Pachycephalosauria

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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. 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 (2006) doubts that either of these species are pachycephalosaurs. Most known pachycephalosaurs lived during the Late Cretaceous Period, in what is now North America and Asia. They were all bipedal, herbivorous animals with thick skulls; in some species the skull roof is domed and several inches thick. The dome may be surrounded by nodes (for instance, Pachycephalosaurus) or spikes (Stygimoloch).

The function of the thickened skull roof has been heavily debated. It has frequently been suggested that individuals may have rammed each other head-on, as do modern-day mountain goats and musk oxen. However, the rounded shape of the skull might tend to result in glancing blows which would damage the neck. Not only that, but the curve of the neck would have kept the head from ever being lowered for such a strike. Other possibilities include flank-butting (a behavior seen in giraffes) or perhaps defense against predators.

Relatively well-known genera include Pachycephalosaurus, Stegoceras, Stygimoloch, and Prenocephale. The inclusion of some supposed basal pachycephalosaurids in this group has been incorrect. Majungatholus, once thought to be a pachycephalosaur, is now recognized as an abelisaurid theropod (possibly synonymous with Majungasaurus). Yaverlandia, another dinosaur initially described as a pachycephalosaurid, has also recently been reclassified as a theropod (Naish in Sullivan 2006).

[edit] Taxonomy

• 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
              • Dracorex
              • Pachycephalosaurus

[edit] References

• 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 Pqachycephalosauridae (Dinosauria: Ornithischia). New Mexico Museum of Natural History and Science Bulletin 35: 347-365.