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Friday, 25 January 2019

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. The it 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 are 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 of 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 weakens 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?

With ,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-apt than the bodies 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

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