Monolophosaurus

''Monolophosaurus'' ( ; meaning "single-crested lizard") is an extinct genus of tetanuran theropod dinosaur from the Middle Jurassic Shishugou Formation in what is now Xinjiang, China.Holtz, Thomas R. Jr. (2011) ''Dinosaurs: The Most Complete, Up-to-Date Encyclopedia for Dinosaur Lovers of All Ages,'
Winter 2010 Appendix.
/ref> It was named for the single crest on top of its skull. ''Monolophosaurus'' was a mid-sized theropod at about long or more.

Discovery and naming

A nearly complete skeleton of a theropod new to science was discovered by Dong Zhiming in 1981, during stratigraphic exploration for the benefit of the oil industry. The fossil was not unearthed until 1984. In 1987, before description in the scientific literature, it was referred to in the press as ''Jiangjunmiaosaurus'', an invalid '' nomen nudum''. In 1992 it was mentioned by Dong Zhiming as ''Monolophosaurus jiangjunmiaoi'', and in 1993 by Wayne Grady as ''Monolophosaurus dongi''. 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. 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. 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, ''et al'' redescribed the holotype, at the time still the only specimen known, in detail.

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. More conservatively, in 2010 Gregory S. Paul renamed ''Guanlong'' into a ''Monolophosaurus'' species, ''Monolophosaurus wucaii'',Paul, G.S., 2010, ''The Princeton Field Guide to Dinosaurs'', Princeton University Press p. 93-94 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.

Description

The type and only known individual has been estimated at . In 2010, Paul estimated the length at 5.5 meters (18 ft) and the weight at 475 kg (1,047 lbs) . In 2016 Molina-Pérez and Larramendi gave a higher estimation of 7.5 meters (24.6 ft) and 710 kg (1,565 lbs).

Several distinguishing traits have been established. The snout on its midline bears a large crest, the front of which is formed by the praemaxillan. 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.

Skull

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.

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.

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. 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. These openings are relatively large below the first four teeth; more to behind, they become smaller and their row curves downwards. From the ninth tooth onwards, the ''foramina'' merge into a groove. A second row of openings runs parallel to the lower jaw edge and ends at the thirteenth tooth position, which is exceptionally far. At the inside of the dentary, the Meckelian groove

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