It seemed unlikely, but when Michael Hammer first discovered the dinosaur dubbed Willo with his son Jeff at Hell Creek Formation, South Dakota, in 1993, the creature's ribs were so well preserved that he wondered if its chest might still contain internal organs. Now after several years of careful study, Willo's new owners--the North Carolina Museum of Natural Sciences--have announced that the answer is yes. Deep within Willo's 66-million- year-old body--on display at the museum and still half buried in sandstone--lies a remarkably well preserved fossil heart. What's more, the structure of this organ would suggest that Willo was most definitely warm-blooded.



Dale Russell, a senior research curator for the museum--together with Paul Fisher, Michael Stoskopf and Reese Barrick from North Carolina State University, Hammer and Andrew Kuzmitz, an amateur paleontologist and family physician--published the results in the April 21 issue of Science.

The key to Willo's heart lay, for these researchers, in medical imaging. Following Hammer's original hunch, Kuzmitz produced two-dimensional computer tomography (CT) scans of the dinosaur's chest before the museum acquired the specimen in 1996. These images were revealing enough to compel Russell, Stoskopf and Fisher to generate three-dimensional composites from them. "Once the computer software put all the 2-D images together into a 3-D model, it became very apparent that, yeah--this was the real deal," Fisher recalls.

To corroborate the findings, Barrick and graduate student William Straight looked for iron--a key component of blood--using x-ray diffraction analyses, and they found it within the heart but not in the surrounding sediments.

What is most telling about the images is that they reveal two chambers, most likely the ventricles, still intact, as well as one tube positioned like an aorta. The heart's upper chambers, the atria, are more fragile than the ventricles and in this animal may well have collapsed. "The images ... strongly suggest it is a four-chambered, double-pump heart with a single systemic aorta, more like the heart of a mammal or bird than a reptile," Russell notes. Indeed, reptiles--though long thought to be closely related to dinosaurs--have very different hearts, which bear two aortas and do not deliver oxygen to the body as effectively.



It is because of Willo's seemingly more evolved, more efficient heart that they suspect the dinosaur must have been warm-blooded, a reversal of some earlier theories. "Willo's ventricles and aorta indicate it had completely separate pulmonary and body circulation systems, which suggests it had a metabolic rate higher than we generally see in living reptiles," Stoskopf says. And among modern animals, those with high metabolic rates are typically warm-blooded.

Willo, nicknamed after the wife of the rancher on whose property it was found, has yet to be formally classified. The 663-pound, 13-foot-long herbivore is a Thescelosaurus, meaning "marvelous lizard." Thescelosaurus lived at the end of the Cretaceous in North America. Ironically enough, Russell and Hammer believe Willo to be T. neglectus, or the "neglected one." "Marvelous? Yes," Russell comments, "but I don't think this one is going to be neglected anymore."

In fact, there is much more to study where Willo is concerned. The creature was most likely buried in wet, oxygen-free sand, and so rather than decay, its soft tissues underwent saponification, a process by which they turn into a soaplike substance and petrify. As a result, it is the only dinosaur of its kind ever found with a complete skull, and it is so well preserved that tendons are still connected to its spine and cartilage to its ribs. Additional imaging hints at other possible fossilized organs. "We got lucky," Russell adds. "If it hadn't been discovered when it was, it could all have eroded within six months."

Fisher points out that they were lucky on another note: "In order to clean up a specimen to display as a skeleton, you'd throw everything away that wasn't bone. It makes you wonder how much stuff we've missed."