Pachycephalosauria

Pachycephalosaurs Temporal range: Middle Jurassic?–Late Cretaceous,
Undescribed pachycephalosaur, informally referred to Pachycephalosaurus wyomingensis
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Dinosauria
Clade:	†Ornithischia
Clade:	†Pachycephalosauria Maryańska & Osmólska, 1974
Family:	†Pachycephalosauridae Sternberg, 1945
Genera
See text.
Synonyms
Homalocephalidae Dong, 1978

Pachycephalosauria (Template:Pron-en, from Greek παχυκέφαλοσαυρος 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 fossils, the skull roof is domed and several inches thick; in others it is flat or wedge-shaped. While traditionally regarded as distinct species or even families, the flat-headed pachycephalosaurs may actually represent juveniles of dome-headed adults. The domes were often surrounded by nodes and/or spikes.

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 adaptive significance of the skull dome has been heavily debated. The popular hypothesis among the general public that the skull was used in head-butting, as sort of a dinosaurian battering ram, was first proposed by Colbert in 1955. This view was popularized in the 1956 science fiction story "A Gun for Dinosaur" by L. Sprague de Camp. Many paleontologists have since argued for the head-butting hypothesis, including Galton in 1970 and Sues in 1978. In this hypothesis, pachycephalosaurs rammed each other head-on, as do modern-day mountain goats and musk oxen.

Anatomical evidence for combative behavior includes vertebral articulations providing spinal rigidity, and the shape of the back indicating strong neck musculature. It has been 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 flank-butting hypothesis.

A histological study conducted by Goodwin and Horner (2004) argued against the battering ram hypothesis. They argued that the dome was "an ephemeral ontogenetic stage," the spongy bone structure couldn't sustain the blows of combat, and the radial pattern was simply an effect of rapid growth. A later biomechanical analysis by Snively and Cox (2008) concluded, however, that the domes could withstand combat stresses. Lehman (2010) argued that the growth patterns discussed by Goodwin and Horner are not inconsistent with head-butting behavior.

Goodwin and Horner (2004) instead argued that the dome functioned for species recognition. There is evidence that the dome had some form of external covering, and it is reasonable to consider the dome may have been brightly covered, or subject to change color seasonally. Due to the nature of the fossil record, however, it cannot be observed if color actually played a role in dome function.

Longrich et al. (2010) argued that species recognition is an unlikely evolutionary cause for the dome because dome forms are not notably different between species. Because of this general similarity, several genera of Pachycephalosauridae were incorrectly lumped together. This is unlike the case in ceratopsians and hadrosaurids, which had much more distinct cranial ornamentation. Longrich et al. argued that instead the dome had a mechanical function, one which was important enough to justify the resource investment, such as combat.

Classification

Diagram showing Dracorex and Stygimoloch as growth stages of Pachycephalosaurus

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.

A 2009 paper proposed that Dracorex and Stygimoloch were just early growth stages of Pachycephalosaurus, rather than distinct genera.

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 follows Sullivan's 2006 classification.

• Family Pachycephalosauridae
  • Alaskacephale
  • Colepiocephale
  • Goyocephale
  • Hanssuesia
  • Homalocephale - possible juvenile form of Prenocephale
  • Prenocephale (incl. Sphaerotholus)
  • Stegoceras (incl. Ornatotholus)
  • Texacephale
  • 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.

Pachycephalosauria	Stenopelix Wannanosaurus Yaverlandia (now thought to be a theropod instead) Goyacephala Goyocephale <font color="white">unnamed Homalocephale Pachycephalosauridae Stegoceras Pachycephalosaurinae <font color="white">unnamed Tylocephale <font color="white">unnamed Sphaerotholus <font color="white">unnamed Stygimoloch Pachycephalosaurus

External links

• TEDx talk by Jack Horner on shape-shifting dinosaur skulls and dinosaur misclassification.

References

• Sereno, P.C., 1986. Phylogeny of the bird-hipped dinosaurs (Order Ornithischia). National Geographic Research 2: 234-256.