Tetrapod Zoology

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Thanks to Monday’s article on the unusual African mosasaur Goronyosaurus, I will admit that I was – quite seriously – considering doing a ‘mosasaur week’, perhaps even a ‘weird mosasaur week’. Alas, I have not had the time. However: brand-new in the journals is Johan Lindgren et al.’s article on the skin of a Plotosaurus bennisoni specimen from the Upper Cretaceous Moreno Formation of central California (Lindgren et al. 2009) [adjacent Plotosaurus life restoration from wikipedia]. The specimen was collected by Anthony Fiorillo in 1993 and belongs to an individual that was about 6.4 m long when complete. The most interesting thing about it is its skin. Let’s see why.

Plotosaurus has already been the subject of much attention recently, as new work has shown that it had a deep, relatively stiff thorax and (apparently) a specialised, semilunate tail (Lindgren et al. 2007). Incidentally, Plotosaurus and Goronyosaurus are apparently sister-taxa within the mosasaurid clade Plotosaurini (Conrad 2008). Preserved in association with the specimen’s bones are patches of skin: as you can see from these images (Fig. 1 from Lindgren et al. 2009), they are beautifully preserved, though don’t be fooled into thinking that you’re looking at a colour pattern (instead, the dark colour represents preserved organic matter) [note that (d) shows the scales of an extant skink for comparison].

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Mosasaur skin is actually not a new discovery: Snow reported it in 1878, Samuel Williston illustrated it in his 1914 book Water Reptiles of the Past and Present [see image below], and various workers have drawn attention to it in specimens of the derived mosasaurid mosasaurs Tylosaurus and Platecarpus* (Snow 1878, Martin & Rothschild 1989). Preserved skin has also been reported in basal mosasaurs like Vallecillosaurus from Mexico (Smith & Buchy 2008). Having said all that, it’s rarely discussed in the literature and all too few people know about it. The new skin specimen is preserved three-dimensionally. You might predict that derived mosasaurs, being fully aquatic and strongly streamlined, would have smooth, scale-less skin. Though there is still a great deal of uncertainly, a naked, scale-less epidermis has also been suggested for other Mesozoic marine reptiles, like ichthyosaurs and plesiosaurs. Ichthyosaurs apparently did lack scales, but we’re still waiting for more information on plesiosaurs (apparently, the New Zealand cryptocleidoid Kaiwhekea includes skin impressions [Arthur Cruickshank pers. comm.], but I don’t think these have ever been published). In fact some mosasaur workers have indeed suggested that at least some mosasaurs were naked-skinned, simply because it has been assumed that an absence of scales might improve hydrodynamic efficiency. Well, forget it: it seems that all mosasaurs were scaly.

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* Lindgren et al. (2009) state that the Plotosaurus skin is ‘the first documented squamation in a mosasaurine mosasaur’ and also ‘the first record of skin in an advanced member of the Mosasauroidea’. I’m somewhat confused by the second of these claims, given that skin preservation has previously been reported in taxa that are indeed ‘advanced’ within Mosasauroidea.

The skin in the new Plotosaurus is covered in scales, though the scales are tiny for the size of the animal (approximately 2 mm x 2 mm). They’re rhomboidal, and arranged in an alternating, overlapping pattern. What’s perhaps most interesting is that the scales have keeled surfaces: a central keel, running along the long axis, is flanked on either side by lower keels. Longitudinal grooves separate all of the keels (Lindgren et al. 2009). Keels have been mentioned on mosasaur scales before, but I don’t think that multiple keels have been noted. Some of the Plotosaurus scales lack any of this surface ornament, and the authors suggest that these scales might have belonged to the animal’s ventral surface.

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So, the scales possessed parallel grooves and keels on their surfaces, all of which were aligned with the body’s long axis. The most interesting thing about Lindgren et al.’s new paper is that they note the similarity this has with similar ridge and groove systems seen on the skin surfaces of certain other aquatic vertebrates. They note that the dermal denticles of pelagic sharks [like those shown in the adjacent image] possess a superficially similar ornamentation, and that – in the sharks – this seems to have a hydrodynamic role: by ‘channelling’ the boundary layer of liquid across the animal’s epidermis, the ridges and grooves reduce microturbulence and hence lower overall drag. Some work (which I haven’t seen) demonstrates this experimentally (Raschi & Tabit 1992). I’ve also heard of work on the cetacean epidermis which, similarly, demonstrates that microscopic, sagitally aligned ridges and grooves have an identical, drag-reducing function. I heard about this research on TV years ago and have been unable to discover whether it’s been published or not. The relevant TV programme (which would have been screened in the early or mid 1990s) explained how efforts were now underway to use this system on military submarines. Does anyone know any more about any of this? There is, at least, some published research showing that cetacean skin is microscopically grooved (Purves 1963 and references therein) [dermal ridge pattern in a dolphin shown below, from Purves (1963)].

