The specimen has been identified as a member of the Thescelosaurus, meaning ''marvelous lizard,'' genus. Its species has not been determined. For the time being, the dinosaur usually goes by the nickname Willo, after the wife of the rancher on whose land it was found.
When Mr. Hammer saw a huge lump of stone beneath the well-preserved ribs, he said he suspected that the chest cavity might still hold some internal organs. The first C.T. scan by Dr. Kuzmitz seemed to show a heart inside the stony shell, but the two-dimensional images left room for doubt. Then imaging specialists at North Carolina State's College of Veterinary Medicine used new software to produce three-dimensional pictures.
''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 thing,'' said Mr. Fisher, director of the imaging resource facility. ''You could see both ventricles and the aorta.''
The ventricles are adjacent chambers in the lower heart. In this case, the scientists said, they were separated by an iron-rich wall, which they said was another clue appearing to confirm this to be a heart. A single tubular structure resembling an aorta led from the left ventricle, they said. The two upper chambers, the atria, were not distinguishable, the scientists said, though they felt certain of their existence when the animal was alive.
Modern reptiles, which are coldblooded, have three-chambered hearts, with two aortas and only one ventricle; crocodiles have two ventricles but they are incompletely separated. Thus, oxygenated blood from the lungs and deoxygenated blood from the rest of the body become mixed in these reptilian hearts, reducing the overall oxygen content of blood returned to the body and limiting the metabolic rates and activity of these animals.
''A single systemic aorta communicating with the left ventricle greatly reduces the risk of shunting and can be considered a means of more efficiently supporting prolonged periods of high activity,'' the discovery team said in its report.
What makes the discovery especially surprising and puzzling is that the heart resembles a mammal's or bird's but it belonged to an ornithischian, or bird-hipped, dinosaur, one of the two main lineages of these great reptiles. Despite the name, these dinosaurs were far removed from those that were presumed by many paleontologists to have been ancestors of birds; these ancestors were presumed to be theropods, members of the other main lineage known as the saurischian, or lizard-hipped, dinosaurs.
It is therefore possible, Dr. Russell said, that dinosaurs of both lineages -- not just the bird ancestors -- had advanced hearts and high metabolisms. This physiology may have evolved independently or it could stretch all the way back to a common ancestor, some 235 million years ago.
''This means our entire conception of dinosaurs may have to be revised,'' said Dr. Norell of the American Museum of Natural History.
The discovery team said in its report, ''Whether high metabolic rates and advance hearts arose once or more than once among dinosaurs remains an open question.''
Photos: What paleontologists say is the heart of a dinosaur, encased in a natural sarcophagus of stone, is on display at the North Carolina Museum of Natural Sciences. The heart is about the size of a grapefruit, the scientists say. (Jim Page, North Carolina Museum of Natural Sciences, via Associated Press); (Ed Heck/North Carolina Museum of Natural Sciences)(pg. A1) Map of South Dakota highlighting the site of fossil: A fossilized heart could change theories of dinosaur physiology.