Monday 27 September 2021

A tale of plesiosaur tails: vertical fins or horizontal flukes?

The giant elasmosaurid Albertonectes vanderveldei forages for invertebrates and small prey deep underwater. From this posterolateral view, the vertical tail fin is unmissable - but should I have drawn it as a horizontal fluke instead? Welcome to yet another challenge for restoring the life appearance of plesiosaurs!
It’s fair to assume that, when painting or sculpting plesiosaurs, most palaeoartists favour attention to their heads, necks and flippers, these being the most characteristic parts of their anatomy and the components most of our audience will focus on. Plesiosaurs are, of course, famously tricky animals to restore with any degree of certainty, their body plans being entirely unlike anything alive today, their mechanism of swimming - underwater flight with two sets of flippers - being the source of much debate and controversy, and their fossil record giving us few direct insights into their soft-tissue anatomy. So here’s more good news: in recent years another part of plesiosaur anatomy has joined their flippers and necks in artistic and scientific contention, their tails.

Plesiosaur tails have traditionally been little more than afterthoughts in palaeoartworks: cone-shaped structures tapering from the body without much in the way of interesting features. In the last decade or so, however, research interest in plesiosaur caudal anatomy and the resurrection of certain historic observations (e.g. Dames 1895; Smith 2007, 2013; Wilhem 2010; Sennikov 2015, 2019; Otero et al. 2018) has seen plesiosaurs with more interesting tails making regular palaeoartistic appearances. This artistic shift initially showed plesiosaur tails having fish- or ichthyosaur-like vertical fins but, more recently, newer research has made a case for another configuration, horizontal tail flukes (note the terminological distinctions), and such reconstructions are now also appearing with regularity. These fins and flukes, it must be stressed, are not mere whimsy or speculation but based on osteological correlates for some kind of extensive skin structure around the tail tip, and we should clarify that no-one thinks these represent a hitherto unappreciated propulsive organ - plesiosaurs do not suddenly have five ‘engines’ for swimming. Instead, these fins or flukes surely represent devices to act as some kind of rudder or stabilising aid, perhaps helping to offset the impact of swimming around with those giant necks and heads.

Recent skeletal reconstructions of plesiosaurs (or parts thereof) showing different interpretations of caudal rudder anatomy. It is increasingly common to see these features in plesiosaur skeletals and this compilation image could be much bigger, but I've restricted it to those from papers specifically making cases for fins or flukes.

One thing is clear: the evidence for tail rudders in plesiosaurs is pretty strong in several plesiosaur clades and any credible palaeoart of these animals should show them with some kind of fin, flipper or fluke at the tail tip. But, clearly, our current conflicting interpretations of plesiosaur tail anatomy can’t both be correct. I’ve gone back and forth on these ideas in my plesiosaur art over the last couple of years and decided that it was time I looked into this in more detail. What is the evidence for tail fins or flukes in plesiosaurs, and which is most compellingly argued for? Can we even make a call on this topic at the moment? Let's find out.

One of several plesiosaur artworks I've drawn recently showing a fluked tail, rather than (as was my previous preference) a fin. This is Plesiosaurus dolichodeirus scavenging a pterosaur carcass, because plesiosaurs are meant to eat pterosaurs in palaeoart, consarnit.

Something old is new... etc. etc.

First, let's briefly familiarise ourselves with the history of this controversy. It may seem that notions about plesiosaur caudal fins and flukes are part of the Brave New World of changing up old reconstructions of fossil reptiles for radical new ones, but that’s not the case here. Actually, osteological features and soft-tissue remains evidencing caudal rudders in plesiosaurs were identified in the 19th century, and even the dichotomy of interpretation between fins vs. flukes is over 100 years old. Our modern discussions of these concepts are a revival of relatively early investigations into plesiosaur functional morphology and life appearance.

The holotype of Seeleyosaurus guilelmiimperatoris, famous for having a soft-tissue outline around its tail. This specimen was the only plesiosaur fossil on record with direct bearing on the fin vs. flipper controversy, but the tail soft-tissues have been painted over and are no longer accessible for study. Yeah, frustrating, right? From Dames (1895).

