Pachycephalosauria

Pachycephalosauria (Greek for 'thick headed lizards') is a clade of ornithischian dinosaurs. Well-known genera include Pachycephalosaurus, Stegoceras, Stygimoloch, and Dracorex. Most lived during the Late Cretaceous Period, in what is now North America and Asia. They were all bipedal, herbivorous/omnivorous animals with thick skulls. In some species the skull roof is domed and several inches thick; in others it is flat or wedge-shaped. The dome may be surrounded by nodes (for instance, Pachycephalosaurus), spikes (Stygimoloch), or both (Dracorex).

Candidates for the earliest known pachycephalosaur include Ferganocephale adenticulatum from the Middle Jurassic Period of what is now Kyrgyzstan and Stenopelix valdensis from the Early Cretaceous Period of what is now Germany, although Sullivan doubted that either of these species are pachycephalosaurs.

Paleobiology
The function of the thickened skull roof has been heavily debated. It has been frequently asserted that individuals may have rammed each other head-on, as do modern-day mountain goats and musk oxen. It is also suggested that pachycephalosaurs could make their head, neck, and body horizontally straight, in order to transmit stress during ramming. However, in no known dinosaur can the head, neck, and body be oriented in such a position. Instead, the cervical and anterior dorsal vertebrae of pachycephalosaurs show that the neck was carried in an "S"- or "U"-shaped curve.

Also, the rounded shape of the skull would lessen the contacted surface area during head-butting, resulting in glancing blows. Other possibilities include flank-butting, defense against predators, or both. The relatively wide width of pachycephalosaurs (which would protect vital internal organs from harm during flank-butting) and the squamosal horns of the Stygimoloch (which would have been used to great effect during flank-butting) add credence to the former.

Classification
Most pachycephalosaurid remains are not complete, usually consisting of portions of the frontoparietal bone that forms the distinctive dome. This can make taxonomic identification a difficult task, as the classification of genera and species within Pachycephalosauria relies almost entirely on cranial characteristics. Consequently, improper species have historically been appointed to the clade. For instance, Majungatholus, once thought to be a pachycephalosaur, is now recognized as a specimen of the abelisaurid theropod Majungasaurus. And Yaverlandia, another dinosaur initially described as a pachycephalosaurid, has also recently been reclassified as a coelurosaur (Naish in Sullivan 2006). Further complicating matters are the diverse interpretations of ontogenetic and sexual features in pachycephalosaurs.

Taxonomy
The Pachycephalosauria was first named as a suborder of the Order Ornithischia by Teresa Maryańska and Halszka Osmólska in 1974. They included within it only one family, the Pachycephalosauridae. Later researchers, such as Michael Benton, have ranked it as an infraorder of the Suborder Cerapoda, which unites the ceratopsians and ornithopods. In 2006, Robert Sullivan published a re-evaluation of pachycephalosaur taxonomy. Sullivan considered attempts to restrict Maryańska and Osmólska naming of Pachycephalosauria redundant with their Pachycephalosauridae, since they were diagnosed by the same anatomical characters. Sullivan also rejected attempts by Paul Sereno, in his phylogenetic studies, to re-define Pachycephalosauridae to include only "dome-skulled" species (including Stegoceras and Pachycephalosaurus), while leaving more "basal" species outside that family in Pachycephalosauria. Therefore, Sullivan's use of Pachycephalosauridae is equivalent to Sereno and Benton's use of Pachycephalosauria.

Sullivan diagnosed the Pachycephalosauridae based only on characters of the skull, with the defining character being a dome-shaped frontoparietal skull bone. According to Sullivan, the absence of this feature in some species assumed to be primitive led to the split in classification between domed and non-domed pachycephalosaurs; however, discovery of more advanced and possibly juvenile pachycephalosaurs with flat skulls (such as Dracorex hogwartsia) show this distinction to be incorrect. Sullivan also pointed out that the original diagnosis of Pachycephalosauridae centered around "flat to dome-like" skulls, so the flat-headed forms should be included in the family.

The following taxonomy represents Sullivan's 2006 classification.


 * Family Pachycephalosauridae
 * Alaskacephale
 * Colepiocephale
 * Goyocephale
 * Hanssuesia
 * Homalocephale
 * Prenocephale (incl. Sphaerotholus)
 * Stegoceras (incl. Ornatotholus)
 * Tylocephale
 * Tribe Pachycephalosaurini
 * Dracorex - possible juvenile form of Pachycephalosaurus
 * Pachycephalosaurus
 * Stygimoloch - possible juvenile form of Pachycephalosaurus
 * Pachycephalosauridae incertae sedis (uncertain placement)
 * Wannanosaurus
 * Nomina dubia (doubtful names)
 * Gravitholus
 * Ferganocephale
 * Heishansaurus (probably an ankylosaur)
 * Micropachycephalosaurus
 * “Stegoceras” bexelli

Note that Sullivan considered Stenopelix (sometimes assigned to the Pachycephalosauria as the most basal member) not diagnostic enough to be assigned to this group, and placed is as Ornithischia incertae sedis.

Phylogeny
The cladogram presented here follows an analysis by Williamson and Carr, 2002.