Sunday, June 28, 2015

Pachycephalosaurus: Beast of the Week

This week we shall be checking out a well-known dinosaur with an iconic skull.  This dinosaur is one of my personal all time favorites.  I will never forget seeing it's awesome representation in The Lost World: Jurassic Park in the movie theater when I was only eight years old.  The way it smashed that changed me.  Say hello to Pachycephalosaurus!

Pachycephalosaurus was the largest known member of the pachycephalosaurid family at about fifteen feet long from beak to tail.  In life, it was most likely a plant-eater, but some have suggested it may have supplemented its diet with meat in the form of insects and possibly carrion.  The genus name, Pachycephalosaurus, translates to "Thick Head Lizard" in reference to the dinosaur's skull, which is almost ten inches thick in the biggest specimens.  Pachycephalosaurus lived during the very end of the Cretaceous period, between 70 and 66 million years ago, in what is now the Western United States, specifically Montana, South Dakota, and Wyoming.  In fact, its full genus and species name is Pachycephalosaurus wyomingensis, in reference to this.  When alive Pachycephalosaurus would have shared its environment with Tyrannosaurus, Anzu, Quetzalcoatlus, Anatotitan, Ankylosaurus, Triceratops, Troodon, Stygimiloch, and Dracorex.

Life reconstruction of a male Pachycephalosaurus by Christopher DiPiazza.

Sadly, none of Pachycephalosaurus' body beyond the skull has ever been discovered.  However, we have a good idea of what the whole animal looked like thanks to more complete skeletons of other kinds of Pachycephalosaurids it was closely related to.  It was most likely a biped, with short front arms, each ending in a five-fingered hand.  The hips were probably relatively wide with a thick tail base.  The end of the tail would have been stiffened with ossified tendons. (bony rod-like structures found in the tails of many dinosaurs)

Pachycephalosaurus' head was its trademark feature, however.  Like I said earlier, its skull was extremely thick, and made of solid bone, encasing a relatively small brain, forming a round, dome-shaped cranium.  Around the back of the head head, over the eyes, and on the top of the snout, were a series of short, knobby horns.  The mouth was tipped with a blunt beak and the teeth were small and leaf-shaped, most likely for shredding tough plants.  The eye sockets of Pachycephalosaurus skulls are relatively large, and face forward.  This indicates that this dinosaur would have had good vision and was able to see depth quite well in life.  The heavy skull would have been held up by a short, muscular neck in life.

Pachycephalosaurus skull on display at the American Museum of Natural History in New York.

The exact purpose of Pachycephalosaurus' thick skull has been debated much over the years.  The most common idea, especially in the beginning, was that rival Pachycephalosaurus would have ran into each other, head first, like modern day rams do, to establish dominance.  This idea became so popular, that you would be hard pressed to find paleo-art of Pachycephalosaurus not running with its head down, about to ram something in just about every toy, book, or other visual representation of the dinosaur.  More recently, however, there have been paleontologists who have challenged this, saying the shape of Pachycephalosaurus' skull would not have allowed it to have sustained crashing into other hard objects at high speeds without causing serious injury.  Many believed that the large domes of Pachycephalosaurus were mostly for display within the species.  Was it really possible that those thick skulls were just for show, though?  It seems unlikely considering how thick they were.  (Something that isn't entirely visual.)  Finally in 2013, a study on the skulls of many pachycephalosaurids discovered evidence of injuries that had healed over on the skulls of many individual specimens.  In fact, the kind of bone that the skulls of pachycephalosaurids were made of seems to have been especially good at healing and repairing itself after damage was inflicted on them.  This supported the idea, once again, that Pachycephalosaurus and its kin were indeed using their skulls as weapons.  They may have more likely swung their heads into each other at close quarters, instead of charging into each other from a running distance.  You can observe modern giraffes (like in the video below), and many other animals that sport weapons on their heads, combating each other in the same way.  If this indeed was the method used for Pachycephalosaurus fighting, the small horns on the sides of the head could have been utilized as weapons, enforced by the weight of the thick skull. Some still argue that using their skulls as weapons would have injured Pachycephalosaurus in life too much and was thus, is still an unrealistic idea or real life.  These folks need to remember, however, that many animals do get injured quite a lot when fighting rivals.  In fact, many species of animals become permanently maimed, or even die fighting peers for dominance.  If a Pachycephalosaurus did hurt itself or die in combat with another back in the Cretaceous, it would have just been another example of natural selection in the works. Evolution is never perfect.  If it was nothing would ever go extinct!

Pachycephalosaurus is believed to have changed its form drastically as it matured into an adult by certain paleontologists.  The other two, slightly smaller, pachycephalosaurids, that shared its environment, Dracorex (sported lots of horns but no dome) and Stygimoloch (sported horns and a small dome), are believed by some to have actually been juvenile and subadult forms, respectively, of adult Pachycephalosaurus.  If this is the case, Pachycephalosaurus would have had no dome at all when young, and its horns would have become shorter, being absorbed into the growing thickness of the skull as it matured.  It is important to note, however, that there is no known example of this in any kind of living vertebrate.  Since the number of pachycephalosaurid specimens on the fossil record is still very limited, this hypothesis still needs a lot more evidence to be verified and these three dinosaurs may have indeed been different species after all.  It is an interesting idea to consider, however.

From left to right: Dracorex, Stygimoloch, and Pachycephalosaurus.  Paintings by Christopher DiPiazza.

That is all for this week!  As always feel free to comment below or on our facebook page!


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.

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

Peterson, J. E.; Vittore, C. P. (2012). Farke, Andrew A, ed. "Cranial Pathologies in a Specimen of Pachycephalosaurus". PLoS ONE 7 (4): e36227. doi:10.1371/journal.pone.0036227.

Peterson JE, Dischler C, Longrich NR (2013) Distributions of Cranial Pathologies Provide Evidence for Head-Butting in Dome-Headed Dinosaurs (Pachycephalosauridae). PLoS ONE 8(7): e68620. doi:10.1371/journal.pone.0068620

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.

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