which period was that?
Late Cretaceous period
Thescelosaurus
Thescelosaurus ( ; ancient Greek θέσκελος- ("-") meaning "godlike", "marvellous", or "wondrous" and σαυρος ("") "lizard") was a genus of small ornithopod dinosaur that appeared at the very end of the Late Cretaceous period 
how many years ago was the thescelosaurus in existence? More than 66 million years ago
wiki:5919225
Thescelosaurus [SEP] Introduction
Thescelosaurus ( ; ancient Greek θέσκελος- ("-") meaning "godlike", "marvellous", or "wondrous" and σαυρος ("") "lizard") was a genus of small ornithopod dinosaur that appeared at the very end of the Late Cretaceous period in North America. It was a member of the last dinosaurian fauna before the Cretaceous–Paleogene extinction event around 66 million years ago. The preservation and completeness of many of its specimens indicate that it may have preferred to live near streams. This bipedal ornithopod is known from several partial skeletons and skulls that indicate it grew to between 2.5 and 4.0 meters (8.2 to 13.1 ft) in length on average.