Pachycephalosauria

The pachycephalosaurs (Pachycephalosauria, gr. "thick-headed lizards") are an infraorder of marginocephalous ornithischian dinosaurs, which lived since the Early Cretaceous to the Late Cretaceous (approximately 90 to 66 million years ago, from the Berriasian to the Maastrichtian), in what is now North America and Asia. They were bipedal, herbivorous/omnivorous animals with thick skulls. In some fossils, the cranial vault is dome-shaped and several centimeters thick; in others it is flattened or wedge-shaped. Although traditionally considered distinct species or even families, flat-headed pachycephalosaurs may represent juveniles of dome-headed adults. The domes were often surrounded by nodules and/or spines.

Candidates for the first known pachycephalosaurs include Ferganocephale adenticulatum from the Middle Jurassic period of present-day Kyrgyzstan and Stenopelix valdensis from the Early Cretaceous of Germany, although it has been proposed that these species may not be pachycephalosaurs.

Paleobiology

The adaptive significance of the cranial dome has been much debated. The popular hypothesis among the general public, that the skull was used for lunging, like a kind of dinosaurian battering ram, was originally proposed by Colbert, 1955. This idea was popularized in the 1956 science fiction story 'A Gun'. for Dinosaur" by L. Sprague de Camp. Since then many paleontologists have favored the idea of charging, including Galton, 1970 and Sues, 1978. In this hypothesis, pachycephalosaurs charged each other head to head, as modern rams and musk oxen do.

Anatomical evidence for this behavior included vertebral joints that provided rigidity to the spine, and the shape of the back that suggested strong neck musculature. It had been suggested that pachycephalosaurs could align completely horizontally. their heads, necks and bodies in order to transmit the forces of the collision during the fight. However, no known dinosaur could actually orient its body in such a position. Instead, the cervical and anterior dorsal vertebrae of these dinosaurs show that the neck was generally oriented in an "S"- or "U"-shaped curve.

Likewise, the rounded shape of the skull could minimize the contact area during head contact, resulting in glancing blows. Other possibilities include lunging at the animal's flanks, defending against predators, or both. The relative width of the pachycephalosaurs' body (which could protect vital internal organs during confrontations) and the scaly horns of Stygimoloch (which could be used to amplify the effect of the blow) gave credence to the hypothesis of flank attacks.

A histological study by Goodwin and Horner, 2004 argued against the ram hypothesis. They claimed that the dome was 'an ephemeral ontogenetic stage', since the spongy bone structure could not withstand the blows of combat, and the radial pattern is simply an effect of rapid growth. Subsequent biomechanical analyzes deSnively and Cox, 2008 and Snively and Theodor, 2011 concluded that domes could withstand the stresses of combat. Lehman, 2010 stated that the growth patterns discussed by Goodwin and Horner are not inconsistent with head-butting fighting.

Goodwin and Horner, 2004 insisted that the function of the dome was the recognition of members of the same species. There is evidence that the dome had some kind of external covering, and it is reasonable to consider that it may have had a striking covering or changed color seasonally. But due to the nature of the fossil record, it cannot be determined whether color actually played a role. role in the function of the dome. Longrich, Sankey and Tanke, 2010 indicated that the recognition of members of the species is an unlikely evolutionary cause for the development of the dome since its shapes do not vary appreciably between species. Because of this widespread similarity, several genera of Pachycephalosauridae were incorrectly grouped together. This is unlike the case of ceratopsians and hadrosaurids, which have a very varied cranial ornamentation. Longrich et al. They pointed out that the dome must have had a mechanical function, one that was important enough to justify the investment of resources in it, such as combat.

New findings published in the journal Journal Plos One on July 16, 2013 added evidence to the idea of lunging in pachycephalosaur species. Of the 100 domes studied, 20 percent showed signs of wounds, all consistent with combat behavior. Pathologies include holes where the bone became infected due to wounds originating in the skin. These additional findings imply that pachycephalosaurids may have used their heads for both display and defense as in many modern animals.

