Archive for the ‘palaeobiology’ Category
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).
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.
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.
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.
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:
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.
2. Getting into tight spots. Helpful for hunting in reefs, crevices, and kelp forests.
3. Sexual selection. The equivalent of a peacock’s tail – the longer and more brightly coloured the better.
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?
5. Bottom feeding. Hunting in soft sediment. I’m not sure how the long neck really helps here – maybe something akin to number 1?
6. A snorkel. An air supply for staying submerged for prolonged periods of time.
7. Surprise, mother flapper!
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.
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.
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.
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…
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.
An exciting new paper published this week in the journal Science (Vol. 333, p.870-873) provides the first direct evidence for live birth in plesiosaurs, and may have implications for plesiosaur behaviour (O’Keefe & Chiappe, 2011).
Whether plesiosaurs laid eggs or gave birth to live young has been a topic of speculation for nearly 200 years. They have sometimes been portrayed crawling out of the water to lay eggs in the manner of sea turtles, and while palaeontologists have long suspected that plesiosaur anatomy is incompatible with movement on land, empirical evidence either way has been lacking.
The new evidence comes in the form of a fossil plesiosaur skeleton with a fetus preserved in the body cavity. Both individuals have diagnostic characteristics indicating they are the same species, the small individual displays embryonic features and is in the correct position to be a fetus, and there are no signs of it being eaten (bite marks or acid wear). These numerous lines of evidence confirm that this fossil represents a mother and her unborn fetus. This demonstrates that plesiosaurs did not lay eggs and were therefore able to lose their ties with land and spend their entire lives in the ocean.
The newly described fossil plesiosaur is a polycotylid (Polycotylus), one of the last types of plesiosaurs to evolve. It was discovered in Late Cretaceous rocks in Kansas, USA. Polycotylids were highly derived plesiosaurs with torpedo-shaped body outlines and wing-like flippers, a relatively short neck (as far as plesiosaurs go) and a very short tail. They were almost penguin-like in general appearance and also similar to penguins, they would have been fast and agile swimmers.
An unusual aspect of this fossil is the size of the fetus. Most viviparous reptiles give birth to a brood of several small individuals. In contrast, this new fossil shows that at least some plesiosaurs gave birth to a single very large individual, much like whales do today. Many other marine reptiles including ichthyosaurs and mosasaurs gave birth to live young, but this study suggests that plesiosaurs differed in that they invested energy and time into a single individual. This sort of reproductive strategy is often associated with gregarious behaviour and parental care, so the authors of the paper suggest that maybe plesiosaurs were excellent parents too. This hypothesis is fascinating although it would be quite unusual for reptiles.
Fossils of basal sauropterygians (pachypleurosaurs and nothosaurs), close relatives of plesiosaurs, also show that they gave birth to broods of several small live babies, so it is unclear when the evolutionary shift in reproductive strategy occurred in the sauropterygian lineage. It is certainly possible that the first plesiosaurs were more like their ancestors in terms of reproductive behavior. More fossils will ultimately be required to fill in the bigger picture, but for now, it is wonderful to be able to say with certainly that plesiosaurs gave birth to live young.
O’Keefe, F. R. & Chiappe, L.M. 2011. Viviparity and K-selected life history in a Mesozoic marine reptile. Science, 333, 870-873.
Monstertalk is a new sceptical podcast focussing on all things cryptozoological. The most recent episode (episode 004) delves into the idea that plesiosaurs may still be alive today, lurking in lochs and lakes around the world – the so called Plesiosaur Hypothesis. I was interviewed as a guest on this episode and took part in a long discussion about plesiosaur palaeobiology. I’ll admit that I was hesitant to be interviewed at first because I don’t want to get too bogged down or involved in the living plesiosaurs ‘debate’.
The word debate goes into inverted commas because very few cryptozoologists really take the plesiosaur hypothesis seriously, any discussion on the topic is less of a debate and more of a debunk. The plesiosaur hypothesis is really only pushed by 1. the occasional fundamentalist creationist under the (false) impression that a living plesiosaur would somehow discredit evolution (which it obviously wouldn’t), and 2. the media. The media’s fascination with Nessie is especially irksome, no plesiosaur-based science news story in the popular press is self explanatory or interesting enough, it seems, without the inevitable comparison with a fabled creature that doesn’t look like a plesiosaur anyway. This perpetuates the public’s only frame of reference for plesiosaurs as Nessie and does nothing for palaeontology and even less for science education.
It was partly with this in mind that I decided to accept the invitation to be interviewed on Monstertalk, but moreover it was an excellent opportunity to talk about plesiosaur palaeontology and the real mysteries surrounding these fascinating creatures. Far more interesting than those mythical lake monsters I think, and I hope that’s how the podcast came across. I enjoyed the experience although I still haven’t had the courage to listen to myself twittering on. This episode is available to download as a free MP3 here: http://traffic.libsyn.com/skeptic/004_Monstertalk.mp3 and the show notes are here: http://www.skeptic.com/podcasts/monstertalk/09/08/24/. I highly recommend listening to the other episodes too. Enjoy!