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. 

  

Entelodont relationships

While skimming Wikipedia for information about the precise name of the group that includes both entelodonts and pigs (suids), I discovered that entelodonts are not considered to be as closely related to pigs as I originally thought.

Since the entelodonts look pretty much like over-sized warthogs, I assumed they would all be part of the family Suidae. But, it turns out they are at different ends of the cetartiodactyl clade.

Cetartiodactyla is a group containing the artiodactyls, or even-toed ungulates, and cetaceans, such as whales, sea cows and relatives. (The story of how they came to be classified together is a fascinating one, but that will have to be for another time.) I made a very simple cladogram (a type of evolutionary tree diagram) over Cetartiodactyla based on what I could find on Wikipedia:

So, according to this, the entelodonts would be more closely related to cetaceans and hippos than to pigs and boars (Suina is a more inclusive group than the family Suidae that I mentioned before: Suina includes Suidae, which are Old World pigs, and the small family Tayassuidae, which are New World pigs).

Still, entelodonts look so similar to warthogs; they even have fairly similar jugal (cheekbone) processes, which is the topic of this investigation.

 

A warthog skull, showing the pronounced outgrowths of the cheekbone 

(just under the eye socket), similar to those of the entelodonts (and ceratopsians), albeit broader. 

Picture from http://www.africahunting.com/before-after-hunt/7615-warthog-dipped-skull-opinions-wanted.html

Perhaps this is yet another case of convergent evolution, although maybe more intimate, as the warthogs are at least closer to the entelodonts than either are to the ceratopsian dinosaurs. Also, this makes me think I ought to check if there may be more cases of similar cheekbone anatomy within the cetartiodactyl clade. The more animal groups we compare with, the more accurate the picture will be – more complicated, surely, but more accurate.

Note from an article about a new ceratopsian

Well… it was new in 2006… but I read the article not long ago, and first now thought of what it means.

The discovery was a fairly complete specimen of a new primitive ceratopsians, named Yinlong downsi (gen. et sp. nov., which is short for new genus and species, in latin… and, again, it was new when the article was published). The article A basal ceratopsian with transitional features from the Late Jurassic of northwestern China describes the find and attempts to place it within a larger context. From what I gathered when reading it, the article proposes two main ideas, apart from describing the fossil, but only one of has a bearing on the cheekbone case.

The thing about the ceratopsians is that their heads evolved ‘forward’ into a more specialised form, while their bodies evolved ‘backward’ to a more basic body plan. Based on some advanced analysis of the sequence the characters were developed within the Ceratopsia, the authors suggest that they developed their specialised skull form before their bodies evolved into a more primitive shape.

The key word here is before, though. Regardless of in which ‘direction’ they evolved, the point is that the skulls developed earlier. It means that they became what they became because their heads started changing to suit their environment. The body changed appropriately too, but it followed later.

I interpret this as emphasising that the skull is indeed the important feature of the ceratopsians. And, the peculiar jugal (cheekbone) protrusion is one of the first skull features they evolved! It seems it was there from the very beginning, and perhaps was one of the things that made them gradually more and more successful, until rivalled only by one other group of dinosaurs, the hadrosaurs (duck-billed dinosaurs).

Sadly, the article does not go into any detail about the jugal of Yinlong, so it has not done much more than further spur my conviction that that bone played a significant role in making the ceratopsians what they were.

However, I have finished reading another scientific article, one that goes into detail about the skull form and function of another basal ceratopsian, Psittacosaurus gobiensis. It is much more important for my cheekbone investigation, but also very complicated, and, as usual, not directly meant to be relevant, so I need to pick out the evidence and puzzle them together in a way that helps me understand just what it means. That will be for the next time…!