Pachycephalosaurus

*Taxobox
name = "Pachycephalosaurus"
fossil_range = Late Cretaceous


image_caption = Skull of "Pachycephalosaurus" at the Oxford University Museum of Natural History.
regnum = Animalia
phylum = Chordata
classis = Sauropsida
superordo = Dinosauria
ordo = Ornithischia
infraordo=Pachycephalosauria
familia = Pachycephalosauridae
genus = "Pachycephalosaurus"
genus_authority = Brown & Schlaikjer, 1943
subdivision_ranks = Species
subdivision =
*"P. wyomingensis" (Gilmore, 1931 [originally "Troodon" "wyomingensis"] ) (type)
synonyms =
*"Tylosteus" Leidy, 1872

"Pachycephalosaurus" (pronEng|ˌpækiˌsɛfələˈsɔrəs, meaning "thick headed lizard", from Greek "pachy-/παχυ-" "thick", "cephale/κεφαλη" "head" and "saurus/σαυρος" "lizard") is a genus of pachycephalosaurid dinosaur. It lived during the Late Cretaceous Period (Maastrichtian stage) of what is now North America. Remains have been excavated in Montana, South Dakota, and Wyoming. It was an herbivorous or omnivorous creature which is only known from a single skull and a few extremely thick skull roofs. This dinosaur is monotypic, meaning the type species, "P. wyomingensis", is the only known species. "Pachycephalosaurus" was one of the last non-avian dinosaurs before the K–T extinction event. Another dinosaur, "Tylosteus" of western North America, has been synonymized with "Pachycephalosaurus".

Like other pachycephalosaurids, "Pachycephalosaurus" was a bipedal omnivore with an extremely thick skull roof. It possessed long hindlimbs and small forelimbs. "Pachycephalosaurus" is the largest known pachycephalosaur, measuring nearly 4.5–5 meters (15 –16 ft) in length and up to 2000 kilograms (4400 lb) in weight.

The thick skull domes of "Pachycephalosaurus" and related genera gave rise to the theory that pachycephalosaurs used their skulls in intraspecific combat. This theory has been discredited in recent years.

Description

The anatomy of "Pachycephalosaurus" is poorly known, as only skull remains have been described.cite journal| last =Sullivan| first =Robert M.| authorlink =| coauthors =| title =A taxonomic review of the Pachycephalosauridae (Dinosauria:Ornithischia) | journal =Late Cretaceous vertebrates from the Western Interior. New Mexico Museum of Natural History and Science Bulletin| volume =35| issue =| pages =347–366| publisher =| location =| date =2006| url =http://www.nmnaturalhistory.org/science/bulletins/35/sci_bulletin35_43.pdf| doi =| id =| accessdate =2007-11-15] "Pachycephalosaurus" is famous for having a large, bony dome atop its skull, up to 25 cm (10 in) thick, which safely cushioned its tiny brain. The dome's rear aspect was edged with bony knobs and short bony spikes projected upwards from the snout. The spikes were probably blunt, not sharp.cite journal |last=Carpenter |first=Kenneth |authorlink=Kenneth Carpenter |year=1997 |url=http://rmg.geoscienceworld.org/cgi/content/abstract/32/1/19 |title=Agonistic behavior in pachycephalosaurs (Ornithischia: Dinosauria): a new look at head-butting behavior |journal=Contributions to Geology |volume=32 |issue=1 |pages=19–25 |format=pdf]

The skull was short, and possessed large, rounded eye sockets that faced forward, suggesting that the animal had good vision and was capable of binocular vision. "Pachycephalosaurus" had a small muzzle which ended in a pointed beak. The teeth were tiny, with leaf-shaped crowns. The head was supported by an "S"- or "U"-shaped neck.

"Pachycephalosaurus" was probably bipedal and was the largest of the pachycephalosaurid (bone-headed) dinosaurs. Using data from other pachycephalosaurids, it has been estimated that "Pachycephalosaurus" was approximately the length of a large car, perhaps around 4.6 meters (15 ft) long,cite journal| last = Montague| first = R.| title = Estimates of body size and geological time of origin for 612 dinosaur genera (Saurischia, Ornithischia)| journal = Florida Scientist| volume = 69| issue = 4| pages = 243–257| publisher = | location = | date = 2006| url = http://apt.allenpress.com/perlserv/?request=get-abstract&doi=10.1043%2F0098-4590%282006%29069%5B0243%3AEOBSAG%5D2.0.CO%3B2| doi = | id = | accessdate = 2008-06-09| year = 2006] and had a fairly short, thick neck, short fore limbs, a bulky body, long hind legs and a heavy tail, which was likely held rigid by ossified tendons.cite journal| last = Organ| first = Christopher O.| coauthors = Adams, Jason| title = The histology of ossified tendon in dinosaurs | journal = Journal of Vertebrate Paleontology| volume = 25| issue = 3| pages = 602–613| publisher = | location = | date = 2005| url = http://www.oeb.harvard.edu/faculty/edwards/people/postdocs/documents/JVP_25_3_602-613.pdf| doi = | id = | accessdate = 2008-06-10]

