thescelosaurus--one of the so-called bird-hipped dinosaurs. Despite the name, these animals form a separate lineage from the lizard-hipped dinosaurs that may have given rise to birds, says Russell. The researchers say the one-aorta heart may have existed in the common ancestor of all dinosaurs or evolved several times in different dinosaur lineages. "Im putting my money on the double evolution," says Russell.

The researchers urge other fossil hunters to look for traces of internal organs. "In the old days, this would have just been [regarded as] a hard rock in the way, and I would have destroyed it," says Hammer.

"We may have been throwing away the best parts," says Stoskopf.

The discovery is likely to fuel the debate about whether dinosaurs were warm- or cold-blooded animals, but "mostly what it does is show us that the best discoveries are made when we least expect them," says Horner.