Monolophosaurus

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Monolophosaurus
Temporal range: Middle Jurassic, 165Ma
Monolophosaurus skull.jpg
Mounted skeleton in Japan
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Superorder: Dinosauria
Order: Saurischia
Suborder: Theropoda
(unranked): Tetanurae
Superfamily: Megalosauroidea[1]
Genus: Monolophosaurus
Zhao & Currie 1993
Species
  • M. jiangi Zhao & Currie 1993 (type)

Monolophosaurus (/ˌmɒnɵˌlɒfəˈsɔrəs/ MON-o-LOF-ə-SAWR-əs;[2] meaning "single-crested lizard") is a genus of tetanuran theropod dinosaur from the Middle Jurassic Shishugou Formation in what is now Xinjiang, China.[3][4] It was named for the single crest on top of its skull.

Discovery and naming[edit]

Mounted skeletons of Monolophosaurus and Tuojiangosaurus, Field Museum, Chicago, Illinois. The two taxa are not from the same formation

A nearly complete skeleton of a theropod new to science was discovered by a Canadian-Chinese expedition in 1981. The fossil was unearthed until 1984. In 1987, before description in the scientific literature, it was referred to in the press as Jiangjunmiaosaurus, an invalid nomen nudum.[3][5] In 1992 it was mentioned by Dong Zhiming as Monolophosaurus jiangjunmiaoi,[6] and in 1993 by Wayne Grady as Monolophosaurus dongi.[7] These latter names also lacked a description and therefore were nomina nuda as well.

In 1993/1994, Zhao Xijin and Philip John Currie named and described the type species Monolophosaurus jiangi. The generic name is derived from Greek μόνος, monos, "single", and λόφος or λόφη, lophos/lophè, "crest", in reference to the single crest on the snout. The specific name refers to Jiangjunmiao, an abandoned desert inn, near which the fossil was found.[3][8] Jiangjunmiao means "the temple (miao) of the general (jiangjun)"; local legend has it that a general was buried here.

The holotype, IVPP 84019, was discovered in the Junggar Basin, in layers of the Wucaiwan Formation dating from the Bathonian-Callovian. It consists of a rather complete skeleton including the skull, lower jaws, vertebral column and pelvis. The rear of the tail, the shoulder girdle and the limbs are lacking. It represents an adult or subadult individual.[3] The type specimen was restored with plaster to be used in a travelling exhibit. Its left side was encased in foam which has hindered subsequent study. A reconstruction was made of the missing elements to create casts of complete skeletal mounts. In 2010, two studies by Stephen Brusatte e.a. redescribed the holotype, at the time still the only specimen known, in detail.[8][9]

In 2006, Thomas Carr suggested that Guanlong, another theropod with a large, thin, and fenestrated midline crest and from the same formation, was in fact a subadult individual of Monolophosaurus. Usually Guanlong had been considered a proceratosaurid tyrannosauroid, but Carr had performed an analysis in which both specimens clustered and were allosauroids.[10] More conservatively, in 2010 Gregory S. Paul renamed Guanlong into a Monolophosaurus species, Monolophosaurus wucaii,[11] presuming the taxa might be sister species. In 2010, Brusatte e.a. rejected the identity, pointing out that the Guanlong holotype was actually a fully adult individual.[8]

Description[edit]

Restoration

The type and only known individual has been estimated at five metres (16.5 ft).[4] In 2010, Paul estimated the length at 5.5 metres, the weight at 475 kilogrammes.[11]

Several distinguishing traits have been established. The snout on its midline bears a large crest, the front of which is formed by the praemaxillae. It continues to behind over the nasals and lacrimals; its rear touches the frontals. The top of the crest runs parallel to the upper jaw edge. The ascending branches of the praemaxillae each have a forked rear. The side of the praemaxilla features a deep groove running from an opening in the ascending branch towards an opening below the nostril. Within the depression around the upper rear side of the nostril two pneumatic openings are present, of unequal size. The rear branch of the lacrimal, above the eye socket, has a distinctive hatchet-shaped process pointing upwards. The combined frontals are rectangular and elongated with a length:width ratio of 1.67.[8]

