Plesiosaurs on the rocks: the terrestrial capabilities of four-flippered marine reptiles

Maurice Wilson's charming c. 1958 painting of plesiosaurs coming ashore, from the Daily Mail Boys Annual (note "boys": girls aren't allowed to be interested in prehistory. Get out of our tree house!). Such depictions were commonplace until a few decades ago, but are all but extinct now. So what's happened to the idea that plesiosaurs could venture onto land?

Until comparatively recently it was not uncommon to see depictions of plesiosaurians* on rocks and beaches as if they had hauled themselves from the sea like breeding sea turtles or basking pinnipeds. Such restorations have a long history. Some of the earliest marine reptile palaeoart shows plesiosaurians on beaches or very shallow water, and we've even seen land-based plesiosaurians in feature films and documentaries, including notable sequences in When Dinosaurs Roamed the Earth (1969) and Walking with Dinosaurs (1999, below).

*Why 'plesiosaurians' rather than 'plesiosaurs'? Though a common vernacular, the term 'plesiosaurs' is potentially confusing as it could either refer to a number of marine reptile clades (e.g. plesiosauroids, plesiosaurids) or body plans (plesiosauromorphs). 'Plesiosauria' has a less ambiguous meaning as it specifically refers to the clade encompassing rhomaleosaurids, pliosauroids and plesiosauroids, so it might be a preferable catch-all term this marine reptile clade.

Today, it's much rarer to see plesiosaurians depicted outside of the aquatic realm. For... reasons, I'm restoring a number of marine reptiles at the moment, so I've been wondering if it would be acceptable to revive artwork of these creatures on rocky shores, beaches and other coastlines, if only to bring some variation to my marine scenes. As usual, this inquiry began with a literature crawl. Because 19th century palaeoart suggests palaeontologists once imagined these animals as routinely emerging from the waves, I expected marine reptile papers to be full of discussion about the terrestrial prospects of plesiosaurians, perhaps with an in-depth analysis of the concept bringing an end to the artistic tradition of depicting them on land. The transition from imagining plesiosaurians as semi-aquatic to fully aquatic seems to have happened organically, however: if there's a watershed paper or significant debate behind this, I've missed it. Richard Ellis' 2003 book Sea Dragons - perhaps the closest thing we have to an all-encompassing introductory review of marine reptiles - seems to confirm my independent findings, portraying plesiosaur terrestrial abilities as highly doubtful, but also a question without a firm answer in scientific literature.

Jurassic plesiosaur Cryptocleidus sits around the coasts of the Oxford Clay Sea in 1999's Walking with Dinosaurs. Uploaded to Youtube by user MARTINEZZZ365.

The idea of plesiosaurians leaving water has been strongly tied to historic uncertainty about their reproductive habits. It stands to reason that, if plesiosaurians laid eggs, they must have somehow dragged their way out of the sea to construct their nests (Taylor 1981, 1986). The notion that plesiosaurians could have given birth to live young is pretty old (e.g. Seeley 1896) but scant evidence of their reproductive strategies prevented dismissal of land-based nesting habits until relatively recently. We now have evidence of live birth in plesiosaurian relatives (nothosauroids, Sander 1988; Renesto et al. 2003; Cheng et al. 2004) as well as a true plesiosaurian (a polycotylid, O'Keefe and Chiappe 2011), and so we needn't imagine plesiosaurians hauling out onto beaches to lay eggs, turtle-style.

But I'm going to keep pulling at this thread. While their capacity to give birth to live offspring eliminates the behavioural necessity for leaving water, it does not, in itself, demonstrate that plesiosaurian anatomy was functionally incapable of land-based locomotion, or that they did not leave the sea to find refuge or seize prey - orca style - from shorelines. After all, viviparity does not mean that seals, sea otters or even manatees have committed to a fully aquatic life (a voluntarily beached manatee deserves a citation - see Motani et al. 2015). Is there a cogent functional argument for why plesiosaurians might struggle out of water that will let me (and others) escape painting nothing but blue and green pictures?

How plesiosaurians might have moved on land

Our discussion will be aided by first outlining what we might realistically expect of a walking or crawling plesiosaurian. No one, for instance, predicts that plesiosaurians could stride around like sea lions or the plesiosaur in When Dinosaurs Roamed the Earth. Their flipper skeletons were essentially inflexible so they were incapable of being articulated into a walking limb. This precludes walking on their hands and feet in the way that eared seals can. They were also likely incapable of bouncing along in the manner of true seals, where the flexibility of the spine is used to 'hump' their way forward while powerful, gripping claws pull and steer them around. Plesiosaurian bodies were pretty rigid - their robust gastralia and ribs are sometimes superficially compared to turtle shells - and likely incapable of the twisting and bending necessary to bounce their way over shorelines. And in lacking claws, the only contribution their flippers could make to crawling would be crude pushing and lifting actions.