The concept of plesiosaur caudal fins was first broached by Richard Owen, who proposed that lateral compression of the terminal tail vertebrae of Archaeonectrus rostratus indicated a caudal fin (Owen 1865). Owen’s observation had little impact on reconstructions of plesiosaurs at the time, but seeming confirmation of his prediction arrived a few decades later when, in 1895, a specimen of the Jurassic plesiosaur Seeleyosaurus guilelmiimperatoris was described with a soft-tissue outline around much of tail tip (Dames 1895). To date, this specimen remains the only fossil on record that provides direct confirmation of a caudal fin or fluke but, alas, it can no longer be investigated or even validated as the soft tissue component of the fossil has been painted over (BUT - see update at the end of the post). This presents a problem deeper than merely obscuring the body outline. Many marine reptile specimens were often ‘improved’ with forged soft-tissues and realigned bones by 19th century preparators such that anything especially amazing and interesting - like the only known plesiosaur tail fin outline, for instance - really needs verification from modern researchers to be accepted as genuine. I’m not aware of any plesiosaur experts who consider the Seeleysoaurus soft-tissues especially suspicious, but this inescapable caveat hangs around any discussion of this specimen: we can only put so much stock in any interpretation of it, modern or historic. This said, scholars of the late 1800s certainly regarded the body outline as genuine, leading to a few late 19th and 20th-century palaeoartworks in which Seeleyosaurus and other plesiosaurs sported tall, diamond-shaped tail fins. Among the most famous of these were the reconstructions published in Wilhelm Dames' 1895 Die plesiosaurier der süddeutschen Liasformation and the generic pliosaurid reconstruction published by Newman and Tarlo (1967).

Neither finned nor fluked plesiosaurs really caught on in historic palaeoart, but finned reconstructions almost did. Here's a rarely seen example: Woodward's (1896) reconstruction of "Plesiosaurusmacrocephalus, from A Guide to the fossil reptiles, amphibians, and fishes in the Department of Geology and Palaeontology of the British Museum (Natural History).

Not everyone accepted Seeleyosaurus as evidence of caudal fins in plesiosaurs, however. Both Fraas (1910) and Wegner (1914) felt that their caudal skeletons were indicative of a horizontal fluke, citing the absence of a tail bend, the dorsoventral flexibility of the vertebrae, and the size of the caudal ribs as evidence of this feature. I’m not aware of any historic reconstructions showing this configuration (this is not to say that none exist, of course) and assume that fluked plesiosaurs gained even less traction in technical literature and artwork than their finned counterparts. This left most academic and artistic consideration of plesiosaur caudal anatomy assuming featureless, tapering tails for the next century, despite the continued cataloguing of peculiar caudal anatomy in plesiosaurs - most notably, their fused terminal tail vertebrae, recalling the pygostyles of birds (see review by Smith 2013). Today, such structures are considered part of the evidence package for a caudal rudder, but this significance seems to have been mostly overlooked in older publications.

Evidence for fins, evidence for flukes

The story of plesiosaur caudal rudders is thus one of three parts: an initial set of pro-rudder proposals and observations; a period of relative disinterest in the idea; and our modern era of belated engagement with those original hypotheses. This latter stage began about 10-15 years ago when observations made by Owen and Dames were resurrected and augmented by modern plesiosaur experts to make a case for plesiosaur tail fins. Perhaps the most in-depth investigations supporting finned tails to date are those by Benjamin C. Wilhelm (2010) and Adam Stuart Smith (2007, 2013), both of whom looked at the tails of Jurassic plesiosaurs and identified features also seen in swimming animals with vertical caudal fins. Using the relatively completely known tails of the cryptoclidids Cryptoclidus and Muraenosaurus, Wilhelm (2010) catalogued a suite of anatomies correlating with vertical fin lobes, including a relatively large neural spine close to the tail tip (the 17th caudal vertebra in Cryptoclidus); several neural spines with expanded ends; a shift in the orientation of the neural spines at the tip of the tail (from posteriorly-directed to anteriorly-directed) and lateral compression of the terminal caudal vertebrae (see diagram, above). These features recall the finned tails of mosasaurs, Triassic ichthyosaurs and thalattosuchians (e.g. Lindgren et al. 2013; Renesto et al. 2020), where enlarged neural spines mark the start of a caudal fin and shifting neural spine orientations characterise the vertebrae embedded in the fin itself. As with tail-finned reptiles and fish, plesiosaur tails had increased flexion in two regions, both at the tail base and immediately anterior to the enlarged and reorientated neural spines. A slight vertical peduncle - a thinning of the tail structure to minimise its drag profile when being moved through water - was also identified anterior to the enlarged neural spine region. By analogy with the preserved soft-tissues of other marine reptiles, Wilhelm’s study allowed for a prediction of the possible fin outline for cryptoclidids: a small triangular lobe with a concave posterior margin on the upper side of the tail (above). This is quite different from the diamond-shape interpreted by Dames and others from the Seeleyosaurus fossil, but Wilhem’s thesis argues that a large ventral lobe is not suggested by that fossil and we can infer - if Wilhelm’s reconstruction is correct - that only a small amount of material is missing from the dorsal margin (see diagram, below).

Advocates for plesiosaur tail fins highlight similarities between their tail anatomy (see illustrations, above) and those of certain finned marine reptiles, such as early ichthyosaurs and mosasaurs. There are indeed obvious similarities, although the plesiosaur fin supports are clearly far less developed than the examples shown here - a reflection, almost certainly, of tails being adapted for locomotion vs. those adapted for ruddering. Images from Renesto et al. (2020) and Lindgren et al. (2013).

