Creature Feature 12

Hello, everyone! Sorry about the lack of Sci-Day last week, but as I said on the Facebook page a combination of exhaustion and a cramped schedule made it impossible to get anything ready. Anyways, for today's Creature Feature we'll be looking at the dome-headed ornithischian Pachycephalosaurus wyomingensis!

Pachycephalosaurus wyomingensis model, work in progress.

Pachycephalosaurus was a bipedal ornithiscian dinosaur, though its postcranial anatomy is actually poorly known due to lack of description of such remains (Sullivan, 2006). However, there is a lot known about the very distinctive head - it had a thick dome up to 25 cm thick, with bony knobs protruding from the posterior end and short bony spikes jutting out from the snout. In life, these spikes were likely blunt (Carpenter, 1997).

Pachycephalosaurus had a relatively short skull, with large, rounded eye sockets that faced forward - this suggests that it had well-developed binocular vision. Like many other ornithischians, Pachycephalosaurus had a short snout ending in a pointed beak, with teeth placed in the rear of the jaw. Based on what is known of the postcranial anatomy of other, related pachycephalosaurids, the neck was short and thick, adopting an "S" or possibly "U"-shaped curve in life. It likely had short forelimbs, long hind legs, and a bulky body, counterbalanced by a heavy tail that may have been held rigid by ossified tendons (Organ and Adams, 2005). It was the largest member of the Pachycephalosaur family, at an estimated 4.5 meters in length and weighing in at approximately 450 kilograms (Paul, 2010).

While it was originally thought that Pachycephalosaurs would have used their domes like extant musk oxen or bighorn ship (lining up with their bodies horizonal, and headbutting each other), this is no longer believed to be the case. There are several reasons why this idea is disputed, the foremost of which being that the structure of the skull roof would be unable to sustain the stresses from the impact of such ramming. Additionally, there has not been any evidence of scarring or other damage to the domes of preserved Pachycephalosaurus models; however, recent analyses may provide such evidence (Peterson and Vittore, 2012). Lastly, the articulation of the neck and rounded shape of the skull would be unsuitable for direct head-butting (Carpenter, 1997). However, there is some evidence of adaptations in other species of Pachycephalosaur that would allow for headbutting (Snively and Theodor, 2011). There has been an alternative proposal to the direct headbutting behavior originally proposed for intraspecific combat in Pachycephalosaurs. This alternative is that rather than direct head butting, Pachycephalosaurs would strike rivals on the flank, with the thick body of the animals protecting vital organs from serious injury (Sues, 1978) (Carpenter, 1997).

Perhaps the most interesting thing about Pachycephalosaurus is its hypothesized growth. It has been proposed that the species Dracorex hogwartsia and Stygimoloch spinifer actually represent progressive growth stages of Pachycephalosaurus. The spikes and protrusions from the skull show a great degree of plasticity; additionally, the former two 'species' are only known from juvenile specimens, whereas Pachycephalosaurus is exclusively known from adult specimens. Such evidence, combined with the fact that all three lived in the same place at the same time, has led many to believe that the former two are synonymous with Pachycephalosaurus (Horner and Goodwin, 2009). If this is true, Pachycephalosaurus would have lost the large spikes of Stygimoloch and grown a larger dome with age. Additional research has supported this hypothesis, and has proposed that flat skulled Pachycephalosaurs actually represent juveniles of dome headed adults (Longrich et al., 2010).

"Stygimoloch" spinifer model, work in progress. Compare to Pachycephalosaurus and you will see a great degree of similarity.

This possibility is particularly interesting because such radical changes in anatomy might be widespread in dinosaurs, and there may actually be many species currently described that are actually just growth stages of a single species. However, only further evidence will let us know for sure whether or not this is the case.

I hope this week's Creature Feature has taught you a little more about this fascinating creature!

Acknowledgements:

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.

Carpenter, Kenneth. 1997. Agonistic behavior in pachycephalosaurs (Ornithischia: Dinosauria): a new look at head-butting behavior. Contributions to Geology 32 (1): 19-25.

Organ, Christopher O.; Adams, Jason. 2005. The histology of ossified tendon in dinosaurs. Journal of Vertebrate Paleontology 25 (3): 602-613.

Paul, Gregory S. 2010. The Princeton Field Guide to Dinosaurs. Princeton University Press: Princeton, NJ. p. 244.

Peterson, J. E.; Vittore, C. P. 2012. Farke, Andrew A, ed "Cranial Pathologies in a Specimen of Pachycephalosaurus. PLoS ONE 7 (4): e36227.

Snively, E; Theodor, J.M. 2011. Common Functional Correlates of Head-Strike Behavior in the Pachycephalosaur Stegoceras validum (Ornithischia, Dinosauria) and Combative Artiodactyls. PLoS ONE 6 (6): e21422.

Sues, H. D. 1978. Functional Morphology of the dome in pachycephalosaurid dinosaurs. Neues Jahrbuch für Geologie und Paläontologie. Monatshefte 1978, 459-472.

Horner J .R.; Goodwin, M. B. 2009. Extreme cranial ontogeny in the Upper Cretaceous Dinosaur Pachycephalosaurus. PLoS ONE 4(10): e7626.

Longrich, N. R.; 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.