Pachycephalosauria

From Wikipedia, the free encyclopedia
Jump to: navigation, search
Pachycephalosaurs
Temporal range: Late Cretaceous, 90–66Ma
Pachycephalosaurus skull.JPG
Cast of a Pachycephalosaurus wyomingensis skull, Oxford University Museum of Natural History
Scientific classification e
Kingdom: Animalia
Phylum: Chordata
Clade: Dinosauria
Order: Ornithischia
Clade: Marginocephalia
Suborder: Pachycephalosauria
Maryańska & Osmólska, 1974
Family: Pachycephalosauridae
Sternberg, 1945
(conserved name)
Type species
Troodon wyomingensis
Gilmore, 1931 (conserved name)
Genera

See text

Synonyms

Homalocephalidae Dong, 1978

Pachycephalosauria (/ˌpækɨˌsɛfəlɵˈsɔriə/; 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 the flat-headed pachycephalosaurs are traditionally regarded as distinct species or even families, they may actually represent juveniles of dome-headed adults.[1][2] The domes were often surrounded by nodes and/or spikes.

Candidates for the earliest known pachycephalosaur include Ferganocephale adenticulatum from Middle Jurassic Period strata of Kyrgyzstan and Stenopelix valdensis from Early Cretaceous strata of Germany, although R.M. Sullivan has doubted that either of these species are actually pachycephalosaurs.[1]

Paleobiology[edit]

Head-butting behavior[edit]

Restoration of head butting Pachycephalosaurus

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 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 1970 and Sues 1978. In this hypothesis, pachycephalosaurs rammed each other head-on, as do modern-day mountain goats and musk oxen.

Pachycephalosaurus based on the "Sandy" specimen in the Rocky Mountain Dinosaur Resource Center, Woodland Park, Colorado

Anatomical evidence for combative behavior includes vertebral articulations providing spinal rigidity, and the shape of the back indicating strong neck musculature.[3] 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.[4]

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 & Horner 2004 argued against the battering ram hypothesis. They argued that the dome was "an ephemeral ontogenetic stage", the spongy bone structure could not sustain the blows of combat, and the radial pattern was simply an effect of rapid growth.[5] Later biomechanical analyses by Snively & Cox 2008 and Snively & Theodor 2011 concluded, however, that the domes could withstand combat stresses.[3] Lehman 2010 argued that the growth patterns discussed by Goodwin and Horner are not inconsistent with head-butting behavior.[6]

Goodwin & 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.[5] Due to the nature of the fossil record, however, it cannot be observed if color actually played a role in dome function.

Longrich, Sankey & Tanke 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 have sometimes been 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,such as combat, one which was important enough to justify the resource investment.[2]

New findings published in the Journal Plos One on July 16, 2013 further evidence head-butting in species of pachycephalosaurs. Of the 100 domes studied, 20 percent showed signs of healed injuries, all consistent with combative behavior. The pathologies included pitting where the bone had become infected from wounds that originated in the skin. These findings further imply that pachycephalosaurids may have been using their heads for both display and defense as many modern animals do.[7]

Diet[edit]

With respect to their diet, Mallon et al. (2013) examined herbivore coexistence on the island continent of Laramidia, during the Late Cretaceous. It was concluded that pachycephalosaurids were generally restricted to feeding on vegetation at, or below, the height of 1 meter.[8]

Paleopathology[edit]

Peterson et al. (2013) studied cranial pathologies among the Pachycephalosauridae and found that 22% of all domes examined had lesions that are consistent with osteomyelitis, an infection of the bone resulting from penetrating trauma, or trauma to the tissue overlying the skull leading to an infection of the bone tissue. This high rate of pathology lends more support to the hypothesis that pachycephalosaurid domes were employed in intra-specific combat.[9] The frequency of trauma was comparable across the different genera in this family, despite the fact that these genera vary with respect to the size and architecture of their domes, and fact that they existed during varying geologic periods.[9] These findings were in stark contrast with the results from analysis of the relatively flat-headed pachycephalosaurids, where there was an absence of pathology. This would support the hypothesis that these individuals represent either females or juveniles,[10] where intra-specific combat behavior is not expected.

