Stygimoloch ("demon from the river Styx") is a dubious genus of the pachycephalosaur family living in the Late Cretaceous period. It was distinguished from other pachycephalosaurs by the row of spikes on the back of its head, behind the dome characteristic of the family. Jack Horner has presented evidence in 2009 (and others after him) that suggests that Stygimoloch might be the juvenile form of Pachycephalosaurus. However, more recent evidence has suggested that Stygimoloch actually presents a different species of Pachycephalosaurus (P. spinifer).