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Champsosaurs and Metriorhynchids

November 11, 2012

Metriorhynchus by BBC’s CGI technicians. As an accurate depiction of Metriorhynchus, it falls short of several features, like the short forelimbs, the more anguiform like body, et cetera. However, I find it a decent model for a generic simoedosaurid, aside from the small temporal fenestrae and the tail fin.

More interesting than knowing about obscure things is how you came to be aware of them. Just knowing is nowadays an easy, instantaneous task, but my forays into prehistoric knowledge came a long time ago, when I was a little boy, and sometimes it was a genuine adventure.

Nothing rings more true to me than my journey of learning about the obscure fully aquatic sauropsids that are choristoderes. I was always a prehistory geek, but no group of animals has generated so much fervour in learning than these humble, now dead reptiles.

And it all started with a massive misunderstanding.

A crocodile with a shark tail

Inaccurate depiction of Metriorhynchus. One of my first taste of critters like these was a similar picture depicting a Geosaurus. Only it had legs. And was on a beach. Eating a lobster. Srsly.

As a kid interested in paleontology, it would be beyond pathetic to not know about ichthyosaurs and plesiosaurs. However, I can pride myself in that being aware of the more obscure clades of marine sauropsids occured very early in my life.

Back in 2000, my mom got an educational game about Mesozoic marine life. It was a temporary borrowing from the school, and to this day I miss it terribly, even if it does have hilariously outdated models (case in point, a Hesperornis walking like a penguin with a massive generic-theropod tail). From that game, I was aware of critters few if any book until then ever had information about, including Protosuchus and Ptychodus.

Anyways, most important were the lineage of marine crocodillians known as metriorhynchids, known for their scute-less skins and ichthyosaur like tails. My first taste of these animals were quite inept CGI models (a pliosaur-like Metriorhynchus and a champsosaur like Geosaurus), but they did the job of captivating my interest. Alongside the pterosaurs (Pteranodon and Pterodactylus), they were my favourite animals in the game.

A few years passing, and I’ve seen refferences of marine crocs in the Walking with Dinosaurs book and a few encyclopedias, so I knew that those animals were real. Then came one book I had borrowed from the school library, that also featured Metriorhynchus, that had one interesting snippet of information. I cannot recall the exact quote, but it went along the lines of this:

[marine crocodyllians] disappeared eventually on the Palaeocene.

Back then, I was quite surprised, as it pretty much stated that there were marine crocodyllians living in the Cenozoic! On other words, as our earliest ancestors climbed the trees escaping from the crushing gastornithid beak, marine crocodiles swam around, ruling the sea like kings before finally giving way to whales.

This, combined with previous books I got stating the existence of marine crocs that died out in the KT event, incited my curiousity a lot. Back then, I was only aware of Metriorhynchus and Geosaurus, so I was desesperate to learn about those other ichthyosaur like crocodiles living in ancient seas, that appearently lasted as recently as the appearence of the first horses.

Eventually, many, many years later, I would learn that both probably reffered to dyrosaurid crocodillians, which defenitely lasted longer – the most recent taxa died out in the Eocene-Oligocene extinctions -, and weren’t as jaw dropping, looking like big gharials, but back then it inspired dreams of metriorhynchids lording the plesiosaur/mosasaur-less oceans while mammals were eaten by terror birds.

The paradox, the museum and the wardrobe

Steneosaurus, a thalattosuchid crocodylomorph. This, and not the dolphin things above, was the dominant body plan for marine crocodillians. At least the anguiform body is cool.

I had internet for a long period of time, but only in around 2005 did I actually use it. When I did, I indulged my curiousity imediately, satisfying my needs for information. The fact that my sexuality began to blossom back then helped.

Predictably, knowledge on prehistoric animals came first. I found my old pal Metriorhynchus in BBC’s Sea Monsters, much to my glee. Now that I had a source for all the information availiable in the world, learning about the late living sea crocs was on my list.

Once I discovered Wikipedia, I reminded myself to explore this topic. To my surprise, I discovered that metriorhynchids died out by the early Cretaceous. Needless to say, I was shocked. Only much latter did I learn about the Marine crocodylomorph category page, but by then the adventure was over.

