how many years ago was the thescelosaurus in existence?
More than 66 million years ago
Thescelosaurus
It was a member of the last dinosaurian fauna before the Cretaceous–Paleogene extinction event around 66 million years ago.
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Thescelosaurus [SEP] Introduction
There was much discussion over whether the remains were of a heart. Many scientists now doubt the identification of the object and the implications of such an identification.