Sunday, July 6, 2014

Pliosaurus kevani - Business in the Front, Party in the Back

So a recent paper on the biting and feeding mechanics of Pliosaurus kevani has been published, Funtional anatomy and feeding biomechanics of a giant Upper Jurassic pliosaur from Weymouth Bay, Dorset, UK. Brian Switek wrote about it here and you can access the full paper here. As usual I encourage you to read Brians' article as well as the original paper before you go further.

Using bite force estimates, finite element analysis, and beam theory the authors arrived at an interesting ecomorphological compromise reflected in the massive (nearly 2 meter) skull of the Weymouth pliosaur. The back of the skull was indicative of tremendous power and bite force, indeed Pliosaurus kevani ranks right up there with the supreme chompers of all time in terms of bite force. However the front of the skull is decidedly less robust, narrower, and suited for less extreme bite forces. Intuitively this makes sense, even in our own mouth if you really want to chomp down on something you use your rear molars for maximum bite force leverage. Crocodiles also test significantly higher in rear tooth bite pressure and when pulverizing tough prey will preferentially use the rear of the jaw. The front of the skull reflects a compromise of sorts, sacrifice strength for a more efficient streamlined profile and quicker strike that allows for a more generalized foraging modus operandi of both large and small prey items - which is supported by bite marks, stomach remains etc etc.

So far so good, I find this analysis makes a lot of sense. But here is where the paper, which Brian echoes in his own article, falls down for me a bit. The authors suggest that due to the more gracile build of the front of the jaw large prey could neither by shaken violently or twisted (death roll style) for fear of mechanical breakage. Instead large prey, the size of which the authors arbitrarily limit to one-half the length of the pliosaur for reasons I do not fully grasp, was tattered to bits by the rearward part of the mouth until small enough to swallow whole. A feeding methodology, mind you, that no modern vertebrate uses exclusively to dismember large prey. Just think about that for a second. You have a large body in your mouth and start chomping on it until smaller parts start coming off. Do you release the large body to retrieve and swallow the smaller bits? Remember that you are in water so stuff will start sinking and floating away as well... all in all it just seems a little cumbersome and awkward a feeding strategy.

First of all I want to address the claim of the front of the skull being weak and fragile. If you follow the lines on the graph of the pliosaur (solid black) they line up rather closely with those of C. niloticus (nile crocodile) in green.

Especially pertinent is the last graph Log J on the bottom row (size corrected) which measures torsional resistance, i.e. death rolling, where the pliosaur follows the nile crocodile pretty much tit for tat. Basically what can be gleaned is that the skull is not quite so strong as alligators and caimans, but stronger than gharials and pretty much equal to that of a nile crocodile. I do not think weak or fragile are the best terms to describe the jaw, or even the tip of the jaw which still had to be able to withstand several thousand pounds of bite force.

So I do not know about you but I would not characterize nile crocodiles as particularly dainty processors of food, nor do I think they hesitate to shake, twist, tear, gnash, chomp, and clamp onto their prey with reckless abandon. And they certainly do not follow any kind of dictum of only preying on animals 1/2 of their own length.

Beam theory and finite element analysis aside, just look at the damn skull. It is a massive skull and even though the jaw tapers at the end it is still an impressive organ of destruction.

We have to remind ourselves that what animal, apart from another adult Pliosaur, would offer the kind of resistance to break such a jaw? Ammonites, smashed to bits. Plesiosaurs... please, throttled to death. Ichthyosaurs, mince meat. Leedsichthys, mobile feasts. Again back to modern crocodiles, which pulverize, twist, and shake prey, they do actually sometimes have the tips of their jaws broken off. A longirostrine jaw is actually not the ideal morphology for resisting torsion. But this breakage is usually the result of a fight with a bigger croc. Or maybe, as the picture below is rumored to be from a fight with a hippo, but not verifiable. The point is modern nile crocs have mechanical weakness in their skulls but that does not stop them from death rolling, thrashing prey about, and generally being an archosaurian bad ass.

And I think the same held true for Pliosaurus kevani. There was a bit of functional weakness in the skull, but likely the only animal to exploit such weakness was another Pliosaurus kevani.

