Chasmosaurus ( ) 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 and a weight of —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.
Discovery and species Chasmosaurus belli
In 1898, at Berry Creek, Alberta, Lawrence Morris Lambe of the Geological Survey of Canada made the first discovery of Chasmosaurus remains; holotype NMC 491, a parietal bone that was part of a neck frill. Although recognizing that his find represented a new species, Lambe thought this could be placed in a previously known short-frilled ceratopsian genus: Monoclonius. He erected the new species Monoclonius belli to describe his findings. The specific name honoured collector Walter Bell.Lambe, L.M., 1902, "New genera and species from the Belly River Series (mid-Cretaceous)", Geological Survey of Canada Contributions to Canadian Palaeontology 3(2): 25–81
However, in 1913, Charles Hazelius Sternberg and his sons found several complete "M. belli" skulls in the middle Dinosaur Park Formation of Alberta, Canada. Based on these finds, Lambe (1914) erected Protorosaurus ("before Torosaurus"),Lambe, L.M., 1914, "On the forelimb of a carnivorous dinosaur from the Belly River Formation of Alberta, and a new genus of Ceratopsia from the same horizon, with remarks on the integument of some Cretaceous herbivorous dinosaurs", The Ottawa Naturalist 27(10): 129–135 but that name was preoccupied by the Permian reptile Protorosaurus, so he subsequently created the replacement name Chasmosaurus in February 1914. The name Chasmosaurus is derived from Greek χάσμα, khasma, "opening" or "divide" and refers to the very large parietal fenestrae in the skull frill. Lambe now also assigned a paratype, specimen NMC 2245 found by the Sternbergs in 1913 and consisting of a largely complete skeleton, including skin impressions.Lambe, L.M., 1914, "On Gryposaurus notabilis, a new genus and species of trachodont dinosaur from the Belly River Formation of Alberta, with a description of the skull of Chasmosaurus belli", The Ottawa Naturalist 27: 145–155 Apart from the holotype and paratype several additional specimens of C. belli are known. These include AMNH 5422, ROM 843 (earlier ROM 5499) and NHMUK R4948, all (partial) skeletons with skull.
Putative additional species Since that date, more remains, including skulls, have been found that have been referred to Chasmosaurus, and several additional species have been named within the genus. Today several of these are considered to only reflect a morphological variation among the known sample of Chasmosaurus belli skulls.
In addition to Monoclonius belli, later Chasmosaurus, Lambe's 1902 study named Monoclonius canadensis. This species would later be given the distinct genus Eoceratops by Lambe in 1915. In 1990, Thomas Lehmen would refer to this the genus Chasmosaurus, forming the binomial Chasmosaurus canadensis. It was supposedly distinguished by its longer postorbital horns than other speciesT.M. Lehman, 1990, "The ceratopsian subfamily Chasmosaurinae: sexual dimorphism and systematics", In: K. Carpenter and P. J. Currie (eds.), Dinosaur Systematics: Perspectives and Approaches, Cambridge University Press, Cambridge, pp. 211–229 Similarly, in 1933 Barnum Brown named Chasmosaurus kaiseni, honouring Peter Kaisen and based on skull AMNH 5401, also distinguished by its very long brow horns.Brown, B., 1933, "A new longhorned Belly River ceratopsian", American Museum Novitates 669: 1–3 A 2016 analysis of Chasmosaurus specimens by James Campbell and colleagues found that postorbital horn size was too variable to be considered a distinguishing trait, and due to lacking the back of the parietal bone (forming the back of the frill), both species were considered indeterminate beyond the genus Chasmosaurus.Campbell, J.A., Ryan, M.J., Holmes, R.B., and Schröder-Adams, C.J. (2016). A Re-Evaluation of the chasmosaurine ceratopsid genus Chasmosaurus (Dinosauria: Ornithischia) from the Upper Cretaceous (Campanian) Dinosaur Park Formation of Western Canada. PLoS ONE, 11(1): e0145805.
Richard Swann Lull named an unusual, short-muzzled skull, specimen ROM 839 (earlier ROM 5436) collected in 1926, as Chasmosaurus brevirostris in 1933, with the name referring to its short snout.Lull, R.S., 1933, A revision of the Ceratopsia or horned dinosaurs. Memoirs of the Peabody Museum of Natural History 3(3): 1–175 This was seen as a junior synonym of C. belli by Lehman, whereas Campbell and colleagues considered it an indeterminate specimen only referrable to the genus Chasmosaurus.
