Triceratops' Dirty Little Secret

What in the world is this Styracosaurus doing? Read on to find out! Image source: http://www.markwitton.com/shop/4588356872/scavenging-styracosaurus/9400728

‘Anthropomorphisation’ is a good word, and not just because you feel absolutely badass if you manage to spell it right first time. Summed up in those seven syllables is the inherent human condition to project our own sentiments onto other, non-human things. Ever called the weather miserable? Said that a doorway looks menacing? Well done then, folks: you’re keen anthropomorpisers. One of our favourite anthropomorphic acts is to cast roles for animals – it’s like hiring actors for a pantomime. We choose our favourite species and root for them through good times and bad, designate some baddies to boo and hiss at whenever they appear. It’s not always as straightforward as siding with the peaceful herbivores and disliking voracious carnivores: lions are the embodiment of nobility, after all. Robins are garden companions, hyenas are stupid, dirty, cowardly scavengers, foxes are clever and chimpanzees are clowns. Of course, very often these roles bear little semblance to reality: it’s well known that lions are actually infanticidal, chimps can be almost sadistically violent and hyenas are skilled, practised hunters.

The same thing happens to fossil animals, too. Dinosaurs, being everyone’s favourite extinct critters, are subject to this far more than anything else. Other fossil groups – things like brontotheres, or mosasaurs, say - aren’t given enough screen time to develop real personalities, but every kid in the world knows that Diplodocus is one of the good guys, but harassed by the merciless Allosaurus. Likewise, the armoured stegosaurs and ankylosaurs are goodies too, defending themselves against bullying theropods. I wonder if this attitude comes about because, when you’re four years old and doing the ‘dinosaur thing’ that all children do, the world is very objective and idealistic: things are good or bad, and that’s it. Other kids are either your friend or your enemy, and this is projected onto everyone’s favourite Mesozoic archosaurs.

Of course, whatever the reasoning behind it, there is no clearer anthropomorphisation of non-avian dinosaurs than the two late Cretaceous A-listers Tyrannosaurus and Triceratops. Tyrannosaurus epitomises all that can be dangerous in a predatory animal: it’s an enormous killing machine that, I was assured by numerous authors, could hold the eight-year old child reading their words in its open jaws. Triceratops, on the other hand, was the guardian of the dinosaur age. Look at it: it’s got a shield strapped to the back of its head and three spears to ward off enemies with – it’s like a Mesozoic knight. How many pictures are there of herds of Triceratops facing down a voracious Tyrannosaurus, protecting their babies with defensive rings of horns and frills? Loads, that’s how many. Triceratops is the nicest dinosaur going, the one we would like to ride to school. What’s more, we don’t have to worry about being eaten by Triceratops - it might be 10 m long, but it only eats plants....

Of course, there’s a ‘but’ coming up. If your inner child wishes to preserve this idealistic image of Triceratops and his spiky friends, you may wish to stop reading now. Alternatively, to find out that horned dinosaurs may have had a much darker side to their personality, please read on.

Y’see, there is good reason to assume that Triceratops and his chums, known as ceratopsians in the business, were not strictly herbivores. There’s two lines of evidence for this: we’ll get to the direct stuff in a moment, but in the meantime we need to check out the business end of ceratopsian ingestion: their jaws.

One of the most basal ceratopsians we know of is a little critter found in some abundance across Eastern Asia, Psittacosaurus. Its name literally means ‘parrot lizard’, and with good reason: all ceratopsians have a large, deep and very often highly recurved beak, somewhat in the manner of a parrot (this, of course, means that, with one or two exceptions, all ceratopsians have toothless jaw tips). A beaked non-avian dinosaur is not unusual, but ceratopsian beaks are incredibly deep and robust compared to the flattened, spatulate bill of hadrosaurs or the slender croppers of other beaked dinosaurs. Furthermore, despite the immense side of their heads, ceratopsian bills are very, very tapered: nothing like the shovelling mouths of ankylosaurs or hadrosaurs. I don’t think anyone’s ever worked out exactly how strong a ceratopsian bite was, but their beaks seem to indicate that they were capable of producing a decent amount of bite force: certainly the degree of beak curvature produces greater mechanical advantage than a flattened or procumbent beak. Add a sharpened proteinaceous sheath to this, and you’ve got a jaw tip you would not want to stick your hand into.

