Tetrapod Zoology

Tetrapod Zoology

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Plesiosaurs and the repeated invasion of freshwater habitats: late-surviving relicts or evolutionary novelties?


Did big, pelagic rhomaleosaurids give rise to dwarfed, estuarine/lagoonal/freshwater leptocleidids? It's an appealing model. Rhomaleosaurid image (Dmitry Bogdanov) in public domain; leptocleidid by Darren Naish.

Time to talk about another recently published paper I was involved in: this time, the looooong awaited Journal of Systematic Palaeontology paper ‘A new leptocleidid (Sauropterygia, Plesiosauria) from the Vectis Formation (Early Barremian-early Aptian; Early Cretaceous) of the Isle of Wight and the evolution of Leptocleididae, a controversial clade’ (Benson et al. 2012a). Wow, what a long title. Incidentally, the spelling inconsistency right there in the title (we have ‘Early Barremian’ but ‘early Aptian’) was introduced after proof stage by the publishers*. Anyway, those who know about British plesiosaur research will be aware of the project concerned, since it’s been the stuff of legend since the mid-1990s at least.

* I initially thought that this was a typo, but I've since been told that it reflects the fact that the Barremian has official 'Early' and 'Late' subdivisions while the Aptian does not.

Shepherds Chine, as it looked in August 2012. Photo by Darren Naish.

Back in 1995, Warwick Fowler (father of Denver Fowler, these days a well-known palaeontologist based at the Museum of the Rockies) discovered a block of rock containing several plesiosaur bones at Shepherds Chine* at Brighstone Bay on the Isle of Wight. A ‘chine’ is the local, Isle of Wight name for a gulley that contacts a beach; Shepherds Chine (shown here) has a stream at its bottom, and the block concerned was actually discovered in the stream. A second block was discovered close by, at a different date, by then University of Portsmouth student Langan Turner, and a third was found by local collector Hazel Underwood. When united at the local Dinosaur Isle palaeontology museum, it was evident that all three blocks joined together. The partial, articulated skeleton of a small plesiosaur had been found. Actually, a fourth block (not mentioned in the paper) is known as well. It’ll get into the technical literature eventually.

* We opted for ‘Shepherds Chine’ in the paper, rather than ‘Shepherd’s Chine’. Both are in use and I’m not sure which is ‘correct’.

The sea-ward end of the stream at the bottom of Shepherds Chine, as it looked in August 2012. Sometimes - as here - the stream is dammed by a gravel bank and doesn't meet the sea. The blocks containing the leptocleidid were in, or close to, this stream. Look how much sediment is present in the sea close to shore. Photo by Darren Naish.

Langan worked on the specimen for an undergraduate project produced under Dave Martill, and Langan and I were putting together a paper on the specimen round about 2005. Due to my PhD thesis (on theropod dinosaurs, not plesiosaurs), and a list of other things, I simply had to neglect the project and it sat, unloved and un-worked on, for years. It took renewed interest in the specimen from the University of Cambridge’s Roger Benson to get things moving again – Roger approached me with the idea of collaboration in 2010, and the end result is the Benson et al. (2012a) paper we just published. So – what does it say?

The initial hypothesis for the Shepherds Chine plesiosaur is that it was a new specimen of Leptocleidus superstes, a Wealden plesiosaur reported from the Barremian Upper Weald Clay Formation of East Sussex by Andrews (1922). The L. superstes holotype consists of a partial skull, articulated cervical, pectoral and anterior dorsal vertebrae, and various bones from the pectoral region and forelimb. It was recently redescribed by Kear & Barrett (2011). The whole animal was probably about 3 m long.

My sketches of the Leptocleidus superstes holotype (though the skull isn't shown here). The vertebrae are in good shape but much of the coracoids is reconstructed in plaster. See Kear & Barrett (2011) for a modern description.

Similar species – also included in the taxon Leptocleidus – have been discovered in Cretaceous sediments in South Africa (L. capensis) and Western Australia (L. clemai) (Andrews 1911, Cruickshank 1997, Cruickshank & Long 1997). A few additional leptocleidid taxa are known, though note that their inclusion in Leptocleididae was not necessarily supported in the original descriptions and is mostly due to the conclusions of Ketchum & Benson (2010, 2011) and Benson et al. (2012a). Nichollssaura borealis is known from an excellent, semi-complete skeleton from the Albian Clearwater Formation of Alberta (Druckenmiller & Russell 2008a, 2009). It was originally named Nichollssia, but this proved preoccupied by an isopod crustacean.

