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 Leptocleiduswas 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.