Monolophosaurus (meaning “single-crested lizard”) is a genus of tetanuran 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. Monolophosaurus was a mid sized carnosaurian carnivore at about 5 metres long.
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. 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 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, 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.
The type and only known individual has been estimated at five metres (16.4 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.
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, 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. Benson (2008, 2010) placed Monolophosaurus in a clade with Chuandongocoelurus that is more basal than Megalosauridae and Spinosauridae in the Megalosauroidea. Later, Benson et alii (2010) found the Chuandongocoelurus/Monolophosaurus clade to be outside of Megalosauroidea and Neotetanurae, near the base of Tetanurae. A 2012 phylogeny found Monolophosaurus and Chuandongocoelurus, while not sister taxa, to form a group outside more derived groups at the base of Tetanurae.