WASHINGTON -- A modern medical X-ray of a dinosaur fossil named Willo suggests the extinct animal may have had a four-chambered heart typical of warm-blooded animals instead of the simpler heart of cold-blooded reptiles.
If verified by other studies that a rock-hard mass in the fossilized chest is a heart, it will mark the first time that scientists have been able to study the cardiac system of dinosaurs.
A four-chambered heart would mean that the blood circulatory system of dinosaurs was much more advanced than previously believed, that the animals were more tolerant of temperature extremes, and that they were capable of rapid and sustained movement typical of modern birds and mammals.
"This challenges some of the most fundamental theories about how and when dinosaurs evolved," said Dale A. Russell, senior research curator at the North Carolina Museum of Natural Sciences and a paleontologist at North Carolina State University.
A report of the discovery appears Friday in the journal Science.
The discovery came in the 66-million-year-old remains of a member of a group of dinosaurs known as Tescelosaurus, or "marvelous lizard." The precise species has not been identified, but researchers have called it "Willo" in honor of the wife of a rancher who owns the discovery site in South Dakota.
Russell said the animal was about the size of a pony and weighed about 660 pounds. A long bony tail gave it a total length of about 13 feet. It had short legs and was a plant eater, he said.
Michael Hammer, a co-author of the study, found the nearly intact dinosaur fossil in Harding County, S.D., in 1993. The specimen was recovered without disturbing the dark mass in the chest cavity.
It was suspected that the mass could be soft tissue that somehow fossilized with the animal's bones. Usually dinosaur specimens bear no trace of soft tissue, which usually decays before it can become fossilized.
Dr. Andrew Kuzmitz, an Ashland, Ore., physician and amateur paleontologist, later examined the specimen with a CT scan, a form of medial X-ray that gives details of internal structure. He said seven cardiologists looked at the images and identified the object as a heart with separated pumping chambers similar to the human heart.
Paul Fisher, director of the NC veterinarian school, enhanced the CT scan data into three-dimensional images and the presence of a four-chambered heart became obvious, he said.
"You could see both ventricles (lower heart chambers) and the aorta (a major artery)," said Fisher, the first author of the study.
Fisher said that two more human cardiologists and two veterinarian experts have looked at the images and all agree the chest mass is the fossil of a four-chambered heart.
"This is a landmark discovery in the field," said Jack Horner, a dinosaur expert at Montana State University and the Museum of the Rockies in Bozeman. "This means that dinosaurs were warm-blooded. That's a fantastic. It's way cool."
Horner said that because of the South Dakota discovery, he and other researchers will now start doing CT scans on any intact dinosaur fossils they find.
"There are several around like that and I think we'll all start looking at them," said Horner.
Among the effects of the heart discovery, said Horner, is a boost for the idea that birds evolved from dinosaurs, a theory that is becoming more widely accepted by paleontologists. Birds also have four-chambered hearts.
Such hearts have two chambers that collect and pump blood to the lungs, and two chambers that collect oxygen-enriched blood from the lungs and pump it throughout the body. Most reptile hearts have three chambers, two that collect blood from the body and from the lungs, and one that pumps the blood throughout the body. This means the reptilian blood is less oxygen-rich. Less oxygen in the blood slows metabolism and causes the cold-blooded animals to be sluggish when chilled and to have less physical endurance.