Pachycephalosaurus
Pachycephalosaurus | |
---|---|
Cast of the "Sandy" specimen, Royal Ontario Museum | |
Scientific classification | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Phylum: | Chordata |
Clade: | Dinosauria |
Clade: | †Ornithischia |
Clade: | †Pachycephalosauria |
Family: | †Pachycephalosauridae |
Tribe: | † Pachycephalosaurini
|
Genus: | †Pachycephalosaurus Brown & Schlaikjer, 1943 |
Type species | |
†Pachycephalosaurus grangeri Brown & Schlaikjer, 1943
( Junior synonym of P. wyomingensis) | |
Species | |
| |
Synonyms | |
Synonyms of P. wyomingensis
|
Pachycephalosaurus (
Pachycephalosaurus was among the last species of non-avian dinosaurs on Earth before the Cretaceous–Paleogene extinction event. The genus Tylosteus has been synonymized with Pachycephalosaurus, as have the genera Stygimoloch and Dracorex, in recent studies.[4][5]
Like other pachycephalosaurids, Pachycephalosaurus was a
History of discovery
Remains attributable to Pachycephalosaurus may have been found as early as the 1850s. As determined by
In 1890, during the Bone Wars between Othniel Charles Marsh and Edward Drinker Cope, one of Marsh's collectors, John Bell Hatcher, collected a partial left squamosal (YPM VP 335) later referred to Stygimoloch spinifer near Lance Creek, Wyoming in the Lance Formation.[12][13][14] Marsh described the squamosal along with the dermal armor of Denversaurus as the body armor of Triceratops in 1892, believing that the squamosal was a spike akin to the plates on Stegosaurus.[14] The squamosal spike was even featured in Charles Knight’s painting of Cope’s ceratopsid Agathaumas, likely based on Marsh’s hypothesis.[13] Marsh also named a species of now-dubious ankylosaur Palaeoscincus in 1892 based on a single tooth (YPM 4810), also collected by Hatcher from the Lance.[15] The tooth was named Palaeoscinus latus, but in 1990, Coombs found the tooth to be from a pachycephalosaurid, possibly even Pachycephalosaurus itself.[16] Hatcher also collected several additional teeth and skull fragments while working for Marsh, though these have yet to be described.[17][18][19]
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.[20] 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 which was not corrected until 1945 by Charles M. Sternberg.[21]
In 1943, Barnum Brown and Erich Maren Schlaikjer, with newer, more complete material, established the genus Pachycephalosaurus. They named two species: Pachycephalosaurus grangeri, the type species of their new genus, 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 DMNS 469, a dome and a few associated elements from the Lance Formation of Corson County, South Dakota.[22] They also referred the older species "Troodon" wyomingensis to their new genus. Their two newer species have been considered synonymous with P. wyomingensis since 1983.[23]
In 2015, some pachycephalosaurid material and a domed parietal attributable to Pachycephalosaurus were discovered in the Scollard Formation of Alberta, implying that the dinosaurs of this era were cosmopolitan and did not have discrete faunal provinces.[24]
Description
The anatomy of Pachycephalosaurus itself is poorly known, as only skull remains have been described.[10] Pachycephalosaurus is famous for having a large, bony dome on top of its skull, up to 25 cm (10 in) thick, which safely cushioned its brain. The dome's rear aspect was edged with bony knobs and short bony spikes projected upwards from the snout. However, the spikes were probably blunted, not sharp.[25]
The skull was short and possessed large, rounded eye sockets that faced forward, suggesting that the animal had binocular vision. Pachycephalosaurus had a small muzzle that ended in a pointed beak. The teeth were tiny, with leaf-shaped crowns. The head was supported by an S- or U-shaped neck.[25] Younger individuals of Pachycephalosaurus might have had flatter skulls and larger horns projecting from the back of the skull. As the animal grew, the horns shrunk and rounded out as the dome grew.[4][5]
Pachycephalosaurus was
Classification
Pachycephalosaurus gives its name to
Pachycephalosaurus is the most famous member of Pachycephalosauria, even if it is not the best-preserved member. The clade also includes
In 2010, Gregory S. Paul proposed that, while Stygimoloch and Dracorex possibly represent different growth stages of Pachycephalosaurus, Stygimoloch might represent a different species, P. spinifer.[27] This idea has been regarded as a way of interpretation by Mark Witton and Thomas Holtz.[31][32] A phylogenetic analysis from 2021 by Evans and colleagues accepted the validity of the genus Stygimoloch on the basis of it being found in later rock layers than Pachycephalosaurus, but agreed with the consensus that Dracorex represents an ontogimorph instead of a distinct taxon.[33] However, David Evans himself noted in a Twitter post that he and his colleagues would also consider Stygimoloch as P. spinifer.[34]
