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Thursday, 30 May 2013

Wesserpeton evansae: making 'albanerpetontid' a household name

Two Wesserpeton evansae get in each other's faces, because that's what albanerpetontids did. Note this is an updated version of the 2013 press release work referred to below. Prints of this image are available here.
You could be forgiven for thinking otherwise, but the Mesozoic wasn't just the remit of dinosaurs, pterosaurs, marine reptiles and token cool crocodiles. Many other interesting animals shared the world with these famous species, including some that most of us have never heard of. Tuesday of this week saw the (open access) publication of one such animal, the Wessex Formation albanerpetontid Wesserpeton evansae by Steve Sweetman (University of Portsmouth) and James Gardner (Royal Tyrrell Museum of Palaeontology) (2013). Many readers will be familiar with the Wessex Formation or the larger geological unit it is part of, the Wealden Supergroup, because of its frequent mentions as Britain's top dinosaur-bearing deposit. I'm sure many of us are not overly familiar with albanerpetontids, however. This isn't too surprising. To my knowledge, albanerpetontids have never featured prominently in any palaeoart or publications geared towards popular audiences and their existence is largely known only to specialists. The world's naivety to these animals was broken yesterday when Steve and James, with a little help from my painting above, finally told the world why they should add albanerpetontids to their list of cool fossil animals.

Alba-who?
Albanerpetontids are small-bodied amphibians that were fairly common components of terrestrial environments until relatively recently. The youngest members of their clan perished at the end of the Pliocene - about 2.5 million years ago - after an evolutionary run of 160 million years and attaining a wide geographic distribution across North America, Europe, Africa and central Asia. Their general lack of mention in popular press would have you believe otherwise, but they can actually be relatively common fossils. Remains of Wesserpeton are, after crocodiles, the most abundant microvertebrate in the Wessex Formation. Despite their relative abundance, their relationships to other lissamphibians have been debated because many of their fossils are exceptionally scrappy. Traditionally, they have been thought of as caudatans (salamanders) or at least very close relatives. Recent discoveries of complete and articulated albanerpetontid fossils (below) have suggested otherwise however, proposing that they are closely related to a clade containing frogs and salamanders, but not members of any extant amphibian group (McGowan 2002). 

LH 6020, holotype specimen of Celtedens ibericus, a complete albanerpetontid from Lower Cretaceous deposits of Las Hoyas, Spain. Note the 'halo' of scales around the fossil. Snout-vent length of this specimen is 59.5 mm. From McGowen (2002).
The anatomy of albanerpetontids is fairly conservative. They look more-or-less like small salamanders with short limbs and long bodies, but also possess mandibles which interdigitate anteriorly, fused frontals (bones of the skull roof) and relatively flexible necks because of a mammal-like articulation between the skull and neck. They also had bony scales under their skin, a condition which contrasts with the typically thin and delicate skin of most other amphibians. It seems that they spent most of their time burrowing through leaf litter in search of small arthropod prey, with their scaly skin possibly preventing dessication and likely reducing the typical amphibian need for wet or moist environments (but see comments below by the good David Marjanović). Fossils suggest that scales stretched across most of their bodies (we went the whole hog in our reconstruction and covered our Wesserpeton entirely) and onto their eyelids. We thought about these eyelids a fair bit for our painting. The few available depictions of albanerpetontids show animals with eyes perpetually covered with scales, leaving only very small, beady eyes to see with. Steve and I noted that these animals actually have very large orbits however, suggesting that their eyes were probably reasonably sized. It seemed counter-intuitive to possess large eyes and then cover them in soft-tissue, so our reconstruction assumes that the eyelids only partially covered the eyeballs.

Lower jaws of Wesserpeton evansae showing typical (A) and pathological (B) anatomies. From Sweetman and Gardner (2013). 
Small man syndrome
Initially, our plan for the press release painting was to show a single animal reclining in some leaf litter or something equally simple, but Steve suggested early on that we could work in an interesting component of Wesserpeton behaviour. Many Wesserpeton jaws show signs of trauma (above) after being broken during violent acts. The exact cause of this damage is still being looked into and will form the subject of a later paper, but a good preliminary explanation is that Wesserpeton was a vicious species which routinely fought among themselves. Modern salamanders, such as these giants, bite the heads of their opponents before wrestling with each other, twisting and somersaulting with one another to settle disputes over territory and mating access. It's not difficult to imagine such acts taking their toll on the jaws of Wesserpeton, and we thought it would be cool to show this in a press image. Preliminary attempts at rendering this struggled to show the general appearance of the animals however, as their bodies were twisted and their heads obscured by jaws. How could we show the aggressive nature of this animal without actually showing them fighting?

