Pachycephalosaurus

*Pachycephalosaurus wyomingensis*

* Jurassic Park Institute Artwork
* Timothy Bradley Artwork
* Stan Winston Artwork
* Robert F Walters Artwork

**Pachycephalosaurus** is an extinct genus of ornithschian dinosaurs. It was a bipedal herbivore with an extremely thick skull roof, long legs and short arms. It lived during the Late Cretaceous lived with other North American dinosaurs such as *Triceratops* and *Tyrannosaurus*. The fossil can be found in both the United States and Canada.

The thick skull domes of *Pachycephalosaurus* and related genera gave rise to the theory that pachycephalosaurs used their skulls to fight each other. This theory has been disputed in the past few years.

## Description

The anatomy of *Pachycephalosaurus* is not well known, as only its skull has been described.  *Pachycephalosaurus* is famed for its large dome on top of its skull, made of up to 10 in thick bone, which kept its tiny brain safe. The dome's rear aspect was edged with bony knobs and short bony spikes sticking up from the snout. The spikes were likely blunt, not sharp.

The skull was short, with large, rounded eyes that faced towards the front, suggesting that the animal had good sight and was capable of binocular vision. *Pachycephalosaurus* had a small snout which ended in a sharp beak. The teeth were tiny, with leaf-shaped crowns. The head was supported by an "S"- or "U"-shaped neck.

*Pachycephalosaurus* was likely bipedal and was the largest in the group family. It measured 15 to 16 feet long, stood 6 feet tall which is the same height as Deinonychus, and weighed 990 pounds. Based on other dome-headed dinosaurs, it probably had a short, thick neck, short fore limbs, a strong body, long hind legs and a thick tail, which was likely held stiff by ossified tendons.

## History

Remains attributed to *Pachycephalosaurus* may have been found since the 1850s. As judged by Donald Baird, in 1859 or 1860 Ferdinand Vandiveer Hayden, an early fossil collector in the North American West, picked up a piece of bone close to the head of the Missouri River, from what is now known to be the Lance Formation in Montana.  This specimen was described by Joseph Leidy in 1872 as from the dermal plate of a reptile or an armadillo-like animal.  Its true nature was not found until Baird restudied it a century later and identified it as a bone from the back of the skull of *Pachycephalosaurus*, including a set of bony knobs that matched those found on other specimens of *Pachycephalosaurus*.  Since the name *Tylosteus* predates *Pachycephalosaurus*, according to the International Code of Zoological Nomenclature it should be used. In 1985, Baird successfully petitioned to have *Pachycephalosaurus* used in lieu of the old *Tylosteus* as the latter name had not been used for more than 50 years, was based on undiagnostic materials, and had poor information.   This may not be the end; Robert Sullivan suggested in 2006 that the fossil is more like the bone of *Dracorex* than that of *Pachycephalosaurus*. The could be of little importance, though, if *Dracorex* is deemed a youn *Pachycephalosaurus*, as has been recently proposed.

*P. wyomingensis*, the type and sole valid species of *Pachycephalosaurus* so far, was named by Charles W. Gilmore in 1931. He coined it for a skull from the Lance Formation of Niobrara County, Wyoming. Gilmore assigned his new species to *Troodon* as *T. wyomingensis*. At the time, paleontologists thought that *Troodon*, then known just from teeth, was the same as *Stegoceras*, which had similar teeth. Therefore, what are now known as pachycephalosaurids were assigned to the family Troodontidae, a wrong idea not corrected until 1945, by Charles M. Sternberg.

## Classification

The pachycephalosaur *Dracorex* may actually be a *Stygimoloch* or *Pachycephalosaurus* in which the dome and horns are not well-developed, either because the animal was a juvenile or a female. This idea was backed up at the 2007 annual meeting of the Society of Vertebrate Paleontology. Jack Horner of Montana State University showed proof, from study of the skull of the one existing *Dracorex* specimen, that this dinosaur may well be a young form of *Stygimoloch*. Plus, he showed data that shows both *Stygimoloch* and *Dracorex* may be young forms of *Pachycephalosaurus*. Horner and M.B. Goodwin published their findings in 2009, that showed the spike/node and skull dome bones of all three 'species' have extreme plasticity, and that both *Dracorex* and *Stygimoloch* are just known from young specimens while *Pachycephalosaurus* is just known from adult specimens. These notes, as well as the fact that all three forms lived in the same time and place, lead them to think that *Dracorex* and *Stygimoloch* were just young *Pachycephalosaurus*, which lost spikes and grew domes as they aged. A 2010 study by Nick Longrich and colleagues also backed up the theory that all flat-skulled pachycephalosaurs were young, suggesting that flat-skulled forms like *Goyocephale* and *Homalocephale* represent juveniles of dome-skulled adults.

