March 3, 2014 | 77
Between the later part of the Triassic and the very end of the Cretaceous, the seas of the world (and some of its rivers, lakes and estuaries as well) were inhabited by the remarkable group of swimming reptiles known as the plesiosaurs. All plesiosaurs – so far as we know – were predators, the shapes of their teeth and jaws indicating that they preyed variously on swimming and benthic invertebrates, on fish, and also on other aquatic reptiles. Some were macropredators that attacked and ate other plesiosaurs as well as ichthyosaurs, swimming crocodyliforms and turtles. Members of several lineages famously possessed long or ridiculously long necks, the flexibility and function of which have long been the subject of debate. Others (referred to as pliosaurs) had long, superficially crocodile-like skulls, and usually combined these with relatively short necks.
In overall body form, all plesiosaurs were generally alike. Their skeletons possess large, plate-like limb girdles mostly positioned on the ventral surface of the body, and there are two pairs of slender, wing-like paddles and a relatively short tail. An interlocking basket of heavy-boned belly ribs (or gastralia) fill in the space between the pectoral and pelvic girdles and presumably helped keep the body stiff during life. There’s no question that plesiosaurs were limb-propelled swimmers. The question is: what sort of limb-propelled swimming did they use? Were they ‘rowers’, ‘fliers’, some combination of the two, and did they use their forelimbs and hindlimbs synchronously, asynchronously, or what? These issues have been much discussed in the literature (Robinson 1975, Tarsitano & Riess 1982, Halstead 1989, Riess & Frey 1991, O’Keefe 2001a, Carpenter et al. 2010) and are the subject of current investigation.
When it comes to what we really know about plesiosaurs, the literature includes a fair bit about the possible biomechanics of swimming, but is generally focused on anatomy, systematics and phylogeny. Understanding plesiosaur evolution is difficult since substantial convergence and re-evolution of similar body plans and skull shapes seems to have been a pervasive theme; the idea that very long necks and fairly short necks re-evolved independently on some or several occasions has long been popular (Bakker 1993, Carpenter 1997, O’Keefe 2001b, 2002, Ketchum & Benson 2010, Benson et al. 2013), meaning that the conventional versions of the elasmosaur and pliosaur groups are not monophyletic. The most recent work indicates that virtually all Jurassic lineages became extinct round about the Jurassic-Cretaceous Boundary, with a single surviving lineage – the Xenopsaria – then undergoing a major burst of radiation during the Cretaceous, giving rise to new versions of Jurassic-style body plans (Benson & Druckenmiller 2014).
What about the biology, behaviour and lifestyles of these amazing animals? Invaluable studies of jaw biomechanics, tooth form and stomach contents give us insights into what and how plesiosaurs ate, and on how they collected and processed their food (Massare 1987, Taylor 1987, 1992), and some extremely interesting ideas on breathing and olfactory behaviour have been proposed (Cruickshank et al. 1991, Buchy et al. 2006). Trough-like feeding traces preserved on an ancient sea floor provide possible data on plesiosaur foraging behaviour (Geister 1998), and the amazing discovery of a proportionally enormous baby preserved within the body of its mother has given us substantial food for thought as goes the reproductive biology and social lives of these animals (O’Keefe & Chiappe 2011) [see the links below for more on those discoveries and what they might mean].
In general, however, we of course know very little about plesiosaur biology and behaviour, and I’d say that most questions we might ask can only be informed by inference: by extrapolating or guessing based on what we see in living reptiles and other animals. Remember that, when it comes to interesting questions about the behaviour of long-extinct animals (especially weird ones without precise modern analogues), we’re always constrained by a frustrating lack of information.
Nevertheless, a long interest in both plesiosaurs and the amazing behaviour of extant marine tetrapods means that I’ve long had thoughts about diving behaviour, feeding ecology, social behaviour, intentional beaching and so on in plesiosaurs – I even have a half-finished manuscript done on this subject. So it was fitting that, back in October 2012, Daniel Loxton contacted me to ask if I’d like to be the technical consultant on his new book, Plesiosaur Peril. As with his previous works in the same series – Ankylosaur Attack (Loxton 2011) and Pterosaur Trouble (Loxton 2013) – Plesiosaur Peril is a fantastically illustrated children’s book that uses the fictional story of an individual animal to paint a picture of the lifestyles of ancient animals, using scientific data where possible.
