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Friday, 24 February 2017

Plesiosaur palaeoart: thoughts for artists

Jurassic plesiosauroid Plesiosaurus dolichodeirus with a controversially dipped left hindfin. Nothing like a little drama to start a blog post.
Among the first animals to feature prominently in palaeoart were plesiosaurs, those four-flippered marine sauropterygians that need no introduction to anyone who's reading a blog focused on prehistoric life. Some plesiosaur depictions are among the most spectacular palaeoart of all: their arcing spinal columns, toothy faces and the moodiness intrinsic to seascapes are wonderful ingredients for palaeoartists to play with, leading to two centuries of plesiosaurs as dependably gripping art subjects.

Despite their popularity among artists, the theory we apply to our plesiosaur reconstructions has not been significantly 'modernised' in the way that it has for other prehistoric species, most obviously Mesozoic dinosaurs, pterosaurs or fossil mammals. A number of authors and artists have produced solid foundations for the reconstruction of the latter animals - libraries of skeletal references, assessments of gait and stance, heightened awareness of common soft-tissues, etc. - and their life appearances are now more uniformly reconstructed and prone to fewer obvious errors. This has yet to happen for plesiosaurs, however. Modern skeletal reconstructions are few, references for muscle layout and soft-tissue data are fewer, and discussions over aspects of their life appearance are rare.

I was recently commissioned to produce two studies of two Early Jurassic plesiosaurs - one of the plesiosauroid Plesiosaurus dolichodeirus (above) and another of the pliosaurid Attenborosaurus conybeari (below). I cannot claim any expertise in plesiosaur science, but when reviewing art-relevant literature on these animals it struck me that many familiar elements of plesiosaur palaeoart oppose our soft-tissue data, modern muscle studies and flipper arthrology, as well as the generalities of vertebrate anatomy. I'm sure others have noticed these issues before me, but their prevalence in contemporary plesiosaur art suggests they are not as widely known as they could be. In the interests of stirring conversation on restoring plesiosaurs, I thought I'd share my findings and thoughts here.

Flipper shape and motion

One of the ‘classic’ elements of plesiosaur reconstruction is their distinctive flipper shape: a tight, oar-like profile which hugs the contours of the fin skeletons. However, both muscle studies and soft-tissue data indicate that their limb morphology was quite different to the underlying osteology, and our 'oar-like' depictions are problematic.

Firstly, reconstructions of plesiosaur forelimb musculature show that they were likely powerfully muscled around the shoulders, especially ventrally. Reconstructions of plesiosaur forelimb musculature have been around for almost 100 years and several alternative ideas on the exact configuration are available. They vary from sparingly muscled reconstructions where those massive, plate-like pectoral elements are left mostly free of muscle anchorage (e.g. Carpenter et al. 2010), to models where the entire girdle is swathed in huge muscle attachment sites (Araújo and Correia 2015). The latter seems to reflect the most phylogenetically-informed hypothesis (using data from lizards, crocs and turtles, which seems sensible given on-going uncertainty about plesiosaur ancestry) and - from a purely intuitive perspective - an extensively muscled limb girdle seems more likely than a lightly muscled one. Why develop those huge coracoids if they aren't going to anchor anything?

If the more extensive models of pectoral musculature are correct, we need to consider how the proximal regions of plesiosaur forelimbs would have looked like in life. One key consequence is that, once we link all the pectoral muscles to their insertions on the limb and body, the 'shaft' of the 'oar-shaped' flipper disappears: muscles running along the anterior and posterior region of the humerus fill the pinched, concave regions so that the proximal region is almost as thick as the bony paddle. Much of the proximal humerus becomes buried in muscle dorsally and ventrally too, to the extent that we might imagine the shoulder region was quite bulky in life.

