Pachycephalosaurus

Credit: Futaka Kouhei‬.

*Pachycephalosaurus* is an extinct genus of pachycephalosaurid that lived during the Late Cretaceous in North America.

## History

*Pachycephalosaurus* was found as early as the 1850s. Donald Baird found Ferdinand Vandeveer (1859 or 1860) found a cranial fragment in the head Missouri River, Lance Formation, now called ANSP 8568, and described by Joseph Leidy (1872) as the dermal armour for an armadillo-like animal, which was named *Tylosteus*. Baird, over a century later, found it was actually the squamosal of a dinosaur, since the knobs matched other *Pachycephalosaurus*. Since its first name, *Tylosteus*, would generally be preferred by the International Code of Zoological Nomenclature, Baird (1985) petitioned to conserve *Pachycephalosaurus* since the first name had not been used in 50+ years, has poor geographic and stratigraphic data and is named from undiagnostic material. However, Robert Sullivan (2006) suggested ANSP 8568 is more akin to Dracorex, but it has also been proposed the latter is a growth stage of *Pachycephalosaurus*. *P. wyomingensis* was named by Charles W. Gilmore (1931) from partial skull USMN 12031 of the Lance Formation. Gilmore, however, assigned it to *Troodon*, since *Stegoceras* was also lumped at that time. Charles M. Sternberg (1945) un-assigned all pachycephalosaurids from Troodontidae, correcting the misconception.

Barnum Brown and Erich Maren Schlaikjer (1943) described newer and more complete material to erect *Pachycephalosaurus*, naming *P. grangeri*, *P. wyomingensis* and *P. reinheimeri*. *P. grangeri* is from near-complete skull AMNH 1669 of the Hell Creek Formation. *P. reinheimeri* is from dome and elements DMNS 469 from the Lance Formation. *P. wyomingensis* was reassigned from *Troodon*. Since 1983, the other two species are synonymized with *P. wyomingensis*. Pachycephalosaurid material found in 2015, a domed parietal referable to a juvenile, were found in the Scollard Formation. This indicates dinosaurs of this age were cosmopolitan, lacking a discrete faunal province.

"P. spinifer" is an informal name used informally by paleontology enthusiasts to refer to *Stygimoloch*, as some believe remains assigned to that genus may represent a distinct *Pachycephalosaurus* species. This, however, is unconfirmed but has been academically mentioned .

## Description

*Pachycephalosaurus* is relatively poorly known, since only crania have been recovered. The large dome atop said cranium is up to 25 centimeters (10 inches) thick, protecting the brain. The posterior of the dome is studded with bony knobs, with blunt, dull, short spikes on the snout. It has a short skull with large round orbits that orient forwards, indicating good sight and binocular vision. Its short rostrum ends in a pointy beak and jaws filled with tiny teeth with leaf-like crowns. The head is supported by a U or S-shaped neck.

Juveniles may have had flatter skulls and larger horns. It was bipedal, with a somewhat short, thick neck, short forelimbs, a robust body, long hind limbs and a stocky tail. The tail was probably supported by tendons that are thick and ossified. Many children's books oversize *Pachycephalosaurus* to the size of contemporaneous megatheropods like *Tyrannosaurus*. However, it is unknown where this size started and why.

## Classification

*Pachycephalosaurus* is the namesake for Pachycephalosauria, Pachycephalosauridae and Pachycephalosaurini, of which it is classified. Inside the latter, it is most related to *Alaskacephale*. Evans et al. (2013) find:

Upon the naming of *Sinocephale* in 2021, the authors mention *Stygimoloch* as a separate taxon from *Pachycephalosaurus* due to biostratigraphic data. However, *Dracorex* was excluded on the basis that it is immature growth stage, but of what taxon was left unclear.

## Paleobiology

### Ontogeny

*Dracorex* and *Stygimoloch* are proposed juvenile stages or female versions of Pachycephalosaurus. Jack Horner (2007) at the annual meeting of the Society of Vertebrate Paleontology prepared evidence from analyzed skulls. Horner and M.B. Goodwin (2009) published this, finding the spikes, nodes and domes have extreme plasticity, with *Dracorex* and *Stygimoloch* known only from juvenile specimens while all Pachycephalosaurus are adult. They conclude they are ontogenetic stages of *Pachycephalosaurus*, losing spike length and growing their domes as time passed. Nick Longrich (2010) supported this, finding a similar case in Goyocephale and *Homalocephale*. Baby *Pachycephalosaurus* skulls referred in 2016 were used to support the hypothesis, described by David Evans and Mark Goodwin et al., being identical to suspect genera, fitting *Pachycephalosaurus*' growth curve perfectly. Differences between *Stygimoloch* and *Pachycephalosaurus* are may be due to stratigraphic differences, since few *Stygimoloch* specimens have reliable stratigraphic data. All that is known is that *Stygimoloch* are produced from the Upper and *Pachycephalosaurus* from the Lower Hell Creek.

### Dome

It is hypothesized *Pachycephalosaurus* and related genera are equivalent to bighorn sheep or the muskox, with males butting domes headlong. It is thought they would straighten their neck and body to produce stress; however, some think pachycephalosaurs used their domes in other ways. The primary argument against this is that their domes may not have been able to sustain such a force, and most crania lack the damage they would have sustained from this act. The cervicals and anterior dorsals show that neck was S or U-shaped rather than straight, an unfit position for head ramming, The rounded skull would lessen contact surface, resulting in glancing blows. However, it has been proposed they may have stood parallel or directly to a rival, intimidating them. It has been proposed that if this faild, they could strike their rival on the flank rather than the skull. This is supported by the robust torso which may have protected organs from trauma. Sues (1978) first proposed this, with Kenneth Carpenter (1997) expanding upon this.

