Monday, April 13, 2015

Plesiosaur Machinations VI: WE BITE!!

Plesiosaurs and the Misfits together at last!! I've done this occasionally in the past for this blog where a certain song fits and inspires the subject matter at hand. The song We Bite was one of the last songs recorded by the Glenn Danzig incarnation of The Misftis for the Earth A.D. period of the band and really captures the feral, almost proto thrash intensity of the band at the time. This was a shift from what can best be described as the more Ramones inspired, rockabilly punk rock the band had perfected prior to this unholy last gasp. Unlike many bands The Misfits got more intense, serious, and unhinged with age.

Carnivores Live For Pleasure - WE BITE!
Strike Out Like A Wolf's Endeavor - WE BITE!

Sea and Land J.W. Buel 1889

And I think a similar arms race was in effect with plesiosaur evolution culminating in the mighty elasmosaurids of the Cretaceous period. As I discussed in my last post I think plesiosaurs, and especially elasmosaurs, often times filled their tummies with ammonites wrenched from their shells through either rotational feeding, bashing them on the surface, or group feeding or some combination there of. I don't think it was a coincidence that the largest ammonites coincided with the largest elasmosaurids in the Cretaceous. 

Now some readers might be open to plesiosaurs/elasmosaurs preying on large, squishy cephalopods.... but why stop there?

A funny little social experiment played out in front of me on facebook the other day. A person I follow who has written a paleo inspired fictional novel posted the picture below on his feed.

Now if you are as much of a plesiosaur fanatic as myself you will see that not only is that T- rex skull too small for an adult skull but the purported "pliosaur skull" is actually a blown up Elasmosaurus skull. Yep you heard me right, it is the supposedly "gape limited, small fish only skull" of a long necked plesiosaur. What is interesting is that the picture got several hundred likes and comments on said thread and I had to be "that guy" that let people know that they were looking at a plesiosaur not a pliosaur. This tells me a couple of things - the distinction between the two groups is rather arbitrary and if they are so easily confused on general inspection maybe the line between the two was blurred ecologically as well... If one views pliosaurs as large game hunters with compromises in the skull that also allow exploitation of relatively small prey then maybe one could view long necked plesiosaurs as predators with a penchant for small prey but also the ability to exploit relatively large prey/carcassses on occasion. 

If you are keeping up to date on your pinniped behavioral ecology you probably are aware that for a while now a bit of a slow motion revolution has been occurring revealing that many pinniped species kill and consume prey of a size larger than generally presumed and quite frankly should make you a little leery with swimming with them. There was the recent documentation and publication noting grey seals as significant predators of harbor porpoises and seals. And just recently the publication: Intraguild predation and partial consumption of blue sharks Prionace glauca by Cape fur seals Arctocephalus pusillus pusillus (abstract).

Cape Fur Seal predating Blue Shark (photo Chris Fallows)
It was well known "Air Jaws" photographer Chris Fallows that witnessed the carnivoran on chondrichthyan carnage not one time but twice and on both occasions more than one shark was caught by the seal which ate only the energy rich viscera, especially the liver and stomach, disregarding the rest. From the Smithsonian article summarizing said study: " The behavior also suggests that traditional methods of diet estimation might be missing some key strands in the food web. Ecologists have long assumed that seals consume mainly small fish not exceeding about a foot long. But analyzing seal diets usually entails looking at what's found in their guts or in their feces, which in turn depends on recovering hard parts such as fish ear bones. If seals are selectively eating viscera from sharks - or any other large animal - that slippery evidence would have evaded scientists' detection methods, potentially leading to a biased picture of who's eating whom."

