Surprise images of the insides of a 66 million-year-old dinosaur show it had the heart of a warm-blooded animal, adding to evidence that dinosaurs were not slow and plodding but quick and hungry. Scientists used a computed tomography (CT) scan on a fossil nicknamed "Willo" to show the 66 million-year-old dinosaur had a four-chambered heart more akin to a human than to a lizard.

CT scans use X-rays and computer software to "peel away" layers of tissue, or in this case layers of dirt and fossilised bone, to image inside a body. Their report, published in the journal Science, describes how they used the X-ray technology developed for use by doctors to peek inside the ancient fossil.

"Not only does this specimen have a heart, but computer-enhanced images of its chest 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," Dale Russell, a palaeontologist at North Carolina State University who helped co-ordinate the study, said in a statement.

The fossil, on display at the North Carolina Museum of Natural Sciences, was dug up in 1993 in South Dakota. It was exceptionally well-preserved, so the researchers decided to look at it using CT scans.

"We have been doing pieces of dinosaurs off and on for a long time, mostly looking for internal bony structures or bones damaged in some way like a fight," Paul Fisher, director of the university's veterinary Biomedical Imaging Resource Facility, said in a telephone interview. "This was the first time we were looking for what used to be soft tissue structures."

Fisher said they did not know what they would find. "We were surprised that it is as recognisable as it is ¾ something that had been in the ground for 65 to 67 million years," Fisher said. "We did not think we would put it together in a 3-D model and say, 'Oh my God - it's a heart'. It is one of those things where a picture is worth a thousand words."

The study could transform the way palaeontology is done. Until now, researchers have had to rely on little more than bones, and perhaps the imprint of a feather or some skin in rock, to tell how ancient creatures looked. But in some fossils, soft tissue may remain in a form accessible to CT scans. Ironically, this soft tissue looked like dirt to early palaeontologists who scraped it away to reveal bones for study and display, Fisher said.

Evidence from bones alone at first suggested that dinosaurs were big reptiles- Reptiles are cold-blooded and rely on the sun and outside temperatures to warm up their bodies enough to move. They do not eat as often as mammals, which can regulate their own body temperature and have a faster metabolism.

Scientists have had hints, however, that dinosaurs might be more akin to mammals and to birds ¾ which many believe to be the living descendants of dinosaurs. In January 1999, US and Italian researchers used ultraviolet light to peer inside the chest of a fossil baby dinosaur and found its organs were laid out like a bird’s or mammal's.

And other researchers looking at fossilised bones of dinosaurs have found suggestions that they had many blood vessels inside, again indicative of a warm-blooded metabolism. But no one had hoped for such conclusive evidence.

"It's truly amazing that this animal seems to have had such a highly evolved head. The implications completely floored me," Russell said. "This challenges some of our most fundamental theories about how and when dinosaurs evolved." Reptiles have three-chambered hearts and evolved long before mammals.

The specimen itself is a member of the Thescelosaurus genus, weighed about 300kg and was four metres long. The researchers named it Willo after the wife of the rancher on whose property it was found. Thescelosaurus means "marvelous lizard" and the researchers believe it was a member of the T. neglectus species.

More information is available at http://www.dinoheart.org. - Reuters