One 2012 study finds the cranial pathologies were likely caused by fighting, with damage in other pachycephalosaurs previously explained as taphonomic or bone absorption as the same. Peterson et al. (2013) found 22% of all domes bear lesions consistent with osteomyelitis This rate lends support to the intraspecific combat theory. Pachycephalosaurs BMR P2001.4.5 has 23 lesions on its frontal, with DMNA 469 having 5. This is comparable to other pachycephalosaurids, despite their size difference, age and dome structure This starkly contrasts flat-headed pachycephalosaurs who have no pathology. This could support the fact that these forms may be females or juveniles; they would not have fought rivals like adult males would have. Histological examination shows their domes are a unique fibrolamellar bones that are comprised of fibroblasts, who help to heal wounds and remodeling. Peterson et al. (2013) concludes that the lesion distribution and frequency of the frontoparietal supports the agnostic hypothesis; this is even further strengthened by CT scanning of *Stegoceras*, *Prenocephale* and some artiodactyls.

### Diet

Pacycephalosaurus has been speculated to be omnivorous due to the large "fangs" that tip the jaws, but this notion has yet to be discussed among academics.

## Paleoecology

Most *Pachycephalosaurus* are from the Lance and Hell Creek Formations of the west United States. In some parts, it was contemporaneous with *Sphaerotholus*, *Draacorex* and *Stygimoloch*, with the last two possibly being growth stages. It lived with *Thescelosaurus*, *Edmontosaurus*, indeterminate lambeosaurs, *Triceratops*, *Torosaurus*, *Leptoceratops*, *Ankylosaurus*, *Denversaurus*, *Edmontonia*, *Acheroraptor*, *Dakotaraptor*, *Ornithomimus*, *Struthiomimus*. *Anzu*, *Leptorhynchos*, *Pectinodon*, *Paronychodon*, *Richardoestesia* and *Tyrannosaurus*.

## Synonyms

### Genus Synonymy

* *Tylosteus*
* *Stygimoloch*
	+ *Stenotholus*
* *Dracorex*
* *Pacycephalosaurus* (lapsus)

### Species Synonymy

* *Troodon wyomingensis*
* *P. grangeri*
* *P. reinheimeri*

## References

* https://books.google.com.tr/books?id=2JVmDwAAQBAJ&pg=PA18&lpg=PA18&dq=Pachycephalosaurus&source=bl&ots=8V8R4CE-8x&sig=ACfU3U0IfYEW9iOtAllbSKe4AHUixT2LsQ&hl=en&sa=X&ved=2ahUKEwjYtciRl771AhUGS\_EDHfy0Czo4ChDoAXoECAIQAw#v=onepage&q=Pachycephalosaurus&f=false [Authorial work about this dinosaur]
* https://books.google.com.tr/books?id=4Bp1BgAAQBAJ&pg=PA7&lpg=PA7&dq=Pachycephalosaurus&source=bl&ots=NM9JmLSsfY&sig=ACfU3U0rqJtVcmVzvX6MFTqPfNVpdP5erQ&hl=en&sa=X&ved=2ahUKEwjYtciRl771AhUGS\_EDHfy0Czo4ChDoAXoECAMQAw#v=onepage&q=Pachycephalosaurus&f=false [Authorial work about this dinosaur]
* https://www.degruyter.com/document/doi/10.7208/9780226622866-043/html
* https://www.britannica.com/animal/Pachycephalosaurus
* https://cdnsciencepub.com/doi/abs/10.1139/cjes-2020-0190?journalCode=cjes
* https://www.nationalgeographic.com/science/article/news-vegetarian-dinosaur-ate-meat-pachycephalosaurus-paleontology [Dietary Discovery and Reinterpretation]
* https://blog.everythingdinosaur.co.uk/blog/\_archives/2018/10/27/pachycephalosaurus-was-it-carnivorous.html [Dietary Discovery and Reinterpretation]
* https://kids.nationalgeographic.com/animals/prehistoric/facts/pachycephalosaurus
* https://www.nhm.ac.uk/discover/dino-directory/pachycephalosaurus.html
* https://dinomuseum.ca/2020/02/25/the-real-pachycephalosaurus/
* https://www.fossils-facts-and-finds.com/pachycephalosaurus.html
* https://mtdinotrail.org/pachycephalosaurus/
* https://eartharchives.org/articles/paleontologists-uncover-the-tiniest-bonehead-dinosaur/index.html
* https://www.researchgate.net/publication/38038817\_Extreme\_Cranial\_Ontogeny\_in\_the\_Upper\_Cretaceous\_Dinosaur\_Pachycephalosaurus [Scientific Studies about Cranial qualities of this animal]
* https://www.encyclopedia.com/reference/encyclopedias-almanacs-transcripts-and-maps/pachycephalosaurus
* https://pdfs.semanticscholar.org/d51d/f120caa4f237a3862240d431c821f7f51eb7.pdf
* https://www.cusd80.com/cms/lib/AZ01001175/Centricity/Domain/1840/Pachycephalosaurus.pdf
* https://twitter.com/AllosaurusJP/status/1490113193817083904
* https://t.co/4PLNN88Jzr