## Paleobiology

    Nearly all *Pachycephalosaurus* fossils came from the Lance Formation and Hell Creek Formation. *Pachycephalosaurus* co-existed with young pachycephalosaurs *Dracorex* and *Stygimoloch*. Other dinosaurs like *Thescelosaurus*, *Edmontosaurus*, *Triceratops*, *Ankylosaurus*, and the theropods *Ornithomimus*, and *Tyrannosaurus* shared its time and place.

Scientists once thought that *Pachycephalosaurus* and its relatives were like bighorn sheep or musk oxen; where males would ram each other with head butts. It was also believed that they would make their head, neck, and body horizontally straight, to transmit stress during the crash. But it is now believed that the pachycephalosaurs could not have used their domes in this way.

First, the skull roof could not have adequately sustained impact associated with such shock. Also, there is no evidence of scars or other harm on fossils of *Pachycephalosaurus* skulls. Furthermore, the dorsal vertebrae show that the neck was carried in an "S"- or "U"-shaped curve, not a straight direction, and thus to weak for direct clash of heads. Lastly, the rounded shape of the skull would lessen the contacted surface area during head-butting, resulting in misses.

It is more likely that the *Pachycephalosaurus* and other pachycephalosaurs hit each other in the flanks. In this setting, an individual may have stood parallel or faced a rival directly, using intimidation displays to rival. If intimidation failed, the *Pachycephalosaurus* would bend its head down and to the side, and strike the rival pachycephalosaur on its flank. This hypothesis is back up by the broad width of most pachycephalosaurs, a trait that would have kept vital organs from harm. The flank-butting theory was first proposed by Sues in 1978, and expanded by Ken Carpenter in 1997.

### Diet

Scientists do not yet know what these dinosaurs ate. Having small, ridged teeth they could not have chewed tough, fibrous plants as effectively as other dinosaurs of the same time. It is assumed that pachycephalosaurs lived on a mixed diet of leaves, seeds, fruit and insects. The sharp, jagged teeth would have been very effective for shredding plants.

## JPInstitute.com Description

This was a real bonehead. The skull of Pachycephalosaurus was 10 inches thick on top. For years, scientists thought it was used primarily for head-butting contests, sort of like mountain goats do today, but recent studies of the skeletons of related dinosaurs show that its neck might have broken if it tried that.

A skeleton of the Pachycephalosaurus has never been found, so most of what is known about it is only from the skull and its close relatives. There are several strange characteristics, in addition to its thick skull. It lived at the very end of the dinosaur age, when most dinosaurs were fairly advanced, yet it still had five fingers, a primitive characteristic. Also, the shape of its teeth was somewhat primitive, similar to those of the Stegosaurus which had lived over 100 million years earlier. These were clearly plant-eater teeth, but it also had front teeth that could have been used like a meat-eater.

## Dinosaur Field Guide Description

Pachycephalosaurus (Thick-headed lizard") is the last and most famous member of the Pachycephalosauria, or thick-headed dinosaurs. In the 1970s, paleontologist Peter Galton proposed that male pachycephalosaurs used their domed heads as battering rams, like bighorn sheep. The idea caught the public's imagination. In The Lost World: Jurassic Park, you can even see the dome-headed pachycephalosaurs doing head butts (of course, these are genetically engineered dinosaurs and not necessarily the exact same ones that lived 70 million years ago!). But by the 1990s, scientists began to question Galton's head-butting theory. It was pointed out that animals who do butt heads have a wide surface area where the heads come into contact to prevent "head slippage." This happens when two animals butt heads at high speed and do not hit straight on. They risk breaking their necks when their heads suddenly snap to one side. Pachycephalosaurus has a domed, or rounded, head, which would minimize surface contact and therefore increase the risk of head slippage. This throws doubt on the idea of any high-speed head-butting between pachycephalosaurs, but it does not exclude "head-pushing" or "head-ramming" against non-pachycephalosaurs. It just happens that if a pachycephalosaur lowered its head and charged at a theropod, the impact would be right at the level of the theropod's head or pelvis- the perfect place to stop an attacker. Since
Pachycephalosaurus had a skull up to 9 inches (23 cm) thick, guess who'd lose?

### Fun Facts

Males had larger and more ornamented bumps on the back of their skulls than females.

### Trivia

The teeth of Pachycephalosaurus are smaller than the first teeth of a human baby!