Classification and systematics

clade| style=font-size:75%;line-height:75%
label1=Pachycephalosauria
1=clade
1="Stenopelix"
2="Wannanosaurus"
3="Yaverlandia"
label4=unnamed
4=clade
1="Goyocephale"
label2=unnamed
2=clade
1="Homalocephale"
label2=Pachycephalosauridae
2=clade
1="Stegoceras"
label2=Pachycephalosaurinae
2=clade
label1=unnamed
1=clade
1="Tylocephale"
2="Prenocephale"
label2= unnamed
2=clade
1="Sphaerotholus"
label2= unnamed
2=clade
1="Stygimoloch"
2="Pachycephalosaurus"
The cladogram presented here follows an analysis by Williamson and Carr, 2002.cite journal|last=Williamson|first=T.E.|coauthors=Carr, T.D.|date=2002|title=A new genus of derived pachycephalosaurian from western North America|journal=Journal of Vertebrate Paleontology|volume=22|issue=4|pages=779–801|doi=10.1671/0272-4634(2002)022 [0779:ANGODP] 2.0.CO;2|year=2002] One genus listed above, "Yaverlandia", is no longer considered a pachycephalosaurian.cite journal |last=Naish |first=Darren |coauthors=and Martill, David M. |year=2008 |title=Dinosaurs of Great Britain and the role of the Geological Society of London in their discovery: Ornithischia |journal=Journal of the Geological Society, London |volume=165 |pages=613-623]
"Pachycephalosaurus" gives its name to the Pachycephalosauria, a clade of herbivorous ornithischian ("bird hipped") dinosaurs which lived during the Late Cretaceous Period in North America and Asia. Despite their bipedal stance, they were likely more closely related to the ceratopsians than the ornithopods.cite journal| last = Pisani| first = Davide| coauthors = Yates, Adam M., Langer, Max C. and Benton, Michael J.| title = A genus-level supertree of the Dinosauria| journal = Proceedings of the Royal Society B Biological Sciences| volume = 269| issue = 1494| pages = 915–921| publisher = | location = | date = 2002| url = http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1690971| doi =| id = | accessdate = 2008-06-09| pmid = 12028774]

"Pachycephalosaurus" is the most famous member of the Pachycephalosauria (though not the best-preserved member). The clade also includes "Stenopelix", "Wannanosaurus", "Goyocephale", "Stegoceras", "Homalocephale", "Tylocephale", "Sphaerotholus" and "Prenocephale". Within the tribe Pachycephalosaurini, "Pachycephalosaurus" is most closely related to "Dracorex" and "Stygimoloch", although these may be juvenile forms of "Pachycephalosaurus".