Summary diagrams of plesiosaur pectoral musculature based on Araújo and Correia (2015), with some of my own input on the body outlines (middle and right). Left shows a schematic plesiosaur skeleton (based on Rhomaleosaurus) and a 'traditional' soft-tissue outline, traced from Araújo and Correia (2015). Middle shows the superficial dorsal pectoral musculature predicted by their study - note that it embiggens the pinched proximal region of the flipper by bulking out the anterior and posterior humeral regions. Right shows how data from plesiosaur soft-tissues - see below - changes the flipper shape even further.
In this respect their limb anatomy might look more similar to that of modern tetrapod swimmers – such as whales, seals and turtles – than we typically reconstruct it. We might draw particular comparison to pinnipeds, where a noticeable bulge can be seen at the junction between the forelimb and the torso. The size of plesiosaur pelvic girdles probably indicate a similar muscular condition for the hindlimb and we might assume that they weren't slender-necked, 'oar-shaped' fins either.

Holotype specimen of Seeleysaurus guilelmiimperatoris. Note soft-tissue outlines behind the right forelimb and tail. If you'd like to see these tissues in person, you're too late - the body outlines of this specimen were painted over years ago. Bummer. From Dames (1895).
But these are not the only tissues which distort the outline of the flippers. Fossils of plesiosaur body outlines are very rare, but three specimens (the holotypes of Seeleyosaurus, Hydrorion and Mauriciosaurus - see Dames 1895, von Huene 1923 and Frey et al. 2017) preserve soft-tissues that considerably augment their flipper shape. All three show deep wedges of soft-tissues tapering along the back of the fin skeleton to the flipper tip, with Mauriciosaurus showing tissues - though their shape isn't entirely clear - also present behind the proximal limb regions. There is sufficient consistency across these specimens to suggest expanded paddle tissues were common, and maybe even widespread, in plesiosaurs and, for artists, augmenting our plesiosaur flipper skeletons with these trailing edge tissues should be our standard approach to their restoration.

Hydrorion brachypterygius and its soft-tissue forelimb impressions (the dark, grainy textures behind the fins). From von Huene (1923).
Moving on, artists might also want to note that ideas about highly restricted motion of plesiosaur flippers are being revised. Traditionally, authors such as Carpenter et al. (2010) have argued for limited motion at both the shoulder and hip limb joints, resulting in what I like to call the 'sinking rowing boat' pose: depictions of plesiosaurs with limbs projecting just a little off the horizontal, regardless of what they're up to. Restricted fore- and aft motion seems likely given the elongate shape of limb girdle joints, but whether the vertical movement of the limbs was restricted to tight arcs - perhaps as shallow as a 54° total range - is being challenged (e.g. Liu et al. 2015). Plesiosaur limb girdles were evidently highly cartilaginous in life and estimating their joint motion challenging - most of the information we desire to determine some sense of joint mobility is long gone. But if we assume they had more than the slimmest covering of cartilage in the girdle limb joints - which seems sensible, given the huge size of the girdle joints and their poor match for the limb bone shape - we can assume wide arcs of motion to both limb sets before disarticulation. The exact range of movement remains an open question - unpublished studies hint at even greater motion than other 'wide arc' research, such as Liu et al. (2015) (thanks to Darren Naish for advance word on this) - but artists should not feel confined to the 'rowing boat' pose that we've seen plesiosaurs depicted in for decades. With my artist hat on, I find this very welcome news. Plesiosaurs with limbs perpetually stuck out sideways can look a little static even in the hands of great artists, and their limited poseability has not made them the most interesting subjects to reconstruct. Wider arcs of motion allow plesiosaurs to be depicted in more complex and dynamic poses, and to convey a greater range of behaviours - pirouetting around corners with dipped fins, beating their flippers to attain high speeds, dropping their limbs because they're being lazy... all sorts of stuff. Well done, science, you've made at least one artist a happy person.