Umoonasaurus demoscyllus is an Australian leptocleidid from the Aptian-Albian Bulldog Shale (Kear et al. 2006), the best specimen of which is the opalized skeleton long known simply as ‘Eric’. Finally, Brancasaurus brancai from the Wealden Supergroup upper Bückeberg Formation of Germany, named in 1914 and originally described as an elasmosaurid (Wegner 1914), also appears to be a leptocleidid.

Skull of Brancasaurus brancai, from Wegner (1914). I don't think anyone has ever confused it with Brachiosaurus brancai (now Giraffatitan brancai), the sauropod, but you never know.

Intriguingly, most of these smallish, Lower Cretaceous plesiosaurs are from freshwater and marginal marine environments: not from pelagic habitats typically considered the haunt of plesiosaurs. Umoonasaurus is a high-latitude animal from a cool climate that seemingly lived alongside other plesiosaurs as well as platypterygiine ichthyosaurs (Kear et al. 2006).

A new name for the Shepherds Chine leptocleidid

The Shepherd Chine leptocleidid, as display at Dinosaur Isle Museum on the Isle of Wight, in 2012. No part of the skull is known. The reconstructed head isn't bad, but it's way too shrink-wrapped. Photo by Darren Naish.

The Shepherds Chine specimen shares a list of features with L. superstes, so was initially assumed to be a new member of this species. Sad to say, we were also inspired by the tradition of referring all Wealden plesiosaur specimens to this taxon. However, it differs from L. superstes in several key characters and is from a different stratigraphic basin. It’s now obvious that the Shepherds Chine animal is different enough from L. superstes, and indeed from Leptocleidus as a whole, to warrant recognition as a distinct taxon: its clavicles are more gracile than those of Leptocleidus, its coracoids are different in shape, and its neural spines are shorter front-to-back.

It also exhibits a remarkable asymmetric condition of the neural spines where they alternate from having transversely compressed apices to apices that are expanded to the right. Bizarrely, this isn’t unique to the Shepherds Chine animal; it’s seen in a few rhomaleosaurids (including Eurycleidus arcuatus, Meyerasaurus victor, ‘Rhomaleosaurus megacephalus and Avalonnectes arturi) and in Leptocleidus and Brancasaurus as well (Benson et al. 2012a, b). So far as I know, nobody has any idea how this remarkable configuration relates to function, evolution or behaviour.

The two more informative blocks containing the Vectocleidus pastorum holotype. We have much of the dorsal vertebral column of the animal and at least part of its pectoral skeleton. From Benson et al. (2012a).

Anyway, in view of all this, we opted to recognise the Shepherds Chine leptocleidid as a new taxon. It’s Vectocleidus pastorum, meaning approximately ‘Vectis Formation leptocleidid of the shepherds’ (Benson et al. 2012a). I know that ‘cleidus’ actually refers to the clavicle, but we decided to treat it as something of a name ‘root’ for members of Leptocleididae. The ‘of the shepherds’ thing obviously refers to Shepherds Chine.

The phylogenetic position of Vectocleidus within Leptocleididae is uncertain, as it moved around quite a bit in the phylogenetic analysis with no particular position being preferred (Benson et al. 2012a). We otherwise found Umoonasaurus, Brancasaurus and Nichollsaura to be successively more distant to Leptocleidus.

Leptocleidids: where within the plesiosaur radiation?

The big question about leptocleidids concerns their position within plesiosaur phylogeny. This is actually fairly contentious and several different positions have been proposed for the group. Note that I’m keen to avoid producing a full and lengthy discussion of plesiosaur phylogeny here. The ‘traditional’ view of leptocleidids – if there can be said to be such a thing – is that they are archaic plesiosaurs, closely related to rhomaleosaurids and hence part of Pliosauroidea (Andrews 1922), and also that they’re close to the ancestry of the polycotylids.

Best known (and probably largest) member of Rhomaleosauridae: Rhomaleosaurus cramptoni. This is the cast at the NHM, London (not all the limbs represent those originally found with the specimen). Photo by Hector E. Rivera, from Adam Smith's Plesiosaur Directory.

Modern phylogenetic studies generally find Plesiosauria to consist of a Plesiosauroidea and a Pliosauroidea. Note that these clades don’t correspond exactly to the ‘plesiosauromorph’ and ‘pliosauromorph’ body shapes, since the pliosauromorph polycotylids are sometimes found to be part of Plesiosauroidea, specifically being close to cryptoclidids in the studies concerned (O’Keefe 2001, 2004, Ketchum & Benson 2010, Benson et al. 2012a). Adding further complexity is the fact that some studies now find rhomaleosaurids to be distant relatives of pliosaurids, and in fact outside of Neoplesiosauria, the clade that includes pliosaurids and plesiosauroids (Benson et al. 2012b).