Now go back and read that last paragraph and replace the word seal with the word plesiosaur and you will see exactly what I am getting at. If plesiosaurs were leaving behind the hard parts of large prey (and it  should be tacitly obvious why they would do so due to possible choking) you will likely get a view of prey remains not too dissimilar to the small prey bias we see in plesiosaur stomach remains. And from a very small sample size of just a few preserved stomach remains that we have - which were biased from the start arguably - evolves the dogma that you will see repeated in just about every wikipedia page, every professional or popular article on plesiosaurs, every artistic depiction, and every animated feature: that "plesiosaurs (long necked) were a threat to nothing larger than a small squid or fish - specialists on exclusively small prey of the size that they could swallow whole in one bite and nothing larger." 

What do the nuts and bolts of plesiosaur skull anatomy suggest about plesiosaur (again when I say plesiosaur let us just assume I am talking about long necked varieties for simplicities sake) feeding ecology? Turns out that there is a rather extensive study of the jaw closing musculature and finite element analysis of the skull of the elasmosaur Libonectes morgani. An elasmosaurid from the Turonian stage of the early Late Cretaceous discovered in Texas, the preserved skull of Libonectes is generally considered the most complete elasmosaurid skull found to date. Almost as important is how creepy the picture of its excavation is below, almost as if it was dug out of a recent deposit representing some Lovecraftian beast brought to the light of day.

Libonectes morgani c/o T.W. Tidwell SMU College
Now what did the paper I talk about say? Well first of all the name of the paper is Plesiosaur Mastication: A biomechanical analysis of the skull and adductor chamber muscles in the Late Cretaceous Plesiosaur Albertonectes Paleontologica Electronica April 2013 (Araujo & Polcyn) and is online (link). The paper is pretty technical and I had to read it a couple of times to get the gist. There are several conclusions that the author come to best summarized in the exert from the abstract below:

Our results show that a larger physiological cross-section of the adductor muscles is achieved by an enlarged supratemporal fenestra which although it reduces mechanical performance of the skull, it is offset by increased strength of a taller parietal crest and temporal bar, given the loading is largely symmetrical, the lateral components are offsetting yielding a vertical force vector. This arrangement also increases the length of the adductor musculature and thus the total muscle mass. We propose that the reduced pterygoid flange indicates a diminished role for the pterygoideus muscle, reflecting a shift of the majority of the bite force to the adductor mandibulae externus, pseudotemporalis, and adductor mandibulae posterior muscles. Reduction of the pterygoideus falsifies the dual adductor system hypothesis, in which kinetic inertia and static pressure coexist.

Ok I do not disagree with anything they say there as far as anatomy and the results of their FEA. Probably the most important aspect of their paper is that Libonectes did not have a dual adductor system like crocodilians and that the pterygoideus muscle was diminished in importance compared to the muscles of the temporal region.  

So there was a shifting of importance away from pterygoideus muscles (i.e the big muscles at the rear of the lower jaw in crocodiles that make a big bulge) towards muscles of the temporal region (i.e. the notable forehead muscles of many predators). 

Araujo & Polcyn 2013.

As the temporal hole got larger in elasmosaurs this was associated with increased height of parietal crest and temporal bar that offset the mechanical compromise of the larger hole. This increased bite strength as well as mechanical performance. In other words the architecture of the whole temporal region grew not only to accommodate larger temporal muscles but to increase mechanical performance. And I do have to mention that Araujo et. al. did an exemplary job of running the stress & strain tests under the assorted variables at hand such as size of temporal hole, with or without various muscles etc etc that does convincingly show that these factors influence how the skull behaves mechanically.

At this point I should mention that I only give finite element analysis a little bit of leverage in terms of telling me what an animal can or can't do. The reason I don't put full faith in drawing strong conclusions from FEA is that the simulations treat the skull as if it is in a vacuum - always remember that a skull is attached to an animal. Muscles, tendons, skin, fat, bone, and ligaments can absorb and redistribute stress and strain throughout the body. FEA does not account for this. And when you have a long neck with 60 or 70 neck vertebrate that is a significant part of the body that can absorb and redistribute stresses incurred at the head. (BTW red means high stress and blue means low stress in the skull). What I do read from the FEA analysis here is that blue dominates, pressure is equally distributed across the biting portion with a strong vertical bite force. Nowhere in the paper do the authors address how much force Libonectes could bite with and this is unfortunate.