## Links

http://web.archive.org/web/20040214161030fw\_/http://www.jpinstitute.com/dinopedia/dinocards/dc\_pachy.html#

## References

1. ↑ Sullivan, Robert M. (2006). "A taxonomic review of the Pachycephalosauridae (Dinosauria:Ornithischia)". *Late Cretaceous vertebrates from the Western Interior. New Mexico Museum of Natural History and Science Bulletin* **35**: 347–366. Retrieved 2010-11-10.
2. ↑ Carpenter, Kenneth (1 December 1997). "Agonistic behavior in pachycephalosaurs (Ornithischia: Dinosauria): a new look at head-butting behavior" (pdf). *Contributions to Geology* **32** (1): 19–25.
3. ↑ Paul, Gregory S. (2010). *The Princeton Field Guide to Dinosaurs*. Princeton, NJ: Princeton University Press. p. 244. ISBN 978-0-691-13720-9.
4. ↑ Organ, Christopher O.; Adams, Jason (2005). "The histology of ossified tendon in dinosaurs". *Journal of Vertebrate Paleontology* **25** (3): 602–613. doi:10.1671/0272-4634(2005)025[0602:THOOTI2.0.CO;2]. Retrieved 2008-06-10.
5. ↑ Baird, Donald (1979). "The dome-headed dinosaur *Tylosteus ornatus* Leidy 1872 (Reptilia: Ornithischia: Pachycephalosauridae)". *Notulae Naturae* **456**: 1–11.
6. ↑  Leidy, Joseph (1872). "Remarks on some extinct vertebrates". *Proceedings of the Academy of Natural Sciences of Philadelphia, 1872*: 38–40.
7. ↑ Baird, Donald (1979). "The dome-headed dinosaur *Tylosteus ornatus* Leidy 1872 (Reptilia: Ornithischia: Pachycephalosauridae)". *Notulae Naturae* **456**: 1–11.
8. ↑ ICZN Opinion 1371, "*Pachycephalosaurus* Brown & Schlaikjer, 1943 and *Troodon wyomingensis* Gilmore, 1931 (Reptilia, Dinosauria): Conserved." *Bulletin of Zoological Nomenclature*, **43** (1): April 1986.
9. ↑  Glut, Donald F. (1997). "Pachycephalosaurus". *Dinosaurs: The Encyclopedia*. Jefferson, North Carolina: McFarland & Co. pp. 664–668. ISBN 0-89950-917-7.
10. ↑ Sullivan, Robert M. (2006). "A taxonomic review of the Pachycephalosauridae (Dinosauria:Ornithischia)". *Late Cretaceous vertebrates from the Western Interior. New Mexico Museum of Natural History and Science Bulletin* **35**: 347–366. Retrieved 2010-11-10.
11. ↑ Stokstad, Erik (2007). "SOCIETY OF VERTEBRATE PALEONTOLOGY MEETING: Did Horny Young Dinosaurs Cause Illusion of Separate Species?". *Science* **318** (5854): 1236. doi:10.1126/science.318.5854.1236. PMID 18033861.
12. ↑ Gilmore, Charles W. (1931). "A new species of troodont dinosaur from the Lance Formation of Wyoming". *Proceedings of the United States National Museum* **79** (9): 1–6.
13. ↑ Glut, Donald F. (1997). "Troodon". *Dinosaurs: The Encyclopedia*. Jefferson, North Carolina: McFarland & Co. pp. 933–938. ISBN 0-89950-917-7.
14. ↑ Erik Stokstad,"SOCIETY OF VERTEBRATE PALEONTOLOGY MEETING: Did Horny Young Dinosaurs Cause Illusion of Separate Species?", *Science* Vol. 18, 23 Nov. 2007, p. 1236; http://www.sciencemag.org/cgi/content/full/318/5854/1236
15. ↑ 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
16. ↑ 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*, . doi:10.1016/j.cretres.2009.12.002
17. ↑ Sullivan, Robert M. (2006). "A taxonomic review of the Pachycephalosauridae (Dinosauria:Ornithischia)". Late Cretaceous vertebrates from the Western Interior. New Mexico Museum of Natural History and Science Bulletin 35: 347–366. Retrieved 2010-11-10.
18. ↑ Template:Cite journal
19. ↑ 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.
20. ↑ Template:Cite journal
21. ↑ Carpenter, Kenneth (1 December 1997). "Agonistic behavior in pachycephalosaurs (Ornithischia: Dinosauria): a new look at head-butting behavior" (pdf). Contributions to Geology 32 (1): 19–25.
22. ↑ Template:Cite book