P is for Pachycephalosaurus

This week in our Albertan dinosaurian alphabet, we are going to talk about the (debatable) Albertan dinosaur Pachycephalosaurus. It lived during the Late Cretaceous (70-65 million years ago) of western North America, and specimens have been found in Montana, Wyoming, and possibly Alberta. The name means "thick-headed lizard", in reference to the very thick (up to 25 cm) domed skull found in these dinosaurs. One possible specimen has been found in Alberta, and has been referred to from the British Museum of Natural History. However, this record is very dubious [1]. This means that this dinosaur may not be found in Alberta after all. Because of this, I'll go through a short description of Pachycephalosaurus, why it's interesting, then talk about some similar ones that are found in Alberta. 

Strangely enough, this dinosaur is known only from portions of the skull: no postcranial remains have been found, meaning most of its body information comes from other closely related species. The skull is much larger than other related species, suggesting it stood as tall as 1.5 m high (with it's head leaning forward), a length of 4.5 m, and weighing 450 kg. Pachycephalosaurus has a very distinctive skull that makes this dinosaur famous. As mentioned previously, it had an extremely thick skull (up to 25 cm) that had a smooth dome-shape to it. At the back of the skull were several bony spikes and knobs, which are also found around the sides and even on its face. Like other pachycephalosaurids, it had sharp leaf-shaped teeth that would have been successful at eating tough fibrous plants, insects, and seeds. 

Artists impression of Pachycephalosaurus by Nobu Tamura

Pachycephalosaurus is perhaps most famous for what it may have done with its hard, domed head. Originally, it was thought that pachycephalosaurs used their round heads to smash into each others' heads, much like bighorn sheep or musk oxen today. This is the idea that has been picked up by popular media, and in fact appears in many books and movies. However, more recent studies have suggested that this would have been impossible. For example, one study suggests that the amount of force this would create would collapse the skull roof, which would obviously cause problems with the brain, although another study has suggested this is not the case [2-3]. Furthermore, the neck vertebrae are unable to lock in a straight position, which would result in a very unstable structure during collisions. Finally, the smooth, rounded surface of the dome would result in glancing blows, rather than direct contact with each other, which would be of further danger to the animal. Although these animals were likely not butting their heads against each other, they were likely using these thick skulls to butt something softer like the flank of other animals. This could have been used to intimidate another individual in something like a mating competition. 

So what about pachycephalosaurids from Alberta? First of all, a nearly complete specimen of Stegoceras was found in southern Alberta, and is actually housed at the University of Alberta. It was much smaller than Pachycephalosaurus, at 2 m long and weighing only an estimated 60 kg. Another possible Albertan pachycephalosaurid is Stygimoloch, which is found primarily in the US with a possible find in Alberta. It is smaller than Pachycephalosaurus, and has much pointier spikes sticking off of its skull. Some authors believe that Stygimoloch, along with another pachycephalosaur Dracorex (which has a species named after the Hogwarts castle from Harry Potter) actually represent a growth series with Pachycephalosaurus being the adult [4]. In fact, some of you may remember Dr. Jack Horner talking about this 2 summers ago at Jurassic Forest! 

Proposed growth series of Pachycephalosaurus. (G, H) represent the youngest in the series, Dracorex; (E, F) represent an incomplete specimen of Stygimoloch, slightly older; and the top two rows are specimens previously attributed to Pachycephalosaurus. Image from Horner and Goodwin [4].

I hope you enjoyed learning about this interesting dinosaur. Next week, we'll be on Q, which will bring us another non-Albertan dinosaur. 

Other 'P' dinosaurs from Alberta:
Prismatoolithus - a dinosaur egg genus
Prosaurolophus - a hadrosaur 
Parasaurolophus - a hadrosaur
Pachyrhinosaurus - a ceratopsian
Prenoceratops - a ceratopsian
Panoplosaurus - an ankylosaur

References:
1. Ryan, M.J., and Russell, A.P. 2001. Dinosaurs of Alberta. In D.H. Tanke and K. Carpenter eds. Mesozoic Vertebrate Life. Indiana University Press.
2. Carpenter, K. 1997. Agonistic behaviour in pachycephalosaurs (Ornithischia, Dinosauria); a new look at head-butting behaviour. Rocky Mountain Geology 32: 19-25.
3. Snively, E., and Cox, A. 2008. Structural mechanics of pachycephalosaur crania permitted head-butting behaviour. Palaeontologia Electronica 11: 3A. Freely downloadable here
4. Horner, J.R., and Goodwin, M.B. 2009. Extreme cranial ontogeny in the Upper Cretaceous dinosaur Pachycephalosaurus. PLoS ONE 4: e7626. Freely downloadable here