Although working with more fragmentary material, Smith (2007, 2013) found similar features to those reported in Benjamin Wilhelm’s thesis, as well as further evidence of finned tails. This included lateral compression at the tail tip of Rhomaeleosaurus, as well as peculiar ‘node’ vertebrae that might indicate a zone of flexion or even a slight downturn of the tail tip - another feature of finned tails. A wedge-shaped vertebra was noted in another rhomaleosaurid, Macroplata, which might indicate a slight tail downturn, although this taxon curiously lacks laterally compressed distal vertebrae. Agreeing with Benjamin Wilhelm’s thesis, as well as Wilhelm and O’Keefe’s (2010) examination of further caudal material, Adam's study concluded that the flexibility evidenced in the proximal tail region would allow the caudal fin of plesiosaurs to augment steering and stability.

A few years after these works reignited interest in plesiosaur caudal rudders, artistic and academic champions of fluked plesiosaurs also resurrected, retooled and expanded the initial observations and arguments made by Fraas and Wegner (e.g. Sennikov 2015, 2019; Otero et al. 2018). At the core of these proposals were ideas that plesiosaur tails were more convergent with those of whales and manatees than with swimming reptiles or fish, and thus suited to dorsoventral motion and sporting horizontally-aligned soft-tissues at their tips. Some researchers have endorsed this idea by including fluked reconstructions in their papers (e.g. Sachs et al. 2016). 

Were plesiosaur tails reptilian variants of sirenian-like caudal anatomy? Again, there are similarities, especially in the great width of the vertebrae. In lieu of decent sirenian skeletons online, I've borrowed these Florida manatee (Trichechus manatus) skeleton replica images from Bone Clones.

Some of the evidence for plesiosaur flukes is the same as that used for finned reconstructions, such as the two zones of flexion in the plesiosaur caudal skeleton, and the recognition of a distinctive, often stiffened vertebral region at the end of the tail (Sennikov 2015, 2019). This is representative of fluked and finned tails having functional similarity, each essentially being the same thing operating in different planes. Fluke-specific evidence includes the presence of long caudal ribs along much of the tail length which, in making some vertebrae wider than tall, are suggested to create a relatively wide, flat tail with restricted lateral movement (Sennikov 2015, 2019; Otero et al. 2018). Otero et al. (2018) noted that the tips of the caudal ribs of Aristonectes are fibrous for potential attachment of extensive soft-tissue, potentially implying a much wider tail than shown by osteology alone (something seemingly confirmed by soft-tissue data of Mauriciosaurus). The absence of convincing downturned tail tips has also been flagged up, even for taxa with ‘node’ vertebrae (Otero et al. 2018), and has been negatively compared against the finned condition of ichthyosaurs and marine crocodylomorphs (Sennikov 2019). The small size or absence of chevrons, and the low, variable orientation of caudal neural spines, are features thought to have allowed the fluke to move vertically and independently of the rest of the tail (Otero et al. 2018; Sennikov 2019). The plesiosaur torso - a pachyostotic, relatively inflexible trunk - has also been regarded as similar to that of sirenians and thus potentially indicative of a fluked tail (Sennikov 2019). For the fluke model to be correct, of course, the traditional interpretation of Seeleyosaurus having some kind of tail fin has to be wrong. Sennikov (2019) states that this is indeed the case, and provides an alternative view where the Seeleyosaurus soft-tissues represent a horizontally-aligned structure within which the vertebrae have fallen over. The asymmetry of the fluke as preserved is explained as a result of vertebral displacement, soft-tissue decay and incomplete fossilisation.

Problematic as it is, proposals of plesiosaur rudder shape have to contend with the now-lost Seeleyosaurus data to some extent. Some fin models posit that not much has happened to the caudal region of the Seeleyosaurus holotype, whereas the fluke model requires a little more modification. While the fin model seems simplest, the fluke version hardly seems implausible.

Fins vs. flukes, head to head (er... tail to tail?)