True seals, such as the grey seal (Halichoerus grypus), are far less terrestrially proficient than the eared seals (sea lions, fur seals etc.) and have to bounce or drag themselves around when ashore. Large claws on their flippers help in this activity (and are also useful for scratching). Grey seal cow photo by Georgia Witton-Maclean.

If plesiosaurians could leave the water at all, we have to imagine something more akin to turtle locomotion: using their flippers to push and pull themselves along while lying on their bellies. Their weight and a likely inability to clear their bodies entirely from the ground predicts that much of their energy would go into overcoming drag incurred by their wide bodies and tails. Terrestrial plesiosaurians would need to make full use of their powerful flipper downstroke muscles (soundly evidenced by the enormous muscles of their chest and the underside of their hips; see Carpenter et al. 2010; Araújo and Correia 2015) to lift their bodies and propel themselves forward. The exact motion of their flippers remains controversial, but is likely to have been a wingbeat-like action (Taylor 1986; Carpenter et al. 2010; Liu et al. 2015; Muscutt et al. 2017) that may have been enough to shove plesiosaurians over shorelines. The picture we're building here is of a slow and laborious means of locomotion. If plesiosaurians did intentionally leave water, they almost certainly visited locations inaccessible to terrestrial predators.

Polycotylid Dolichorhynchops bonneri demonstrating a fairly typical plesiosaurian torso and flipper construction. At face value, the retention of four limbs and stout limb girdles looks like terrestrial locomotion shouldn't be too hard for these guys - they certainly look more terrestrially capable than many other marine tetrapods. From Carpenter et al. (2010). Scale bar is 1 m.

I also think we should rule out raw body size as a compelling reason to doubt land-based locomotion in plesiosaurians. There's no reason to regard plesiosaurians as atypically heavy compared to other marine animals (Everhart 2000, Henderson 2006) and, indeed, their general lack of pachyostosic skeletons might make them lightweight compared to some other aquatic tetrapods (e.g. Street and O'Keefe 2010). Many species were probably within the mass ranges of living species known to transition between land and sea. The known maximum limit for this lifestyle is set by bull elephant seals which, according to Wikipedia, reach 3,000 - 4,000 kg. We don't have many plesiosaurian mass estimates to compare this figure to, but a few noteworthy values are Everhart's (2000) predicted mass of 2.8 tonnes for a 9 m long plesiosauroid, and Henderson's (2006) 217 kg 3 m Cryptocleidus. Truly giant plesiosaurians - 10 and 11 m long individuals - are beyond the masses of big elephant seals (Henderson 2006), but this still leaves plenty of small and mid-sized species at or below the mass threshold of marine species that we know can venture onto land, assuming they have the right adaptations. For me, our question is most appropriately addressed through assessment of anatomy and functional morphology, not a priori judgements about size.

Scrutinising the model

The bar we've set for plesiosaurian terrestrial locomotion is thus pretty low: even if they can only shamble up a beach we could consider our conditions met. But how feasible is even this laborious means of getting around on land? To cut to the chase: not very. There are multiple aspects of plesiosaurian anatomy that probably precluded even very basic terrestrial capabilities.

Plesiosaurian flippers, for instance, seem poorly suited for use on coastal substrates. Semi-aquatic species such as turtles, seals and terrestrially-roaming fish have a degree of jointing or articulation in their forelimbs which transforms them from flippers or paddles into walking limbs (Mazouchova et al. 2013, also see this post on the potentially amphibious ichthyosaur Cartorhynchus). A jointed limb performs considerably better on loose substrates (such as those common on beaches, mudflats and other shoreline locations) because it enables greater control of force distribution as animals move. Immobile flippers tend to skim over or dig into sand or mud, while jointed limbs can respond to yielding substrates to maximise lift and propulsive forces. Where substrates have already been disturbed, fixed-shape flippers can struggle to get any purchase at all (Mazouchova et al. 2013).

The evolution of the plesiosaurian flipper - represented here with early sauropterygians (A - B) and true plesiosaurians (C - D) involves the development of tightly fitting bones and removal of joint mobility. This makes for a superior flipper for an underwater flier, but compromises their terrestrial capacity. Image and caption from Storrs (1993).

Assuming these findings apply to plesiosaurians - and there's no reason they shouldn't - their effectively immobile flippers present a major barrier to terrestrial activity. Plesiosaurian flippers lack both obvious bony joints or significant cartilaginous regions that would allow them to flex, so they best fit those modelled flippers which skid around or dig into the substrates they're meant to traverse. We must consider that their flippers are married to animals that are already encumbered by large size and weight, as well as the additional difficulty of drag forces operating on their bodies as they moved forward. It is not hard not to imagine a beached plesiosaurian like a heavy vehicle stuck in sand, spinning its wheels as it tries to move forward.