Below is a cladogram modified from Evans et al., 2013.[35]
Pachycephalosauria |
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Below is a cladogram from Evans et al., 2021.[33]
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Paleobiology
Growth
Aside from Pachycephalosaurus itself, two other pachycephalosaurs were described from the latest Cretaceous of the northwestern United States: Stygimoloch spinifer ("thorny
The discovery of baby skulls assigned to Pachycephalosaurus that were described in 2016 from two different bone beds in the Hell Creek Formation has been presented as further evidence for this hypothesis. The fossils, as described by David Evans and Mark Goodwin et al are identical to all three supposed genera in the placement of the rugose knobs on their skulls, and the unique features of Stygimoloch and Dracorex are thus instead morphologically consistent features on a Pachycephalosaurus growth curve.[5]
It has been noted that morphological differences between Stygimoloch and Pachycephalosaurus may also partly be due to slight stratigraphic differences. The few Stygimoloch specimens that have reliable stratigraphic data were all collected from the upper part of the Hell Creek Formation, whereas Pachycephalosaurus morphs were all collected from the lower part.[39] This has also led to suggestions that Stygimoloch might represent its own species, P. spinifer.[27][31][32] In their 2021 redescription of Sinocephale bexelli, Evans and his colleageues treated Stygimoloch (but not Dracorex) as a separate taxon based on their phylogenetic analysis.[33] However, Evans himself has noted that he and his colleagues support the idea of P. spinifer.[34]
Dome function
It has been widely hypothesized for decades that Pachycephalosaurus and its relatives were the ancient, bipedal equivalents of
The primary argument that has been raised against head-butting is that the skull roof may not have adequately sustained impact associated with ramming, as well as a lack of definitive evidence of scars or other damage on fossilized Pachycephalosaurus skulls. However, more recent analyses have uncovered such damage (see below).[40][41] 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 that it might have been unfit for transmitting stress from direct head-butting. Lastly, the rounded shape of the skull would lessen the contacted surface area during head-butting, resulting in glancing blows.[25]
Alternatively, Pachycephalosaurus and other pachycephalosaurids may have engaged in flank-butting during 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 on its flank. This hypothesis is supported by the relatively broad torso of most pachycephalosaurs, which would have protected vital organs from trauma. The flank-butting theory was first proposed by Sues in 1978 and expanded upon by Ken Carpenter in 1997.[25]
In
Histological examination reveals that pachycephalosaurid domes are composed of a unique form of fibrolamellar bone
Diet
Scientists do not yet know what these dinosaurs ate. Having very small, ridged teeth, they could not have chewed tough, fibrous plants like flowering shrubs as effectively as other dinosaurs of the same period. It is assumed that pachycephalosaurs lived on a mixed diet of leaves, seeds, and fruits. The sharp, serrated teeth would have been very effective for shredding plants.[47][48] It has also been suspected to a degree that it may have included meat in its diet. The most complete fossil jaw shows that it had serrated blade-like front teeth, reminiscent of those of carnivorous theropods.[49]
Paleoecology
Nearly all Pachycephalosaurus fossils have been recovered from the
See also
References
- JSTOR 4523163. Retrieved November 13, 2020.
- ^ "Definition of pachycephalosaurus | Dictionary.com". www.dictionary.com. Retrieved February 22, 2020.
- ^ Colbert, Edwin H. (Edwin Harris); Knight, Charles Robert (1951). The dinosaur book: the ruling reptiles and their relatives. New York: McGraw-Hill. p. 152.
- ^ PMID 19859556.
- ^ S2CID 131282984.
- ^ a b Baird, Donald (1979). "The dome-headed dinosaur Tylosteus ornatus Leidy 1872 (Reptilia: Ornithischia: Pachycephalosauridae)". Notulae Naturae. 456: 1–11.
- ^ Leidy, Joseph (1872). "Remarks on some extinct vertebrates". Proceedings of the Academy of Natural Sciences of Philadelphia: 38–40.
- ^ ICZN Opinion 1371, "Pachycephalosaurus Brown & Schlaikjer, 1943 and Troodon wyomingensis Gilmore, 1931 (Reptilia, Dinosauria): Conserved." Bulletin of Zoological Nomenclature, 43 (1): April 1986.
- ISBN 978-0-89950-917-4.
- ^ a b c Sullivan, Robert M. (2006). "A taxonomic review of the Pachycephalosauridae (Dinosauria:Ornithischia)" (PDF). Late Cretaceous Vertebrates from the Western Interior. New Mexico Museum of Natural History and Science Bulletin. 35: 347–366. Retrieved November 10, 2010.
- ^ S2CID 36443204.
- .
