Stygimoloch was a small pachycephalosaur whose thick skull bristled with bony knobs and spikes. It may have engaged in head-butting, either for defense or to establish a pecking order during mating. Its name derives from the river Styx ("stygi") and the name of a Semitic god, Moloch. In Greek mythology, the Styx was a mythical river flowing through the underworld near Hades (Hell). Stygimoloch's remains were discovered in the Hell Creek formation of Montana and Wyoming. According to myth, children were fed to the god Moloch. However, even if there had been any youngsters around in the Cretaceous, they would have been in no danger of being eaten by Stygimoloch; it was a vegetarian.It has been suggested that Stygimoloch could be a specimen representing one growth stage of the species Pachycephalosaurus. Differences in the pattern of spikes and the size of the skull dome were originally considered to be differences between species, but are now thought to represent changes between the juvenile and adult of one species. This issue represents one of the recurring challenges in paleontology -- the proper identification of species that could be another sex or a juvenile individual of a particular species.
In the early stages of the production of Dinosaur, Pachycephalosaurus, Stygimoloch's larger and well known cousin, was originally planned to co-star in the film with its smaller relative. But ultimately, only the Stygimoloch made it to the final cut.