A Hard Heart

Warm-blooded dinos may have been quite common

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....

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