Monday, October 6, 2014

Thus Spoke Zarafasaura

Seriously I need to start a plesiosaur legal defense fund. What other group of Mesozoic beasties has suffered such a profound and jarring hit to their reputation? Once depicted as worthy and regal denizens of the deep, able to hold their own against the other Mesozoic marine reptiles - everywhere I look now

Hawkins demonic plesiosaurs battling other sea monsters in eternal darkness. nice

all I see are pictures of plesiosaurs getting rag dolled by mosasaurs or decapitated by pliosaurs. Seriously its like everyone in the ocean is just taking out plesiosaurs left and right as if they were blundering insults to Darwinian evolution.

From Hero to Zero

Well I say it is high time we turn back these insults to plesiosaurs. 

Don't get me wrong we actually do know plesiosaurs were preyed upon. I just think all this violence perpetrated upon plesiosaurs glosses over the very real and very impressive predatory arsenal they had at their disposal. You heard me right plesiosaurs had an impressive predatory arsenal.

I think first and foremost this emerging image of plesiosaurs as little more than cannon fodder for the more impressive macropredatory saurians it shared the ocean with has a lot to do with two prevailing notions; that (1) long necked plesiosaurs (plesiosauromorphs) are often considered a bit "samey" and; (2) they are considered gape limited (can't expand their lower jaw out) predators of small prey (i.e. basically fish no longer than 20" for the largest elasmosaurids).

Well with regards to number #1 that they are a bit "samey" this is quickly discredited upon actually looking at the surprising diversity in both tooth/jaw morphology. Yes there were slight, fine toothed plesiosaurs - that potentially even sieved small prey out of the water/sediment - but at the other end of the spectrum there were some impressively toothed and jawed species that speak to a more robust capacity for predation. Which gets me to #2 that plesiosauromorphs were severely limited to a prey base of only smaller fish and cephalopods.

In order to challenge #2 I want to invoke a little heralded elasmosaurid from the Maastrichtian (Upper Cretaceous) pbosphate rocks of Morocco: Zarafasaura oceanis (Paper 2013 Lomax & Wahl). I just want you to take a good look at the skull; the robust mandible; strong piercing dentition; large temporal (jaw closing muscle) area; brevirostrine morphology. This was the bulldog of the elasmosaurids. 

Lomax & Wahl 2013. scale bar 10 cm
Lomax & Wahl 2013. scale bar 20 cm

Are you really confident that this was a mere predator of only small fish and cephalopods - harmless to anything bigger? So confident that you would go for a swim with it? Are you sure that sauropterygian brain would not take an opportunistic lunge at you? Or even worse if it travelled and fed in groups (a distinct possibility given live birth for this group) that you might meet your fate in a spectacularly gruesome Mesozoic version of being "drawn and quartered" by several  20 foot long sauropterygians?

What I think gets left out of the discussion with regards to prey dismemberment in carnivores that operate at 1 g versus carnivores that operate in water is the differing physics of each realm. In the water as food is being shook, rolled, or twisted the water itself acts as resistance to such movement. This higher viscosity of water compared to air allows the consumer to gain leverage and further increase the damage and bodily insults to the food item as it is being shook about violently in any number of ways (yanking, rolling, twisting). This extra bit of leverage that aquatic predators can use to dismember food is put to good use by various sharks, fish, eels, crocodiles and other critters. And as I have discussed before I think a number of long necked plesiosauromorphs may have engaged in food dismemberment strategies - including cooperative feeding/dismemberment and twist/rotational feeding - to increase the size envelop of food items that they can consume.

Neck flexibility is a tricky thing in plesiosaurs. They could not likely do the graceful "S" snake like strike pose but dorso-ventral flexibility of the amount pictured above by Mark Evans is possible give or take a bit depending on how much intervertebral cartilage was present. But for our purposes here the diagram above is useful in imaging how such animals may have used the puncture/pierce type teeth, long neck, and heavy rigid torso to get bite sized chunks off of  carcasses. 

1) Establish a good securing bite.

2) With neck extended and heavy torso acting as fulcrum/pivot point/ballast - yank neck away from food item. If flesh is soft and/or rotten this step might not even be needed. 

3) Conspecifics and/or water viscosity aids in leveraging off a bite size morsel. 

4) Swallow, rinse, & repeat

Is there any evidence for Zarafasaurus oceani engaging in rugged feeding events? Well maybe... In the paper I linked to earlier (Lomax & Wahl, 2013) there is specific mention of "vertical, serration like marks preserved on the inner side of the left dentary" and the "odd, strong rugosity that surrounds the base of the posterior premaxillary teeth". Picture below of both features.