We currently have two main conflicting hypotheses of leptocleidid affinities. Hypotheses 1 is that they are part of traditional Pliosauroidea (“traditional” since it’s taken to include rhomaleosaurids) and are most likely close to rhomaleosaurids, and perhaps with a sister-group relationship to polycotylids. Hypothesis 2 is that they are plesiosauroids, part of the same major clade as cryptoclidids, but also perhaps with a sister-group relationship to polycotylids.

Two competing evolutionary scenarios

Cartoon leptocleidid in cartoon lagoon. By Darren Naish.

The really interesting thing about the competing nature of these hypotheses is that they posit markedly different evolutionary scenarios for the history of the group. According to Hypothesis 1, leptocleidids could be regarded as conservative relicts that have retained their pliosauromorph shape from Jurassic ancestors. The fact that leptocleidids are small plesiosaurs whose fossils come from lagoonal, estuarine and freshwater environments could be interpreted as evidence that they were only able to persist into the Cretaceous by hanging on in marginal, ‘safe haven’ habitats, and by mostly avoiding competition with younger plesiosaurian clades. If this scenario is correct, and if polycotylids really are close kin of leptocleidids, a further interesting spin on things is the possibility that polycotylids started their history in the same sort of non-pelagic, marginal marine habitats before taking to the open oceans and evolving giant size.

According to Hypothesis 2, leptocleidids are not conservative relicts at all, but relatively young evolutionary novelties in which small size, pliosauromorph proportions and life in marginal marine and freshwater environments are all new specialisations that evolved from pelagic, long-necked, plesiosauromorph ancestors.

Our new study includes an evaluation of these competing phylogenetic hypotheses. Hypothesis 1 mostly relies on the fact that leptocleidids possess a number of anatomical features that recall the archaic rhomaleosaurids of the early part of the Jurassic. As described most cogently by the late Arthur Cruickshank (1997), leptocleidids supposedly resemble rhomaleosaurids in having a so-called dorsomedial cleft between the two premaxillae (a groove that runs along the dorsal midline of the snout, in between the eyes and along the anterior part of the snout), in having trough-like excavations on the articular and prearticular bones in the lower jaw, and in having “strong descending flanges” on the postorbital bars (Cruickshank 1997, p. 221).

I used to be an unashamed supporter of this hypothesis: not because I regard it as cool and really interesting (which it is) but, rather, because the anatomical characters involved looked fairly compelling. Alas, re-examination shows either that they are not as compellingly similar as originally thought, or that they are actually more widely distributed, and hence not so important (Benson et al. 2012a). Basically, we think that the anatomical similarities between rhomaleosaurids and leptocleidids have been overstated.

Skull of Leptocleidus capensis, from Cruickshank (1997). Note the raised posterior part of the skull deck, with its anteriorly located spur-like structure. A raised squamosal region is a potential synapomorphy of Leptocleidia (Benson et al. 2012a).

Note also that a close relationship between leptocleidids and polycotylids is well supported: as per Ketchum & Benson (2011), this clade is termed Leptocleidia (other authors have termed it Leptocleidoidea). We report several new characters that support the monophyly of Leptocleidia, the best known of which (or most-oft-commented-on) concerns the raised posterior part of the skull table, sometimes described as forming a sort of ‘cockscomb’-like configuration on the squamosal arch. While the existence of Leptocleidia might be regarded as widely supported among plesiosaur workers (Druckenmiller & Russell 2008b, Smith & Dyke 2008, Ketchum & Benson 2010, 2011, Benson et al. 2012a), it wasn’t supported by Kear & Barrett (2011), and this partly explains why their results are so discordant with ours.

Plesiosaur phylogeny as recovered by Smith & Dyke (2008): note that the leptocleidid + polycotylid clade (here termed Leptocleidoidea) is nested within Pliosauroidea, being closer to Pliosauridae than to Rhomaleosauridae.

Are leptocleidians pliosauroids (in the ‘traditional’, rhomaleosaurid-including sense of the term), or plesiosauroids? This idea that leptocleidids could be plesiosauroids was contested by Kear & Barrett (2011) since Leptocleidus has a palatal morphology and several neck characters that make it seem very different from plesiosauroids. However, the characters concerned are problematic: the relevant palatal character (concerning the presence of a structure called the ectopterygoid flange) is now known to be present in some microcleidid and elasmosaurid plesiosauroids, and the neck characters could very likely be absent in Leptocleidus because they’re linked to the long-necked condition present in most plesiosauroids but obviously absent in Leptocleidus (Benson et al. 2012a).