The news is that Plesiosaur Peril is now out, so go buy it! In the text you’re about to read below, Daniel has kindly let me use several of the amazing, computer-generated illustrations that he, together with Jim W. W. Smith, created for Plesiosaur Peril. I’m sure you’ll agree that they look great. What I’ve also done is feature the exchange of questions and answers that Daniel and I engaged in while he was preparing the book’s text. Hopefully you’ll find this interesting; I also hope that it’ll provide some interesting insight and background into the book’s contents, as well as illustrate what we do know versus what we don’t when it comes to plesiosaur behaviour and biology.
Reconstructing live plesiosaurs – a tricky task
Before we get into that, I’d like to make a few comments about plesiosaur life appearance. While the two creatures that feature in the book – the Middle Jurassic, western European taxa Cryptoclidus and Liopleurodon – are among the best known of plesiosaurs, we have a huge number of questions about even their very appearance. It’s well known these days that plesiosaurs were almost certainly not capable of the erect-necked or swan-necked poses shown in older artwork: the anatomy is against it, it doesn’t work when we consider their buoyancy and pose in the water, and it’s not consistent with what we understand of their ecology and behaviour anyway. While they should therefore be reconstructed with more horizontal neck poses, it doesn’t follow, however, that they were wholly incapable of maintaining a vertical neck posture for a brief period – read on…
Plesiosaur skin texture is still very much an unknown. Plesiosaur expert Arthur Cruickshank once told me that skin impressions associated with the New Zealand plesiosaur Kaiwhekea katiki (one of the controversial aristonectine elasmosaurids, once thought to be late-surviving cryptoclidids) revealed a smooth surface, studded regularly with low, rounded scutes or scales, though so far as I know this has never been confirmed or published.
However, if this one plesiosaur really was like this, it doesn’t mean that they all were (nor would that peculiar skin texture necessarily apply to the whole body). So – were plesiosaurs scaly or smooth-skinned? And if they were scaly, did they have overlapping scales, non-overlapping scales, smooth scales, or ornamented scales? Impressions of a smooth skin have been reported for the Jurassic plesiosaur Attenborosaurus, but the original specimen was destroyed during WWII and no further information is available. [Adjacent image by Ghedoghedo.]
Those of you familiar with the plesiosaur literature will know that there has recently been a resurgence of interest in the actually rather old idea that at least some plesiosaurs possessed a vertical fin on the tail. Smith (2013) showed how the proportions and form of the tail vertebrae in the pliosaur Rhomaleosaurus are suggestive of a tail fin of some sort. Wilhelm (2010) also explored this subject in an unpublished thesis, concluding that members of the Cryptoclidus group likely had a tail fin too, perhaps used as a rudder. Daniel and I discussed the possibility of putting tail fins on the plesiosaurs of Plesiosaur Peril, but ultimately the information came in too late for us to be confident about including it – the Liopleurodon’s tail, at least, does have a small fin, though, as a nod to this possibility.
While I’m here, it’s worth saying that there are a few other bits of plesiosaur life appearance that we might be getting wrong. The general thinking at the moment is that the paddles of live plesiosaurs essentially matched the outlines of the bones. However, one (still, maddeningly, unpublished) plesiosaur fossil from England seems to preserve the impressions of flexible, curved tips to its flippers that aren’t reflected in the underlying osteology. [Image below by Matěj Baťha.]
Also, what gives with the teeth and jaw edges of these animals? As you can see from all the reconstructions shown here, tradition has it that plesiosaur teeth protrude obviously from the jaw edges, unsheathed by lips and with the skin around the teeth adhering tightly to the skull bones. The whole configuration I’ve just described is not implausible, since it’s essentially the condition present in crocodylians (and crocodylians, being mostly aquatic, may well be reasonable models for plesiosaurs in some respects). But, given the fact that ‘lips’ (of a sort) cover the teeth in snakes and lizards – even aquatic ones like sea snakes – we’re left wondering whether plesiosaurs really looked like this. Could ‘lips’ (that is, sheets or bands of skin, not necessarily lips in the flexible, sensitive sense) have covered these crazy teeth? Then again, living snakes and lizards don’t have teeth that protrude or interlock in the same fashion as plesiosaur teeth. For now, we don’t know either way: I’m inclined to think that their teeth really did protrude as we usually show, and that soft tissue did not obscure or cover them. But it’s something to be curious about; something that will only be informed by the discovery of exceptional specimens.