The species Chasmosaurus irvinensis was named in 2001, stemming from the uppermost beds of the Dinosaur Park Formation. It is distinguished by its unembayed parietal bar and the downwardly curled ornamentation along the back of its frill.R.B. Holmes, C.A. Forster, M.J. Ryan and K.M. Shepherd, 2001, "A new species of Chasmosaurus (Dinosauria: Ceratopsia) from the Dinosaur Park Formation of southern Alberta", Canadian Journal of Earth Sciences 38: 1423–1438 This species was given its own genus, Vagaceratops, in 2010. However, a 2019 study by Campbell and colleagues returned Vagaceratops back to Chasmosaurus due to observing that some putative specimens of C. belli show transitional anatomy towards that of C. irvinesis. These transitional specimens were removed from C. belli, and merely considered indeterminate Chasmosaurus. These include the specimens YPM 2016 and AMNH 5402. A 2020 study by Denver Fowler and Elizabeth Freedman Fowler agreed with the evolutionary link between Chasmosaurus and Vagaceratops, but considered the latter an even closer relative and possible ancestor of Kosmoceratops. Consequently, they suggested it should be maintained as a distinct genus from Chasmosaurus, as its placement would probably remain unstable until chasmosaurine relationships are better understood. They also noted that CMN 2245, another former C. belli specimen, represents an additional seemingly transitional specimen between C. belli and Vagaceratops.
In 2010, Nicholas Longrich named the new genus and species Mojoceratops perifania based on fossils previously considered to belong to Chasmosaurus. The species Mojoceratops perifania was based on holotype specimen TMP 1983.25.1 consisting of a partial skull including the parietal and from the paratypes TMP 1999.55.292, an isolated lateral ramus of a right parietal, and NMC 8803, central bar and lateral rami of parietals. Specimens AMNH 5656, NMC 34832 and TMP 1979.11.147, and (tentatively) AMNH 5401 and NMC 1254 were also referred to the genus. All specimens assigned to Mojoceratops were collected from the Dinosaur Park Formation (late Campanian, 76.5–75 ma) of the Belly River Group of Alberta and Saskatchewan, western Canada. The generic name is derived from mojo and the specific name means "conspicuous pride" in Greek, both referring to the skull frill. Longrich noted that both C. canadensis and C. kaiseni seemed likely represent specimens of Mojoceratops, but considered definitive assignment impossible due to the poor preservation of the C. canadensis holotype and lack of the parietal bar in C. kaiseni.
Following the original assignment of the holotype and other skulls to Mojoceratops, several teams of researchers published work questioning the validity of this new genus. In 2011, Maidment & Barrett failed to confirm the presence of any supposedly unique features, and argued that Mojoceratops perifania was a synonym of Chasmosaurus russelli. Campbell and colleagues, in their 2016 analysis of Chasmosaurus specimens, agreed and added that some supposedly unique features, such as grooves on the parietal bone, were actually also present in the holotype of C. russelli and, to various degrees, in other Chasmosaurus specimens. This variability, they argued, strongly suggested that Mojoceratops was simply a mature growth stage of C. russelli. Recently, the referral of Eoceratops, C. kaiseni, and Mojoceratops to C. russelli was considered doubtful as the holotype of C. russelli is actually from the upper Dinosaur Park Formation, according to recent fieldwork.
Former species
Charles Mortram Sternberg added Chasmosaurus russelli in 1940, based on specimen NMC 8800 from southwestern Alberta (lower Dinosaur Park Formation). The specific name honours Loris Shano Russell.Sternberg, C.M., 1940, "Ceratopsidae from Alberta", Journal of Paleontology 14(5): 468–480 Many specimens, primarily from the lower sections of the Dinosaur Park Formation, were referred to C. russelli over time, with the species considered distinguishable from C. belli by the deeper median embayment of its parietal. However, it was later noted C. russelli holotype had unusual characters more reminiscent of ceratopsids such as Utahceratops, indicating it may be unrelated to the rest of the species. Further study of the specimen published in 2026 supported this, and it was moved to the distinct genus Cryptarcus. Other former specimens of "russelli" remain of uncertain classification pending further study.
Thomas Lehman described Chasmosaurus mariscalensis in 1989 from Texas,Lehman, T.M., 1989, "Chasmosaurus mariscalensis, sp. nov., a new ceratopsian dinosaur from Texas", Journal of Vertebrate Paleontology, 9(2): 137–162 which has now been renamed Agujaceratops.
In 1987, Gregory S. Paul renamed Pentaceratops sternbergii into Chasmosaurus sternbergi,Paul, G.S., 1987, "The science and art of reconstructing the life appearance of dinosaurs and their relatives: a rigorous how-to guide", pp 4–49 in: Dinosaurs Past and Present Volume II, Natural History Museum of Los Angeles County but this has found no acceptance. In 2000, George Olshevsky renamed Monoclonius recurvicornis Cope 1889 into Chasmosaurus recurvicornis as its fossil material is likely chasmosaurine;Olshevsky, G. 2000. An Annotated Checklist of Dinosaur Species by Continent. George Olshevsky, Publications Requiring Research, San Diego, 157 pp this species is currently considered to be taxonomically invalid.
Description
Chasmosaurus was a medium-size ceratopsid. In 2010 G.S. Paul estimated the length of C. belli at 4.8 metres, its weight at two tonnes.Paul, G.S., 2010, The Princeton Field Guide to Dinosaurs, Princeton University Press p. 269–270