Aspects of the neck

My experience with the mass-economising, lightweight long necks of terrestrial or volant tetrapods means the extensively developed vertebrae of longer necked plesiosaurs are of great personal interest. Freed of the constraints of mass reduction, their numerous neck vertebrae are short, highly developed elements with long, robust processes - the exact opposite of the long, simplified structures I'm used to dealing with. Assuming plesiosaur necks were constructed like those of other amniotes (below), they likely anchored powerful muscles along their lengths. In particular, their neural spines are very tall and we can assume they bore enhanced musculature associated with lifting and turning the neck - useful features for long necked animals living in a dense fluid medium. Myological reconstructions suggest that the axial column would bear muscles connecting to the pectoral girdle, producing a deep set of tissues at the neck-torso junction (Araújo and Correia 2015, see pectoral myology diagram above). Artists should equip these animals with chunky, powerful 'reptilian' necks rather than svelte, bird-like variants. I do wonder if thick muscles along the neck might have impacted their neck mobility somewhat - another reason to assume long-necked plesiosaurs were only capable of bending their necks into simple curves (e.g. Zammit et al. 2008).

Amniote neck muscle groups and functionality, modelled by the American alligator Alligator mississippiensis. If the same basic rules apply to plesiosaurs, we should expect many species to have huge muscles and very powerful necks. Diagram concept and muscle layout after Snively and Russell (2007).
The neck/skull articulation of plesiosaurs is also of interest. In many taxa, including Plesiosaurus itself, the posterior face of the skull is displaced anteriorly to the jaw joints. This condition is not unique to plesiosaurs, also being found in some other reptiles including living crocodylians. This 'staggering' of the posterior skull margins might minimise any obvious topographic demarcation between head and neck tissues (the head/neck junction is less obvious in crocodylians than it is in many birds and mammals, for instance) as as well as complicate motion at the head-neck joint. The anteriormost cervical vertebrae and their articulation with the skull would be buried by bone laterally and throat tissues (including muscles and hyoid cartilages) ventrally, and we have to wonder if this envelope of material would limit how far the skull could pivot on the neck. The analogous condition in modern crocodylians seems to bear out this prediction, so perhaps we should not be restoring plesioaurs with mammal- or bird-like cocked heads.

Trunk shape - cross section and lateral profile

Plesiosaurs are often restored with a generic, 'barrel-shaped’ trunks. This is appropriate for some taxa, but not all. It must be said that plesiosaur torso shape is an area of on-going research. I recently spoke with a number of plesiosaur experts on this matter and found aspects like rib and gastralia articulation, the vertical position of the pectoral girdle and so on were somewhat contentious (thanks to Richard Forrest, Aubrey Roberts and Mark Evans for their thoughts). The crux of the issue is that, unlike some reptiles (such as birds or pterosaurs), plesiosaur torso skeletons don't slot neatly together in a single, incontrovertible manner, as is evident to anyone who's seen more than one plesiosaur mount in a museum. Understanding their torsos requires precise appreciation of their vertebral rib articulations, knowing their rib and gastralia curvature in three dimensions, and the benefit of fully articulated fossils for reference. This is quite a list of requirements, and one that is only currently met by a fraction of plesiosaur taxa.

Despite this, detailed reconstruction attempts provide reason to think not all plesiosaurs had tubby, barrel-shaped torsos. Close inspection of vertebral rib articulations and the shape of three-dimensionally preserved plesiosaur torso skeletons allowed O’Keefe et al. (2011) to reconstruct some cryptoclidids with tall, barrel-shaped bodies, and others with dorsoventrally compressed ones (below). In some genera, like Tatanectes, this is augmented further by almost flat dorsal ribs. It is difficult to gauge torso cross sectional shapes from just looking at a typical, half-prepared and flattened plesiosaur fossil, but artists should be mindful that not all species will have circular torso sections. Given how important torso shapes are to a reconstruction, we should check research literature carefully to make the most informed call we can on this aspect of restoring their life appearance.