The issues with plesiosaurian limbs do not stop there, as their limb girdles are also ill-equipped for supporting their weight on land. While augmented ventrally to accommodate big downstroke muscles, the upper regions of plesiosaurian shoulder and pelvic girdles are only weakly developed. This isn't unusual for aquatic species as a major role of expanded upper limb girdles - specifically the scapulae of the shoulder, and ilium in the hips - is stabilisation of the limb girdles during terrestrial locomotion (some readers may recall us discussing this recently in context of another marine reptile, Helveticosaurus). But while adequate for life at sea, on land these small girdle elements provide only weak girdle support and thus impede locomotion, and this was probably true for plesiosaurians. Though retaining a connection between the ilia and sacral vertebrae, the articulation is weak and ligamentous, and thus unsuited to weight-bearing (O'Keefe and Chiappe 2011). Similarly, their small scapulae leave little space for muscles associated with stabilising the shoulder against the body, and the shoulder is poorly braced for terrestrial locomotion (see Rieppel 1989 for discussion, also Araújo and Correia 2015). We thus have flippers ill-suited to land-based locomotion attached to limb girdles which are maladapted to weight-bearing. These are not the features of animals that were regularly hauling themselves onto shorelines.

Pelvis of Brancasaurus brancai in lateral (A) and medial (B) view, from Carpenter et al. (2010). Note the extremely narrow ilium: this structure articulates with the vertebrae overlying the hip to act as an important brace for the pelvic girdle during terrestrial locomotion, so its small size does not bode well for the prospects of plesiosaurs leaving water.

And there are further impacts from the reduction of the scapula to be explored. The muscles that elevate our heads and necks are anchored to our scapulae as well as our vertebrae, so the size of bones making up our shoulders dictates how large muscles important to neck elevation and control (e.g. the trapezius, levator scapulae) can be. Plesiosaurians famously have large necks and/or heads, but lack the weight-saving adaptations seen in similarly-proportioned land animals, such as pneumatic tissues and reduction or displacement of neck musculature towards the body (e.g. Taylor and Wedel 2013, also see this discussion of the lifestyle of Tanystropheus). Their necks and heads must have thus been heavy compared to those of large necked or big-headed terrestrial reptiles**, so we might expect substantial shoulder bones to anchor massive neck elevators if they were routinely leaving water. This casts their tiny scapulae as the exact opposite of what we might expect if these animals were routinely crawling around on land.

**To put some numbers on this, Henderson (2006), modelled plesiosaurian heads and necks with tissue densities of 1.05 g/l, about 1.5 times heavier than a value we assume for a bird or bird-like dinosaur.

The pectoral region and musculature of Rhomaleosaurus, modified from Araújo and Correia (2015). While the humerus is well muscled, the scapula is proportionally tiny compared to the chest, which has implications for shoulder stability and carriage of the head and neck.

Plesiosaurians could have developed an alternative approach to supporting the weight of their heads and necks out of water, such as bulking up the muscles surrounding their neck vertebrae. If so, we could predict structures equivalent to withers (elevated vertebral spines over the shoulders related to expanded neck musculature) in species with particularly large heads and necks. Such structures are not to be found plesiosaurians however, so I don't think this idea is compelling. This being so, it looks like neither the shoulders or anterior trunk vertebrae provide sufficient space for the powerful neck muscles needed to elevate their necks and heads on land for sustained periods. The impact of struggling to lift their heads for long intervals could include further impedance to their locomotion (additional drag, difficulty overcoming obstacles) as well as exposing their facial tissues to abrasion and other damage.

Ventral view of rhomaelosaurid Meyerasaurus victor, as figured by Smith and Vincent (2010). Note that the extensive bones of the chest, hips and gastric regions are entirely unfused: like many secondarily marine animals, plesiosaurian skeletons were likely far more cartilaginous than their terrestrial ancestors. Scale bar is 1 m.

A final, but no less significant, consideration for our inquiry is the high volume of cartilage associated with the plesiosaurian skeleton. A glut of unfused bones and loosely fitting contours between closely associated elements betrays a skeletal system held together primarily through extensive amounts of cartilage tissue. This includes areas of relevance to land-based locomotion, such as the shoulder and hip joints, the limb girdles and the region between the gastralia and ribs. In water, these softer connections wouldn't cause any issue, but on land the flexibility and softness of cartilage might weaken skeletal support and strength. The hypothesis outlined above posits that land-based plesiosaurians are essentially moving around with brute force, shoving their large masses around with motions of their flippers against large drag forces. Under this scenario, a relatively rigid and uncompromising skeletal frame would be ideal, whereas one with large volumes of cartilage would make movement less efficient.

What about small-bodied plesiosaurians?

Without any aspects of plesiosaurian anatomy looking incompatible with terrestrial activity, we might need to play a biomechanical get-out-of-jail-free card to prevent abandonment of this concept altogether: small body size. Inescapable rules of scaling mean that a given bauplan, expressed at smaller size, can perform biomechanical feats impossible for bigger individuals. Might small plesiosaurian species or juveniles exploit greater relative tissue strength ratios, lower body masses and improved muscle power:weight ratios to haul themselves onto land, leaving only larger plesiosaurians confined to water?