Do you speak salamander? Common body postures used to communicate between individuals of the red-backed salamander Plethodon cinereus. We took panel B as our primary inspiration for the Wesserpeton evansae PR image. Figure and caption from Jaeger (1984).
The solution came in the form of postural language borrowed from modern red-backed salamanders (Jaeger 1984, and above).Most animals will attempt to intimidate their rivals with ritualised postures which enhance their perceived size and strength before coming to physical blows, and there's no reason to think that little albanerpetontids were any different. We decided to use the postures of modern salamanders in our image, setting one of our animals as a dominant pose with a raised trunk and tail, and the other crouched and submissive. In doing so, we implicated the violent nature of this species (enhanced by the larger animal getting in the face of the smaller, just like most douches with attitude problems) but maintained the ability to show their anatomy. Entirely coincidentally, Darren Naish and John Conway recently spoke about incorporating animal postures into palaeoart in Tetrapodcats (sic) episode 6, which makes for interesting listening if you're thinking about making your restored extinct animals communicate more fluidly.

Finally, a quick word on the body size of Wesserpeton. We've mentioned it was small, but how small? The answer is tiny. As in, 35 mm snout-vent length tiny. This thing really puts the 'micro' in 'micropalaeontology'. We prepared another set of press images to show what this means in real life (available in different colours to suit whatever occasion you're at where you want to discuss the size of Wesserpeton):

The United Colours of Wesserpeton, which is dwarfed by the palm of your hand no matter what colour you are.  For some reason, this image makes me want to imagine a world without lawyers.
I'm no expert on this sort of thing, but I'll wager that Wesserpeton is one of the smallest, if not the smallest tetrapod species known from the Wessex Formation, and probably one of the smallest tetrapods in the fossil record. It's fossils were only recovered through bulk sampling tonnes of plant debris bed from the Wessex Formation, horizons rich in plant and vertebrate remains deposited after sheetflood events, and would be almost impossible to find via surface prospecting. Those of you with excellent memories may recall that Steve's ongoing analyses of these beds have revolutionised our understanding of the Wessex palaeobiota, of which Wesserpeton is just one discovery among many. 

And that will have to do for now. Next week: back to the world of pterosaurs with pterosaur mummies, as promised last week before Wesserpeton face-wrestled its way into centre stage. My plan from here on is to have some sort of run-up to the publication of my book, Pterosaurs: Natural History, Evolution, Anatomy on June 23rd, so be sure to stick around if wing membranes are your thing.

References
  • Jaeger, R. G. 1984. Agonistic behavior of the red-backed salamander. Copeia, 309-314.
  • McGowan, G. J. 2002. Albanerpetontid amphibians from the Lower Cretaceous of Spain and Italy: a description and reconsideration of their systematics. Zoological Journal of the Linnean Society, 135(1), 1-32.
  • Sweetman, S. C., and Gardner, J. D. 2013. A new albanerpetontid amphibian from the Early Cretaceous (Barremian) Wessex Formation of the Isle of Wight, southern England. Acta Palaeontologica Polonica, 58, 295–324.

13 comments:

  1. Great idea to publish an article about Lissamphibians ! They are overshadowed with marine reptiles by Dinosaurs.
    I love the scale in the painting,a nice add with the colors...

    I hope that one day someone will write a book about Marine Fauna in the mesozoïc...But first, there are Pterosaurs !

    Oliver

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    1. Hi Oliver,

      Thanks for the kind words. The scale on the image was something that took some time to get right because the animals are so very, very small (I still think we're a bit off actually, but don't tell anyone). I modelled the cycad base on the right, for instance, on a small cycad I have at home and was astounded when each segment proved to be about a third as long as the animals themselves. Environments at that scale are so very different to what we're used to that it took a while to start thinking about them in the right way. We tend to think of leaf litter as a flat carpet on the ground for instance, but to a tiny Wesserpeton that litter would be a complex, three-dimensional world with distinct tiers and horizons. It was actually quite fun to tackle something on such a different scale for a change, what with so many other Mesozoic species being pretty durned big.