Discovery and history

Remains attributable to "Pachycephalosaurus" may have been found as early as the 1850s. As determined by Donald Baird, in 1859 or 1860 Ferdinand Vandiveer Hayden, an early fossil collector in the North American West, collected a bone fragment in the vicinity of the head of the Missouri River, from what is now known to be the Lance Formation in southeastern Montana.cite journal |last=Baird |first=Donald |year=1979 |title=The dome-headed dinosaur "Tylosteus ornatus" Leidy 1872 (Reptilia: Ornithischia: Pachycephalosauridae) |journal=Notulae Naturae |volume=456 |pages=1–11] This specimen, now ANSP 8568, was described by Joseph Leidy in 1872 as belonging to the dermal armor of a reptile or an armadillo-like animal.cite journal |last=Leidy |first=Joseph |authorlink=Joseph Leidy |year=1872 |title=Remarks on some extinct vertebrates |journal=Proceedings of the Academy of Natural Sciences of Philadelphia, 1872 |pages=38–40] Its actual nature was not found until Baird restudied it over a century later and identified it as a squamosal (bone from the back of the skull) of "Pachycephalosaurus", including a set of bony knobs corresponding to those found on other specimens of "Pachycephalosaurus". Because the name "Tylosteus" predates "Pachycephalosaurus", according to the International Code of Zoological Nomenclature it should be preferred. Baird successfully petitioned to have "Pachycephalosaurus" used instead of older "Tylosteus" because the latter name had not been used for over fifty years, was based on undiagnostic materials, and had poor geographic and stratigraphic information.cite book|chapter=Pachycephalosaurus |last=Glut |first=Donald F. |authorlink=Donald F. Glut |title=Dinosaurs: The Encyclopedia |year=1997 |publisher=McFarland & Co |location=Jefferson, North Carolina |pages=664–668 |isbn=0-89950-917-7] This may not be the end of the story; Robert Sullivan suggested in 2006 that ANSP 8568 is more like the corresponding bone of "Dracorex" than that of "Pachycephalosaurus".cite journal| last =Sullivan| first =Robert M.| authorlink =| coauthors =| title =A taxonomic review of the Pachycephalosauridae (Dinosauria:Ornithischia) | journal =Late Cretaceous vertebrates from the Western Interior. New Mexico Museum of Natural History and Science Bulletin| volume =35| issue =| pages =347–366| publisher =| location =| date =2006| url =http://www.nmnaturalhistory.org/science/bulletins/35/sci_bulletin35_43.pdf| doi =| id =| accessdate =2007-11-15] The issue is of uncertain importance, though, if "Dracorex" actually represents a juvenile "Pachycephalosaurus", as has been recently proposed.cite journal |last=Stokstad |first=Erik |year=2007 |title=SOCIETY OF VERTEBRATE PALEONTOLOGY MEETING: Did Horny Young Dinosaurs Cause Illusion of Separate Species? |journal=Science |volume=18 |pages=1236 |url=http://www.sciencemag.org/cgi/content/full/318/5854/1236 |doi=10.1126/science.318.5854.1236] "P. wyomingensis", the type and currently only valid species of "Pachycephalosaurus", was named by Charles W. Gilmore in 1931. He coined it for the partial skull USNM 12031, from the Lance Formation of Niobrara County, Wyoming. Gilmore assigned his new species to "Troodon" as "T. wyomingensis".cite journal |last=Gilmore |first=Charles W. |year=1931 |title=A new species of troodont dinosaur from the Lance Formation of Wyoming |journal=Proceedings of the United States National Museum |volume=79 |issue=9 |pages=1–6] At the time, paleontologists thought that "Troodon", then known only from teeth, was the same as "Stegoceras", which had similar teeth. Accordingly, what are now known as pachycephalosaurids were assigned to the family Troodontidae, a misconception not corrected until 1945, by Charles M. Sternberg.cite book|chapter=Troodon |last=Glut |first=Donald F. |authorlink=Donald F. Glut |title=Dinosaurs: The Encyclopedia |year=1997 |publisher=McFarland & Co |location=Jefferson, North Carolina |pages=933–938 |isbn=0-89950-917-7]

In 1943, Barnum Brown and Erich Maren Schlaikjer, with newer, more complete material, established the genus "Pachycephalosaurus" and made "T. wyomingensis" its type species. They also named two more species: "Pachycephalosaurus grangeri" and "Pachycephalosaurus reinheimeri". "P. grangeri" was based on AMNH 1696, a nearly complete skull from the Hell Creek Formation of Ekalaka, Carter County, Montana. "P. reinheimeri" was based on what is now DMNH 469, a dome and a few associated elements from the Lance Formation of Corson County, South Dakota.cite journal |last=Brown |first=Barnum |authorlink=Barnum Brown |coauthors=and Schlaikjer, Erich M. |year=1943 |title=A study of the troödont dinosaurs with the description of a new genus and four new species |journal=Bulletin of the American Museum of Natural History |volume=82 |issue=5 |pages=115–150 |url=http://digitallibrary.amnh.org/dspace/bitstream/2246/387/1/B082a05.pdf |format=pdf] These later two species have been considered synonymous with "P. wyomingensis" since 1983.cite journal |last=Galton |first=Peter M. |authorlink=Peter Galton |coauthors=and Sues, Hans-Dieter |year=1983 |title=New data on pachycephalosaurid dinosaurs (Reptilia: Ornithischia) from North America |journal=Canadian Journal of Earth Sciences |volume=25 |pages=462–472]

At the 2007 annual meeting of the Society of Vertebrate Paleontology, Jack Horner of Montana State University presented evidence that "Dracorex" and "Stygimoloch" may be juvenile forms of "Pachycephalosaurus".