Mounted skeleton

The holotype skull has a length of eighty centimetres. It is as such rather flat but this is obscured by the presence of a large snout crest occupying about three quarters of the skull length, reaching the level of the eye sockets. Sprouting from the praemaxillae at the snout tip, the crest is largely formed by the nasal bones. Transversely it has a triangular cross-section with a broad base and a more narrow top; this does not form a ridge, however, but has a flat upper surface. The nasal crest side is very rugose with a series of bosses and swellings. The nasal bone contributes to the upper rear part of the depression around the antorbital fenestra. This area shows a number of pneumatic openings or pneumatopores, where diverticula of the air sacks entered the bone. In the front two small foramina are present, more to the rear two large horizontal oval openings. CAT-scans showed that internally the nasal bone is heavily pneumatised, with large air chambers. Also the jugal bone is pneumatised. The lacrimal is I-shaped. It has an ascending branch forming the vertical rear edge of the crest; due to the triangular cross-section this branch is inclined towards the midline of the skull. The upper outer side of this branch forms a rectangular boss. Behind the eye socket, on the postorbital another, smaller, horn is present. The frontal bones do not contribute to the crest; they are unique among Theropoda in having a combined rectangular instead of triangular shape, due to the posterior position of the crest rear.[8]

Profile view of skull cast

The praemaxilla has a narrow ascending branch, forming the front of the crest. The rear of this branch is forked and embraces a lateral point of the nasal, a feature not recognised in the original description of 1994. At the base of the branch a small opening is present. A larger opening is located below the nostril. Both openings are connected by a distinct groove, curving around the underside of the nostril. The function of this unique trait is unknown. The praemaxilla bears four teeth. The maxilla bears thirteen teeth. The maxilla has a short depression around the lower front of the antorbital fenestra. Within this area a smaller hollowing is located, closed at the inside, perhaps representing the fenestra promaxillaris, of which it has the usual position, or the fenestra maxillaris, the normal identity of a single opening.[8]

In the braincase, the channel of the nervus trigeminus, the fifth brain nerve, is not bifurcated. The palatine bone is pneumatised, as shown by the presence of a pneumatopore.[8]

In the lower jaw, the external mandibular fenestra is rather small for a basal tetanuran. The holotype shows eighteen teeth in the right dentary, seventeen in the left dentary; such an asymmetry is not rare among large theropods. A row of foramina is present below and on the outer side of the tooth row.[8]

Classification[edit]

Monolophosaurus was originally termed a "megalosaur" and has often since been suggested to be an allosauroid. Smith et alii (2007) was the first publication to find Monolophosaurus to be a non-neotetanuran tetanuran,[12] by noting many characters previously thought to be exclusive of Allosauroidea to have a more wider distribution. Also, Zhao et alii in 2010 noted various primitive features of the skeleton suggesting that Monolophosaurus could be one of the most basal tetanuran dinosaurs instead.[9] Benson (2008, 2010) placed Monolophosaurus in a clade with Chuandongocoelurus that is more basal than Megalosauridae and Spinosauridae in the Megalosauroidea.[1][13] Later, Benson et alii (2010) found the Chuandongocoelurus/Monolophosaurus clade to be outside of Megalosauroidea and Neotetanurae, near the base of Tetanurae.[8]

Digital restoration

The following cladogram shows a 2010 analysis of the Megalosauroidea.[14]

Megalosauroidea


Xuanhanosaurus




Marshosaurus




Condorraptor



Piatnitzkysaurus








Chuandongocoelurus



Monolophosaurus




Spinosauridae


Baryonyx



Suchomimus





Irritator



Spinosaurus





Megalosauridae





Paleobiology[edit]

The type specimen (IVPP 84019) had its tenth and possibly eleventh neural spines fractured. They are fused together. A series of parallel ridges on one of the specimen's dentaries may represent tooth marks.[15]

References[edit]