Ok, on to the Q&A. This reflects part of the back-and-forth that went on between Daniel and myself as he did his research for the book. The first part of the discussion focuses on the long-necked Cryptoclidus, a slender-toothed cryptoclidid known from Middle Jurassic strata in England, France and perhaps Russia and elsewhere. There is some confusion about the total length of Cryptoclidus. All of the good specimens indicate total lengths of 3-4 m but several authors have said that these are juveniles or subadults, and that the true adult length was more like 6-8 m. Anyway, after looking at the possible behaviour and lifestyle of Cryptoclidus, we move on in the second part of the discussion to discuss the large, robust-skulled pliosaurid Liopleurodon.
Q: Would Cryptoclidus have engaged in spyhopping behaviour?
A: I wonder how interested plesiosaurs would have been in seeing things in air. But, then, you can say the same thing about cetaceans (note also that the apparently flattened eyeballs of plesiosaurs mean that their eyesight in air was perhaps not great). As is now reasonably well known, raising the whole neck upwards from the horizontal may not have been possible at all, but, yes, vertical spyhopping remains plausible. Maybe plesiosaurs occasionally wanted to check for flocks of foraging pterosaurs, the vicinity of land and so on.
Q: Did Cryptoclidus eat stones?
A: Yes, we know that plesiosaurs did this, and in fact you might imagine that they went to special places to collect the right sorts of stones (after all, in some marine environments, stones and pebbles are rare). Imagine the possibility of plesiosaurs making treks to special pebbly beaches (and hence into very shallow water) to find and swallow stones. Of incidental interest is that stones can also be carried in the roots (and attached sediment) of floating trees.
Q: Any possibility that Cryptoclidus might have dug or foraged for prey like clams or crabs?
A: The slim teeth of Cryptoclidus suggest that this animal wasn’t habitually able to eat or subdue anything tough-shelled or chunky, so shrimps and thin-shelled bivalves are probably at the extreme end of what it was capable of subduing. However, grubbing around in the sediment and reaching into burrows are totally plausible. In fact, there is some evidence suggesting that long-necked plesiosaurs
were mostly may sometimes have behaved as ‘benthic grazers’, reaching down to pick at prey from the seafloor (McHenry et al. 2005).
Q: Was Cryptoclidus social? Would it have travelled in groups?
A: Given how widespread social interactions are in reptiles of all sorts, I’m inclined to think that anything goes, more or less. Some lizards, crocodylians and turtles really are social, hanging out in groups or pairs deliberately (examples: spiny-tailed goannas, gopher tortoises), others have long-term monogamy and pair-bonding (examples: American alligators, Shingleback skinks). Some data indicates that plesiosaurs had complex social lives, and I certainly consider it plausible that extended social bonds were present in these animals. Read on.
Q: Any indication of pack-hunting behaviour in plesiosaurs?
A: If these animals moved in groups – or found themselves in groups fortuitously or incidentally – then co-operative hunting of a sort is plausible, since even animals that don’t live together will co-operate to herd or cluster prey. I’m not sure I’d want to call this pack-hunting, but social hunting of a sort seems plausible and perhaps likely. Imagine benthic-foraging plesiosaurs flushing prey towards others, or a group of them hemming prey in, and moving together so that the prey gets bunched up. There’s no direct fossil evidence for this, but the fact that some living crocodylians practise co-operation (Gans 1989, Yamashita 1991, King et al. 1998) provides an analogy.
Q: Do you think juvenile plesiosaurs stayed with their parents? Could these animals have lived in family groups, or pods?
A: The pregnant polycotylid described by O’Keefe & Chiappe (2011) shows that baby plesiosaurs were enormous relative to their mothers (as in, more than 33% the mother’s length). Enormous babies indicate substantial maternal investment, and – in living reptiles – are suggestive of parental care, and also such things as kin recognition and strong social bonds. The giant polycotylid baby actually suggests that plesiosaurs practised extended parental care, with babies staying with their mothers for a long time: as in, months to years. I emphasise that this is speculative, and that what went for polycotylids might not have worked for Cryptoclidus too.
