Chasmosaurus (pronounce /ˌkæzməˈsɔrəs/ KAZ-mo-sawr-us) is a genus of ceratopsid dinosaur from the Upper Cretaceous Period of North America. Its name means 'opening lizard', referring to the large openings (fenestrae) in its frill (Greek chasma meaning 'opening' or 'hollow' or 'gulf' and sauros meaning 'lizard'). With a length of 5 - 6 meters (16 - 20 ft) and a weight of 3.6 tonnes (4.0 short tons), Chasmosaurus was a ceratopsian of average size. Like all ceratopsians, it was purely herbivorous. It was initially to be called Protorosaurus, but this name had been previously published for another animal.