The cheekbone case: Briefly about Ceratopsia

A quick background of the horned dinosaurs, the ceratopsians might be in order. In order to understand the cheekbone case in its broader context, I mean. What I am trying to say is: if you have a clear picture of these animals, many of the conclusions drawn in the investigation will probably make more sense.

I will try to keep it simple and broad, though I find many details incredibly fascinating and there is the risk of slipping into details about tooth shape and wear patterns… I hope to avoid this!

Ceratopsia includes the famous Triceratops and many other dinosaurs with impressive horns on their skulls, and, more characteristically, a large neck frill or neck shield that extends from the rear of their head toward their backs.

A fleshed-out drawing of a Triceratops, probably the most famous of the ceratopsian dinosaurs. 

Image from http://www.guardian.co.uk/science/2009/feb/07/dinosaurs-evolution-bipeds-quadrupeds

Like most land vertebrates, the ceratopsians walked on four legs – they were quadrupedal. Interestingly, however, their ancestor as well as many primitive ceratopsians were probably able to walk comfortably on two legs (bipedal), so there must have been a reason for the ceratopsians to evolve toward walking on all fours. A simple explanation is that the ceratopsians needed huge guts to process their food: plant material is much more difficult to extract energy from than flesh. Being big and heavy, they were probably sluggish most of the day – until something bothered or threatened them!

Since the ceratospians walked on four legs, both their legs grew sturdy and strong rather than slim and flexible, their hands and feed got broad, and their claws became blunt and hoof-like. Having perfect balance, they did not need a particularly long or stiff tail.

(In other words, their body was not very unique or interesting. That does not mean it is not important! Just that there is little to say about it unless you dig into a deeper level of anatomy. I do not think that is necessary at the moment; if any aspect of their bulk anatomy becomes relevant to the investigation, I think it will be more appropriate to mention it when it comes to that.)

I feel I should at least mention that there has been quite some debate about how these animals moved, in particular regarding whether they were cursorial – could run, to charge at an enemy like a rhino – or if they defended themselves by standing firm in place like a phalanx army. Though some studies favour the cursorial idea, I personally find it difficult to accept simply because the ceratopsians had considerably shorter forelimbs compared to their back legs. If they tried to run, their longer hind legs would eventually ‘catch up’ with the front limbs and they would topple over. (I also have a hard time accepting the computer models they create to calculate the optimal way of moving for the animal and assume that that would be how it moved. However, my dispute with this type of investigation is more philosophical than anything, and I realise that going on about that now would be stepping way out of line. Maybe another time!)

Let us move on to the heads of the Ceratopsia. Facial horns and a neck frill are their most distinguishing features, but there is an immense variety in their design, and probably also in their function.

Drawings of the skulls of some of the centrosaurines, a group of advanced ceratopsians, giving a taste of the tantalising diversity of horn and frill shapes and sizes in the Ceratopsia. Image from http://ceratopsiansrevealed.blogspot.se/2011/07/centrosaurine-or-chasmosaurine.html

The neck frill is an outgrowth of a certain skull bone called the parietal, which is located somewhere around the back of the skull roof (the exact position differs between different types of animals, as many skull bones do… and also I do not really know exactly where it is on any animal really, hahaha!). The horns are pointy outgrowths that extend from several different bones, typically somewhere on the snout, along the frill rim, or above the eyes. The horns were bony, but probably covered on the outside by keratin, the same protein that makes our hair and nails, and also covers the beaks of birds and is likely to have covered the beaks of many other dinosaurs, including the ceratopsians (more about the beaks later). If I am not mistaken, a keratin cover protects the bone from everyday wear (the replaceable keratin is worn down instead of the bone), and also enables the horn to get a sharper point.

But, not all ceratopsians had horns, and not all had neck frills either! The ones that had both horns and frills are grouped as ceratopsids; those with only a neck frill are called protoceratopsids; and the psittacosaurids were the primitive forms with neither frill nor horns.

A simplified family tree of the Ceratopsia, showing the gradual acquisition of typical features: first the neck frill and later the facial horns. Image sources: psittacosaurid (left): http://www.wesleym.info/finalproject/psittacosaurus/ ; protoceratopsid (centre): http://tsjok45.wordpress.com/2012/10/02/dinosauricon-pq/ ; 

ceratopsid (right): http://ba-k.com/threads/830106-Plantean-posibilidad-para-matrimonios-gays-en-Sonora

If I am not mistaken, the protoceratopsids and ceratopsids together form the Neoceratopsia, the group containing all ceratopsians with a neck frill. So, Neoceratopsia includes all ceratopsians except the psittacosaurids, the most primitive. Thus, Neoceratopsia is a subset within the Ceratopsia (and Ceratopsidae forms a subset within the Neoceratopsia).

