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Investigating plesiosaur swimming using computer simulations

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One of the many areas of controversy in plesiosaur palaeobiology is the topic of how they swam. The question goes back almost 200 years to the 1820s when the first complete plesiosaurs were described from the Jurassic cliffs of Lyme Regis, UK. Plesiosaur swimming is a particularly difficult topic to study for a number of reasons. Plesiosaurs are extinct so there are no modern descendants, they have a unique body plan with no modern analogues, and swimming animals tend not to leave a good trace fossil record. All of this leaves us in a sticky predicament, but there are avenues we can explore to come to a greater understanding of plesiosaur locomotion.

Previous researchers have tried to answer the question of how plesiosaurs swam by conducting detailed osteological analyses, while others have approached it experimentally using robotics, or humans with fabricated paddles. These studies have started to settle on some consensus but there is still some uncertainty. The topic can also be explored experimentally through computer simulation and I was fortunate enough to be involved in such a study in collaboration with colleagues at the Georgia Institute of Technology. Our findings were published today in the open access journal PLOS Computational Biology (Liu et al. 2015).

Meyerasaurus swimming

Rendering of the Meyerasaurus victor plesiosaur model used in the study by Liu et al 2015.

What did we do?

We built a full-size, 3D, virtual plesiosaur, placed it in a simulated fluid, and gave it articulated joints so that it could propel itself through the fluid. We ran thousands of simulations to find the optimal ranges of limb motion and gaits – those that moved the animal forward the furthest. We did so multiple times under different specified parameters to see how different available ranges of joint motion effected the results. To investigate the potential contribution of the different limbs, some of the simulations used all four limbs while others used the forelimbs or hind limbs only. You can check the open access paper for the technical details.

What did we find?

We generated a lot of results in the form of videos. The simulations with the most plausible ranges of motion have a flying stroke with a large up-down component. This is essentially a form of underwater flight similar to penguins and turtles. One of the key questions we wanted to explore was how the forelimbs and hindlimbs moved relative to each other. Our results were inconsistent in this regard, which is significant in itself. The ‘forelimb-only’ simulations are just as fast as simulations using all limbs, which implies that the forelimbs were the powerhouse in plesiosaur swimming while the hindlimbs were more passive, at least during steady cruising.  However, in ‘hindlimb-only’ simulations, where the hind-limbs were asked to do all of the work, the rear flippers flail around a lot but the motion isn’t transferred into thrust or forward motion. Instead, in these simulations the whole plesiosaur rocks around the centre of body mass – apparently a rear-drive plesiosaur is a no-goer. This physical constraint probably explains why no other animals have adopted this unusual body plan, and it also explains why the gait is so variable in our simulations – the hind limbs provide so little thrust during cruising that how they move relative to the forelimbs is irrelevant.

plesiosaur swimming with all four limbs in large range

An example of one of the simulations. This one shows optimal swimming in a simulation of all four limbs using the widest joint range parameters. The parameters in this particular simulation are probably beyond the biologically possible limits of the joints, but other simulations had more conservative ranges (see the paper for all the videos).

 

plesiosaur swimming with all four limbs in large range - tip traces

To help us understand the limb strokes we traced the tips of the limbs. This one shows the limb tip traces for the above simulation, viewed posterolaterally. The hind-limbs in this simulation used only a small proportion of the available range.

 

Out of curiosity (and not included in the paper), we also manually simulated some specific limb strokes as hypothesised by previous plesiosaur researchers: rowing,  figure-of-8 flying, and modified flying. However, none of these manual simulations were as efficient as the best simulations found in our study through optimisation.

Does the method work?

Can we be sure that the method works and how do we know? Without a time machine we can never be completely certain that simulations of extinct organisms are correct. However, we can test the methods by applying them to models of animals for which their swimming is already known. In this case, our method was applied to several modern day animals including a turtle, a fish, and a frog (Tan et al. 2011). In each modern day animal the simulations were consistent with the biological reality, which suggests that the virtual reality is mirroring actual reality. This gives us confidence in our method.

New questions raised

Our simulations may shed light onto some old questions, but they raise new ones. If the hind limbs weren’t used for steady swimming, why are they so similar in shape and size to the forelimbs? Our study focussed entirely on propulsion but not on steering or stability, so we suggest that the hindlimbs may have helped the animal change direction more efficiently. Another alternative is that the rear flippers may not have been used in steady cruising – the sort of swimming our method focussed on – but may have instead been used for sudden short-lived bursts of speed. This sort of behaviour would be unstable over long distances (and so our method would reject it), but the hind flippers may have helped the plesiosaur lunge at prey or avoid a larger predator.

Plesiosaur tip traces

Tip-traces of the most efficient limb strokes resulting from simulations with all four limbs. The top simulation explored a narrow range of available motion, the middle simulation explored a medium range of available motion, and the bottom simulation explored a wide range of available motion (probably exceeding the biological limits of joint motion).

