Illustration of the carnosaur Monolophosaurus
Monolophosaurus (/ˌmɒnɵˌlɒfəˈsɔrəs/ MON-o-LOF-ə-SAWR-əs; meaning "single-crested lizard") was a genus of theropod dinosaur from the Middle Jurassic Shishugou Formation in what is now Xinjiang, China. It was named for the single crest on top of its skull. The type and only known individual is estimated at 5 metres (16.5 ft). The area that Monolophosaurus was found showed signs of water, so it is possible that this dinosaur lived on the shore of lakes or ocean. The Monolophosaurus jiangi IVP 84019 had its 10th and possibly 11th neural spines fractured. The tenth is fused to the eleventh. A series of parallel ridges on one of the specimen's dentaries may represent tooth marks.
Discovery and naming[]
A nearly complete skeleton was unearthed in 1984. At first, before description in the scientific literature, it was known in the press as "Jiangjunmiaosaurus", a nomen nudum.
In 1993 Zhao and Currie named the type species Monolophosaurus jiangi; the species name refers to Jiangjunmiao ("an abandoned desert inn") near which the holotype IVPP 84019 was found. Monolophosaurus was originally termed a "megalosaur" and has often since been suggested to be an allosauroid. Carr (2006) even suggested that the "proceratosaurid" "tyrannosauroid" Guanlong was a subadult Monolophosaurus and therefore an "allosauroid", by noting both taxa have a large, thin, and fenestrated midline crest, but this is probably not the case.
Smith et al. (2007) was the first publication to find Monolophosaurus to be a non-neotetanuran tetanuran, by noting many characters previously thought to be exclusive of Allosauroidea to have a more wider distribution. Also, Zhao et al. (2009) noted various primitive features of the skeleton suggesting that Monolophosaurus could be one of the most basal tetanuran dinosaurs instead.[10] Benson (2008, 2010) placed Monolophosaurus in a clade with Chuandongocoelurus that is more basal than Megalosauridae and Spinosauridae in the Megalosauroidea. Latter, Benson et al. (2010) found the Chuandongocoelurus/Monolophosaurus clade to be outside of Megalosauroidea and Neotetanurae, near the base of Tetanurae.
Description[]
The type and only known individual has been estimated at five metres (16.5 ft). In 2010, Paul estimated the length at 5.5 metres, the weight at 475 kilogrammes.
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.
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 at the level of the third tooth extends to the front into two superimposed narrow slits. The rear of the lower jaw shows a unique combination of a kinked suture between the angular and the surangular, and the basal trait of the surangular reaching the rear jaw edge. The rather small foramen surangulare posterior is not overhung by a thick bone shelf, which is rare among large theropods.
The vertebral column consists of nine cervical vertebrae, fourteen dorsals and five sacrals. The number of tail vertebrae is unknown. The cervical vertebrae of the neck are strongly pneumatised. They possess pleurocoels at their sides and their insides are hollowed out by large air chambers. The neural spines of the cervical vertebrae are narrow in side view and decreased in width towards the rear: those of the eighth and ninth vertebrae were rod-like. At least the first three dorsal vertebrae of the back have pleurocoels as well. The dorsals are connected by robust hyposphene-hypantrum complexes. From the sixth vertebra onwards the neural spines abruptly become wider. The neural spines of the sacral vertebrae are not fused into a supraneural plate. The tail base is slightly oriented downwards. The caudal vertebrae of the tail base also show hyposphene-hypantrum complexes.
In the pelvis, the ilium has a slightly convex upper profile. Its front blade has a pending, hook-shaped point. The edge of the base of the front blade is incised by a groove. The ilium has some basal traits. The process to which the pubic bone is attached, has two facets, one directed to below, the other obliquely pointing to the front, instead of a single facet. Also basal is the fact that the hip joint is overhung by a hood-shaped extension of the antitrochanter; the front of this hood reaches further to below and to the outer side. There is no clear brevis shelf. The pubic bones and the ischia resemble each other in having a "foot" and being per pair connected via bony skirts, pierced by a foramen.
Classification[]
Gallery[]