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  2. Hey Mark, interesting post. What exactly do you mean by scales (confusing to me since its an amphibian)? Are they the same sort of structures seen on reptiles or did they just have rougher textured skin like toads and certain newts?

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    1. Hi Chris,

      The scales are true scales, and not just leathery skin. You can just about make out the individual scales in the complete Celtedens image above, particularly around the neck. I guess a good analogy here is feathers on reptiles. Sounds weird because modern reptiles are scaly, but birds have one or two things to say about that.

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  3. This was great, but I have only one slight fudge. If Wesserpeton had scales, is there any possibility that with large eyes came large spectacle scales, such as those on large geckos? If that's the case, then you could have some serious fun with BIG eyes.

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    1. Hi txtriffidranch,

      Thanks for the comment and interesting idea. I don't think there's any reason to infer spectacle lenses in albanerpetontids yet, however. I could be wrong, but there's no indication from their fossils to indicate they had them. The scales we have preserved within the eye of Celtedens are small and hexagonal - basically the same as those seen on the rest of the animal - and these probably represent scales from eyelids. There's nothing else preserved there to indicate a transparent lens scale also sat within the orbit (would be interesting to know what the preservation potential of that sort of structure is - I suspect it's not high), so I'd hesitate in making the assumption that there was one. Caution is particularly warranted because the spectacle lenses aren't ubiquitous across scaly animals, just across a few groups like snakes and geckos. Thus, there's probably no reason to infer their presence just because albanerpetontids were scaly.

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  4. Excellent to see albies getting this sort of attention. Wesserpeton is indeed "one of the smallest tetrapods in the fossil record": it might be beaten by the Paleogene mammal Batodonoides (1.3 g, and probably less than 30 mm long), plus there are tons of fossil tadpoles.

    On the 'spectacle scale' suggestion, some albanerpetontid fossils seem to preserve evidence for distinct eyelids.

    Darren

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    1. Hey Darren, thanks for the comment.

      "Excellent to see albies getting this sort of attention."

      Certainly makes a change from ornithodirans hogging the palaeo press limelight. Working on this image has me wondering if other unusual extinct species would do well in press work, but the scientists studying them don't think to put them out there. Maybe pterosaur and dinosaur scientists are simply exposure hungry and obsessed with seeing their own names in print? No, that can't be true.

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  5. David Marjanović2 June 2013 at 08:43

    with their scaly skin preventing dessication and likely reducing the typical amphibian need for wet or moist environments

    Oh no. These aren't sauropsid scales. Sauropsid scales are thickenings of the keratinized layer of the epidermis. What albanerpetontids retain are plates of bone in the dermis, you know, fish scales. Fish scales do not protect from desiccation.

    How big the eyes were is one question, but eyelids must have been present: all terrestrial lissamphibians have eyelids.

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    1. Thanks for the comment, David. I was unable to find reference to the exact composition of albanerpetontid scales and tried to be as vague as possible with their description in the post. I've tweaked the text to make it more accurate now.

      "Fish scales do not protect from desiccation."

      OK, that's a very interesting comment. The albanerpetontid literature - at least the bits that I've read, but I claim no expertise here - says quite the opposite! Do bony scales really have no effect whatsoever on dessication rates when compared to thin, naked skin? I mean, is this a case of their resistance to dessication being overstated in some places, or is it a complete non-starter?

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  6. David Marjanović4 June 2013 at 13:12

    Well, maybe they have some effect, but they're inside the skin. The epidermis and part of the dermis lie above them and are thus unprotected.

    The albanerpetontid literature is written by herpetologists who aren't familiar with either "fish" or with all those Palaeozoic tetrapods that retained scales... they know squamates and lissamphibians.

    There are extant terrestrial amphibians (most newts for instance) that have a thick keratinized top layer of the epidermis, like amniotes. It's of course possible that the albanerpetontids had such a thing, too. But it's independent of scales.

    I forgot to mention that many caecilians retain such scales, too, but those are very poorly mineralized.

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    1. Thanks again, David, this is all very interesting to hear. I've now linked to this discussion in the text above. Sounds like the dessication resistance and burrowing ideas need something of a rethink.

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