The future

We hope to explore the above questions about the function of the hind limbs in the future. There’s plenty of scope for other related studies on different plesiosaurs or other extinct swimming animals. For example, we selected a plesiosaur with a generalised morphotype for this study, but plesiosaurs as a group are highly variable. We’d like to look at some of the more extreme morphotypes in the future, the long-necked elasmosaurids and short-necked pliosaurids, to see how the proportions of the body impact the simulations. We also focussed all of our attention (and computing power) on how the limbs move, because that was our main focus. However, we acknowledge that the tail and neck may also have been important in locomotion. This is something else we hope to explore in the future. In the meantime, every journey must start with a first step, or – in this case – a first flap.

Liu S, Smith AS, Gu Y, Tan J, Liu CK, Turk G. (2015) Computer Simulations Imply Forelimb-Dominated Underwater Flight in Plesiosaurs. PLoS Comput Biol 11(12): e1004605. doi:10.1371/journal.pcbi.1004605

Tan J., Gu Y., Turk G., and Liu C. K. 2011. Articulated swimming creatures. ACM Transactions on Graphics, 30(4), 58:1–58:12.

Written by Adam S. Smith

December 18th, 2015 at 7:42 pm

Resurrecting the Unfortunate Dragon

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The five metre-long holotype specimen of ‘Plesiosaurus’ megacephalus, from the Jurassic of Street-on-the-Fosse, Somerset, was one of several plesiosaurs once displayed in the Bristol Museum and Art Gallery. As one of the earliest plesiosaurs to evolve it is an important species for understanding the early history of the group. Sadly, the fossil skeleton was destroyed along with many other important specimens when the museum was struck by a bomb during the Second World War. This destroyed fossil material is sometimes referred to today as the ‘ghost collection’.

Atychodracon skeleton

Historical photograph of the holotype skeleton (BRSMG Cb 2335) of Atychodracon megacephalus (Stutchbury, 1846). Bristol City Museum & Art Gallery. Length of skeleton equals 4960 mm.

Thankfully, all was not lost. Moulds had been taken from some of the fossils before the war, and in the case of ‘Plesiosaurus’ megacephalus, multiple casts of its skull and forelimb were produced prior to its destruction. These were deposited in the collections of several museums, including the British Geological Survey (BGS), Keyworth; Natural History Museum, London; and Trinity College, Dublin.

The casts provide a valuable resource that I was able to use to describe ‘Plesiosaurus’ megacephalus in an article published this year in the open access journal Palaeontologia Electronica (18.1.20A p.1-19). The study focused on the casts held in the BGS, but was also facilitated by The Bristol Museum and Art Gallery who provided historical photographs of the ‘ghost collection’ from their archives. The photo (above) shows how the entire fossil skeleton appeared before it was destroyed. The BGS also produced three-dimensional digital laser scans of the casts as part of their JISC-funded ‘GB3D fossil types online’ project. The resulting virtual models are free to view or download (here) and can be rotated on screen or 3D-printed.

Atychodracon skeleton

Three dimensional scan with texture (colour) removed of plaster cast (BGS GSM 118410) of the holotype (BRSMG Cb 2335) skull of Atychodracon megacephalus (Stutchbury, 1846) in ventral (palatal) view. Scale bar equals 100 mm.

The skeleton of ‘Plesiosaurus’ megacephalus is distinct enough from all other plesiosaurs, including Rhomaleosaurus and Eurycleidus, to warrant a new genus name. I called it Atychodracon, meaning ’Unfortunate Dragon’, in reference to the unfortunate destruction of the original fossil material. This project also demonstrates that casts of fossils, and 3D laser scans, can provide valuable data for palaeontologists – they can be described, measured, and coded into analyses. When original fossil material has been lost, damaged or destroyed, the scientific value of casts increases even further. This study is the first publication to make use of the publicly available repository of 3D laser scans provided by the BGS. The Bristol Museum and Art Gallery is now investigating the possibility of using physical representations of their ‘ghost collection’ in future exhibitions, to bring long lost fossils such as Atychodracon ‘back to life’.

Find out more by checking out the article at Palaeontologia Electronica.

Atychodracon skeleton

Plaster cast (BGS GSM 118410) of the ventral surface of the right forelimb of the holotype of Atychodracon megacephalus (Stutchbury, 1846) (BRSMG Cb 2335). 1, three dimensional scan with texture (colour) removed, 2, photograph, 3, interpretation.

