Stygimoloch

Stygimoloch
Stygimoloch spinifer skull (Museum für Naturkunde, Berlin)
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Sauropsida
Superorder:	Dinosauria
Order:	Ornithischia
Suborder:	Marginocephalia
Infraorder:	Pachycephalosauria
Family:	Pachycephalosauridae
Genus:	Stygimoloch Galton & Sues, 1983
Species
S. spinifer Galton & Sues, 1983 (type)

Stygimoloch (meaning "horned devil from the river of death") is a putative genus of pachycephalosaurid dinosaur from the end of the Cretaceous period, roughly 65 million years ago. It is currently known from the Hell Creek Formation and Lance Formation of the Western Interior (United States), where it lived alongside Tyrannosaurus and Triceratops.

The type species, S. spinifer, was described by British vertebrate paleontologist Peter Galton and German paleontologist Hans-Dieter Sues of the National Museum of Natural History in 1983. However, Jack Horner et al. suspect that it is a juvenile Pachycephalosaurus.

It is a relatively large pachycephalosaur, with the skull being about 46 centimeters long (18 in). Among North American pachycephalosaurs, only Pachycephalosaurus is larger. Unlike other pachycephalosaurs, the domed skull is relatively small, slightly flattened from side to side, and pear-shaped; even when isolated this unusual dome can easily be distinguished from the broader, larger domes of Pachycephalosaurus.

File:Stygimoloch.jpg

Life reconstruction of Stygimoloch.

While the dome is reduced in size, the ornamentation over the skull is more elaborate than in any other pachycephalosaur. Short, conical hornlets covered the nose, and the back corners of the skull bore an enormous pair of massive, backward-pointing spikes, up to 5 centimeters in diameter (2 in) and 15 centimeters long (6 in); these are surrounded by two or three smaller spikes. The function of this unusual ornamentation is unknown. Even if other pachycephalosaurs did butt heads (which is a subject of continuing debate), the small dome of Stygimoloch suggests that this behavior was not as important. Instead, the skull ornament might have functioned for display, may have been used for self-defense, or perhaps were locked together and used in shoving matches, like the horns of deer. More likely, however, is that the squamosal horns were used to inflict pain during flank-butting.

Profile view of the skull of Stygimoloch.

The pachycephalosaur Dracorex may actually be an individual of Stygimoloch or Pachycephalosaurus in which the dome and horns are not well-developed, either because the animal was a juvenile or a female. This consideration was supported at the 2007 annual meeting of the Society of Vertebrate Paleontology. Jack Horner of Montana State University presented evidence, from analysis of the skull of the single existing Dracorex specimen, that this dinosaur may well be a juvenile form of Stygimoloch. In addition, he presented data that indicates that both Stygimoloch and Dracorex may be juvenile forms of Pachycephalosaurus.