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The obvious inference to make from the parallel grooves and ridges on the tiny Plotosaurus scales, then, is that they served an identical hydrodynamic function (Lindgren et al. 2009). Perhaps the next step is to test this by making models and testing them in flume tanks (real or virtual). What improvement or advantage might drag-reducing scales make on a mosasaur, if any? And would the structures really enhance performance as their supposed analogues do in sharks?

Indeed, we may be jumping ahead too far, and Lindgren et al. (2009) also note that the keels on the scales may have reduced shininess, and hence helped mosasaurs to conceal themselves from prey (and from other predators). They further note that scales may also have helped protect these animals from predators, rivals and ectoparasites. An anti-parasite function is not obvious to me, perhaps it’s something to do with the regularity with which scales can be shed? [Platecarpus image below from wikipedia].

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There is one final confound to all this, and this is the phylogenetic perspective: given that keeled scales are ancestral for anguimorphs (the major squamate clade to which mosasaurs and other platynotans belong), are mosasaur scales, in fact, nothing special? And what about the complicating factor that unkeeled scales have been reported from one basal mosasaur? Lindgren et al. (2009) provide answers on this: it seems that multiple keels are derived relative to the primitive anguimorph condition (in which case, mosasaur scales might be highly specialised). Meanwhile, the unkeeled scales reported in one basal mosasaur (Vallecillosaurus) come from the animal’s ventral surface and hence do not demonstrate that keeled scales were definitely absent in this taxon.

So, it’s an interesting paper that proposes an interesting idea. As for me: having just written an article of over 1000 words, I have failed failed failed again.

For a previous article on the skin of Mesozoic marine reptiles see The skin of ichthyosaurs.

Refs – -

Conrad, J. L. 2008. Phylogeny and systematics of Squamata (Reptilia) based on morphology. Bulletin of the American Museum of Natural History 310, 1-182.

Lindgren, J., Jagt, J. W. M. & Caldwell, M. W. 2007. A fishy mosasaur: the axial skeleton of Plotosaurus (Reptilia, Squamata) reassessed. Lethaia 40, 153-160.

- ., Alwmark, C., Caldwell, M. W. & Fiorillo, A. R. 2009. Skin of the Cretaceous mosasaur Plotosaurus: implications for aquatic adaptations in giant marine reptiles. Biology Letters doi:10.1098/rsbl.2009.0097

Martin, L. D. & Rothschild, B. M. 1989. Paleopathology and diving mosasaurs. American Scientist 77, 460-467.

Purves, P. E. 1963. Locomotion in whales. Nature 197, 334-337.

Raschi, W. & Tabit, C. 1992. Functional aspects of placoid scales: a review and update. Australian Journal of Marine and Freshwater Research 43, 123-147.

Smith, K. T. & Buchy, M.-C. 2008. A new aigialosaur (Squamata: Anguimorpha) with soft tissue remains from the Upper Cretaceous of Nuevo León, Mexico. Journal of Vertebrate Paleontology 28, 85-94.

Snow, F. H. 1878. On the dermal covering of a mosasauroid reptile. Transactions of the Kansas Academy of Science 6, 54-58.

Comments

  1. #1 Carpworld
    April 17, 2009

    Great article, Darren. AFAIK, modern military subs do indeed have drag reducing coatings that also absorb incident sound energy and are tailored for specific parts of the ship which emit different frequencies. The drag reduction comes from micro-pits and compliant coatings to react to pressure fluctuations and they can also release a polymer which can further reduce drag. I seem to recall they can get the drag down by 60% with all of this. The coatings were indeed based on the study of dolphins and sharks. I like submarines!

  2. #2 Carpworld
    April 17, 2009

    Sorry, that should be *boat* not *ship*, my bad.

  3. #3 Dartian
    April 17, 2009

    As for me: having just written an article of over 1000 words, I have failed failed failed again.

    Not only that, but you have violated your ‘tetrapods only’-principle with that picture of shark scales. You’re losing it, man.

