Fossilized dinosaur heart. Chest cavity of Willo, the first dinosaur with a fossilized heart. The fossilized structures (dark brown) are embedded in the surrounding rock (grey). The heart is part of the circular mass at centre, and has been found to be similar to those of birds and mammals, rather than those of reptiles, suggesting that Willo may have had a high metabolic rate and might have been warm-blooded. Willo is a Thescelosaurus, a plant- eating dinosaur from 66 million years ago. He was discovered in the USA in 1993. For 3-dimensional computed tomography (CT) scans revealing the anatomy of this fossil, see images E446/382-383.
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