5 individuals, including nearly complete adult skeleton. The largest and in some way the most primitive of the ceratosaurs. Ceratosaurus ("horned lizard") somewhat resembled its more advanced rival Allosaurus. The first skeleton of Ceratosaurus was discovered in 1883/4 by M.P.Felsch at the same quarry in which the fine skeleton of Allosaurus was excavated in Fremont County, Colorado. 

A large part of the skeleton of this theropod was recovered which provided O.C.Marsh with very good information with which to describe another new type of theropod, which was both smaller and clearly different from Allosaurus.   

Ceratosaurus also shows several subtle differences from that of Allosaurus. In particular the hand has four well-- developed fingers, unlike Allosaurus which has only three. Ceratosaurus skeleton was found to possess the remains of a narrow row of bony plates which seem to have run down the middle of the back. Compared with Allosaurus. Ceratosaurus would seem to have been a smaller, more lightly built and agile predator.

Ceratosaurus may be defined as that Neoceratosauria taxon having: premaxilla-maxila fenestra; lacrimal fenestra; rostral prong of angular contacts dentary-splenial concavity; humerus sigmoid in cranial view; anterior blade of ilium dorsoventrally expanded; metatarsals proximally fused