Monolophosaurus (/ˌmɒnoʊˌlɒfoʊˈsɔːrəs/ MON-oh-LOF-oh-SOR-əs;[1] meaning "single-crested lizard") is an extinct genus of tetanuran theropod dinosaur from the Middle Jurassic Shishugou Formation in what is now Xinjiang, China.

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.

In 1993/1994, Zhao Xijin and Philip John Currie named and described the type species Monolophosaurus jiangi.

The holotype IVPP 84019 was discovered in the Junggar Basin, in layers of the Wucaiwan Formation dating from the Bathonian-Callovian.

[7][8] 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.

In 2010, Brusatte et al rejected the identity, pointing out that the Guanlong holotype was actually a fully adult individual.

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.

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.

This area shows a number of pneumatic openings or pneumatopores, where diverticula of the air sacks entered the bone.

CAT scans showed that internally the nasal bone is heavily pneumatised, with large air chambers.

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.

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 rather small foramen surangulare posterior is not overhung by a thick bone shelf, which is rare among large theropods.

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.

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.

Smith et al. (2007) was the first publication to find Monolophosaurus to be a non-neotetanuran tetanuran,[11] by noting many characters previously thought to be exclusive of Allosauroidea to have a wider distribution.

Also, Zhao et al. in 2010 noted various primitive features of the skeleton suggesting that Monolophosaurus could be one of the most basal tetanuran dinosaurs instead.

[15] Chuandongocoelurus Coelurosauria Monolophosaurus Spinosauridae Megalosauridae Piatnitzkysauridae Asfaltovenator Metriacanthosauridae Allosauridae Neovenator Chilantaisaurus Megaraptora Eocarcharia Concavenator Acrocanthosaurus Carcharodontosaurinae A 2023 examination of Irritator challengeri found Monolophosaurus to be a sister taxon to Spinosauridae as part of a larger, monophyletic Carnosauria.

Monolophosaurus Metriacanthosauridae Lourinhanosaurus Aorun Allosaurus The type specimen (IVPP 84019) had its tenth and possibly eleventh neural spines fractured.