Behind this lies a sophisticated and totally unique chewing mechanism. Ceratopsians are one of two dinosaur groups that learnt to properly chew their food, but they have their own unique slant (almost quite literally) to it. Like their fellow masticators, the ornithopods, ceratopsians had constantly-replenishing batteries of teeth lining their jaws, meaning that when worn teeth could be shed and replaced by fresh ones. However, chewing ornithopods have pleurokinetic skulls – that is, the cheek region of the upper jaw can bulge ever so slightly when the lower jaw is adducted, meaning the food between their teeth is ground and torn laterally as they masticate. By contrast, ceratopsian jaws are not pleurokinetic and can only operate in the vertical plane. In fact, the tooth wear on ceratopsian teeth shows that the teeth occluded exactly in this manner. Because their cheek region is absolutely stuffed solid with teeth, their dentition essentially acts like a set of shears, chopping foodstuffs rather than grinding it. This method of mechanical digestion is not seen in any other animal and, as we’ll find out in a moment, is a little odd.

Operating this battery of food slicing mayhem is a half-decent set of jaw muscles. Now, I should point out that those reconstructions often wheeled out with ceratopsian jaw muscles extending onto their frill are outdated and very likely wrong. The bone surface texture of ceratopsian frills doesn’t show features you’d expect from muscle anchorage and, besides, most of these frills have dirty-big holes in them: you can’t anchor big jaw adductor muscles to nothing but soft-tissue. However, this does not mean the real regions of jaw muscle attachment are anything to be sneezed at: rather, ceratopsians have large, robust coronoid processes (that is, an upright extension of bone on the lower jaw) that would allow for anchorage of big external adductor muscles. Conversely, the sites for anchoring the internal adductor musculature aren’t huge (except for in some basal forms), but the jaw joint certainly is: it’s like the sort of hinge you’d see on a drawbridge. Such a structure would not be needed if ceratopsians had weak, flimsy bites.

So, the take home message here is that ceratopsians have powerful, shearing bites. Further physical breakdown of foodstuffs would take place in a stone-filled gizzard, as known in Psittacosaurus. But what would they be gizzarding? Well, herein lies the problem. Ceratopsian jaws seem over-engineered for pruning leaves or cropping ferns, and their teeth appear better suited to slicing rigid, fibrous food into chunks than mechanically breaking down cellulose. Whatever they were feeding on, they were selective feeders: their beaks are far too narrow to harvest food en masse. It has been suggested that ceratopsians were connoisseurs of angiosperm trees, dining on fibrous branches as well as leaves and flowers. From the large size of the gut cavities in these animals, it does appear that vegetative matter of some kind made up a reasonable percentage of their diet – plants need big guts to break them down. However, amongst modern animals, the most successful shear-chewers do not eat plants. No sir, they eat meat.

Check out the teeth in your cat or dog's mouth later on: you’ll see they’re perfectly adapted for sliding past each other, scissor-style, for slicing meat into chunks. This presents us with the intriguing possibility that the bizarre dentition of ceratopsians was set-up for a similar purpose and, what’s more, that powerful beak takes on a whole new meaning if you imagine it being used to rip a carcass to pieces. All of a sudden, our Cretaceous crusader is taking on a rather dark image: eating meat involves aggression to get at the spoils, maybe even enough hostility to – gasp – kill another animal. Should we risk upsetting generations of kids for this hypothesis? Well, yes, for the reasons mentioned above.

So, rather than placid herbivores, we should imagine ceratopsians as bad-tempered, pig-like animals, chasing dromaeosaurs off their kills, rooting around for small, defenceless vertebrates and maybe, should they come across a wounded, frail hadrosaur, pulling the poor thing apart with their monstrous beaks while it’s still breathing. Of course, plant-matter was probably still on the menu, but maybe as part of a much richer diet. If such a hypothesis is true, this casts a whole new light on ceratopsian skull lesions and pathologies: maybe these fellas had short-tempers when it came to queuing for access to food. It is extremely tempting to compare the horns and graven faces of ceratopsians with the tusked, sculpted skulls of modern pigs; simply adding fuel to the fire.

So, brutish, omnivorous ceratopsians, then – undeniably controversial, but surely quite possible. I’m not sure if this idea is out in the technical literature yet (although Mark Witton did a piece on it years back and Darren Naish has talked about it too), but it’d be interesting to see what the palaeontological community makes of it. The idea of pleasant, herbivorous ceratopsians is just as entrenched in professional palaeontology at it is in kids books, so it could be rejected outright on the basis that we’ve known what ceratopsians ate for years. Personally, I find the idea of ceratopsians acting like giant Cretaceous warthogs far more interesting than them standing around chewing trees, but that’s just me. Anyway, to anyone daft enough to still be reading (well done if you are), I think I’d best stop there. Good night, folks. Don’t have three-horned nightmares.