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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

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

Why did elasmosaurids have such a long neck?

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It was once common knowledge that elasmosaurid plesiosaurs were bendy-necked beasts that swanned about near the surface, striking snake-like at slippery prey. It is now common knowledge that their necks were relatively rigid rod-like structures, the function of which remains something of a mystery. The truth, with regard to flexibility at least, is probably somewhere in between. The most recent study to provide estimates of flexibility in elasmosaurid necks gives ranges of motion in the region of 75–177° ventral, 87–155° dorsal, and 94–176° lateral, depending upon the thickness of cartilage present between adjacent vertebrae (Zammit et al. 2008). Visually, that looks something like this:

Elasmosaurid neck flexibility

Ranges of elasmosaurid neck motion as estimated by Zammit et al. 2008.

Elasmosaurids weren’t the completely stiff-necked creatures they’re sometimes made out to be — even a tiny amount of flexibility between vertebrae adds up when you have 70+ neck bones. But why did plesiosaurs have such a long neck in the first place? This is a difficult question to answer because 1. plesiosaurs are extinct and left behind no living descendants, and 2. there are no other extant aquatic long-necked organisms to provide analogues. To my knowledge (and correct me if I’m wrong) there are no long-necked fish, cetaceans, sea turtles, or any other long-necked organisms that spend their entire life underwater. At least not to the extent seen in plesiosaurs.

Elasmosaurids were weirdos, but they maintained this long-necked bauplan for 135 million years, so they were successful weirdos. The long neck also evolved independently in different plesiosaur lineages, some cryptoclidids have extremely long necks too, for example. This all indicates a strong selection pressure (or pressures) driving the evolution of the long neck in plesiosaurs, despite the great risk involved in exposing such a delicate part of the anatomy in an ocean filled with super-predators. The long neck was therefore obviously doing something(s) useful. However, we can only really guess what.

Here are the top possible functions for the long neck in elasmosaurids (I’ve ruled out those possibilities that would require flexibility greater than the estimates given above). Some of these ideas are reasonable and have been suggested before, while others are, ahem, unreasonable and quite ridiculous.

1. Stealth device. Fish are stupid. The long neck provided distance between the bulky body of the plesiosaur and the unsuspecting prey.

Elasmosaurid neck function

2. Getting into tight spots. Helpful for hunting in reefs, crevices, and kelp forests.

Elasmosaurid neck function

3. Sexual selection. The equivalent of a peacock’s tail – the longer and more brightly coloured the better.

Elasmosaurid neck function

4. Food storage. Hamsters have cheeks, plesiosaurs had necks. This might not be as ridiculous as it sounds. Leatherback turtles do something similar (despite their incredibly short necks) by having an extended oesophagus that wraps around the stomach. Their prey (usually jellyfish) is held in place in the oesophagus by backwards-pointing projections (papillae) while excess water is expelled. After temporary storage in the oesophagus the morsels are transported to the stomach. Perhaps elasmosaurids were jelly fish specialists too?

Elasmosaurid neck function

5. Bottom feeding. Hunting in soft sediment. I’m not sure how the long neck really helps here – maybe something akin to number 1?

Elasmosaurid neck function

6. A snorkel. An air supply for staying submerged for prolonged periods of time.

Elasmosaurid neck function

7. Surprise, mother flapper!

Elasmosaurid neck function

8. Energy saver. Moving costs energy, so a long neck might allow the plesiosaur to feed, slumped on the sea bed, hardly moving its body in the process.

Elasmosaurid neck function

9. Electrogenic organ. Plesiosaur necks housed electrocytes and so longer necks create higher voltage electric fields. For electrolocation (sensing prey), elecrofishing (stunning prey to be consumed at leisure), and/or electric defence (to protect from pliosaurs and mosasaurs). This hypothesis comes from here, and was raised to my attention by Darren Naish.

Elasmosaurid neck function

10. Wrench of death. Grab and twist – for pulling ammonites out of their shells. Originally suggested here – thanks again to Darren Naish for reminding me. Twist feeding has also been suggested for short necked pliosaurs, for which it makes morse sense to me.