- ^ a b Greenfield, Tyler (December 8, 2020). "Armor for Agathaumas". Incertae Sedis. Retrieved 2022-03-20.
- ^ a b Marsh, O. C. (1891). I.—The Gigantic Ceratopsidæ, or Horned Dinosaurs, of North America 1. Geological Magazine, 8(5), 193-199.
- S2CID 130167326.
- ISBN 0-521-36672-0
- ^ "Paleobiology Collections Search". collections.nmnh.si.edu. Retrieved April 21, 2022.
- ^ "Paleobiology Collections Search". collections.nmnh.si.edu. Retrieved April 21, 2022.
- ^ Gilmore, Charles W. "Pachycephalosaurus wyomingensis; YPM VP 003272; North America; USA; Wyoming; Niobrara County". collections.peabody.yale.edu. Retrieved April 21, 2022.
- .
- ISBN 978-0-89950-917-4.
- ^ Brown, Barnum; Schlaikjer, Erich M. (1943). "A study of the troödont dinosaurs with the description of a new genus and four new species" (PDF). Bulletin of the American Museum of Natural History. 82 (5): 115–150.
- doi:10.1139/e83-043.
- S2CID 129253322.
- ^ a b c d Carpenter, Kenneth (December 1, 1997). "Agonistic behavior in pachycephalosaurs (Ornithischia: Dinosauria): a new look at head-butting behavior" (pdf). Contributions to Geology. 32 (1): 19–25.
- ^ Black, Riley (October 28, 2009). ""Bone-Headed" Dinosaurs Reshaped Their Skulls". Smithsonian Magazine. Retrieved February 11, 2023.
- ^ ISBN 978-0-691-13720-9.
- PMID 24802911.
- S2CID 85983799. Archived from the original(PDF) on August 29, 2008. Retrieved June 10, 2008.
- PMID 12028774.
- ^ a b Witton, Mark [@MarkWitton] (July 26, 2018). "Not quite: Pachycephalosaurus and Stygimoloch are both genera, each with their own species (P. wyomingensis and S. spinifer). There's discussion over whether S. spinifer is the same as P. wyomingensis or, at least, should be considered P. spinifer" (Tweet) – via Twitter.
- ^ a b Holtz, Thomas R. Jr. [@TomHoltzPaleo] (July 29, 2020). "Dracorex definitely seems to be a juvenile. But new work is finding that the classic Stygimoloch specimens are from high in the Hell Creek, and classic Pachycephalosaurus are low. So Stygimoloch (or Pachycephalosaurus) spinifer might be the descendant of P. wyomingensis" (Tweet) – via Twitter.
- ^ S2CID 244227050.
- ^ a b Evans, David [@DavidEvans_ROM] (November 14, 2021). "I think we said in an SVP talk that we'd consider Stygi a species of Pachycephalosaurus - P. spinifer. HSnt been peer reviewed yet, but that's where the data are leaning in my opinion" (Tweet) – via Twitter.
- PMID 23652016.
- ^ Erik Stokstad,"SOCIETY OF VERTEBRATE PALEONTOLOGY MEETING: Did Horny Young Dinosaurs Cause Illusion of Separate Species?", Science Vol. 18, November 23, 2007, p. 1236; http://www.sciencemag.org/cgi/content/full/318/5854/1236
- ^ a b Horner J.R. and Goodwin, M.B. (2009). "Extreme cranial ontogeny in the Upper Cretaceous Dinosaur Pachycephalosaurus." PLoS ONE, 4(10): e7626. Online full text Archived May 1, 2011, at the Wayback Machine
- .
- PMID 29166406.
- S2CID 84961066.
- ^ PMID 22558394.
- ^ PMID 23874691.
- .
- ^ eid REH (1997) Histology of bones and teeth. In: Currie, PJ and Padian, K, editors. Encyclopedia of Dinosaurs. Academic Press, San Diego, CA. 329–339.
- ^ 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. Retrieved December 4, 2012.
- PMID 21738658.
- ISBN 978-0-520-24209-8.
- ^ "The Real Pachycephalosaurus". Philip J. Currie Dinosaur Museum. February 25, 2020. Retrieved August 18, 2021.
- ^ "Vegetarian dinosaur may have actually eaten meat, skull suggests". Science & Innovation. October 24, 2018. Archived from the original on November 8, 2018. Retrieved May 7, 2019.
- ISBN 0-520-24209-2.
External links
- Pachycephalosaurus in the Dinodictionary Archived May 10, 2012, at the Wayback Machine
- Pachycephalosaurus wyomingensis from National Geographic Online
- TEDx talk by Jack Horner on shape-shifting dinosaur skulls and dinosaur misclassification.
- Data related to Pachycephalosaurus at Wikispecies
- Media related to Pachycephalosaurus at Wikimedia Commons