Having briefly reviewed this evidence, can we see which of these models seems strongest? I stress my word choice in that sentence: our understanding of plesiosaur soft-tissues is not yet sufficient to make claims of absolute certainty, so any conversation on them should be peppered with appropriate caveats and considerations. This is a case where it’s much easier to be a researching scientist, where “we don’t yet know” is a perfectly acceptable response, than a palaeoartist, where you have to come down on one side of a debate or another. So, with the proviso that I’m not sure we can really make a definitive call yet… and with some urging of readers to check the papers mentioned above for themselves to make up their own minds... and with appropriate caution… and having now run out of ways to stall writing this sentence... I find myself... more persuaded by arguments for a tail fin at present. The fin hypothesis seems to explain more of the peculiar anatomy of plesiosaur tail tips than its rival, and it presents an overall simpler, and thus more likely correct, interpretation of our available data. The highlighted similarities between plesiosaur tails and those of certain other fin-tailed swimming reptiles are prudent observations not yet accounted for in the fluke model and, when thinking about the evolution of such features, it seems a shorter developmental distance for a plesiosaur to evolve a fin than a fluke. I’m especially thinking of the switch from primarily lateral to vertical spinal flexion, the former being ancestral to reptiles and thus common to many in swimming reptile species. Significant vertical caudal flexion has developed among diapsids of course, in birds, and some swimming birds even use their tail fans in a rudder-like fashion (Felice and O’Connor 2014), so we shouldn’t rule out this capability for plesiosaurs entirely. Moreover, this is not to say that the observations from Team Fluke are redundant. Maybe the wide, potentially muscular tails of plesiosaurs were indeed capable of an unusual amount of vertical motion, even with a fin? If, indeed, plesiosaur tails operated as ruddering aids, some surely had a big job on their hands: there’s no way moving those giant necks and heads didn’t have a tremendous rotational impact when swimming and their tails may have had an important role in keeping their owners on course. A weedy tail with limited mobility in either plane might not have been up to that task. It would be neat to see simulations of this sort of thing.

I also agree with Wilhelm’s (2010) assessment of the Seeleyosaurus tail outline, which looks - so far as can be seen in Dames’ (1895) illustrations, more like a vertical structure than a horizontal one. I’m not sure how much stock we should place in interpretations of the drawings of this specimen given that the original soft tissues are now inaccessible, but, for what it’s worth, the relatively neat margins and asymmetry of the tissue outline, the general position of the vertebrae and location of possible ‘fin’ tissues above the tallest neural spines look consistent with a small dorsal lobe (sensu Wilhelm 2010), while a fluked interpretation relies on a fair amount of distortion from decay and disarticulation. But there is certainly plenty of ambiguity around that specimen and I could be convinced otherwise. Needless to say, more plesiosaur specimens with soft-tissues preserved around the tail tip would be incredibly useful in this discussion.

Plesiosauria was a diverse group with a huge array of body proportions, sizes and lifestyles. Did the gigantic pliosaur Kronosaurus queenslandicus share the same caudal rudder shape and size as Cryptoclidus? We need more research to say either way.

Of course, this tentative endorsement of vertical fins comes with many caveats. Despite the antiquity of proposals that plesiosaurs bore a soft-tissue rudder on their tails, these discussions are still in their infancy. Very few studies have directly addressed this topic and our analyses have, thus far, been largely limited to qualitative descriptions and comparisons with other vertebrates. Our taxon sampling is also relatively limited and has not yet incorporated the most extreme examples of plesiosaur size or body plans. I can’t be the only one wondering what the extremely long-necked elasmosaurids and giant pliosaurids were doing with their tails and, given the innumerable studies linking tail shape to ecology in swimming animals, it’s not crazy to assume plesiosaurs of different lifestyles might have had differently shaped rudders. We may even already be finding evidence of such variation, it being observed (for example) that elasmosaurids may not have had the mobile terminal tail region identified in some cryptoclidids (Otero et al. 2018). Moreover, might plesiosaurs have done something unusual with their tails, such that the fin vs. fluke dichotomy is an oversimplification of a more complex organ befitting their unique swimming style? Could other structures - keels or projecting stabilisers - have augmented a larger fin? It will be interesting to see what further research reveals and, in particular, what quantified comparisons between the caudals of plesiosaurs and those of fluked and flippered animals have to tell us. Many of the features discussed above are relatively subtle - a taller neural spine here, a slight wedging of the centrum there. Careful, quantified comparisons of a suite of features with modern and extinct analogues may help us distinguish whether fins or flukes are the better tail rudder model, or if such analogues are useful guides at all.

And that, with one foot off the fence on this matter, but still not feeling like a conclusion has really been reached, is where we'll leave off for now. Hopefully, more definitive evidence for plesiosaur tail morphology will appear soon but, until then, the best thing artists can do is not take the comments above as gospel: check out the papers discussed here (most are open access or otherwise available online) to make up your own mind which is the better-supported model. I suspect I'm still going to remain uncertain about my plesiosaur tails for some time, but there's comfort knowing that this isn't really something we can be especially confident about until more data is gathered.

UPDATE: 28/09/2021 (the day after posting). In what is clearly good news, it turns out that the paint covering the Seeleyosaurus holotype soft-tissues has been removed and that the specimen is, at time of writing, under study by plesiosaur expert Sven Sachs. You can see a photo of the restored specimen at Sven's website, here. The paint was actually removed some years ago but this has not, to my knowledge, been mentioned in more recent papers discussing this specimen, hence the error in reporting here. Thanks to Sven and others who have pointed this out.

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