Histological examination reveals that pachycephalosaurid domes are composed of a unique form of fibrolamellar bone[11] which contains fibroblasts that play a critical role in wound healing, and are capable of rapidly depositing bone during remodeling.[12] Peterson et al. (2013) concluded that, taken together, the frequency of lesion distribution and the bone structure of frontoparietal domes, lends strong support to the hypothesis that pachycephalosaurids used their unique cranial structures for agonistic behavior.[9]

Classification[edit]

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.[13]

Taxonomy[edit]

The Pachycephalosauria was first named as a suborder of the Order Ornithischia by Maryańska & Osmólska 1974. They included within it only one family, the Pachycephalosauridae.[14] Later researchers, such as Michael Benton, have ranked it as an infraorder of the Suborder Cerapoda, which unites the ceratopsians and ornithopods.[15] In 2006, Robert Sullivan published a re-evaluation of pachycephalosaur taxonomy. Sullivan considered attempts by Maryańska and Osmólska to restrict the definition of Pachycephalosauria redundant with their Pachycephalosauridae, since they were diagnosed by the same anatomical characters. Sullivan also rejected attempts by Sereno 1986, in his phylogenetic studies,[16] 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.[1]

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

Note that Butler et al., 2011 reassigned Stenopelix and Micropachycephalosaurus to the Ceratopsia.

Note that Microcephale (Mycocephale?) and Foraminacephale haven't been described yet and are not present on this list. 21st century in paleontology 20th century in paleontology 19th century in paleontology 2090s in paleontology 2080s in paleontology 2070s in paleontology 2060s in paleontology 2050s in paleontology 2040s in paleontology 2030s in paleontology 2020s in paleontology 2010s in paleontology 2000s in paleontology 1990s in paleontology 1980s in paleontology 1970s in paleontology 1960s in paleontology 1950s in paleontology 1940s in paleontology 1930s in paleontology 1920s in paleontology 1910s in paleontology 1900s in paleontology 1890s in paleontology 1880s in paleontology 1870s in paleontology 1860s in paleontology 1850s in paleontology 1840s in paleontology 1830s in paleontology 1820s in paleontology Stygimoloch Pachycephalosaurus Dracorex Wannanosaurus Tylocephale Texacephale Stegoceras Sphaerotholus Prenocephale Homalocephale Hanssuesia Gravitholus Goyocephale Colepiocephale Amtocephale Alaskacephale Acrotholus 21st century in paleontology 20th century in paleontology 19th century in paleontology 2090s in paleontology 2080s in paleontology 2070s in paleontology 2060s in paleontology 2050s in paleontology 2040s in paleontology 2030s in paleontology 2020s in paleontology 2010s in paleontology 2000s in paleontology 1990s in paleontology 1980s in paleontology 1970s in paleontology 1960s in paleontology 1950s in paleontology 1940s in paleontology 1930s in paleontology 1920s in paleontology 1910s in paleontology 1900s in paleontology 1890s in paleontology 1880s in paleontology 1870s in paleontology 1860s in paleontology 1850s in paleontology 1840s in paleontology 1830s in paleontology 1820s in paleontology

Phylogeny[edit]

The cladogram presented here follows an analysis by Williamson & Carr 2002.[19]

Pachycephalosauria

Stenopelix



Wannanosaurus



Yaverlandia (now thought to be a theropod instead)


Goyacephala

Goyocephale


unnamed

Homalocephale


Pachycephalosauridae

Stegoceras


Pachycephalosaurinae
unnamed

Tylocephale


unnamed

Sphaerotholus


unnamed

Stygimoloch



Pachycephalosaurus










Cladogram after Longrich, Sankey and Tanke (2010).[20]

