Chasmosaurus
| Chasmosaurus Temporal range: Late Cretaceous (Campanian), | |
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| C. belli skeleton, Royal Ontario Museum specimen 843 | |
Scientific classification  | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Dinosauria |
| Clade: | †Ornithischia |
| Clade: | †Ceratopsia |
| Family: | †Ceratopsidae |
| Subfamily: | †Chasmosaurinae |
| Genus: | †Chasmosaurus Lambe, 1914 |
| Type species | |
| †Chasmosaurus belli Lambe, 1902 | |
| Species | |
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| Synonyms | |
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Chasmosaurus (/ˌkæzmoʊˈsɔːrəs/ KAZ-moh-SOR-əs) is a genus of ceratopsid dinosaur from the Late Cretaceous Period in North America. Its given name means 'opening lizard', referring to the large openings (fenestrae) in its frill (Greek chasma, meaning 'opening', 'hollow', or 'gulf'; and sauros, meaning 'lizard'). With a length of 4.3–4.8 metres (14.1–15.7 ft) and a weight of 1.5–2 tonnes (1.7–2.2 short tons)—or anywhere from 2,200 to nearly 5,000 lbs—Chasmosaurus was of a slightly smaller to "average" size, especially when compared to larger ceratopsids (such as Triceratops, which were about the size of an African bush elephant).
It was initially to be called Protorosaurus, but this name had been previously published for another animal. All of the excavated specimens of Chasmosaurus were collected at the Dinosaur Park Formation, Dinosaur Provincial Park, Alberta, Canada. For many years, Chasmosaurus russelli was considered a second species. However, in 2026 its type specimen was given the separate genus Cryptarcus, with other prior "C. russelli" specimens being of uncertain taxonomy.