Written by Adam S. Smith

October 18th, 2015 at 12:01 am

Monograph on Rhomaleosaurus thorntoni

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Many readers will be familiar with the giant plesiosaur on display in the marine reptiles gallery of the Natural History Museum, London. This is a cast of the 7 metre long holotype of Rhomaleosaurus cramptoni, the original of which is housed in the National Museum of Ireland (Natural History) and formed the basis for my PhD thesis back in (time flies!) 2007. However, The Natural History Museum, London, also has its very own massive (also ~7 m long) and quite real Rhomaleosaurus type specimen to rival the ‘Dublin Pliosaur’ in size. NHMUK PV R4853, the mighty Rhomaleosaurus thorntoni, is from the Toarcian (Lower Jurassic) of Northamptonshire. It was donated to the museum prior to 1922 but has never been described and figured in its entirety before.

Rhomaleosaurus cramptoni

Rhomaleosaurus cramptoni cast on display in the NHM, London

 

My newest paper, co-authored with Roger Benson (Smith & Benson 2014), provides a detailed description and photographic atlas of the entire skeleton of Rhomaleosaurus thorntoni, and it was published by the Palaeontographical Society just in time for me to distribute copies to colleagues at the SVP annual meeting in Berlin last November (2014). Few monographs of this kind, i.e. a comprehensive treatments of a single taxon, exist for plesiosaurians, especially up-to-date ones, so the paper should prove useful. The monograph includes 35 photographic plates depicting, essentially, every bone in the skeleton from multiple angles. We describe the skeleton in detail and figure the more complicated elements as interpretive illustrations. It’s just a bigger than average descriptive paper, really, but one that has been many years in the making (even more than it usually takes!). I’ve been waiting for the published monograph to be listed on the Palaeontographical Society publications page prior to posting this article, but since it is not yet forthcoming I decided to post this anyway. I’ll update this blog entry with a link to the volume once it is listed. [Edit – here is the link]:

Rhomaleosaurus ilia

Rhomaleosaurus thorntoni ilia (Plate 33 from Smith & Benson [2014])

The entire manuscript, including the photographs and figures, is completely new: this is not a rehash of my PhD thesis on Rhomaleosaurus. The skeletal reconstruction is brand new as well and I hope that it comes to replace my previous reconstruction of Rhomaleosaurus in time, which I was never completely satisfied with (figured in Smith [2007], Smith & Dyke [2008], and Smith [2013]). It is important to highlight that the new reconstruction represents R. thorntoni specifically, which we demonstrate is a distinct species, whereas the previous reconstruction represented Rhomaleosaurus sp. using R. cramptoni where possible and R. thorntoni as a proxy where not. As such, the original reconstruction was a mishmash of two different species, with related scaling errors. Most of the differences apparent between the new and old reconstructions are, however, due to stylistic improvements and a greater attention to detail, rather than genuine anatomical differences between R. cramptoni and R. thorntoni. The lateral view, especially, had some perspective issues with the ribs and limbs, which are corrected in the new reconstruction. There is still some margin for error in the proportions of the tail and neck in the new reconstruction because these are incomplete in the holotype (and only known specimen) of R. thorntoni, but I’m much more satisfied with it.

Rhomaleosaurus thorntoni

Rhomaleosaurus thorntoni reconstruction. Scale bar = 1m.

There is some doubt over the systematic position of rhomaleosaurids. They are traditionally regarded as pliosaurs, but they might not really be included within that clade, so for this reason we refrained from referring Rhomaleosaurus to Pliosauroidea in the title. We don’t include a cladistic analysis in our monograph to investigate this question, but we do summarise all previous ones and identify areas of relationship consensus within the clade Rhomaleosauridae. More cladistic work is required to confirm whether rhomaleosaurids are an early plesiosaurian offshoot, or pliosaurs proper.

Rhomaleosaurid cladograms

Ingroup relationships of rhomaleosauridae according to different researchers (text-fig 11 from Smith & Benson [2014])

So, where’s the PDF? Sadly, there isn’t one, and this has been discussed and debated in some detail over at SV-POW (here). I say ‘there isn’t one’, but what I really mean is that distribution of the PDF is forbidden, since a beautiful PDF does exist (I was annotating it in the final proof stages). I was hopeful that permission would be granted for me to share the final PDF along with the hard copies provided for authors to distribute, but it was not to be. Of course, I’m disappointed about the barrier this puts between my research and potential readers, and I’m concerned about the impact this might have on it being cited. However, the hard copy is a quality publication, which can be thought of as more of a book than a paper. Those individuals that require it for research purposes can always request one from me directly – I can’t make promises but drop me an email if you have a serious interest ([email protected]).

The Palaeontographical Society funded some of my visits to the Natural History Museum to see the fossil material and this influenced my decision to select the Monograph of the Palaeontographical Society as a publication venue for this work. Plus, the format suits such an exhaustive treatment. I’d  like to thank the editor, Yves Candela, who made a significant contribution to the volume and coordinated the whole process.

Update: The monograph is now available for sale from the Pal Soc website here.