As another analogy let me offer you up the story of the Tomistoma schlegilii - better known as the false gharial. Like the gharial this relatively narrow snouted croc was long assumed to be pretty much a specialist of fish, avoiding larger vertebrates due to limitations imposed by its skull. Except the Tomistoma did not get the memo and is reported to dine on birds, reptiles, deer, and primates (including the bipedal type primate). In one particularly gruesome account a very large 5 meter female Tomistoma killed and ate a grown adult man which was pulled from the archosaurs stomach (picture above) in addition to an adult proboscis monkey and adult long-tailed macaque (reference croc specialist group newsletter). I wonder how the Tomistoma dismembered said human quarry? Did it pulverize the body to soften it up and then swallow? Engage in a little bit of death rolling? Perhaps slap the body against the surface of the water or perhaps the bank of the shore? My bet would be probably a little bit of all three. And that is also how  I bet Pliosaurus kevani would have dealt with its meals. An initial grasp with the quick biting end of the jaw, then ratcheting the prey towards the back of the skull several pulverizing bites to splinter shell, break bone, and collapse rib cages. Then, once dead, depending on the size of the quarry a couple of good thunks against the surface of the water, or if too heavy grab an extremity or soft spot and death roll off a bite sized piece. We can't rule out, given the likely habit of live birth in sauropterygian plesiosaurs, some type of social cohesion in which case rotational feeding was likely extremely probable. You know, just like nile crocs pulling apart a zebra today. Also check out how similar the lower jaw of the Tomistoma is to the lower jaw of Pliosaurus kevani.

Foffa D, Cuff AR, Sassoon J, Rayfield EJ, Mavrogordato MN, Benton MJ. Functional anatomy and feeding biomechanics of a giant Upper Jurassic pliosaur (Reptilia: Sauropterygia) from Weymouth Bay, Dorset, UK, (2014) Journal of Anatomy

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Tristan Stock said...

Great post! I agree with all your main points. However, although I am tempted to believe in a marine pliosaur badass, I also have to trust my modern analogue gut and say that these pliosaurs probably mostly fed on smaller prey items during day-to-day life. Why risk attacking a large 7 meter plesiosaur which can bite back when you can munch on a smaller and less threatening fish? Nevertheless, when they wanted to be a badass and take on a Leedsichthys or other huge prey item, I'm sure they most definitely were.

Away from pliosaurs, when I read through the paper, I was most surprised by high resistance numbers for Baryonyx, which despite a small dip towards the middle, stayed up right next to the Nile Crocodilie and Pliosaurus kevani the whole time. I'm guessing this agrees with Cuff and Rayfield's earlier estimates last year.

Cuff AR, Rayfield EJ (2013) Feeding Mechanics in Spinosaurid Theropods and Extant Crocodilians. PLoS ONE 8(5): e65295. doi:10.1371/journal.pone.0065295

Duane Nash said...

Thanks for comment. I agree smallish cephalopods, fish, and vertebrates were probably the bread and butter of their diet.

As for the Baronyx that is interesting. Sometimes I think we do spinosaurids a disservice when we refer to them as "mere fish eaters". If you are going after huge lungfish, great white sized coelecanths, and saw fish that top 20' then you are a burly predator.


BK said...

Agreed with the part on Tomistoma, pliosaurs and spinosaurs. Thin jaws aren't always weak jaws.

BK said...

Agreed with the part on Tomistoma, pliosaurs and spinosaurs. Thin jaws aren't always weak jaws and fish are just as hard as anything else.

Unknown said...

Fascinating post, although I do agree that the main diet of these guys may have consisted of smaller vertebrates and cephalopods.

One thing, do you plan to write about pliosaurs size? I mean, with all the hype surrounding these charismatic carnivores, especially the good old BBCesque Liopleurodon, I wonder if you'd be interested to dig into this field?

I think the current maximum size determined for any pliosaur has been put at around 13m and 20 tonnes by C.R. McHenry but I wonder if something else has been overlooked and if the claims of >15m pliosaurs are really only myths.

Duane Nash said...

Thanks for comments. No plans as yet on giant pliosaurs though I will touch on pliosaurs in general again...

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