A Hard Heart

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After an hour's digging in the blazing sun in September 1993 along fossil-rich Hell Creek, in northwest South Dakota, even veteran dinosaur hunter Michael Hammer was astonished by what he saw. Sticking out of the sandstone was part of an animal's backbone--four vertebrae--apparently exposed by heavy summer rain. Its legs and tail had been washed away. Still, Hammer recognized it as a rare, largely intact skeleton of a Thescelosaurus, a parrot-beaked vegetarian that lived 66 million years ago.

As he poked around in the chest cavity, he unearthed a reddish-brown grapefruit-size clump that scientists last week identified as the creature's heart. That in itself was a paleontological first, since soft-tissue organs are rarely preserved. But as North Carolina State University researchers report in Science, X-ray scans revealed something even more significant: a four-chambered heart and an aorta, rather than the less metabolically efficient three-chambered hearts of snakes and crocodilians. That, says paleontologist Dale Russell, is the strongest evidence yet that dinosaurs, unlike their reptile kin, were warm blooded, like birds and mammals.

To be sure, one of the major branches of the dinosaur family, the theropods--ancestors of birds--have long been suspected of being warm blooded. But Thescelosaurus ("marvelous lizard") belongs to a different branch, known as ornithiscians. That suggests that more than a few of the dinosaur lineages may have been warm blooded--pointing the way to a major revision of our understanding of the animals that ruled the earth for some 150 million years.