    (Just kidding, of course. The post is impressive as usual, and that shark scale image is awesome. It looks like a fleet of Star Wars spacecraft or something.)

    scales may also have helped protect these animals from predators, rivals and ectoparasites. An anti-parasite function is not obvious to me, perhaps it’s something to do with the regularity with which scales can be shed?

    It may or may not be pertinent to note here that there’s at least one species of tick, Amblyomma nitidum, which is found exclusively on laticaudid sea snakes – which, of course, have scaly skins (Heatwole, 1999).

    Reference:

    Heatwole, H. 1999. Sea Snakes, UNSW Press, Kensington.

  4. #4 Sordes
    April 17, 2009

    Very interesting. Did you know that the evolved mosasaurs from Specworld had already such keeled sclales similar to the denticles on shark-skin?

  5. #5 Lilian Nattel
    April 17, 2009

    But so interesting!

  6. #6 Matt
    April 17, 2009

    Since Plotosaurus had similar scales to a shark and skull morphology like an icthyosaur. Should reconstructions of Plotosaurus be given caudal fin and a tail like a shark? They seem to be filling in the ecological niche left over by the icthyosaurs. This was discussed by Lindgren(2007) at the 2nd Mosasaur meeting.

  7. #7 Darren Naish
    April 17, 2009

    Matt: Lindgren et al. (2007) – cited above – presented evidence for a vertical lobe on the tail fin in Plotosaurus, and noted a similarity with some ichthyosaurs. No evidence for a dorsal fin, but then you need soft tissue outlines for this.

    Many thanks for comments, especially to Carpworld Nik.

  8. #8 anon
    April 17, 2009

    - “having just written an article of over 1000 words, I have failed failed failed again.”

    Dude, why don’t you break up these “long” articles into 2 or 3 sections and post them on separate days? If you posted Monday-Wednesday-Friday, you could write one article at the beginning of the week and then have the rest of the week free to work on your other stuff.

    (Yeah, that would be a “light” posting schedule, but that’s the idea, right?)

  9. #9 jck
    April 17, 2009

    I think this sort of texture is also used in the suits that Olympic swimmers wear.

  10. #10 deang
    April 17, 2009

    As for me: having just written an article of over 1000 words, I have failed failed failed again.

    It is because you are lazy, an epic failure.

    Seriously, though, this was another very informative post. Thank you.

  11. #11 Bec
    April 17, 2009

    If it helps, I enjoyed reading it so much I didn’t even notice its length. Fascinating stuff.

  12. #12 Hai~Ren
    April 17, 2009

    Thanks for another very informative post, Darren.

    Dartian: A marine tick??!

    Mosasaurs are really one of my favourite groups of animals. If the only marine deposits we had were from the Jurassic and Early Cretaceous, who could have guessed that a group of sea-going lizards would quickly replace the ichthyosaurs, plesiosaurs and pliosaurs as the dominant marine predators?

  13. #13 AnJaCo
    April 17, 2009

    …scales may also have helped protect these animals from … ectoparasites. An anti-parasite function is not obvious to me…

    My thought would be that an ectoparasite would need an ‘opportunity’ to latch on to its host. Less surface turbulence/greater laminar flow would seem to make this a bit more difficult. The little buggers would flow right by with less chance to grab on. Sort of like trying to grab the brass ring on a merry-go-round that is spinning like mad.

    And:
    Dartian, do those marine ticks latch on to the snakes when they are on land? If not, now I have something else to look for on my torso when I leave the ocean. Oh wait, I am nowhere near the tropics.

  14. #14 LeeB
    April 18, 2009

    Something that springs to mind, if the keels reduce drag then why have unkeeled scales on their ventral surface, surely drag would affect the ventral surface as much as the dorsal surface.

    And producing more blogs on weird mosasaurs is no bad idea, there are lots out there such as Prognathodon kianda, Kourisodon, Igdamanosaurus, Carinodens, Pluridens etc.

    LeeB.

  15. #15 Carlos
    April 18, 2009

    “Very interesting. Did you know that the evolved mosasaurs from Specworld had already such keeled sclales similar to the denticles on shark-skin?”

    Sort of. In the original version they had them, but they then were replaced by soft skin like that of ichthyosaurs. But, given this post, the staff might wish to give mosasaurs scales again

  16. #16 David Marjanović
    April 18, 2009

    What? We got rid of the scales? Surprises me. I’ll have to check (and repair it).

