Chasmosaurus

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Chasmosaurus
Temporal range: Late Cretaceous, 76.5–75.5Ma
Academy of Natural Sciences, Philadelphia - IMG 7414.JPG
C. belli skeleton, Academy of Natural Sciences
Scientific classification e
Kingdom: Animalia
Phylum: Chordata
Clade: Dinosauria
Order: Ornithischia
Family: Ceratopsidae
Subfamily: Chasmosaurinae
Genus: Chasmosaurus
Lambe, 1914
Species

Chasmosaurus (/ˌkæzmɵˈsɔrəs/ KAZ-mo-SAWR-əs) 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 4–5 metres (13–16 ft) and a weight of 2 tonnes (2.2 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. All specimens of Chasmosaurus were collected from the Dinosaur Park Formation of the Dinosaur Provincial Park of Alberta, Canada. C. russelli comes from the lower beds of the formation while C. belli comes from middle and upper beds.[1]

Discoveries and species[edit]

G. F. Sternberg preparing a C. belli skull

In 1898, Lawrence Morris Lambe of the Geological Survey of Canada made at the Berry Creek the first discovery of Chasmosaurus remains, holotype NMC 491, a parietal bone that was part of a neck frill.[2] Although recognizing that his find represented a new species, Lambe thought this could be placed in a previously-known short-frilled ceratopsian genus: Monoclonius.[2] He erected the new species Monoclonius belli to describe his finding.[2] The specific name honoured collector Walter Bell.[3]

However, in 1913, Charles Hazelius Sternberg and his sons found several complete "M. belli" skulls in the middle Dinosaur Park Formation of Alberta, Canada.[4] In January 1914, Lambe named all these finds as a separate genus: Protorosaurus, the name indicating an ancestry to Torosaurus.[5] However, this name quickly was shown to be preoccupied by a Permian reptile, Protorosaurus Meyer 1836. Therefore Lambe created the replacement name Chasmosaurus in February 1914.[2] It 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.[6]

C. russelli, Royal Tyrrell Museum

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.[2] Today some of these are considered to only reflect a morphological variation among the known sample of Chasmosaurus belli skulls;[2] others are seen as valid species of Chasmosaurus or as separate genera. In 1933 Barnum Brown named Chasmosaurus kaiseni, honouring Peter Kaisen and based on skull AMNH 5401, differing from C. belli in having very long brow horns.[7] This form is perhaps related to Chasmosaurus canadensis ('from Canada') named by Thomas M. Lehman in 1990.[8] The latter species, originally Monoclonius canadensis Lambe 1902, had been described as Eoceratops canadensis by Lambe in 1915. Eoceratops and the long-horned Chasmosaurus kaiseni are now provisionally thought to be exemplars of Mojoceratops.[9] Richard Swann Lull in 1933 named an unusual, short-muzzled skull, specimen ROM 839 (earlier ROM 5436) collected in 1926, as Chasmosaurus brevirostris, "with a short snout".[10] This has been seen as a junior synonym of C. belli.[8] 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.[4][11] Thomas Lehman described Chasmosaurus mariscalensis in 1989 from Texas,[12] which has now been renamed Agujaceratops.[13] The most recently described species is Chasmosaurus irvinensis named in 2001,[14] which stems from the uppermost beds of the Dinosaur Park Formation. This species was given its own genus, Vagaceratops, in 2010.[15]

Skull of C. belli ROM 5436, type of C. "brevirostris", on display at the Royal Ontario Museum, Toronto

In 1987, Gregory S. Paul renamed Pentaceratops sternbergii into Chasmosaurus sternbergi,[16] 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;[17] this is a nomen dubium.

Today, only two species are seen as valid: C. belli and C. russelli. They show small differences in morphology and a difference in stratigraphy, as C. russelli is found in the older lower Dinosaur Park Formation and C. belli in the middle Dinosaur Park Formation of the Campanian,[18] together spanning a time period of 76.5 to 75.5 million years ago.[4] Apart from the holotype and paratype several additional specimens of C. belli are known. These include AMNH 5422, AMNH 5402, ROM 843 (earlier ROM 5499) and NHMUK R4948, all (partial) skeletons with skull; and YPM 2016, a skull. Apart from its holotype C. russelli is known from its paratype CMN 8803, a skull frill; CMN 41933, a rear skull frill; RTMP 81.19.175, the right side of a skull and CMN 2280, a partial skeleton with skull found by the Sternbergs in 1914.