Q: Assuming that parental care was at play, would an adult have defended a juvenile from predators? If so, how?
A: If there’s a strong bond between a mother and her baby, then, yes, I should imagine that the mother would defend her baby from attackers. Firstly, the juvenile might well travel close to the mother’s body so as to be concealed from predators (as occurs in whales). Sharks and mid-sized pliosaurs that came to attack the baby could well have received nasty flipper-smacks from the mother. When it comes to the danger posed by giant pliosaur predators, however, I suppose fast retreat would be the only way of escaping. If that’s so, we have to wonder whether babies could swim as fast as adults.
Q: What do you think about the possibility that these animals might have intentionally beached for safety or escape, like walruses or seals or penguins?
A: I’m inclined to think that adult Cryptoclidus were too large to be able to fully leave the water. In fact, I reckon that plesiosaurs as a whole lack features that might permit terrestrial locomotion. If there are social bonds between juveniles and adults, you wouldn’t expect a juvenile to self-beach while its mother stayed in the water. Like most people who have expressed an interest in this area, I consider it possible, however, that plesiosaurs could self-beach in the way that some killer whales, bottlenose dolphins and wels catfish do: in other words, perhaps some pliosaurs might have self-beached in order to grab prey from the water’s edge. But there’s nothing about Cryptoclidus here that makes it different from a pliosaur: if a Cryptoclidus could self-beach to escape a pliosaur, I would think that the pliosaur could self-beach in the same way.
Q: Do you think Cryptoclidus might have eaten ammonites?
A: I don’t think Cryptoclidus has the right sort of teeth or jaws to do much damage to an ammonite, but I suppose it could grab at the tentacles and maybe attack the soft parts in general if they were protruding. Of course, that all depends on what ammonites looked like when alive (there being a few competing ideas).
We now move to consideration of that famous giant pliosaur, arch-predator Liopleurodon. Reaching 6 m and perhaps twice this, Liopleurodon has a giant, long-snouted skull lined with deeply rooted, conical teeth. It was undoubtedly a powerful predator of other vertebrates, presumably grabbing and dismembering other plesiosaurs as well as ichthyosaurs, crocodyliforms and fish. Indeed, preserved bite marks showing that it sometimes attacked, and presumably fed on, its long-necked relative Cryptoclidus. Understandably, Daniel wanted Liopleurodon as the villain of the piece: the lurking predator that presents the main danger to our cryptoclidid heroes.
Q: Is it plausible that these animals were aggressive to rival individuals?
A: Yes, totally. I like to imagine these animals as combining various behavioural traits of cetaceans, crocs and lizards. As discussed above, plesiosaurs might have had social bonds with ‘friends’ and relatives, but they might been aggressive too, and maybe with some species mostly behaving aggressively towards conspecifics (as per Saltwater crocodiles). There are a few big pliosaur specimens where snout tips and flippers seem to have been bitten by other, similarly-sized pliosaurs, so we may even have direct evidence for this sort of behaviour.
Q: Would pliosaurs like Liopleurodon ever had rested on the sea floor?
A: Not sure about this, since I’m not sure they could hold their breath for hours. I know that some turtles sleep on the seafloor, but my suspicion is that they have much lower oxygen demands than plesiosaurs. I can buy brief settling or foraging on the seafloor, but no extended time spent there. Mind you, trace fossils suggest that these animals ploughed through sediment when foraging, and perhaps also rubbed their bodies against stones or patches of sand for grooming purposes (Geister 1998).
Q: Do you think that pliosaurs were ambush predators?
A: I feel it’s plausible that these giant predators did the same thing that white sharks do: that they lurked in dark water, then rushed upwards towards prey silhouetted at the surface. This is a sort of ambush hunting I suppose. A prey species is therefore vulnerable when at the surface and might try to spend as little time there when in an environment where pliosaurs are present (in the modern world, elephant seals spend as little time at the surface as possible when white sharks are around).