  17. #17 Dartian
    April 18, 2009

    AnJaCo (and Hai-Ren):

    Dartian, do those marine ticks latch on to the snakes when they are on land?

    Correct. The ticks themselves aren’t marine; they attach to laticaudid sea snakes when these come to land*. For how long the ticks survive when the snakes are at sea I don’t know.

    * Fully terrestrial reptiles, as is well known, are often real tick magnets.

  18. #18 Carlos
    April 18, 2009

    @David Marjanovic: Yes you did. Though the mosasaur page is no longer working for me, so I can’t check again

  19. #19 Carpworld
    April 18, 2009

    I was thinking again about this drag reduction business and it makes sense that this adaptation in mosasaurs would be most useful not for energy saving or speed but for stealth. Combined with anguilliform swimming it would make the animal extremely quiet.

  20. #20 Metalraptor
    April 18, 2009

    Carlos is right, the mosasaur page was down.

    “Not only that, but you have violated your ‘tetrapods only’-principle with that picture of shark scales. You’re losing it, man.”

    And the post on the “vertebrates” of Snaiad, don’t forget that.

  21. #21 Tanystropheus
    April 18, 2009

    Fascinating article, Darren!

    Please consider doing a ‘mosasaur week’ whenever you get the time…I for one would very much appreciate it!

  22. #22 R.A.W.
    April 18, 2009

    I have a book on military aircraft developments that mentioned using denticle-like structures back in the ’80s. I don’t know if anything has since come of it.

    Getting laminar airflow over the surface of an aircraft has proven tricky. Little imperfections, like rivets, tend to mess it all up.

  23. #23 DDeden
    April 19, 2009

    Some guy swimming from one isle to another in Hawaii got ‘nibbled’ by a cookie cutter shark; having keeled scales might prevent that and remoras. No doubt hydrodynamic efficiency was critical, perhaps also a form of structural color, as in feathers and moth wings (if it works underwater).

    Humans have subcutaneous muscles (pili) that lift the hair (for warmth, goose pimples), seals lack these. Are the shark denticles or snake scales adjustable by similar tensioning of skin muscles to give a smoother flow at certain speeds? Fab post!

  24. #24 Sordes
    April 19, 2009

    I don´t think that denticle-like sclaes would prevent very much from parasites. The mosaurs had already before scales, and I don´t think there would be that much difference. Furthermore even modern sharks suffer from a lot of parasites on and in their skin. And it also does not prevent them from cookie-cutter sharks. Nearly all megamouth-shark specimens showed typical scars made by the small parasitic sharks.
    The denticle-like surface is already even used to upgrade suits for professional swimmers, and it even works for planes.

  25. #25 John Scanlon FCD
    April 20, 2009

    I tried to fill in some detail on the habits of Laticauda yesterday but my comment was swallowed. Trying again:
    Sea Kraits are aquatic to about the same extent as Galapagos marine iguanas: they spend most of their time on land (basking, sleeping, sloughing, mating, egg-laying) and forage in the sea (or in one species, a land-locked lake) for a few hours at a time (they can sustain longer dives than Amblyrhynchus because the equatorial current warms a lot between the Galapagos and western Pacific). This leads to very high densities on land (cf. seabird colonies) and creates good conditions for ectoparasites, as long as they can survive underwater for a few hours at a time. Ticks are pretty tough. True sea-snakes don’t come out on land and therefore don’t have this problem, but can get encrusted with barnacles and algae, countered by a higher sloughing frequency than most land snakes.

    Also, there are some other basal pythonomorphs with preserved scales, e.g. Carsosaurus marchesettii and Pontosaurus kornhuberi, which have single keels.

  26. #26 Graham Peter King
    April 22, 2010

    Wow.

    I like the way you ‘fail’, Darren!

    Smoother ventral scales makes me wonder.. did mosasaurs ever have to manoeuvre on a substrate (bask on offshore rocks like seals) or even come out onto land? even if only wriggling across a sandbar, perhaps a smooth belly might be advantageous over a ridged one, in terms of reducing friction/abrasion? (though these preserved scales and keels are so small (2mm), that may not be significant).

    I wonder if mosasurs mated whale-style.

    Incidentally would the scales scale up in size with size of an individual, from hatchling/live birth to adult, or would the scales stay the same size and there would just be more of them? How are extant reptiles in this respect?