  1. ^ a b Benson, Roger B. J. (2010). "A description of Megalosaurus bucklandii (Dinosauria: Theropoda) from the Bathonian of the UK and the relationships of Middle Jurassic theropods". Zoological Journal of the Linnean Society 158: 882−935. doi:10.1111/j.1096-3642.2009.00569.x. 
  2. ^ Creisler, Ben (July 7, 2003). "Dinosauria Translation and Pronunciation Guide M". Retrieved August 23, 2010. 
  3. ^ a b c d Zhao, Xi-Jin; Currie, Philip J. (1993). "A large crested theropod from the Jurassic of Xinjiang, People's Republic of China". Canadian Journal of Earth Sciences 30: 2027–2036. Bibcode:1993CaJES..30.2027Z. doi:10.1139/e93-178. 
  4. ^ a b Holtz, Thomas R., Jr. (2007). Dinosaurs: The Most Complete, Up-to-Date Encyclopedia for Dinosaur Lovers of All Ages. New York: Random House. pp. Genus list "last updated 8/1/2008". ISBN 978-0-375-82419-7. 
  5. ^ Holley, David (October 23, 1987). "2nd creature was meat-eater: fossil remains of huge dinosaur found in China". Los Angeles Times. Retrieved August 23, 2010. 
  6. ^ Dong, Z., 1992, Dinosaurian Faunas of China, Ocean Press/Springer-Verlag, Beijing/Berlin. 188 pp
  7. ^ Grady, W., 1993, The Dinosaur Project — The Story of the Greatest Dinosaur Expedition Ever Mounted, Edmonton: Ex Terra Foundation. Toronto: Macfarlane Walter & Ross. 61 pp
  8. ^ a b c d e f g h i Brusatte, Stephen L.; Benson, Roger B. J.; Currie, Philip J.; Zhao, Xijin (2010). "The skull of Monolophosaurus jiangi (Dinosauria: Theropoda) and its implications for early theropod phylogeny and evolution". Zoological Journal of the Linnean Society 158: 573–607. doi:10.1111/j.1096-3642.2009.00563.x. 
  9. ^ a b Zhao, Xijin; Benson, Roger B. J.; Brusatte, Stephen L.; Currie, Philip J. (2010). "The postcranial skeleton of Monolophosaurus jiangi (Dinosauria: Theropoda) from the Middle Jurassic of Xinjiang, China, and a review of Middle Jurassic Chinese theropods". Geological Magazine 147 (1): 13–27. doi:10.1017/S0016756809990240. 
  10. ^ Carr, T. 2006. "Is Guanlong a tyrannosauroid or a subadult Monolophosaurus?", Journal of Vertebrate Paleontology 26:48A
  11. ^ a b Paul, G.S., 2010, The Princeton Field Guide to Dinosaurs, Princeton University Press p. 93-94
  12. ^ Smith ND, Makovicky PJ, Hammer WR, Currie PJ. 2007. Osteology of Cryolophosaurus ellioti (Dinosauria:Theropoda) from the Early Jurassic of Antarctica and implications for early theropod evolution. Zoological Journal of the Linnean Society 151: 377–421.
  13. ^ Benson, 2008. A new theropod phylogeny focussing on basal tetanurans, and its implications for European 'megalosaurs' and Middle Jurassic dinosaur endemism. Journal of Vertebrate Paleontology. 51A.
  14. ^ Benson, R. B. J.; Carrano, M. T.; Brusatte, S. L. (2009). "A new clade of archaic large-bodied predatory dinosaurs (Theropoda: Allosauroidea) that survived to the latest Mesozoic". Naturwissenschaften 97 (1): 71–78. Bibcode:2010NW.....97...71B. doi:10.1007/s00114-009-0614-x. PMID 19826771.  edit
  15. ^ Molnar, R. E., 2001, Theropod paleopathology: a literature survey: In: Mesozoic Vertebrate Life, edited by Tanke, D. H., and Carpenter, K., Indiana University Press, p. 337-363.

Further reading[edit]

External links[edit]