Description[edit]

Restoration of C. belli

Chasmosaurus was a medium-size ceratopid. In 2010 G.S. Paul estimated the length of C. belli at 4.8 metres, its weight at two tonnes; C. russelli would have been 4.3 metres long and weighed 1.5 tonnes.[19] The known differences between the two species mainly pertain to the horn and frill shape, as the postcrania of C. russelli are poorly known. Like many ceratopsians, Chasmosaurus had three main facial horns - one on the nose and two on the brow. In both species these horns are quite short, but with C. russelli they are somewhat longer, especially the brow horns, and more curved backwards. The frill of Chasmosaurus is very elongated and broader at the rear than at the front. It is hardly elevated from the plane of the snout. With C. belli the rear of the frill is V-shaped and its sides are straight. With C. russelli the rear edge is shaped as a shallow U, and the sides are more convex.[19] The sides were adorned by six to nine smaller skin ossifications (called episquamosals) or osteoderms, which attached to the squamosal bone. The corner of the frill featured two larger osteoderms on the parietal bone. With C. russelli the outer one was the largest, with C. belli the inner one. The remainder of the rear edge lacked osteoderms. The parietal bones of the frill were pierced by very large openings, after which the genus was named: the parietal fenestrae. These were not oval in shape, as with most relatives, but triangular, with one point orientated towards the frill corner.

Interestingly, Chasmosaurus specimen NMC 2245 recovered by C.M. Sternberg was accompanied by skin impressions.[2] The area conserved, from the right hip region, measured about one by 0.5 metres. The skin appears to have had large scales in evenly spaced horizontal rows among smaller scales.[2] The larger scales had a diameter of up to fifty-five millimetres and were distanced from each other by five to ten centimetres. They were hexagonal or pentagonal, thus with five or six sides. Each of these sides touched somewhat smaller scales, forming a rosette. Small, non-overlapping convex scales of about one centimetre in diameter surrounded the whole. The larger scales were wrinkled due to straight grooves orientated perpendicular to their edges. From top to bottom, the large scale rows gradually declined in size.[20] Unfortunately, nothing can as yet be learned about the coloration of Chasmosaurus from the known fossil skin impression samples.

Phylogeny[edit]

Skeletal restorations of the two valid species, C. belli (top) and C. russelli (bottom). The restorations share the postcrania

Chasmosaurus was in 1915 by Lambe within the Ceratopsia assigned to the Chasmosaurinae.[21] The Chasmosaurinae usually have long frills, like Chasmosaurus itself, whereas their sister-group the Centrosaurinae typically have shorter frills. Most cladistic analyses show that Chasmosaurus has a basal position in the Chasmosaurinae.

The following cladogram shows the phylogeny of Chasmosaurus according to a study by Scott Sampson e.a. in 2010.[15]

Ceratopidae 

Centrosaurinae


 Chasmosaurinae 

Chasmosaurus




Mojoceratops




Agujaceratops





Utahceratops



Pentaceratops





Coahuilaceratops





Kosmoceratops



Vagaceratops





Anchiceratops




Arrhinoceratops




Ojoceratops



Eotriceratops




Torosaurus




Nedoceratops



Triceratops














Paleobiology[edit]

Depiction of the mega-herbivores in the Dinosaur Park Formation, C. belli on the left

Chasmosaurus shared its habitat, the east coast of Laramidia, with successive species of Centrosaurus. A certain niche partitioning is suggested by the fact that Chasmosaurus had a longer snout and jaws and might have been more selective about the plants it ate.

The function of the frill and horns is problematic. The horns are rather short and the frill had so large fenestrae that it could not have offered much functional defense. Paul suggested that the beak was the main defensive weapon.[19] It is possible that the frill was simply used to appear imposing or conceivably for thermoregulation. The frill may also have been brightly colored, to draw attention to its size or as part of mating display. However, it is difficult to prove any sexual dimorphism. Lull suggested that C. kaiseni, which bore long brow horns, was in fact the male of C. belli of which the females would have short ones.[10] This is contradicted by the possible identity of C. kaiseni with Mojoceratops. In 1927 C.M. Sternberg concluded that of the two skeletons he had mounted in the Canadian Museum of Nature, the smaller one, NMC 2245, was the male and the larger, NMC 2280, the female.[22] However, today the two are referred to different species.

