Tuesday, 25 June 2013

The Enduring Pliosaurus


 
Every now and then a story breaks in the palaeoworld that goes global thanks to the rapid electronic communications of today. More often than not it is some form of new dinosaur that is announced but, on this occasion, it was the turn of a marine reptile named Pliosaurus kevani that took centre stage. This was the culmination of a story that began over ten years ago when the specimen began eroding out of the cliffs at Black Head, near Osmington Mills, Dorset which exposes the famous Late Jurassic Kimmeridge Clay Formation.
Kevan Sheehan (for whom the specimen is now named) collected the majority of the specimen over a period of eight years and, when more of the specimen had been recovered from land that was privately owned, it was clear that the specimen was more than just a little bit special. I first got wind of this early in 2009 and then official confirmation came later in Autumn that year. The skull was truly massive and would need a lot of preparation to enable the necessary research to go ahead.
The man charged with this task was my friend and colleague Scott Moore-Fay who spent a year preparing the beast. We knew he was working on a pliosaur and that he was working on “something special” but we did not realise that it was the Weymouth Bay pliosaur. Every time we met up, either in the field or elsewhere, we would get another titbit of information and when he described the tooth sockets in the jaw that were “…..big enough to fit your fist into” I guessed what it was.
But, as is the way with these things, you do not probe or ask too many questions since it was obvious that the skull would need to be described. As it turns out, the team who took on the task comprised of what can only be described as a who’s who of British marine reptile palaeontologists led by Roger Benson – a real fantasy palaeo-dream team if you like. And what a superb paper they have produced (Benson et al 2013) and many other palaeontologists have remarked that this could be a template for others in how to produce a scientific paper for today.    
Pliosaurs were sauropterygians and members of the clade Plesiosauria of which, unsurprisingly, the other group consists of the long necked plesiosauroids. Pliosaurs were enormous animals characterised by their short necks, large skulls and massive jaws. Some of them grew to enormous proportions, certainly in excess of 12 metres although some of the earlier estimates of size were likely fanciful. Pliosaurs were true macropredators and likely fed on other contemporary marine reptiles although other forms, such as Peloneustes, were of a smaller average size (3 to 4 metres) and likely fed on ammonites, belemnites and fish.
Pliosaurs swam by using their flippers much like a bird would use its wings to fly by utilising them in an up and down motion to produce propulsion via hydrodynamic force. The flippers, despite being composed of multiple bone elements, were surprisingly flexible, terminate to a point distally and are very streamlined. These are very similar to the swimming appendages of many animals of today including whales, turtles and penguins.
Of course, I believe it is fair to say that the actual swimming technique is still not fully understood although we can be very confident in our interpretation.  It would appear that the two sets of flippers would be flapped up and down in a staggered motion so that as the front flippers were raised up the rear flippers would lowered down and then vice versa – a very efficient way of swimming and probably surprisingly quick as well.
Interestingly, pliosaurs had larger rear paddles then their front paddles whilst plesiosaurs had larger front paddles. This is almost certainly to do with their predatory habits – plesiosaurs relying on speed and agility to catch fish and invertebrates while pliosaurs relied on a devastating burst of speed and power to attack their prey