Regarding their diet, Mallon et al. (2013) examined the coexistence of herbivores on the island-continent of Laramidia, during the Late Cretaceous. It was concluded that pachycephalosaurids were generally restricted to feeding on vegetation located one meter or less above the ground.

Systematic

Pachycephalosauria is defined as the most inclusive clade containing Pachycephalosaurus wyomingensis (Brown & Schlaikjer, 1943), but not Triceratops horridus (Marsh, 1889), Heterodontosaurus tucki (Crompton & Charig 1962), Hypsilophodon foxii (Huxley, 1869) and Ankylosaurus magniventris (Brown, 1908).

Many pachycephalosaur remains are not complete, usually consisting of parts of the frontoparietal bone that forms the distinctive dome. This makes their taxonomic identification a difficult task, since the classification of genera and species within Pachycephalosauria depends almost entirely on their cranial characteristics. Consequently, some species have been misclassified in this clade. For example, Majungatholus, once considered a pachycephalosaur, is now recognized as a specimen of the abelisaurid theropod Majungasaurus. And Yaverlandia, another dinosaur initially described as a pachycephalosaurid, has also been reclassified as a coelurosaur (Naish in Sullivan, 2006). Additional difficulties lie in the various interpretations of the ontogeny and sexual traits of these dinosaurs.

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

Taxonomy

Pachycephalosauria was initially named as a suborder of the order Ornithischia by Maryańska and Osmólska, 1974. They included only one family in this, Pachycephalosauridae. Later researchers, such as Michael Benton, reclassified it as a infraorder of the suborder Cerapoda, which includes ceratopsians and ornithopods. In 2006, Robert Sullivan published a reevaluation of the taxonomy of pachycephalosaurs. Sullivan considered Maryańska and Osmólska's attempt to restrict the definition of Pachycephalosauria to be redundant with their Pachycephalosauridae, given that they diagnosed them with the same anatomical characteristics. Sullivan also rejected the classification of Sereno, 1986 in his phylogenetic studies, in which he redefined Pachycephalosauridae to include only species with a high skull dome (such as Stegoceras and Pachycephalosaurus).), meanwhile leaving the most "basal" outside this family in Pachycephalosauria. Therefore, Sullivan's use of Pachycephalosauridae is equivalent to Sereno and Benton's use of Pachycephalosauria.

Sullivan diagnosed Pachycephalosauridae based on cranial characteristics alone, with the defining feature being the dome-shaped frontoparietal bone. According to Sullivan, the absence of these characteristics in some species that were considered primitive led to the division between dome-headed and flat-headed pachycephalosaurs; However, the discovery of more advanced forms and possible juvenile pachycephalosaurs with flattened heads (such as Dracorex hogwartsia) show that this distinction is incorrect. Sullivan also noted that the original diagnosis of Pachycephalosauridae focused on 'flattened to domed' skulls, so flattened head forms should be included in the family.

The following taxonomy follows Sullivan's 2006 classification unless otherwise noted.

• Family Pachycephalosauridae
  • Acrotholus
  • Alaskacephale
  • Amtocephale
  • Colepiocephale
  • Foraminacephale
  • Goyocephale
  • Gravitholus
  • Hanssuesia
  • Homalocephale - possible youth form Prenocephale
  • Prenocephale
  • Sinocephale
  • Sphaerotholus
  • Stegoceras (includes to Ornatotholus)
  • Texacephale
  • Tylocephale
  • Wannanosaurus
  • Tribu Pachycephalosaurini
    • Dracorex - possible youth form Pachycephalosaurus
    • Pachycephalosaurus
    • Stygimoloch - possible youth form Pachycephalosaurus
• Nomina dubia (doubted names)
  • Ferganocephale
  • Heishansaurus (probably an ankylosaurus)

It should be noted that Butler et al., 2011 reassigned Stenopelix and Micropachycephalosaurus to Ceratopsia.

Phylogeny

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

Cladogram according to Longrich, Sankey and Tanke (2010).

Below is a modified cladogram from Evans et al., 2013.