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Pachycephalosaurs Fossil range: Cretaceous
Skull of Pachycephalosaurus from Oxford University Museum of Natural History.
Conservation status
Extinct (fossil)
Scientific classification
Kingdom: Animalia Phylum: Chordata Class: Sauropsida Superorder: Dinosauria Order: Ornithischia Suborder: Cerapoda Infraorder: Pachycephalosauria Maryańska & Osmolska, 1974
Families
Homalocephalidae Pachycephalosauridae

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 first 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 (2006) doubts that either of these species are pachycephalosaurs.

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.^[1]

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.

Most pachycephalosaurid remains are incomplete, 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 theropod (Naish in Sullivan 2006). Further complicating matters are the diverse interpretations of ontogenetic and sexual features in pachycephalosaurs.

Dinosaurs Portal

The Pachycephalosauria clade is divided into two families: Homalocephalidae and Pachycephalosauridae. Homalocephalidae consists of the eponymous Homalocephale and an incertae sedis Ornatotholus. Pachycephalosauridae contains every other classified pachycephalosaurid species. There is no distinct differentiation between Pachycephalosauridae and its parent clade (Pachycephalosauria). Sullivan (2006) suggests that this hierarchy is redundant, and should be revised.

• Suborder Cerapoda
  • Infraorder Pachycephalosauria (= Pachycephalosauridae sensu Sullivan 2006)
    •  ?Micropachycephalosaurus
    •  ?Ferganocephale
    •  ?Stenopelix
    • Wannanosaurus
      • Superfamily Homalocephaloidea
        • Homalocephale
        • Incertae sedis: Ornatotholus
        • Family Pachycephalosauridae (= "Psalisauridae", "Tholocephalidae")
          • Stegoceras
          • Goyocephale
          • Incertae sedis: Gravitholus
          • Subfamily Pachycephalosaurinae (= "Domocephalinae")
            • Tylocephale
            • Prenocephale
            • Sphaerotholus
            • Colepiocephale
            • Hanssuesia
            • Alaskacephale
            • Tribe Pachycephalosaurini (sensu Sullivan 2006)
              • Tylosteus
              • Stygimoloch -- classified tentatively as a juvenile form of Pachycephalosaurus ^[2]
              • Dracorex -- -- classified tentatively as a juvenile form of Pachycephalosaurus ^[2]
              • Pachycephalosaurus

1. ^ Carpenter, Kenneth (1997). "Agonistic behavior in pachycephalosaurs (Ornithischia:Dinosauria): a new look at head-butting behavior" (pdf). Contributions to Geology 32 (1): 19-25. 
2. ^ ^{a b} "Did Horny Young Dinosaurs Cause Illusion of Separate Species?", Science Vol. 18, 23 Nov. 2007, p. 1236.

• Maryańska, T. & Osmólska, H., 1974. Pachycephalosauria, a new suborder of ornithischian dinosaurs. Palaeontologica Polonica 30: 45-102.
• Sereno, P.C., 1986. Phylogeny of the bird-hipped dinosaurs (Order Ornithischia). National Geographic Research 2: 234-256.
• Sullivan, R.M., 2006. A taxonomic review of the Pachycephalosauridae (Dinosauria: Ornithischia). New Mexico Museum of Natural History and Science Bulletin 35: 347-365.