And then something weird happened. That year, I went on a trip to a museum exposition about Mongolian dinosaurs. Said exposition only lasted an year, but it was the best thing that happened to that museum, and I think it eventually returned, several years latter. For the first type, I could see in accurate size the remains of Gallimimus, Velociraptor, Oviraptor and several others, somewhat disappointed that they weren’t larger than an ostrich, but still entertained.

And then I saw this:

Tchoiria skeleton.

Not only was the skeleton similar, but there was a reconstruction that looked exactly like a metriorhynchid, and the museum guide labelled it a “false crocodile”. Since I was used to the term as reffering to non-dinosaur dinosauromorphs, I imeaditely jumped to the conclusion that it was a marine crocodillian, the type of creature I was looking for. At some point latter, I saw a pic of what I assume was a Champsosaurus in an Eocene environment, so I was pretty much sure that my assumption was correct.

I chalked up the Wikipedia page as outdated, the first time I did so. And for a while I was satisfied.

Eventually, I desired to learn more about these late-living “marine crocodiles”, to see how they differed from Metriorhynchus and Geosaurus. But, as much as I tried, I couldn’t find the information I wanted. Wikipedia did not up-date, and not knowing the names of those animals, I couldn’t search for information on them.

I then started to use the internet as a social medium, but  did not bring up the strange marine crocodyllians, even because I soon learn that prehistory nerds did not take kindly to interest in anything other than dinosaurs back then.

Another exposition latter, about Liaoning taxa, I learned about Hyphalosaurus, the strange, nothosaur like reptile. I knew it wasn’t a lizard from the start, but I’ve always learned that nothosaurs – my second best guess – were extinct by the Jurassic, so while it did fascinate me, I eventually forgot about it.

Finally, the truth

A Champsosaurus pic I found. I don’t know who the owner is, so please warn me if you want credit. At any rate, I do have to comment that the limbs should be longer and more flipper-like, though.

Eventually, through Avancna’s pics or some other artists similar in style, I learned about Champsosaurus. Finally, these mysterious “crocodillians” had a name!

It was then, to my greatest surprise, that I learned that these were not crocodile relatives, but in fact part of a strange clade of aquatic reptiles known as choristoderes, that also included the long necked Hyphalosaurus from the second exposition. Suffice to say, I was amazed – I had, until then, thought that dinosaurs, crocodillians, ichthyosaurs, plesiosaurs, lizards, tuataras, turtles and pterosaurs were the only reptiles to survive the Triassic extinctions. And imagine my reaction when I learned that these critters survived until the Pliocene!

I then learned about the dyrosaurids and marine gharials, and the rest is history, as my previous article on choristoderes should indicate.

Comparing

Philydrosaurus pic. What metriorhynchids are to crocs, champsosaurs are to animals like these.

Ultimately, I think it is safe to say that mistaking champsosaurs for late living metriorhynchids is a reasonable error. After all, both groups were externally quite similar: aquatic, crocodile like animals, without scutes and with smooth skin instead, with gharial like snouts – derived metriorhynchids and Simoedosaurus aside -, limbs converted into flippers, massive temporal fenestrae and associated jaw muscles and eyes located forward in the skull rather than dorsally. To say nothing of their fully aquatic lifestyles.

Of course, there were some differences, like the fact metriorhynchids were exclusively marine while champsosaurs preffered freshwater or estuaries, the fact that metriorhynchids produced macropredatory monstruousities similar to an hybrid between a shark and an allosaur – though the crocodile like Simoedosaurus came close -, the fact that metriorhynchids had ichthyosaur like tails, and the fact that metriorhynchids had long, almost serpentine bodies while champsosaurs had shorter, more rotund torsos, implying in turn a more benthic lifestyle as opposed to the certainly pelagic crocodillians. Still, they were less obvious than the similarities.

Overall, both lineages of aquatic reptiles are fascinating in their own right, though personally the choristoderes are my favourites by a long shot.

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