Elasmosaurid neck function

Other suggestions are welcome! Edit – I’ve updated the list with some new suggestions and will add more soon based on the comments posted below…

References

Zammit, M., Daniels, C. B. and Kear, B. 2008. Elasmosaur (Reptilia: Sauropterygia) neck flexibility: Implications for feeding strategies. Comparative Biochemistry and Physiology, Part A, 150, 124–130.

Written by Adam S. Smith

November 23rd, 2014 at 6:28 pm

The Planet Dinosaur plesiosaurs (part 2)

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So, time for more Planet Dinosaur plesiosaurs… In part 1 of we familiarised ourselves with the taxonomy and appearance of the plesiosaurian stars of the fourth episode of the BBC’s Planet Dinosaur, ‘Fight for Life’. Now we’re all set up to pick apart, with ruthless abandon, the animators’ painstaking efforts to bring these animals to life.

Predator X Planet Dinosaur

Predator X. We're going to need a bigger blog. Copyright BBC.

Plesiosaur swimming is an active area of research and frankly we don’t know exactly how they swam. There are some fundamental uncertainties in our understanding of plesiosaur locomotion, in particular, how did the four limbs move relative to each other? Sure, we might also be off in our estimations of dinosaur gaits too, but at least we have the footprint evidence to understand the basics. For example, theropod dinosaurs like Allosaurus, which also appeared in this episode, put one foot in front of the other. Yet the animators still didn’t quite manage to capture this satisfactorily as to my eye the walk cycles in the Planet Dinosaur dinos are pretty wooden. So, with this in mind, what chance did the plesiosaurs have?

Well, I’m probably overstating the point. It is true that we don’t know exactly how plesisoaurs swam, but we do have a good understanding of the limb strokes in general, and we know how they didn’t swim. So let’s start there. Derived aquatic animals generally swim either by undulating their body (e.g. crocs, dolphins etc.) or by keeping their body sturdy and using their limbs (e.g. turtles, penguins). Plesiosaurs are in the latter category and their skeleton is constructed accordingly to resist flexibility. They have tightly articulating dorsal vertebrae, robust ribs, and a strong rigid ventral skeleton consisting of flat plate-like girdles, bridged by a strong mesh of gastralia (belly ribs).

To my distress, some early movement cycles of the Kimmerosaurus included not only a limb-based phase, but also a wriggling  phase with long lateral undulations of the whole body, with the flippers pulled in flush to the flanks. In this regard, the Planet Dinosaur plesiosaurs originally wanted to have their cake and eat it. Thankfully these side-to-side wiggling phases, and also an up-down wiggling phase, didn’t appear in the final show. However, there is still a little too much dorso-ventral flexibility in the plesiosaur bodies on occasion. The Kimmerosaurus sometimes appear rather too bendy, and in one scene the pliosaur pretty much breaks its own spine, but I was pleased to see these deviations kept to a minimum.

It is generally agreed that the plesiosaur flipper stoke was essentially a modified form of underwater flying with a major up-down component and minor back-forth component. They were not used to row and it is unlikely that the flippers could be pulled back to be flush with the flanks of the body, even though they do this every now and again in the show. The limb-stroke of the forelimbs also looks a bit off at times in Planet Dinosaur. It is my understanding that the digital models are rigged with approximate points of flexibility – they don’t have anatomically correct skeletal anatomy inside – so it is only to be expected that the points of joints and range of motion may not always be spot on. It’s also worth bearing in mind than animators are not biologists, yet they’re ultimately responsible for the construction of biological critters.

When I saw the trailer for Planet Dinosaur I noticed the flippers in the pliosaur being pulled back almost flush with the body and feared for the worst. This happens in the scene when the pliosaur comes to blows with the Kimmerosaurus. But this was a one off and for the most part the pliosaur is cruising around with healthy deep limb strokes. Plesiosaurs may have changed their gait depending on the the velocity and maneuverability they wished to achieve, so it is also reasonable to suppose that all four limbs sometimes moved in unison for a burst of speed, as also depicted in the show. Overall, I thought the Kimmerosaurus‘ elegant prancing and dancing, twisting and turning, was wondrous. It would be unfair, and perhaps premature given our current state of understanding, to pick on it too much.