The pliosaurid Thalassiodracon hawkinsi is one of the smallest plesiosaurians known at c. 2 m long, and yet it bears all the same hallmarks of incompetent terrestrial abilities as its larger relatives. Are the virtues of small size enough to justify thinking animals of this size could leave the water? Photo from Wikipedia, by the paleobear, CC BY 2.0.

I must admit that I'm skeptical even here. Anatomically speaking, small plesiosaurians have the same terrestrially-inhibited anatomy as their larger relatives: inflexible flippers, high cartilage volumes, and low weight-bearing capabilities for the body, head and neck. Smaller size could make all the difference, but size alone is of ambiguous significance to predictions of extinct animal behaviour. It's clearly a factor in what fossil species could and couldn't do, but it should not overshadow better-established relationships between form and function when considering extinct animal lifestyles (see also: flight in giant pterosaurs). So while small size theoretically makes terrestrial locomotion more achievable for an aquatic animal, it's ultimately circumstantial evidence for this behaviour and not an especially compelling counterargument.

And as a final, closing thought on this, it's also worth considering that plesiosaurians were generally large-bodied creatures. Genuinely small species (such as the c. 2 m long Thalassiodracon hawkinsi) are rare and even their offspring were large (e.g. 1.5 m calf lengths in 4.7 m long mothers - O'Keefe and Chiappe 2011). Mesozoic shorelines were probably not swarming with small plesiosaurians even if they did have superior terrestrial capabilities, because by and large plesiosaurians weren't small creatures.

Bring on the blue paints

So, should palaeoartists get back to painting plesiosaurs out of water? Sadly, no. Any notion that plesiosaurians were capable of hauling themselves onto land is not only unnecessary in light of what we know of their reproductive biology, but also contradicts much of what we understand about the functional morphology of semi-aquatic animals. Their four-flipped construction looks a little more terrestrially-capable than the body of a whale or ichthyosaur, but I suspect an accidentally beached plesiosaurian would be in just as much trouble as these more classically-shaped marine forms. History shows that we can make beached plesiosaurians look half convincing in art, but it's a hollow victory: the science is not on our side.

I still find it odd that there isn't a more detailed discussion of plesiosaurian - or maybe broader sauropterygian - terrestrial capability in marine reptile literature. After all, many clades generally regarded as relatives or even ancestral to plesiosaurians are regarded as semi-aquatic (e.g. nothosaurs, Helveticosaurus) and there has to be an interesting story there regarding the progressive abandonment of land. Hopefully, studies along these lines will be performed before we're much older. But for the time being, I'll be restoring all my plesiosaurians in water, where they almost certainly belonged. I'll leave you with this painting of Pliosaurus kevani in their rightful habitat.

Pliosaurus kevani large and small, at home in the sea. Junior is particularly happy that it doesn't have to venture out onto land, probably because it's always freezing cold getting out of the water, and pliosaurids were surprisingly wimpy about that sort of thing. #palaeofact

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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, 1-34.
• Cheng, Y. N., Wu, X. C., & Ji, Q. (2004). Triassic marine reptiles gave birth to live young. Nature, 432(7015), 383.
• Ellis, R. (2003). Sea dragons: predators of the prehistoric oceans. University Press of Kansas.
• Everhart, M. J. (2000). Gastroliths associated with plesiosaur remains in the Sharon Springs Member of the Pierre Shale (Late Cretaceous), western Kansas. Transactions of the Kansas Academy of Science (1903), 64-75.
• Henderson, D. M. (2006). Floating point: a computational study of buoyancy, equilibrium, and gastroliths in plesiosaurs. Lethaia, 39(3), 227-244.
• 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 computational biology, 11(12), e1004605.
• Mazouchova, N., Umbanhowar, P. B., & Goldman, D. I. (2013). Flipper-driven terrestrial locomotion of a sea turtle-inspired robot. Bioinspiration & biomimetics, 8(2), 026007.
• Motani, R., Jiang, D. Y., Chen, G. B., Tintori, A., Rieppel, O., Ji, C., & Huang, J. D. (2015). A basal ichthyosauriform with a short snout from the Lower Triassic of China. Nature, 517(7535), 485.
• Muscutt, L. E., Dyke, G., Weymouth, G. D., Naish, D., Palmer, C., & Ganapathisubramani, B. (2017). The four-flipper swimming method of plesiosaurs enabled efficient and effective locomotion. Proc. R. Soc. B, 284(1861), 20170951.
• O’Keefe, F. R., & Chiappe, L. M. (2011). Viviparity and K-selected life history in a Mesozoic marine plesiosaur (Reptilia, Sauropterygia). Science, 333(6044), 870-873.
• Renesto, S., Lombardo, C., Tintori, A., & Danini, G. (2003). Nothosaurid embryos from the Middle Triassic of northern Italy: an insight into the viviparity of nothosaurs? Journal of Vertebrate Paleontology, 23(4), 957-960.
• Rieppel, O. (1989). Helveticosaurus zollingeri Peyer (Reptilia, Diapsida) skeletal paedomorphosis, functional anatomy and systematic affinities. Palaeontographica Abteilung A, 123-152.
• Sander, P. M. (1988). A fossil reptile embryo from the Middle Triassic of the Alps. Science, 239(4841), 780-783.
• Seeley, H.G. (1896) On a pyritous concretion from the Lias of Whitby. Annual Report of the Yorkshire Philosophical Society, 1895, 20–9.
• Storrs, G. W. (1993). Function and phylogeny in sauropterygian (Diapsida) evolution. American Journal of Science, 293(A), 63.
• Street, H. P., & O'Keefe, F. R. (2010). Evidence of pachyostosis in the cryptocleidoid plesiosaur Tatenectes laramiensis from the Sundance Formation of Wyoming. Journal of Vertebrate Paleontology, 30(4), 1279-1282.
• Taylor, M. A. (1981). Plesiosaurs-rigging and ballasting. Nature, 290, 628-629.
• Taylor, M. A. (1986). Marine reptiles: Lifestyle of plesiosaurs. Nature, 319(6050), 179-179.
• Taylor, M. P., & Wedel, M. J. (2013). Why sauropods had long necks; and why giraffes have short necks. PeerJ, 1, e36.