 Pachycephalosauridae 


Stegoceras




Colepiocephale



Gravitholus





Texacephale



Hanssuesia




Sphaerotholus? brevis




S. goodwini




S. edmontonensis



S. buchholtzae







Alaskacephale



Pachycephalosaurus




Stygimoloch



Dracorex






Tylocephale



Prenocephale




Homalocephale



Goyocephale



Wannanosaurus







Below is a cladogram modified from Evans et al., 2013.[21]

 Pachycephalosauria 

Wannanosaurus yansiensis


 Pachycephalosauridae 


Colepiocephale lambei



Hanssuesia sternbergi



Stegoceras novomexicanum



Stegoceras validum





Goyocephale lattimorei




Homalocephale calathocercos




Tylocephale gilmorei




"Prenocephale" brevis




Amtocephale gobiensis





Acrotholus audeti



Prenocephale prenes






Alaskacephale gangloffi



Pachycephalosaurus wyomingensis





Sphaerotholus buchholtzae



Sphaerotholus goodwini












Notes[edit]

  1. ^ a b c Sullivan 2006
  2. ^ a b c d e f g Longrich, Sankey & Tanke 2010
  3. ^ a b Snively & Cox 2008
  4. ^ Carpenter 1997
  5. ^ a b Goodwin & Horner 2004
  6. ^ Lehman 2010
  7. ^ NBC News: "Boneheaded dinosaurs butted heads"
  8. ^ Mallon, Jordan C; David C Evans; Michael J Ryan; Jason S Anderson (2013). [tp://link.springer.com/article/10.1186/1472-6785-13-14 "Feeding height stratification among the herbivorous dinosaurs from the Dinosaur Park Formation (upper Campanian) of Alberta, Canada"]. BMC Ecology 13. 
  9. ^ a b c Peterson JE, Dischler C, Longrich NR (2013) Distributions of Cranial Pathologies Provide Evidence for Head-Butting in Dome-Headed Dinosaurs (Pachycephalosauridae). PLoS ONE 8(7): e68620. doi:10.1371/journal.pone.0068620
  10. ^ Longrich NR, Sankey J, Tanke D (2010) Texacephale langstoni, a new genus of pachycephalosaurid (Dinosauria: Ornithischia) from the upper Campanian Aguja Formation, southern Texas, USA. Cretaceous Research 31(2): 274–284.
  11. ^ Reid REH (1997) Histology of bones and teeth. In: Currie, PJ and Padian, K, editors. Encyclopedia of Dinosaurs. Academic Press, San Diego, CA. 329–339.
  12. ^ Horner JR, Goodwin MB (2009) Extreme Cranial Ontogeny in the Upper Cretaceous Dinosaur Pachycephalosaurus . PLoS ONE 4(10): e7626. Available: http://www.plosone.org/article/inf o%3Adoi%2F10.1371%2Fjournal.pone. 0007626. Accessed 2012 Dec 4.
  13. ^ Horner & Goodwin 2009
  14. ^ Maryańska & Osmólska 1974
  15. ^ Benton 2004, pp. 472
  16. ^ Sereno 1986
  17. ^ Watabe, Tsogtbaatar & Sullivan 2011
  18. ^ a b Stokstad 2007
  19. ^ Williamson & Carr 2002
  20. ^ Longrich, N.R., Sankey, J., and Tanke, D. (2010). "Texacephale langstoni, a new genus of pachycephalosaurid (Dinosauria: Ornithischia) from the upper Campanian Aguja Formation, southern Texas, USA". Cretaceous Research 31 (2): 274–284. doi:10.1016/j.cretres.2009.12.002. 
  21. ^ Evans, D. C.; Schott, R. K.; Larson, D. W.; Brown, C. M.; Ryan, M. J. (2013). "The oldest North American pachycephalosaurid and the hidden diversity of small-bodied ornithischian dinosaurs". Nature Communications 4: 1828. doi:10.1038/ncomms2749.  edit

References[edit]

External links[edit]