Cryptoclidid torsos in cross section, with (over conservative) soft-tissue outlines. Modified from O'Keefe et al. (2011).
It is not only the cross section of plesiosaur trunks which are of artistic interest. Neural spine height is not always consistent along the dorsal column, with genera like Attenborosaurus having much taller vertebrae towards the anterior end of the torso. I don't think we know much about the torso cross section of this animal yet, but its vertebral proportions alone imply a proportionally deep shoulder region and a ‘tear-drop’ profile in lateral aspect. This may have been translated into soft-tissue depth in life: deep neural spines over the shoulder might betray a well developed m. latissimus dorsi, a forelimb elevator muscle that could be beneficially augmented for a swimming animal. Interestingly, Attenborosaurus has larger forelimbs than hindlimbs, and it's not entirely daft to wonder if its big shoulder vertebrae and their possible role in beefing out the shoulder muscles reflect forelimb-dominated swimming (see Liu et al. 2015). That's a discussion for another day, of course: the take home for artists here is to pay attention to those trunk vertebrae, and think about how they might influence the long-axis trunk symmetry.
Attenborosaurus conybeari, Jurassic equivalent of those top-heavy gym users who forget about working their legs.


And finally... so long, shrink-wrapping

A recurrent theme in this post has been the idea of plesiosaur skeletons being deeply buried in soft-tissues of varying kinds. One of the most amazing plesiosaur fossils known to date, recently described from Cretaceous deposits of Mexico (Frey and Stinnesbeck 2014; Frey et al. 2017), clearly vindicates this theory. This specimen is the holotype of Mauriciosaurus fernandezi, which preserves a near-continuous body outline to give us an unprecedented glimpse of its life appearance. Much of the soft-tissue includes belly and lateral body wall skin impressions (tiny, 12 x 2 mm rectangular scales arranged in rows along the animal), but even more surprising is how much soft-tissue there is: by gum, this was a tubby creature, particularly around the tail. Even the thinnest regions of the outline are a good 50 mm wide, and some parts are considerably deeper. Frey et al. (2017) ascribe much of this depth to fatty, subdermal adipose tissue, including the caudal mass. Many living reptiles have extensive fat deposits around their tails (as discussed for prehistoric animals in this post) and it would not be surprising if plesiosaurs used this adaptation to streamline their shape. As noted by Frey et al. (2017), the preserved torso shape is not dissimilar to those of highly pelagic turtles or penguins.
Line drawing of Mauriciosaurus fernandezi holotype, redrawn from Frey et al. (2017). This specimen is extra special for reminding us of the finest Queen song of all time.
Whether these plump tails were the case for all plesiosaurs remains to be seen. Frey et al. (2017) note that the caudal vertebrae of Mauriciosaurus has small processes for muscle attachment, and may have been weakly muscled in life. This might be predicted, as a tail encased inside a deep cone of fat is unlikely to have been capable of much movement even if it was strongly muscled (although, that said, some living marine mammals are very flexible despite their deep fatty tissues). However, other plesiosaurs - including, for easy reference, the Hydrorion depicted above - do have large caudal sites for muscle attachment - might they have moved their tails about more freely? Given the compelling evidence for caudal fins or rudders in several plesiosaur species (Dames 1895; Wilhem 2010; Smith 2013 - check out Brian Switek's post if you need a quick primer) it might make sense for some species to maintain mobile tails to aid steering. We should note that the partially preserved tail tissues of Seeleyosaurus are not as chunky as those of Mauriciosaurus: they're thick, sure, but not obviously part of a wide, wedge-shaped mass, perhaps suggesting a more easily moved structure. Hopefully, more plesiosaur soft-tissues will turn up soon to give us more insight on this matter.

As a final point on the Mauriciosaurus fossil, we can now add plesiosaurs to the list of fossil taxa with specimens directly opposing 'shrink-wrapping' palaeoartistic conventions. It joins fossils of dinosaurs (Mesozoic and beyond), pterosaurs, mammals, early archosauromorphs and many others in suggesting the soft-tissues of long extinct creatures were no less extensive than those of modern species. As with living taxa, their skeletons were mostly placed well inside their bodies, not just under the surface of a thin skin. There's no doubt that soft-tissue depth is going to vary across animal bodies and between species, but it's increasingly difficult to defend reconstructions where bodies tightly hug skeletal contours, where facial tissues are sucked into every skull cavity, and where the depth of fats and integuments are not factored into the restorative process. 'Shrink-wrapping' is one of the few aspects of palaeoart that is testable against fossil data, and it is not winning out.