As mentioned earlier the paper discounts the dual adductor system of crocodilians due to the underdeveloped pterygoid attachments in Libonectes (and, by extension, other sauropterygians built the same way). Following form this they suggest an emphasis on speed rather than force in the jaw closing apparatus in these animals. As evidence they cite the fact that gharials also have diminished importance of the pterydoid muscles. As such the authors favor a prey choice of small teleost fish.

And here is where I disagree. There is a strong argument to be made that the importance of the pterygoid muscular apparatus in crocodilians has a lot to do with putting the bulk of the animal's anatomy under the water in order to conceal the head when approaching prey. Therefore in crocodilians  there is an adaptive constraint to shift the bulk of the jaw closing musculature underwater. Plesiosaurs were under no such constraint and, being obligate piscivores, gharials are under no such constraint as well. In fact the increase of the temporal hole and concurrent heightening of temporal and parietal bars speak to an increase in power and bite force. And if there was an abundance of slow twitch muscles in that ample temporal region we might be looking at a bite that could hold on for a long time...

So I don't really follow this line of logic to argue that Libonectes and plesiosaurs of a similar build were obligate piscivores. For me the strongest indicator of being an obligate piscivore is the shape of the jaw and teeth - and we see this pattern again and again in nature - long narrow jaws, with more or less homodont dentition. Dolphins do this, gharials do this, gar do this. Plesiosauromporph plesiosaurs fall outside of this pattern. In fact some genera of elasmosaurids such as Callawayasaurus, Terminonatator, and Zarafasaura developed increasingly shorter snouts with jagged heterodont dentition!!

Convergence in obligate piscivores. Dolphin, Gharial, and Gar. Credit Ariel Zambelich wired
As difficult as it is to imagine what plesiosauromorphs were it is also important to realize what they were not. To better illustrate this point I drew a hypothetical obligate piscivore plesiosauromorph as I imagine it should look based on what other obligate piscivores do.

This plesiosaur does not exist yet
I gave it a highly felxible mobile neck like an Anhinga, a longirostine jaw, binocular vision for accurate strikes, and rows of needle sharp dentition. A flexible and slim lower jaw that could flex open to assist in swallowing fish.

But it is hypothetical because no known long necked plesiosaur developed this suite of characteristics.

They could not fold their neck back like an Anhinga to dart after quick moving, small prey. Although there was stereoscopic vision in plesiosaurs most species show a tendency for the eyes to be directed upward which suggests capture and stalking from below. They did not evolve longirostine skulls to maximize their reach and speed at snapping up small, slippery prey. Although some species evolved needle like, slender dentition other species evolved jagged, well rooted 2" plus conical daggers that would not look out of place in the jaws of a nile crocodile. Long necked plesiosaurs did not loosen up the skulls of the lower jaws like their relatives snakes and lizards do as well as many fish eaters to help swallow fish. Instead the lower mandible became one solid bone basically sutured up completely and with a noted unification of the mandibular symphysis. I do think one lineage of plesiosaurs became obligate piscivores/small game only speicalists: the polycotlylids. Their jaws converge with other known fish specialists.

All in all the majority of plesiosauromorph plesiosaurs fail in many of the aspects that make obligate piscivores so successful. And I think we should be asking why is this?

The answer I believe is quite simply many of them were not the small prey specialists so often ascribed to them but opportunistic mesopredators able to subsist on a wide prey base. Again I make the analogy towards pinnipeds. Long purported to be exclusively piscivorous but now known to be capable predators of quite large tetrapod prey. And in order to push the envelope even a bit more I drew this depiction below of the Early Jurassic plesiosauromorph Occitanosaurus tournemiensis (usually sunk into Microcleidus) taking down the Early Jurassic pliosauromorph Hauffiosaurus. Both species were in the 3-4 meter range and as is pretty obvious Occitanosaurus was an elasmosaurid before it was cool to be an elasmosaurid and Hauffiosaurus was a polycotylid before it was cool to be a polycotylid. You should also take note of the fact that I am inverting the usual trope of "pliosauromorphs" handing "plesiosauromorphs" their asses.