Paleobiology

Nearly all "Pachycephalosaurus" fossils have been recovered from the Lance Formation and Hell Creek Formation of the western United States. "Pachycephalosaurus" co-existed alongside fellow pachycephalosaurs "Dracorex" and "Stygimoloch".citation| last =Bakker| first =Robert T.| first2 =Sullivan| last2 =Robert M.; Porter, Victor; Larson, Peter; and Saulsbury, Steven J.| authorlink =| coauthors =| editor-last =Lucas,| editor-first =S. G. and Sullivan, R.M.| title ="Dracorex hogwartsia", n. gen., n. sp., a spiked, flat-headed pachycephalosaurid dinosaur from the Upper Cretaceous Hell Creek Formation of South Dakota| journal =Late Cretaceous vertebrates from the Western Interior. New Mexico Museum of Natural History and Science Bulletin| volume =35| issue =| pages =331–346| publisher =| location =| date =2006| url =http://www.childrensmuseum.org/dinosphere/draco_rex/dracorex_hogwartsia.pdf| doi =| id =| accessdate =2007-11-12] Other dinosaurs that shared its time and place include "Bugenasaura", the hadrosaurids "Edmontosaurus" and "Anatotitan", ceratopsids "Triceratops" and "Torosaurus", ankylosaurid "Ankylosaurus", and the theropods "Ornithomimus", "Dromaeosaurus", and "Tyrannosaurus".Weishampel, David B.; Barrett, Paul M.; Coria, Rodolfo A.; Le Loeuff, Jean; Xu Xing; Zhao Xijin; Sahni, Ashok; Gomani, Elizabeth, M.P.; and Noto, Christopher R. (2004). "Dinosaur Distribution". In: D.B. Weishampel, P. Dodson, and H. Osmólska (eds.) "The Dinosauria" (2nd edition). 517–606. ISBN 0-520-24209-2.]

Scientists once suspected that "Pachycephalosaurus" and its relatives were the bipedal equivalents of bighorn sheep or musk oxen; that male individuals would ram each other headlong. It was also believed that they would make their head, neck, and body horizontally straight, in order to transmit stress during ramming. However, it is now believed that the pachycephalosaurs would not have used their domes in this way.Foremost, the skull roof could not have adequately sustained impact associated with such ramming. Also, there is no evidence of scars or other damage on fossilized "Pachycephalosaurus" skulls.cite journal |last=Goodwin |first=Mark |coauthors=and Horner, John R. |year=2004 |title=Cranial histology of pachycephalosaurs (Ornithischia: Marginocephalia) reveals transitory structures inconsistent with head-butting behavior |journal=Paleobiology |volume=30 |issue=2 |pages=253–267 |doi=10.1666/0094-8373(2004)030<0253:CHOPOM>2.0.CO;2] Furthermore, the cervical and anterior dorsal vertebrae show that the neck was carried in an "S"- or "U"-shaped curve, rather than a straight orientation, and thus unfit for direct head-butting. Lastly, the rounded shape of the skull would lessen the contacted surface area during head-butting, resulting in glancing blows.

It is more probable that the "Pachycephalosaurus" and other pachycephalosaurid genera engaged in flank-butting in intraspecific combat. In this scenario, an individual may have stood roughly parallel or faced a rival directly, using intimidation displays to cow its rival. If intimidation failed, the "Pachycephalosaurus" would bend its head downward and to the side, striking the rival pachycephalosaur on its flank. This hypothesis is supported by the relatively broad width of most pachycephalosaurs, a trait that would have protected vital organs from harm. The flank-butting theory was first proposed by Sues in 1978, and expanded upon by Ken Carpenter in 1997.

Diet

Scientists do not yet know what these dinosaurs ate. Having very small, ridged teeth they could not have chewed tough, fibrous plants as effectively as other dinosaurs of the same period. It is assumed that pachycephalosaurs lived on a mixed diet of leaves, seeds, fruit and insects. The sharp, serrated teeth would have been very effective for shredding plants.cite book |last=Maryańska |first=Teresa |coauthors=Chapman, Ralph E.; and Weishampel, David B. |editor=Weishampel, David B.; Dodson, Peter; and Osmólska, Halszka (eds.)|title=The Dinosauria |edition=2nd |year=2004 |publisher=University of California Press |location=Berkeley |isbn=0-520-24209-2 |chapter=Pachycephalosauria |pages=464–477]

References

External links

* [http://www.dinodictionary.com/dinos_p.asp#PACHYCEPHALOSAURUS "Pachycephalosaurus" in the Dinodictionary]
* [http://animals.nationalgeographic.com/animals/prehistoric/pachycephalosaurus-wyomingensis.html "Pachycephalosaurus wyomingensis"] from "National Geographic" Online