The cheekbone case: Introduction

Over a year ago, while watching a tv-show about extinct mammals, an idea struck me about a possible link between the odd cheekbones of the suids – boars and their relatives – and the similarly peculiar and peculiarly similar cheekbones of the ceratopsians, or horned dinosaurs. 

The skulls of an entelodont (extinct relative of boars; top) and a Protoceratops (a well-known early ceratopsian; bottom). Not to scale. Images from: (top) http://www.flickr.com/photos/velocibadgergirl/2072208547/; (bottom) http://www.flickr.com/photos/stevelewalready/3505166117/

I have thought about it from time to time, as a potential topic if we ever got tasked in school to investigate anything of our own choosing. However, I was thinking that it might be more fun to present it here to you.

I have not yet fleshed out the idea with any scientific sources, but I want to do the background reading after having presented the original idea. In that way, I hope to update you with findings from the scientific literature perhaps once a week, and so we may see how that argument develops, from an embryonic stage idea to hopefully a full-fledged hypothesis that may attract the attention of professionals (that is my goal!).

The curious similarity between these two animal groups, which never co-existed (the ceratopsians were extinct 65 million years ago; the suids evolved 30-20 million years ago) and whose ancestors were separated in the evolutionary line more than 300 million years ago, is that they evolved very similar-looking jugals or cheekbones (in mammals, the jugal may also be referred to as the zygomatic). Strictly speaking, it is the processes (outgrowths) of the jugals that I am referring to, not the entire bone, but the similarities are still striking: the processes are very large, lobed and point outward and downward (ventrolaterally, in fancy terms).

This connection ought to be related to some similarity in the diet of the animals, would you agree? However, while the suids are mostly omnivorous generalists (eat more or less any food they can get), ceratopsians are interpreted as strict herbivores, probably specialising on particular types of plants. These are rather different. Maybe, then, it has to do with something else they use their mouths for.

What about attack/defence? It is known that at least Protoceratops (the ceratopsians in the figure above) used its sharp, powerful beak in the best recorded instance of a dinosaur predator attacking a dinosaur herbivore: the famous ‘fighting dinosaurs’ fossil from the Mongolian desert of the Cretaceous shows a Protoceratops attacked by a Velociraptor. 

The ‘fighting dinosaurs’ fossil: the Protoceratops (top left) has its (missing) beak in a firm bite around the arm of the attacking Velociraptor, suggesting that at least this ceratopsians used its jaws in defence against predators. Image from http://www.sciencedaily.com/releases/2007/11/071107074326.htm

The suids naturally used their powerful jaws to subdue prey. In addition, it is known that at least the entelodonts frequently bit one another in fighting among themselves: bite marks, on the skull in particular, from teeth that match the same species, are common, and may have been caused during fights for territory or mating rights. It may further be possible that also ceratopsians used their beaks in territorial or courtship fights – at least in the early, hornless forms.

How could the cheekbone play a role in such use of the jaws? If it has been enlarged to that extent, it probably was important. What was the point of that shape?

In the tv-show I was watching when I got the idea, they argued that the entelodont jugals worked as jaw stabilisers. Entelodonts could open their jaws very widely, but when doing so, they could easily be dislocated by struggling prey. The large jugal processes served as attachment sites for powerful cheek muscles that would keep the jaws in place. Could this have been the same case for ceratopsians?

This is one of the questions where I would need to consult the scientific literature in detail. Were the ceratopsians able to open their jaws so widely? Would that have been advantageous? How?

Earlier, I did some broad thinking about what other animals have long jaws or big beaks, that might provide further insight to this mystery. I have only gotten as far as making a few bullet-point lists:

Big beaks (that open wide?)

• parrots
• toucans
• pelicans
• those big-beaked finches
• terror birds (extinct)
• hesperorniforms (extinct)
• turtles

Long jaws (that open wide?)

• crocodiles
• mosasaurs (extinct)
• pliosaurs (extinct)
• spinosaurids (extinct)
• pterodactyloid pterosaurs (extinct)

I want to add to these lists as this investigation goes along, as well as taking time to look through each of these in detail.

I hope you found this as curious as I do, and that you feel keen to come with your own thoughts and comments. I will warmly welcome and appreciate any feedback you may have, so please, do not hesitate to make your ideas heard. Together, we could make this into something big.