Nevertheless, leptocleidians as a whole have been regarded as pliosauroids by several other authors (Druckenmiller & Russell 2008a, b, Smith & Dyke 2008). The Ketchum-Benson plesiosaur database we used has previously recovered polycotylids as plesiosauroids (Ketchum & Benson 2010, 2011), and that’s the case in our new study: Leptocleidia is the sister-group to Cryptoclididae – forming the clade Cryptoclidia, first named by Ketchum & Benson (2011) – with Elasmosauridae being the sister-group to Cryptoclidia. We bring attention to several additional characters (present in the cervical ribs, scapula and coracoid) that seem to strengthen the view that leptocleidians are plesiosauroids.

The favoured plesiosaur phylogeny from Benson et al. (2012a). Note that leptocleidians are within Cryptoclidia, within Plesiosauroidea. Why is a pliosaurid taxon highlighted? Read on...

Don’t get me wrong: I’m not saying that the monophyly and phylogenetic position of Leptocleidia has now been decisively resolved. Rather, it remains controversial, and I know for a fact that other studies of plesiosaur phylogeny – currently in preparation – disagree with the results reported in Benson et al. (2012a).

Freshwater plesiosaurs

To wrap things up, then, here’s a summary of what I’ve described so far: leptocleidids are probably close relatives of polycotylids, forming with them the clade Leptocleidia. While some studies find leptocleidians to be pliosauroids (and with Pliosauroidea being a clade that includes rhomaleosaurids), we (Benson et al. 2012a) argue that leptocleidians are plesiosauroids, close to cryptoclidids and forming with them the clade Cryptoclidia. If all of this is true, then the appealing idea that leptocleidids are late-surviving descendants or close relatives of rhomaleosaurids is not correct: rather, leptocleidids are novelties that evolved ‘pliosauromorph’ proportions independently of rhomaleosaurids. Furthermore, their adaptation to marginal marine and freshwater environments was a new phenomenon; they were not relicts, “forced under competition to seek refuge in a relatively protected environment in the inshore shallows”, of the sort suggested by Cruickshank (1997, p. 223).

There are quite a few life restorations of leptocleidids. This shows the Australian taxon Leptocleidus clemai, by John Long. From Cruickshank & Long (1997). I don't think the flippers are depicted well.

The idea that leptocleidids – and plesiosaurs in general – invaded freshwater environments is not new (Cruickshank 1997, Kear & Barrett 2011). Numerous plesiosaur fossils from the Lower and Middle Jurassic of China and Australia, and from the Cretaceous of Australia and North America, come from freshwater settings. This might show that plesiosaurs of some or many ordinarily marine species were able or willing to live temporarily or permanently in freshwater habitats, or it might show that species within several or many lineages were freshwater specialists.

Speculative reconstruction of the Ashdown maniraptoran (from Naish & Sweetman 2011): a tiny theropod dinosaur, represented only by a single cervical vertebra. Like Hastanectes, it's from the Wadhurst Clay Formation of the Hastings Group.

Our study provides additional support for the second possibility, since we also show that a second freshwater plesiosaur taxon is present in the Wealden Supergroup: originally named Cimoliasaurus valdensis in 1889 but mostly neglected recently as an alleged nomen dubium, it was argued by Ketchum (2011) to be a distinct taxon. We acted on this, and renamed it Hastanectes valdensis (Benson et al. 2012a). Hastanectes is known from numerous vertebrae as well as various limb and pelvic bones, all of which comes from the Valanginian Wadhurst Clay Formation of the Hastings Group (the same unit, incidentally, that yielded the Ashdown maniraptoran). Hastanectes isn’t a leptocleidid: in fact, the highly distinctive anatomy of its neck vertebrae show that it’s a pliosaurid. Hastanectes may show “that multiple plesiosaurian clades were found in marginal or freshwater habitats, even during the brief earliest Cretaceous interval” (Benson et al. 2012a, p. 15).

There's one thing left to say. We know that plesiosaurs invaded the freshwater realm, and we now know that mosasaurs did as well. Of the other major Mesozoic marine reptile groups, what about ichthyosaurs? Were there freshwater ichthyosaurs? A few specimens have been reported from lagoonal settings and two fragments of vertebrae possibly come from a freshwater setting (Ketchum 2011). We'll come back to this matter in time.

For previous Tet Zoo articles on plesiosaurs and other sauropterygians, see...

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Benson, R. B. J., Evans, M. & Druckenmiller, P. S. 2012b. High diversity, low disparity and small body size in plesiosaurs (Reptilia, Sauropterygia) from the Triassic–Jurassic boundary. PLoS ONE 7, e31838.

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The views expressed are those of the author and are not necessarily those of Scientific American.

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