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.

New takes on the Wealden Supergroup palaeobiota, part 2: Baryonyx, freshwater plesiosaurs, ornithomimosaurs and others

Last week we took a look at some new art of animals from the Wealden Supergroup, the intensively studied, historically important Lower Cretaceous rocks of Southern Britain. We all know the Wealden for celebrity dinosaurs like Iguanodon and Baryonyx, but there's a heap of other interesting animals in there which get relatively little publicity. It's mostly these we're focusing on here, in the second (and final) part of these 'picture of the day'-style posts. 

As before, if you like anything here, remember that you can buy prints of them all from my shop (the Wealden section might be relevant) and its new Facebook outlet. Indeed, if you like my work and are on Facebook, why not 'like' the new Mark Witton Palaeoart page? It's the best place to see when new prints and finished pictures are available.

Baryonyx walkeri: king of the fishers, redux

Baryonyx walkeri, off for a stroll among the crocodyliforms and pterosaurs.

Let's break this post in with a familiar animal: spinosaurid Baryonyx. It's hard to appreciate now how weird this animal seemed back in the 1980s and 1990s. At this point, other spinosaur material was only very poorly known, and laymen and scientists alike found this weird, superficially-crocodile like animal fascinating. Ironically, it's recently turned out that we first collected Wealden spinosaur material centuries ago, but struggled to recognise its significance until more complete remains were unearthed in the 1980s. We now know that Baryonyx can be found throughout a good chunk of upper Wealden stratigraphy and teeth referable to it - or another spinosaurid - are fairly common, at least as Wealden dinosaur fossils go. Baryonyx provided the basic template we'd recognise for all spinosaurid anatomy until last year when, famously, some spinosaurs were proposed to be rather different. It's clear that, whatever is going on with Spinosaurus, Baryonyx retains more conventional hindlimb and pelvic proportions, and may not have been so aquatically adapted as true spinosaurines. In this updated image, B. walkeri is splashing into a body of water while goniopholidid crocodyliforms and gnathosaurine pterosaurs go about their business around it. Note how much larger Baryonyx is compared to the crocs: Baryonyx is the largest theropod in the Wealden Supergroup, by a good margin.

Button-toothed crocs, redux

Bernissartid Koumpiodontosuchus aprosdokiti foraging for molluscs. It's eating a mud snail, Viviparus cariniferus, while tiny (6 mm long) physid gastropods Prophysa crawl over pond scum in the lower left of the image. Dragonflies provide scale, and unnamed tetanurans prowl around the background.

Last year I was lucky enough to provide the first restoration of Kompiodontosuchus aprosdokiti, a small neosuchian crocodyliform common to the Wessex Formation, and perhaps other parts of the Wealden sequence. Koumpiodontosuchus is a bernissartid, a group of small-bodied crocodyliforms with robust, shell-cracking teeth at the back of their jaws. As you'll know if you read my write up last year, these were likely employed in smashing molluscs and insects. The tetanuran theropods in this image are unnamed, but are not thought to be referable to any existing Wealden taxa. We probably need more material of them to consider them nameable, however: recognising that they are different from other Wealden theropods is only half the battle. Modern students of Wealden fossils famously do their best to preserve historic names based on fragmentary bones, but there seems to be an effort to 'future proof' Wealden taxonomy against confusion by only naming well-represented, characteristic animals. I guess I could have chosen one of the better known theropods to play the 'This was the Age of Dinosaurs' card for this PR image, but I think it's good to show that not all large theropods in the Wessex palaeobiota were Neovenator, Baryonyx or Eotyrannus. 