I had some problems with the interactions between the animals and their underwater environment, the water resistance wasn’t always convincing. At times the plesiosaurs seemed to move via some sort of invisible jet propulsion. Either that, or the momentum being generated from the limb stokes was overestimated. This might be put down to limitations in the technology, and I suppose there is a certain amount of intuition required for perfecting these details. similar explanation could also be invoked to explain why some of the dinosaur walk cycles in the series don’t always cut it when it comes to mass and momentum.

A quick note on that characteristic feature of the plesiosaur, the long neck, which was correctly restored as a relatively inflexible structure. Not a swan-like pose in sight, and no graceful necks arching out of the water. Other recent documentaries (e.g. Sea Rex 3D) just couldn’t resist including such outdated but iconic imagery, presumably against better advice. The evidence for plesiosaur feeding habits from the gutter-like trace fossils, or Lebensspuren (for that it their proper name), was discussed by Darren Naish on the old version of his Tetrapod Zoology site. These structures might have been produced by plesiosaurs, they might not, but it is a reasonable suggestion that fits with other evidence from stomach contents (McHenry et al., 2005).

Kimmerosaurus Planet Dinosaur

Kimmerosaurus - The Jurassic vaccume cleaner. Copyright BBC.

On several occasions in the program the plesiosaurs break the surface, where, dowsed in harsh sunlight, I’d be hard pushed to tell whether the splashes are CGI or real (if I didn’t already know). These scenes were really excellently excecuted. The pliosaur comes up for air at one point and snorts out a powerful jet of water spray from its external nostrils. It isn’t certain that plesiosaurs breathed through their tiny external nostrils, especially if they employed the method of underwater olfaction explained in the show (these plesiosaurs want to have their cake and eat it again!), but, well, why not, it looked nice didn’t it!

The scientific exposition sections included some dodgy versions of my Rhomaleosaurus reconstruction as a stand in for Predator X, which was therefore anatomically incorrect for a pliosaurid. I believe that the Kimmerosaurus illustration was also based on my Rhomaleosaurus reconstruction. The tooth mark evidence in the skull of a plesiosaur comes from an elasmosaurid specimen formerly referred to the nomen dubium ‘Woolungasaurus‘, and now known as Eromangasaurus australis (following some confusion resulting from two researchers accidentally stepping on each others toes a little and publishing reviews of the material almost simultaneously – the specimen had a short stint as Tuarangisaurus australis and Eromangasaurus carinognathus) (Kear, 2007). Anyway, it certainly isn’t Plesiosaurus as the reconstruction was mysteriously labelled in the show. The interpretation of the bite marks is pretty speculative as who knows for sure the position of the the animal in the water column? But pliosaurs sometimes ate long-necked plesiosaurs, that is certain.

Predator X Planet Dinosaur

A hungry Predator X wrestles with a sand bank in search for tasty Kimmero-morsels. Copyright BBC.

As scientists, palaeontologists have the luxury of not having to commit to any single hypothesis. We can say without shame that we don’t know, at least not with certainty, how plesiosaurs hunted, how they breathed, how they swam. We can interpret evidence tentatively and you’ll have noticed the frequent use of non-commital language in this review such as ‘probably’ and ‘unlikely’. But this doesn’t help artists, script writers or animators who, charged with the task of restoring these long-dead beasties, are forced to plump for one option of the other (under the format used by Planet Dinosaur at least). With all the pitfalls presented to them along the way, I think the Planet Dinosaur team did a sterling job bringing Predator X and Kimmerosaurus back to life. And no, I’m not being apologetic, or sympathetic, just because I was involved. I’m really enjoying this series and look forward to the other episodes.

References

Kear, B. 2007. Taxonomic clarification of the Australian elasmosaurid genus Eromangasaurus, with reference to other austral elasmosaur taxa. Journal of Vertebrate Palaeontology, 27, 241-246.

McHenry, C., Cook, A., and Wroe, S. 2005. Bottom feeding plesiosaurs. Science, 310, 75.

Written by Adam S. Smith

October 8th, 2011 at 12:34 pm