References:

Smith, A. S. 2007. Anatomy and systematics of the Rhomaleosauridae (Sauropterygia: Plesiosauria). PhD thesis. University College Dublin, 278pp.

Smith, A.S. 2013. Morphology of the caudal vertebrae in Rhomaleosaurus zetlandicus and a review of the evidence for a tail fin in Plesiosauria. Paludicola 9 (3): 144–158.

Smith, A.S. and Dyke, G.J. 2008. The skull of the giant predatory pliosaur Rhomaleosaurus cramptoni: implications for plesiosaur phylogenetics. Naturwissenschaften, 95, 975-980.

Smith A.S. and Benson R.B.J. 2014. Osteology of Rhomaleosaurus thorntoni (Sauropterygia: Rhomaleosauridae) from the Lower Jurassic (Toarcian) of Northamptonshire, England. Monograph of the Palaeontographical Society, London: 168 (642), 1–40, pls 1–35.

Written by Adam S. Smith

January 8th, 2015 at 12:58 pm

Pliosaurus kevani – the Weymouth Bay Pliosaur

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I’ve been rather quiet again recently, however, as coauthor of an article just published in PLOS ONE, I’ve good reason to come out of my shell today. The new paper describes and names the Weymouth Bay Pliosaur, a spectacular almost complete skull over 2m long. As discussed in the open access paper (take a look), the specimen is sufficiently different from all other pliosaurs to warrant a scientific name of its own, Pliosaurus kevani.

Weymouth Bay Pliosaur

Pliosaurus kevani was named to honour Kevan Sheehan, the Osmington Mills café owner who collected most of the skull, piece by piece, over a period of eight years. Kevan collected chunks up to 60 kg each as they weathered out from the Jurassic aged Kimmeridge Clay Formation sea-cliff. The specimen was purchased with funding secured by Dorset County Council’s museum service from the Heritage Lottery Fund Collecting Cultures programme, and Dorset and Devon county councils. It was prepared between 2010 and 2011 by Scott Moore-Fay and went on public display in Dorchester County Museum in July 2011.

Richard Forrest, who was involved with the project from the beginning, first had the idea of putting together a ‘dream team’ of British plesiosaur specialists to study and describe the skull. This is the first collaboration of its kind among plesiosaur researchers (as far as I know), and I feel lucky to have had the opportunity to contribute to it under the driving force of our lead author, Roger Benson.

Weymouth Bay Pliosaur

Roger Benson (left) and Richard Forrest (right) collecting data from the Weymouth Bay Pliosaur – Pliosaurus kevani

The massive skull has a long snout, circular orbits, huge temporal openings for the jaw musculature, and a deep mandible. Large portions of the skull have been crushed flat during fossilisation, so one of my tasks was to reconstruct the skull to show how it might have appeared before it was flattened. After several versions and much input from Mark Evans, I’m pleased with how it turned out, and I think we’ve produced a pretty accurate reconstruction of Pliosaurus. On the basis of this reconstruction I’ve also had a go at restoring the life appearance of P. kevani in profile. Despite its large size and massive teeth, the head is rather gracile.

Weymouth Bay Pliosaur
Weymouth Bay Pliosaur

Pliosaurus belongs to a group of plesiosaurians known as thalassophonean pliosaurs. If you haven’t heard of them before, that’s because the name Thalassophonea, or “sea slayers”, was proposed just this year (Benson & Druckenmiller, 2013) for a natural group of derived giant pliosaurids including Pliosaurus, Liopleurodon, and Kronosaurus. Thalassophoneans were macropredators, that is, giant predators doing the sort of dirty work in the Middle-Late Jurassic and Cretaceous that rhomaleosaurids did in the Early Jurassic. The paper also discusses the evolution of pliosaurids. The earliest thalassophoneans have a long mandibular symphysis, but in later members the symphysis becomes shorter. This trend is related to a shift in the dietary habits of pliosaurs from primarily fish-eaters to macropredators. In conjunction with this trend, we demonstrate that pliosaurids tend to follow Cope’s Rule – they get larger throughout their evolutionary history.

Weymouth Bay Pliosaur

We also name two other new species of Pliosaurus in the paper, P. westburyensis and P. carpenteri, based on material in the Bristol Museum & Art Gallery from Westbury, Wiltshire. Again, there are morphological aspects of these specimens that distinguish them from one another, but don’t justify new genus names. So, add these new species to the existing list of valid Pliosaurus species and we find ourselves with a rather large number of species within a single genus. The others being: P. funkei, also known as Predator X, P. brachydeirus, P. rossicus, and there might be one or two more pending thorough description of the material. Some invalid species of Pliosaurus have recently been sunk too. Future research might show greater generic diversity among these species, but that’s really dependent on the discovery of more satisfactory fossil material.

Written by Adam S. Smith

May 31st, 2013 at 9:21 pm