Welcoming the new Wealden ornithomimosaurs

A flock of Wessex Formation ornithomimosaurs forage in a marshland, while istiodactylid pterosaurs skulk about behind them.

Those keeping their ears to the ground will know that the newest arrivals to the Wealden dinosaur palaeobiota are ornithomimosaurs, commonly known as ostrich dinosaurs. Two specimens show that these animals were present in both the Weald and Wessex basins of the broader Wealden succession, and one of these fossils represents a historic taxon named in 1889: Valdoraptor oweni. Key to identifying ostrich dinosaurs in the Wealden was the discovery of abundant ornithomimosaur remains in France, many of which are so reminiscent of Valdoraptor and other Wealden theropod material that they may represent the same taxon. If you want to know more about these and their relationship to the complex story of Wealden theropods, check out Darren Naish's post on this at Tetrapod Zoology.

The above new painting shows a group of (nameless) Wessex Formation ornithomimosaurs in a well-vegetated marshland, in the rainy season, while istiodactylid pterosaurs mosey about in the background. The abundance of ostrich dinosaurs and juveniles in the middle-right are nods to the frequent recovery of abundant specimens of different levels of maturity at many ostrich dinosaur sites, including the new, French 'Angeac ornithomimosaur'. Note that the wings of the running foreround animal are somewhat swept back: I don't think the more common way of reconstructing ornithomimosaurs with 'dangly arms' looks right. They look like they should be holding shopping bags or something.

Valdosaurus in the forest, redux

Two Wealden dryosaurids Valdosaurus canaliculatus, and a stubborn avialan.

Ornithomimosaurs weren't the only fast runners in Wealden landscapes. Dryosaurids, like Valdosaurus canaliculatus were also fleet-footed animals with powerful, well-muscled hindlimbs, and tiny bodies attached to the front. In this reworked image, two of these 3-4 m long animals are taking it slow through a Wealden woodland. Although Wealden climates were quite warm and arid, leaving much of the landscape looking quite chaparral-like, some relatively upland parts seem to have been more vegetated: it's here that this picture is set. In my mind, these animals always walked with the stooping posture of the foreground animal - as noted last time, I like the idea that prehistoric animals had characteristic postures varying slightly from those we consistently restore in skeletal restorations. Note the avialan on the left of the image, which is a nod to the recovery of bird teeth from Wealden deposits. Anyone who's ever been forced to walk around a stubborn reclined mallard will recognise the situation now facing the Valdosaurus.

Barilium dawsomi in leathers, redux

Barilium dawsoni, a large and very robust iguanodont from Sussex. A flock of 'Ashdown maniraptorans' add scale.

Last time we featured Iguanodon bernissartensis: now it's the turn of the 'other' big Wealden iguanodont, the stratigraphically older, and osteologically chunkier Barilium dawsoni. In this redone painting, I've tried to make the Barilium skin more interesting than just plain old scales, covering the back in small, horny ossicles and creasing the flanks as if the skin is particularly thick, leathery and folded. I think we should be rendering more interesting skin regularly in scaly dinosaur palaeoart, as it seems most extensive dinosaur skin remains show unexpected features - strangle scales, wattles, folds and that sort of thing - which small skin patches mostly cannot record adequately. It's interesting to contrast these skin impressions with homogeneous restorations of scaly dinosaur appearance presented by some, where every species is covered in smooth hide following perfect contours of the underlying tissues: I'm not sure that's what fossils are telling us. As before, the 'Ashdown maniraptoran' provides scale to the bulk of Barilium. For the uninitiated, the Ashdown maniraptoran is seriously small for a Mesozoic dinosaur - maybe about 30-50 cm long. If you find big iguanodonts exciting, be sure to check out this previous post.

Polacanthus redux, again

A Wealden tree vies for attention with Polacanthus foxii, and some tiny birds.

OK, I'm cheating a bit with this one. This redone version of a much older painting has been posted fairly recently, but it seemed a bit remiss to skip this ankylosaur in this run down of recently produced Wealden palaeoart. Polacanthus foxii is, of course, the Wealden's sacral-shield-bearing nodosaurid, shown here strolling around a Cretaceous hillock with some birds for company. Having scratched the completist itch, let's move on, because we've seen this all before.

Accidentally sinister Leptocleidus, redux 

Mother and calf Leptocleidus superstes, a freshwater leptocleidid plesiosaur, explore a river inlet in Lower Cretaceous Sussex. 

Our final stop is in Wealden rivers and estuaries, where Leptocleidus superstes and other species of freshwater leptocleidid plesiosaurs roamed. The new version of this image has added a lot of detail on top of the original, which has inadvertently made the mother and calf Leptocleidus look more sinister than intended - hey, it's not my fault their teeth stick out like that. Back in the original post on these animals I mentioned that pliosaurs may also have been present in Wealden lakes and rivers, but note that this is no longer certain: the Hastanectes valdensis remains once provisionally considered pliosauroid have been placed in Leptocleididae in more recent analyses. That does make for a neater story - it means that leptocleidids retain their dominant role as 'near-shore-or-freshwater' animals, but perhaps a slightly less interesting one.