Lomax & Wahl 2013
Interesting for sure, but nothing definite. Could those vertical serrations suggest habitual feeding of shelled/rugged prey? Ammonites, chondrycthians, turtles or all of the above? Sidedness? Who knows but the rugosities, serrations, and general rugged and robust morphology of the skull and skeleton as a whole suggests a capacity for certain violent feeding activities.

Wiki. CC
But what spurred me on to write about Zarafasaura today, as I have been wanting to write about this guy for a while now, is actually a paper: Mosasaurids (Squamata) form the Maastrichtian phosphates of Morocco: biodiversity, paleobiogeography, and paleoecology based on tooth morphoguilds. Bardett et al. 2014 Gondwana Research Abstract:

Mosasaurid squamates are the most numerically abundant, and taxonomically/ecologically diverse clade of marine amniotes represented in the Maastrichtian Phosphates of Morocco. With few exceptions, they are faunally typical of the Southern Mediterranean Tethys Margin (around palaeolatitude 25°N) and range from the base to the top of the stage. The Moroccan assemblages include at least 7 genera and 10 species representing a broad spectrum of sizes and morphologies that illustrate several ecological trends. Noteworthy is the predominance of Mosasaurinae which are widespread in contemporaneous outcrops worldwide and constitute 80% and 70% of the total genus/species number respectively. In contrast, Halisauromorpha and Russellosaurina (plioplatecarpines) are scarce and tylosaurines are presently unknown. All of the Moroccan mosasaurids exhibit characteristic tooth morphologies and can be placed into resource partitioning morphoguilds indicative of adaptations for piercing, crushing or cutting. Medium to large predators are found to distribute along the ‘Crush’–‘Cut’ axis of the morphoguild projection, and a new ‘Crush–Cut’ guild, previously unrecognised amongst Mesozoic marine amniotes, accommodates severalPrognathodon species. Also of importance is the lack of mosasaurids along the ‘Pierce’–‘Crush’ axis, potentially inferring that these ecological niches were occupied by other marine vertebrates such as selachians and plesiosaurians. In addition, the relative abundance of mosasaurids throughout the Maastrichtian series of the Gantour Basin evidences direct ecological competition or predation phenomena.

Did you catch that sentence towards the end? The one that says "Also of importance is the lack of mosasaurids along the ‘Pierce’–‘Crush’ axis, potentially inferring that these ecological niches were occupied by other marine vertebrates such as selachians and plesiosaurians.

As the only other plesiosaurians known from the Moroccan phospates are dainty toothed polycotylids, who does that leave but Zarafasaura oceanis?

And as I said earlier we need to stem the tide of plesiosaurs getting man handled so here I drew a mob (siblings?) of Zarafasaura oceanis having a go at the carcass of an immature Ocepochellon bouyi. Did they kill it or scavenge it? You decide.

Putting a little of the alien, evil, otherworldly, Ctulhuish aspect back into plesiosaurs.


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Paleo King said...

This is a good and much-needed post. Plesiosaurs all too often get painted as wimps. Obviously they were dangerous enough to survive for millions of years in all the savagery of the Mesozoic seas, so they must have had something going for them.

It's just the same as when typical kids' books depict sauropods as defenseless and weak, when in reality they were armed with thumb and foot claws, deadly weaponized tails (some with clubs, whips and even spikes), flared out hip muscles to kick sideways and whack a bird-limbed theropod into the dirt, and simply the SIZE to be dangerous at any speed.

Even the mouth of something like Giraffatitan was dangerous, those teeth had sharp tips like railroad spikes. And the head of even an immature Giraffatitan was 3 feet long, imagine how big the head was on an "adult" specimen like HMN XV2.

Duane Nash said...

Thanks for comment Nima. Yes we need to shift the paradigm of plesiosaurs back a bit to the original demonic long necked soul suckers so often depicted in earlier paleo-art.

Agreed on sauropods and large herbivores in general. Funny you mentioned megasauropod heads as I was looking at a pic of Arnentinosaurus and well it was perspective trick but it looked big enough to swallow a full grown man. I never thought about the rail road spiked teeth of brachiosaurids but now that you mention it they are impressive.

Working on a post suggesting a full grown Iguanodon bull was an animal you would not want to tussle with.

BK said...

Yay for killer plesiosaurs!

Anonymous said...

Niche partitioning alone can't explain how elasmosaurids like elasmosaurus, thalassomedon grew to the sizes they did without the ability to defend themselves. Yes, it's true their fossils were discovered in areas without large mosasaurs but they just didn't eat fish. Pterosaurs and juvenile mosasaurs have been discovered in stomach contents, plus their mass, sharp teeth and jaws were nothing to scoff at.

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