Again, so much of this hinges on if you buy into a social adaptation for these animals - which is a very defensible argument as I went over before. And if you go down that rabbit hole it does in fact open up other rabbit holes to go down.

Now drowning is a time honored reptilian tradition. It does not have to be quick or easy but drowning  prey will get the job done. Once an animal is bitten and then latched onto - and Occitanosaurus did have the teeth to bite onto stuff - you have to wonder how the body, skull, and teeth will react to that

skull Occitanosaurus

forceful interaction. Again not as a single tooth, skull or muscle - but as an entire animal in which stresses and strains can be absorbed and redistributed throughout the entire body. I don't actually think that the jaws and teeth of pelsiosaurs have to be all that robust, strong, or tough at all to maintain a grip. Pythons and boas do just find latching onto and holding onto large, strong terrestrial prey. And python teeth and jaws are a lot more dainty than most plesiosaur teeth and jaws.

Python skull

In the ocean where animals are often a bit softer not operating at 1 g I see no compelling reason that the skull, teeth, and jaws could bite into and hold onto largish stuff. They had a wide gape, and some of those teeth were pretty impressive and temporal muscle size and strength seems to have increased over time as noted earlier. If the prey was too strong or large the plesiosaur would most likely lose a tooth and disengage before the skull itself suffered damage. Could it be possible that plesiosaurs had involuntary biofeedback processes that would disengage from the bite if it was too straining?

Again though, we have to imagine that if a long necked plesiosaur bit into a large and struggling prey item that as the prey item struggled those stresses would largely pass through the head of the plesiosaur into the long neck and ultimately the stout and strong torso of the animal. Remember that as the neck got closer to the body it got stiffer and heavier in these animals. The plesiosaur would simply roll along with the struggling prey and, especially if mob attacking said prey, simply outlast it.

Occitanosaurus tournemiensis. credit Hinweise link

In short I think the majority of typically long necked "plesiosaurian" plesiosaurs fit this dual functionality approach. Able to subsist on small and large food items this basically "mesopredator" niche faired well for them allowing them to outlast more specialized ichthyosaurs, sea crocodiles, leptocleidids, hypercarnivorous "pliosaurine" pliosaurs, and coexist with mosasaurs.

artist Charles Fuge c/o Love in the Time of Chasmosaurus

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BK said...

I really wish that speculative obligate piscivore plesiosaur existed.

Duane Nash said...

Yeah maybe one day we will find one like that. It just seems that whenever plesiosaurs started tapering the jaws they lost neck length and started to look like a "polycotylid" grade plesiosaur.

Robert Haan said...

Duane ! You're finally back !Been a long boring month without you to light up my day with all things wet and slimy.

Anyway another great post, i was lost abit on some of the technical aspects but its become pretty clear now that long neck plesiosaurs were feasible bruisers of the prehistoric oceans , pretty gratifying for me on a personal level as elasmosaurids, namely Elasmosaurus itself has always held a place in my heart.

Duane Nash said...

Thanks Robert glad I could light up your day!! And to anyone who thinks elasmosaurids were nothing but a threat to small squid and trout sized fish I would ask them are you so sure that you would go for a swim with an Elasmosaurus? Do you feel lucky today, do you?

Austin Major said...

So, in a Jurassic Park/World scenario, they can see us as a potential food?

Duane Nash said...

We would be like a rag doll to any plesiosaur in the water. What could you really do if a large plesiosaur bit and held on. Shake free form that 20 foot long, one ton neck? Not only would we be potential prey, we would be easy prey.

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