It’s plausible that they also used obstacles, debris, plant growth and maybe even sediment clouds in the water to conceal themselves before bursting from cover and pursuing prey. I wonder if reefs (sponge reefs were apparently a big thing in the Jurassic) were ever big enough to provide cover for a pliosaur. There’s some controversy over how old kelp and similar algae are, but if they were present in the Jurassic we might perhaps imagine a pliosaur hiding in a kelp forest… imagine the unlucky prey animal that swims into such a lurking giant. Could pliosaurs have used suspended sediment (and thus reduced visibility) to conceal themselves, perhaps then relying on their sense of smell to locate prey? If so, hunting in the mouth of an estuary might have been a viable strategy that these animals could have exploited. I can’t think of any living animals that deliberately stir up sediment to conceal themselves, but there are whales (Kogia) that produce a cloud of reddish fluid (probably faeces) and then hide inside the cloud. They use this as more of a predator-avoidance strategy than a hunting one, however.
Q: Did pliosaurs use pursuit predation as a prey-catching strategy?
A: My best guess (largely concordant with what other plesiosaur researchers think is likely) is that they remained concealed as best as possible before dashing out (or up) at speed. In other words, there would be a chase, but not necessarily a long one. A prey animal would have to react very quickly – that is, have very good acceleration – to get away, and it may be that pliosaurs were better at accelerating than some of the prey animals they went for. Liopleurodon, for example, has especially big hindflippers (bigger than the foreflippers), so maybe it was a better accelerator than Cryptoclidus.
Q: Is there any evidence for pack hunting in pliosaurs?
A: As per the above comments on Cryptoclidus, social bonds of some sort – involving parental or mating relationships – are now plausible for these animals, so we might imagine twos or threes working together. In lizards and crocs, juveniles and parents recognise each other (that is, exhibit kin recognition) long after their original parental care phase has passed. I’m speculating here but, theoretically, a big juvenile pliosaur of several years old could conceivably meet up with its mother.. and, if she has a new juvenile, we can get a group of 3 animals (or more). A recent study of dwarf caimans described exactly this: a mother was discovered with both brand-new hatchlings as well as juveniles of over 1 yr old, obviously from a previous brood (Campos et al. 2012). Anyway, I think it’s plausible to think of related individuals co-operating.
Q: Could pliosaurs have indulged in breaching behaviour?
A: Crocs do head slaps, white sharks smack their tails on the water surface, and whales breach [adjacent image by Gillfoto]. Based on this data, I think we can regard it as likely that plesiosaurs also used noisy displays at the water surface to communicate signals over distance (this, of course, then raises questions about hearing abilities and so on). These might have been aggressive signals, territorial signals, or signals related to courtship. We can but speculate.
Q: Is basking behaviour likely for pliosaurs?
A: Lounging at the water surface (sleeping/resting) is plausible, either in deep water or shallow water, perhaps even in places where the animal is partly in contact with the sediment (though see caveat above about lying on the sea floor).
Q: Do we know that Liopleurodon preyed on Ophthalmosaurus, the familiar contemporaneous ichthyosaur?
A: There are certainly ophthalmosaur bones marked with pliosaur bite marks – so, yes, Liopleurodon surely did predate on Ophthalmosaurus when it could catch it. I’m not sure if the bite marks we can see on ophthalmosaur bones demonstrate any preference as goes attack style, but it’s been stated that bitten plesiosaur bones tend to belong to the limbs. This has led to the irresistible notion that pliosaurs wrenched the paddles from plesiosaurian prey in order to disable them. There is even some data that supposedly shows a preference for (if I remember correctly) right-sided attacks!
Q: Any possibility of Liopleurodon eating drowned or swimming dinosaurs?
A: I think we can be confident that they did this. There’s even possible evidence for this, since armour plates from an ornithischian dinosaur were apparently found within the stomach contents of a pliosaur (Taylor et al. 1993).
Clearly, there is – as I said above – a huge amount we don’t know about plesiosaur behaviour. It’s possible, perhaps likely, that we will never know about these sorts of things. Plesiosaur Peril is based on evidence as much as is possible, but we obviously had to speculate and extrapolate where appropriate.
I think that Plesiosaur Peril looks amazing and I’m sure that kids and other interested readers will enjoy it very much. And it’s not as if there are many books out there devoted to plesiosaurs already… there are, like, two or three… so a new one, produced for popular audiences, is a wonderful thing. My congratulations to Daniel on getting the book out there and on working so hard to popularise an amazing and somewhat under-appreciated group of animals. Buy Plesiosaur Peril now: Daniel announced its publication here at Skepticblog.
For previous Tet Zoo articles on plesiosaurs and other sauropterygians, see…
Refs – -
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