And that's all for now - I hope you've enjoyed this jaunt back to the ancient Wealden and these revised artworks. I'm sure we'll visit the Wealden again in time. Coming next, probably: walking with non-pterodactyloid pterosaurs.

Marine reptiles behaving badly: freshwater(ish) Wealden plesiosaurs

Mother and calf Leptocleidus superstes, a freshwater leptocleidid plesiosaur, explore a swampy river inlet in Lower Cretaeceous Sussex. Of course, a real swampy scene should probably be drawn showing an impenetrable amount of suspended sediment and goo with, possibly, some plesiosaur-shaped silhouettes, but that would make for a lousy image. Prints of this image are available.

For various reasons, I've recently taken an interest in the plesiosaurs of the Wealden Supergroup. The latter will need no introduction to many readers here, being a very famous succession of Lower Cretaceous sediments which provide one of the best known dinosaur faunas in Europe, along with a diverse array of pterosaurs, crocodilians, amphibians, fish and, well, all sorts of things. The big deal about Wealden plesiosaurs is that they represent - gasp! - freshwater and brackish species rather than the marine variants we're more familiar with. Reading into these animals has been pretty fascinating and resulted in the generation of the following text and images presented here. My hope is that these will one day have a 'proper' home, but they'll have to sit here and wait for the meanwhile. The text below has been targeted at a fairly general audience and may not contain anything new for some readers, and doesn't contain citations. If, however, you're after more Wealden plesiosaurs (and who isn't?) with an authoritative twist, you'll want to be sure to check out this Tetrapod Zoology post and, of course, Adam Stuart Smith's Golden Trilobite Web Award winning-Plesiosaur Directory. If Mesozoic marine animals are your thing, you may also want to check out these posts on Ophthalmosaurus and the Oxford Clay fauna.

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Plesiosaurs are well-known aquatic Mesozoic reptiles characterised by their four large flippers and variably developed necks and heads. Their anatomy is completely unlike that of any other swimming animal, with barrel-shaped bodies tightly locked together by large, plate-like limb girdles which bore robust, powerfully muscled paddles. These flippers, highly modified limbs which are of no use on land, were entirely responsible for propelling plesiosaurs through water, their tails being relatively short and of apparent little assistance in underwater propulsion. This group, more correctly termed 'Plesiosauria’, has long been recognised as falling into two lineages: the pliosauroids and plesiosauroids. Some pliosauroids were super-predators like Liopleurodon and Pliosaurus, animals with likely stretched between 7 - 10 m in length with enormous skulls and jaws. These animals were likely top predators of many marine settings, hunting other large swimming vertebrates. Most pliosauroids bore relatively short necks but, in contrast, several plesiosauroid lineages – including famous species like Elasmosaurus, Cryptoclidus, and Plesiosaurus – developed long necks and small heads, ideal for foraging on relatively small fish and squid. Neck and skull proportions were once taken as a clear indicator of which group a given plesiosaur would belong to, but this idea has fallen from favour as the complexity of plesiosaur evolution has become apparent.

We mostly imagine these reptiles as sea- and ocean-going animals, making their occurrence in freshwater and brackish facies like those of the Wealden seem unexpected. Plesiosauria was a successful and adaptable group however, with a complex evolution that ran for 135 million years from the Late Triassic (c. 200 Ma) to the end Cretaceous (66 Ma) and included acclimatising to waters across the entire planet. Although plesiosaurs are undoubtedly mostly marine, they can be found in freshwater and brackish habitats throughout much of their history. Indeed, it seems that plesiosaurs invaded near-shore and freshwater habits on multiple occasions, although the catalyst of these invasions remains unknown. Did they thrive in environments free of large aquatic predators? Were they exploiting untapped niches and food sources? More data is required to answer these questions.

Skull reconstruction of Leptocleidus capensis, an Early Cretaceous leptocleidid from South Africa. The skull of L. superstes was probably pretty similar to this. Based on illustrations in Cruickshank (1997).

Wealden plesiosaur fossils are not particularly common. Most are isolated vertebrae, fragmentary limb bones and teeth, with only a handful of partial articulated skeletons and skulls known. These remains are important to palaeontologists because the Lower Cretaceous record of plesiosaurs is rather sparse, so Wealden plesiosaurs – rare as they are - provide an important window into this phase of plesiosaur evolution. Plesiosaur remains are span the entire Wealden stratigraphy and occur in both sub-basins, suggesting that they were long-term denizens of Wealden ecosystems. Three species of Wealden plesiosaur are currently recognised, each known by incomplete skeletons: Leptocleidus superstes (Upper Weald Clay Formation, East Sussex; also see the image above), Vectocleidus pastorum (Vectis Formation, Isle of Wight) and Hastanectes valdensis (Wadhurst Clay Formation, Hastings). Some fragmentary Wealden plesiosaur fossils clearly differ from these animals and likely represent additional, poorly known species.