A juvenile Chasmosaurus belli found in Alberta, Canada by Phil Currie et al., reveals that Chasmosaurus may have cared for its young, like its relative, Triceratops, is hypothesized to have done. The juvenile measured 5 feet long and was estimated to be 3 years of age and had similar limb and frill proportions to the adult Chasmosaurus, indicating that Chasmosaurus was not fast moving and that juveniles did not need to be fast moving either to keep pace with adults. The fossil was complete save for its missing front limbs, which had fallen into a sinkhole before the specimen was uncovered. Skin impressions were also uncovered beneath the skeleton and evidence from the matrix that it was buried in indicated that the juvenile ceratopsian drowned during a possible river crossing.[23]

References[edit]

  1. ^ Ryan, M.J. & Evans, D.C. (2005). "Ornithischian dinosaurs". Dinosaur Provincial Park: A Spectacular Ecosystem Revealed. In: Currie, P.J. & Koppelhus, E.B. (Eds) Indiana University Press, Bloomington. pp. 312–348. 
  2. ^ a b c d e f g h Dodson, Peter; et al. "Chasmosaurus". The Age of Dinosaurs. Publications International. pp. 110–111. ISBN 0-7853-0443-6. 
  3. ^ 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
  4. ^ a b c Arbour, V. M.; Burns, M. E.; Sissons, R. L. (2009). "A redescription of the ankylosaurid dinosaur Dyoplosaurus acutosquameus Parks, 1924 (Ornithischia: Ankylosauria) and a revision of the genus". Journal of Vertebrate Paleontology 29 (4): 1117–1135. doi:10.1671/039.029.0405. 
  5. ^ 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
  6. ^ 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
  7. ^ Brown, B., 1933, "A new longhorned Belly River ceratopsian", American Museum Novitates 669: 1-3
  8. ^ a b T.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
  9. ^ Nicholas R. Longrich (2010). "Mojoceratops perifania, A New Chasmosaurine Ceratopsid from the Late Campanian of Western Canada". Journal of Paleontology 84 (4): 681–694. doi:10.1666/09-114.1. 
  10. ^ a b Lull, R.S., 1933, A revision of the Ceratopsia or horned dinosaurs. Memoirs of the Peabody Museum of Natural History 3(3): 1-175
  11. ^ Sternberg, C.M., 1940, "Ceratopsidae from Alberta", Journal of Paleontology 14(5): 468-480
  12. ^ Lehman, T.M., 1989, "Chasmosaurus mariscalensis, sp. nov., a new ceratopsian dinosaur from Texas", Journal of Vertebrate Paleontology, 9(2): 137-162
  13. ^ Lucas SG, Sullivan RM, Hunt AP (2006). "Re-evaluation of Pentaceratops and Chasmosaurus (Ornithischia: Ceratopsidae) in the Upper Cretaceous of the Western Interior". New Mex Mus. Nat. Hist. Sci. Bull. 35: 367–370. 
  14. ^ 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
  15. ^ a b Scott D. Sampson, Mark A. Loewen, Andrew A. Farke, Eric M. Roberts, Catherine A. Forster, Joshua A. Smith, Alan L. Titus (2010). "New Horned Dinosaurs from Utah Provide Evidence for Intracontinental Dinosaur Endemism". PLoS ONE 5 (9): e12292. doi:10.1371/journal.pone.0012292. PMC 2929175. PMID 20877459. 
  16. ^ 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
  17. ^ Olshevsky, G. 2000. An Annotated Checklist of Dinosaur Species by Continent. George Olshevsky, Publications Requiring Research, San Diego, 157 pp
  18. ^ Maidment, S.C.R., and Barrett, P.M., 2011, "A new specimen of Chasmosaurus belli (Ornithischia: Ceratopsidae), a revision of the genus, and the utility of postcrania in the taxonomy and systematics of ceratopsid dinosaur", Zootaxa, 2963: 1-47
  19. ^ a b c Paul, G.S., 2010, The Princeton Field Guide to Dinosaurs, Princeton University Press p. 269-270
  20. ^ Sternberg, C.M., 1925, "Integument of Chasmosaurus belli", Canadian Field-Naturalist 39: 108-110
  21. ^ L.M. Lambe, 1915, "On Eoceratops canadensis, gen. nov., with remarks on other genera of Cretaceous horned dinosaurs", Canada Geological Survey Museum Bulletin 12, Geological Series 24: 1-49
  22. ^ Sternberg, C.M., 1927, "Horned dinosaur group in the National Museum of Canada", Canadian Field-Naturalist 41: 67-73
  23. ^ http://www.nbcnews.com/science/rare-baby-dinosaur-skeleton-unearthed-canada-2D11650637