And that's that, then

I'm sure there's a lot more we could say on restoring plesiosaurs, but this is where we'll have to leave this discussion for now - hopefully this post helps fill the deficit of detailed discussion on plesiosaur life appearance. I must admit that these recent efforts at restoring plesiosaurs have given me a newfound interest in the group, and I wouldn't be surprised if artwork these chaps and their relatives turn up around here soon.

Next time: sharks vs. pterosaurs - who will win? (Spoiler: not the pterosaurs)

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References

  • Carpenter, K., Sanders, F., Reed, B., Reed, J., & Larson, P. (2010). Plesiosaur swimming as interpreted from skeletal analysis and experimental results. Transactions of the Kansas Academy of Science, 113(1/2), 1-34.
  • Dames, W. B. (1895). Die plesiosaurier der süddeutschen Liasformation. Verlag d. Kgl. Akad. d. Wissenschaften.Frey, E., & Stinnesbeck, W. (2014). Plesiosaurs, reptiles between grace and awe. In Dinosaurs and Other Reptiles from the Mesozoic of Mexico (pp. 79-98). Indiana University Press.
  • Frey, E., Mulder, E., Stinnesbeck, W., Rivera-Sylva, H., Padilla-Gutiérrez, J., González-González, A. 2017. A new polycotylid plesiosaur from the early Late Cretaceous of northeast Mexico. Boletín de la Sociedad Geológica Mexicana. 69 (1): 87-134
  • Liu, S., Smith, A. S., Gu, Y., Tan, J., Liu, C. K., & Turk, G. (2015). Computer simulations imply forelimb-dominated underwater flight in plesiosaurs. PLoS Comput Biol, 11(12), e1004605.
  • O’Keefe, F. R., Street, H. P., Wilhelm, B. C., Richards, C. D., & Zhu, H. (2011). A new skeleton of the cryptoclidid plesiosaur Tatenectes laramiensis reveals a novel body shape among plesiosaurs. Journal of Vertebrate Paleontology, 31(2), 330-339.
  • von Huene, F. (1923). Ein neuer Plesiosaurier aus dem oberen Lias Württembergs. Jahreschefte des Vereins für vaterländische Naturkunde in Württemberg, 1923, 3-23.
  • Wilhelm, B.C. 2010. Novel anatomy of cryptoclidid plesiosaurs with comments on axial locomotion. Ph.D thesis, Marshall University, Huntington, WV. USA
  • Zammit, M., Daniels, C. B., & Kear, B. P. (2008). Elasmosaur (Reptilia: Sauropterygia) neck flexibility: Implications for feeding strategies. Comparative Biochemistry and Physiology Part A: Molecular & Integrative Physiology, 150(2), 124-130.

5 comments:

  1. Excellent article, but one minor correction: Radio Gaga is the finest Queen song (though I appreciate that it would hard to shoehorn that into the article).

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  2. Nice article, but some of the references are not in the list at the end.
    It is good to see Mauriciosaurus getting the attention, it is a beautiful specimen.
    The surface covering on it may have interesting things to say regarding friction and the hydrodynamics of it moving through the water.

    Not all marine reptiles are completely smooth and you could wonder if some of them ended up colonised with patches of barnacles like whales; also if there were Remora equivalents in the mesozoic and also small fish following along behind them (surely a safer place to be than the front end).

    LeeB.

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  3. I for one would like barnacled or seaweed infested plesiosaurs

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  4. Nice to see a piece on plesiosaurs! I really appreciated hearing about the musculature and soft-tissue anatomy.

    Have you seen Duane Nash's "Plesiosaur Machinations" series on Antediluvian Salad? I don't recall if any plesiosaur workers have commented on it, but it certainly got me thinking about plesiosaur diversity, behavior, and bioecology.

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    Replies
    1. Duane Nash actually predicted plesiosaurs would be more muscular than usually shown in the machinations.

      And now Mark Witton has just given confirmation of this.

      Delete