Since neck frill and facial horns are not found in all ceratopsians, these iconic characteristics cannot be used to accurately define, or diagnose the group. We need features that all ceratopsians share. Quite conveniently for us, the ventrolateral jugal process that is the focus of the cheekbone case investigation is one of them! Another, more well-known diagnostic character is the rostral bone, which is completely unique to the ceratopsians; since it is exclusive to this group, it is actually the most appropriate diagnostic feature of the Ceratopsia. The rostral is the pointy bone that forms the beak of the upper jaw in ceratopsians.

The skull of a Triceratops with the rostral marked in red. From http://www.junglekey.fr/search.php?query=Marginocephalia&type=image&lang=fr&region=fr&img=1&adv=1

On the picture above you may also notice a very similar bone at the tip of the lower jaw. This is the predentary bone, which forms the beak on the lower jaw of all ornithischian (bird-hipped) dinosaurs. So, while many herbivorous dinosaurs had a predentary, only the ceratopsians ‘completed’ the beak with an equivalent bone in the upper jaw.

This does not mean that the other ornithischians only had half a beak: the vast majority of them instead lost their teeth in the premaxillary bone, the frontmost (anteriormost, in fancy words) bone in their upper jaws. In other words, their upper beaks were made out of a different jaw bone: the rostral in ceratopsians and the premaxilla in other ornithischians.

Why is this important? I am not entirely sure, but the best idea I have may actually have interesting implications to the cheekbone case! It seems to me that the beaks formed by the premaxillary bone rarely, if ever, became very sharp. I have little information to back up this hunch, other than my impression from what I have seen and read about herbivorous dinosaurs so far. But the ceratopsian beak is typically described as sharp, both at the tip and on the edges. It is also likely that the rostral was covered by a keratin sheath in life, just like the horns. (Such a keratinous beak is called a rhamphotheca in scientific jargon.) As with the horns, the cover probably helped protecting the bone and made the beak even sharper.

What could they need a sharp beak for? One explanation is that it was part of their many adaptations to slicing plant material. There are several features of their teeth and jaws that suggest they processed their food by cutting, rather than grinding it as other advanced dinosaurian herbivores did (more about that later).

Another idea is that is was used as a defensive weapon. Think about the psittacosaurids, which had no horns or neck shield for protection; they probably relied heavily on their beaks to fight off predators and rivals. As they evolved into more heavy-bodied forms, they may have needed additional defences, and therefore developed the remainder of their arsenal.

The famous fighting dinosaurs fossil is strong evidence for this. It is the best fossil of an actual fight between a predator and a prey, which happens to be a Protoceratops.

The fighting dinosaurs fossil, showing the last death struggle between a Velociraptor and a Protoceratops. The ceratopsid seems to have used its beak to defend itself from the predator. 

Image from http://www.eos.ubc.ca/courses/Dist-Ed/EOSC116/eosc116-Lesson22.html

The Protoceratops has a firm bite around the wrist of the predator, either intended to cause injury or simply lock the hand so its claws could not be used (in which case it failed pretty badly, because the predator got its sickle-like foot claw clear into the throat of the Protoceratops).

Now, if we accept that the beak was used for attacking predators, and probably also rivals, we have opened up another possibility for the use of the jugal process. Recall from the introduction that it may have served as attachment site for powerful jaw muscles. A sharp beak powered by strong muscles would create a devastating bite, probably very effective  against predators… if they came within range. Furthermore, if the muscles attached to the jugal process were meant to be stabilising the jaws when biting around a struggling animal, just as suggested for the entelodonts (see the introduction), it could suggest that the ceratopsians used their beaks to grab their antagonists, perhaps to crush bones, rather than scratch or tear off flesh with the sharp beak edges.

Jesus, this brief introduction became much longer than I expected. Good thing we only have one main thing left: the teeth. I promised not to go into detail with the teeth, but I must mention the dental battery of the ceratopsids (the most advanced ceratopsians). A dental battery is a compaction of several rows of teeth, tightly concentrated to form a highly efficient surface for chewing plant material. It has only evolved in Ceratopsidae and Hadrosauridae (the duck-billed dinosaurs), another group of very advanced herbivorous dinosaurs. (The detailed design of these dental batteries is rather different between the two groups, so there is little similarity except that there multiple tooth rows are compacted together.) It is arguably the most efficient chewing design ever evolved among herbivores, and it is probably the main reason why these two groups quickly came to dominate the dinosaur herbivore fauna.

Such were the ceratopsians! (Maybe you did not expect that? Really, they were incredibly successful!)