Leptocleidus and Vectocleidus belong to a plesiosaur group known as Leptocleididae, an unusual lineage of Late Jurassic – Early Cretaceous plesiosaurs with necks of short or moderate length and relatively small skulls. This anatomy represents an ‘intermediate’ grade between the short-necked pliosauroids and long-necked plesiosauroids, which has caused some confusion about their relationships to other plesiosaurs. Some suggest they are a ‘relict’ lineage of early, generalised pliosauroids, but other proposal consider them derived plesiosaurids which abandoned long-necked morphologies in favour of a more generalised body plan. Whatever they are, it is noteworthy that all known leptocleidid fossils are known from freshwater, brackish or near-shore environments, suggesting they abandoned the more typical plesiosaur existence of life in open waters and spent much of their time in lakes, rivers and coastlines. This would make leptocleidids comparable to some modern seals (including Baikal seals, several types of ringed seal and harbour seals) and dolphins (such as the Irrawaddy dolphins; Baiji, Chinese river dolphin, and Tucuxi, Amazonian river dolphins) which have abandoned pelagic lifestyles or, at least, make considerable incursions up estuaries and rivers in search of food. Indeed, seals and river dolphins may be the best modern ecological analogues to Wealden leptocleidids. The skulls and jaws of these plesiosaurs were equipped with large jaw muscles and conical, partially serrated teeth, ideally suited to feeding on a small bodied prey. Their diet probably mostly comprised fish, supplemented by opportunistic taking of other, small swimming animals. The four-flippered propulsion system of plesiosaurs may have been ideally suited to navigating complex and tight underwater habits in pursuit of cryptic prey, permitting for excellent manoeuvrability as well as bursts of speed.

Skeletal reconstructions of mother and foetal Polycotylus latippinus, polycotylid plesiosaurs which are not a million miles away, phylogenetically speaking, from leptocleidids. Was this strategy of birthing solitary, large calves found in leptocleidids - and other plesiosaurs for that matter - as well? From O'Keefe and Chiappe 2011; image from here.

At least Leptocleidus was a fairly large animal for the Wealden waterways, attaining body lengths of around 3 m. This size may have dissuaded attacks from even the largest Wealden aquatic and semiaquatic predators, but the same cannot be said for their calves. ‘Calves’ is an appropriate word here: fossils of Late Cretaceous plesiosaurs (which happen to be closely related to leptocleidids) show that at least some plesiosaurs did not lay eggs like many other reptiles, but instead gave birth to a solitary, large and very well developed baby. This reproductive strategy is extremely similar to that of large mammals but is virtually unheard of in reptiles. Although live births are known in many modern lizards and snakes, only a few modern reptiles (various types of skinks) are known to produce a single, large and highly developed offspring. The development of such reproductive strategies in plesiosaurs is therefore rather remarkable (though we must be mindful that we do not know how common this strategy was across Plesiosauria). Both mammals and reptiles that invest heavily in a single offspring are highly social and engage in maternal care, which may indicate that adult plesiosaurs did the same. Perhaps Wealden leptocleidids protected their young from predators, warding off attacks from marauding goniopholidids crocodilians and other plesiosaurs until they were large enough to look after themselves.

Hastanectes valdensis: a possible pliosaur which, even at only 2 m long, is good reason not to paddle in Wealden waterways. Especially if you're a small crocodile.

Such predatory attempts may have been attempted by our third Wealden plesiosaur, Hastanectes (above). Some have suggested that Hastanectes is a pliosaurid rather than a leptocleidid, and closely related to the large, powerful members of this lineage with short-necks and large skulls armed with tusk-like teeth. If so, Hastanectes may represent a small (2 m long) version of these predators. Interestingly, no Hastanectes specimens currently known represent fully-grown animals, suggesting it may have grown somewhat larger. Even at 2 m in length, such a pliosaur would be keen predator of small and medium-sized swimming creatures in Wealden waters, perhaps taking not only fish but also regularly hunting other reptiles. This interpretation of Hastanectes has not gone unchallenged, however: some very recent studies have suggested it represents another Wealden leptocleidid.

A fourth, and largely mysterious type of Wealden plesiosaur is represented by very scant remains indeed. A solitary vertebra from the Hastings Group hints at the presence of a long-necked plesiosauroid within the Wealden. Exactly what sort of plesiosaur this represents however, and how it may have functioned within the Wealden palaeoecosystem, is unknown at present.

References

• Cruickshank, A. R. I. (1997). A lower Cretaceous pliosauroid from South Africa. Annals of the South African Museum 105, 207–226.
• O’Keefe, F. R. & Chiappe, L. M. (2011). Viviparity and K-selected life history in a Mesozoic marine plesiosaur (Reptilia, Sauropterygia). Science 333, 870-873.