Kronosaurus - and that is the great A.S. Romer taking an interest
 
Pliosaurs were a remarkably enduring lineage evolving in the Early Jurassic and lasting until the early Late Cretaceous. The best known of the Cretaceous forms is probably Kronosaurus and the images of the skeleton at Harvard University are amongst the best known representations of a pliosaur throughout the world. But the real heyday for pliosaurs spans the period from the Mid to Late Jurassic and, despite being found throughout the world, the centre for pliosaur fossils and the hub of pliosaur research remains here in the UK.
However, the nature of fossilisation in the ancient seas was very much hit and miss and complete skeletons of marine reptiles are relatively rare despite the vast amount of material that has been recovered over the years – and this is particularly applicable to pliosaurs. Indeed many pliosaurs were named for, what would be considered today, fairly non-descript elements that would have benefitted from being described as Pliosauridae indet.
But instead we have the usual collection of named taxa that should have remained nameless and chief amongst these is Pliosaurus and the authors took this opportunity to review the Late Jurassic Kimmeridgian pliosauroids in the new paper whilst describing fully the new skull.
Specimens of Pliosaurus display seven autapomorphies of which one – the possession of trihedral or subtrihedral teeth – is always of interest to us since pliosaur teeth are the one element from these animals that are most likely to be found (for the other autapomorphies read the paper!). Teeth, and the morphology of the symphisis in pliosaurs, are extremely revealing and represent our best opportunity of identifying certain pliosaurids – at least to the generic level.
In simple terms the symphisis of the lower jaws is the fusion point where the left and right mandibles come together. By counting the amount of alveoli present from this point on you get an extremely good indication of the genus that you are dealing with. I do have a thought about this means of identification, which I will come to shortly, but it has proven itself to be a fairly robust technique.
The alveoli count in the mandibular symphisis, as well as the overall alveolar counts in the maxillary and dentary bones, are extremely important when assessing pliosaur taxonomy but, of course, it is not the only means of identification and there are other differences that need to be considered – and this is where it gets difficult. There is just not enough post cranial material to add sufficient detail to the overall picture and even then, bones such as the cervical vertebrae, appear non-diagnostic in pliosaurs. So as you can see, things are not that simple.  
The skull of Pliosaurus kevani, on the other hand, is exceptional and, at a total length of nearly two metres, is both large and informative. Four autapomorphies were easily sufficient for this specimen to be declared a new species. Unfortunately the symphisis is actually missing to a large degree on the mandibles and so the actual length can only be estimated but some clever calculations have enabled a reasonable guestimate length to be around 400mm.
The authors, by combining their research into P. kevani and their reassessment of other Kimmeridgian pliosaurs have performed a phylogenetic analysis and found that they represent a monophyletic group and suggest that all Kimmeridge Clay Formation pliosaurids are all referable to Pliosaurus since there is insufficient difference in other pliosaur elements that warrant the erection of new taxa. The valid species, therefore, are the type species Pliosaurus brachydeirus and include P. kevani, P. westburyensis, and P. carpenteri. Two other species included in the clade are P. rossicus from Russia and P. funkei from Spitsbergen.
I think there should be a little caveat at this point. We do not have anywhere near a growth series for any form of pliosaur. Even in the Oxford Clay Formation, pliosaur remains are scarce and yet, by way of comparison, Cryptoclidus eurymerus (a plesiosaur) is known more or less from an entire growth series so we know a lot about their morphological changes through their ontogeny. We know very little about pliosaur ontogentic change. For sure, there are some nice specimens of various pliosaurids that have given us significant insight into their taxonomy, anatomy and lifestyle and yet, in real terms, we still know very little.
So we must accept the possibility that even research as thorough as this may well be compromised as and when new specimens of pliosaurids are recovered. And yet I suspect that new pliosaurids will be increasingly hard to come by in the future – especially in the UK now since, except for one or two exceptions, they will only ever be found in coastal locations. And let’s not forget that even in the highly productive Oxford Clay they are not common fossils and it seems reasonable to assume that big pliosaurs were open ocean macropredators that probably only ventured into shallower waters at certain times of the year – perhaps to breed or to take advantage of a seasonal food source.
Having said all that, pliosaur teeth still turn up from time to time and, as mentioned earlier, are quite informative. Firstly, you cannot mistake a true pliosaur tooth since they are often large crowned with a sharp tip and have big roots although from small pliosaurs, such as Peloneustes, they are small and indeed can be sometimes mistaken for marine crocodile or even plesiosaur. You are still considered lucky to find a big intact pliosaur tooth and most do have the tip missing. 
But there is no mistaking big pliosaur teeth and some are instantly identifiable. Liopleurodon teeth are amongst the most recognisable of all. Despite most pliosaur teeth being heavily ribbed, the teeth of Liopleurodon are distinct. Martill (1991) described the teeth as having prominent widely spaced ridges that reached the apex of the tooth with alternating ridges only reaching halfway up the crown. This is absolutely spot on and when you compare the teeth of Liopleurodon with other pliosaurids there can be, in my eyes, no mistaking them. 
The teeth of Pliosaurus are also diagnostic and this is further highlighted in the new paper. Pliosaurus teeth are known to have a flat surface on the labial side of the tooth and this is an agreed autapomorphy of the taxon. Other pliosaurid teeth are a little more difficult to identify in some cases and these are normally those from the very back of the jaws or are unerupted teeth and they are often smaller than the big pliosaur teeth you may be familiar with.

The upright example displays perfectly the flat side of the tooth.
Be that as it may, the teeth of pliosaurs are all impressive and superbly evolved hunting and feeding aids. The wrinkles and ridges of the teeth made them extremely strong and durable and it is clear they were designed to withstand enormous bite pressures and torsion which you would expect from an animal that hunted contemporary plesiosaurs and ichthyosaurs. Although none of the teeth are serrated as you would find in dinosaurs or, indeed, some mosasaurs it is thought that the sharpened ridges on some teeth would have made for a decent cutting edge thus making these teeth very effective cutting tools as well.
A final thought. I find the conclusion that all pliosaurids in the Kimmeridge Clay Formation are referable to Pliosaurus as remarkable. Not that I doubt the new research at all (far from it) but I wonder what happened during the six million years or so from the Callovian stage where we have multiple species of pliosaurids in the Oxford Clay Formation (including Pliosaurus) to only having the one in the Kimmeridgian?
During our work in the Oxford Clay Formation we found it to be generally assumed that, because it is such a well sampled formation, that there are probably no new species to be found. The discovery of Pachychostasaurus back in the nineties surprised most people – but not all (Cruickshank et al 1996). Our personal experiences plus the fact that some Oxford Clay material has been reviewed over the last few years suggests more surprises to come.
Is this the case for the Kimmeridgian? Is it possible that there are still more surprises to erode out of the cliffs? Well maybe but the period that encompasses the Oxfordian stage of the Jurassic is obviously of great importance as far as the evolution of the Kimmeridgian pliosaurs is concerned and is something we will take a look at in the near future.

 

References

Benson RBJ, Evans M, Smith AS, Sassoon J, Moore-Faye S, et al. (2013) A Giant Pliosaurid Skull from the Late Jurassic of England. PLoS ONE 8(5): e65989. doi:10.1371/journal.pone.0065989
A.R.I. Cruickshank, D.M. Martill and L.F. Noè  A pliosaur (Reptilia, Sauropterygia) exhibiting pachyostosis from the Middle Jurassic of England. Journal of the Geological Society.1996; 153: 873-879
Martill, D.M & Hudson, J.D 1991. Marine Reptiles. Fossils of the Oxford Clay. The Palaeontological Association, London, UK, 1, 237-239.

2 comments:

ted said...

Hello. Good post !

Somehow, do you know the size of the teeth in the last picture ?

Mark Wildman said...

The larger teeth are around the 75 - 100mm mark including the root. The smaller crowns are in the 25 - 35mm range. These teeth can be viewed at the Sedgwick Museum in Cambridge - a wonderful museum that makes up in quality what it lacks in size. A must visit museum for anyone's bucket list.

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