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.

Monday, 24 June 2013

Welcome to Tyrannosauroidea Central

 
Thomas Carr has just launched a blog about everything tyrannosaur at Tyrannosauroidea Central and I am absolutely delighted that he has decided to join the Mesozoic Blogosphere. Described by fellow tyrannosaur aficionado Thomas Holtz Jr as ".....probably the best tyrannosaur anatomist on the planet....." this blog should prove to be a gold mine of information as Carr reveals and discusses his research, both past and present, and keeps us up to date on the very latest news regarding all things about my favourite clade.

As a staunch follower of Carr's research and, as you all know, particularly fascinated by Tyrannosauridae, I am genuinely excited by this development and look forward to the blog developing and am particularly interested in his upcoming Daspletosaurus monograph and his books that he has been working on.

Welcome to the Mesozoic Blogosphere Thomas!   

Sunday, 2 June 2013

Shimmering Diamonds in an Ancient Sea


 
Field trip reports are not as plentiful on this blog as they once were. There are a few reasons for this – obviously the closure of Quarry 4 was a big blow and a few of the other quarries we were permitted to visit have now closed or have stopped allowing access. This is an unfortunate consequence of the continuing uncertain economic conditions and a health and safety culture gone mad. I have alluded to this many times before but it is still worth highlighting as our geological and palaeontological treasure chest closes ever tighter.
So when the opportunity arose for a field trip to a classic locality became available, I took the opportunity with both hands, as did three of my colleagues from our research group. The trip was a joint venture organised by the Natural History Museum (London) and the Geologists’ Association and was certain to be well attended but it was definitely a trip that you wanted to be on – especially as the site was now earmarked for a housing development. This was very likely to be the last ever permitted site visit.
Keymer Tileworks Quarry is located in the heart of the Weald at Burgess Hill in West Sussex. The tile works has existed in one form or another since the late 1500’s but the current site was migrated to from Ditchling Common when the clay reserves there became exhausted. This was certainly a long drawn out affair since it took 80 years to move the production facilities from 1860 to 1940!
The quarry itself is Late Hauterivian in age and appears to almost straddle the boundary between the Lower and Upper Weald Clay Formations and is thus slightly older than the Bluff (Barremian) – around 130 million years old. Today’s working quarry is not the original workings on the site as measured by Cook and Ross (1996) but there is obviously still a recognisable lithostratigraphic sequence.
Intriguingly, the sedimentary sequence in the quarry is very similar to the Bluff and typical of the Weald in as much it that it represents terrestrial through fluvial to lacustrine conditions (Batten & Austen, 2011) and, as a result contains a very similar fossil assemblage. These include a strictly non-marine fauna of both plants and animals and include sharks, bony fish, crocodiles, turtles, dinosaurs as well as insects, crustaceans and molluscs. Plants include ferns, conifers and horsetails.
As we pulled into the car park there was already a large group of workers there getting their equipment ready.  I recognised quite a few people from the Bluff, a few from the NHM and other groups as well and, as I had already forecast, others continued to arrive. However, it was not that large a group that it was unmanageable and after a briefing regarding the usual health and safety rules, as well as a brief introduction to the quarry, we were soon on our way in.
The tile works was significantly different from the brick works at the Bluff. Before being mothballed the Bluff was a 24/7 fully automated works but here the tiles are all handmade and a lot of the old fashioned out buildings and wooden crates holding the tiles seemed to reflect this traditional method of manufacture. I found it rather refreshing in today’s modern and efficient manufacturing industry that there are still places like this around and it is also tinged with a degree of sadness that the tile works here at Keymer’s has been selected for extinction.
Anyhow I digress. We skirted around the tile works and then slowly descended into the quarry area. As I looked to the right I could see the remnants of the original workings, now either overgrown or flooded with one particular area now almost completely consisting of a glutinous quicksand - a modern day predator trap if ever I saw one. Slowly we approached the raised ramparts of a small quarry and as I looked down there was a very small excavation that was flooded in the bottom – a pump was in situ to keep the water under control. This could not possibly be the quarry – or could it?
Indeed it was. At first I thought it was just a small offshoot from the main quarry but it wasn’t – this was it. I cannot stress how small this site was when compared to the big open sky quarries I was used to. It was like a duck pond with raised sides and I thought that after about an hour tops I would be winging my way home cursing the fact that I had travelled over 80 miles for very little. But first impressions were, to say the least, deceiving.

After the initial shock regarding the size of the quarry the next thing that struck me was the astonishing beauty of the exposed clays, silts and sandstones which were a beautiful mixture of reds, blues, greens, greys and even purple. This was further exaggerated by the succession dipping to the south west and it is another regrettable fact that this section of the quarry has still not been measured properly– nor is it likely to be.
 
We made our way into the quarry and I arrived with others on the south east face to start prospecting. Prospecting is perhaps the wrong descriptive word to use in this case for as we looked at the light grey/green clay layer that lay directly below a reddish-brown layer we could see fish scales and bones weathering out – loads of material. It was apparent that there had been nobody collecting here for quite a while and the continual erosion throughout this extraordinarily hard Winter had done its job.
Several of us immediately lay down and got to work. It was great fun as careful examination of the clay in combination with a gentle raking of the clay produced many scales of various fish which included lepidotids and Lissodus. Vertebrae, despite not being as common as scales, were still plentiful and ranged in size from about 8mm in length to those that could barely be made out with the naked eye. Odd fish teeth and bone made up the other most collected fossils.
The abundance was astounding and you could not fail to find material and the closer you got to the clay the more you would find. The scales were absolutely beautiful and collecting them was strangely compelling and I got the feeling that everybody felt like a kid in a sweetshop as you picked them up one after another – they were like shimmering diamonds in an ancient sea.
Whilst this was going on, the team from the NHM were busily digging into the same microvertebrate bed to remove bulk samples for sieving. They were attempting to remove as much material as possible since this was meant to be the last ever visit to this quarry and, as the bags slowly built up, I actually thought they might struggle to drive it all away. My colleagues Mark and Simon were also part of the team that was continuously beavering away and filling up the sacks with matrix.   
As the day wore on the microvertebrate bed was stripped bare of its fossils and the group began to slowly spread out leaving the NHM crew to their toil. Not only was this active quarry being prospected but others had started to spread out across the site. Those particularly interested in fossil insect remains started to inspect the randomly spread siltstones that had been excavated over the years while others started to scour the older excavations which were only available in small sections due to either being overgrown or flooded.
We decided to stick to the current quarry and continue the search but due to the size of the quarry it was already difficult to find an untouched spot. But this is part of the fun and one of the spots that I had earmarked earlier was a small mound of clay from the same fish bed that had actually been used as a spot where many people dumped their bags, ruck sacks and equipment. As they dispersed the mound became available for prospecting and we soon moved onto the spot.
This turned out to be an inspired choice and we recovered many more scales from the mound and I was also pleased to find a disassociated fish spine and there appeared to be quite a bit of it. I carefully removed the larger parts before recovering the smaller parts of the spine which were obviously quite delicate. Careful excavation of the clay with a dental pick revealed no more parts of the spine and, satisfied there was no more to be recovered, we decided to move on.
On the south west side of the quarry was a well-known reddy-brown clay with a history of exposing vertebrate material. Unfortunately, this had seen better days and was covered in some form of crust that rendered any excavation unlikely in the time available. But there had been the odd rock fall and the spoil had been well searched by others prior to our arrival. Immediately parallel to this however was another exposure of the microvertebrate bed of which some had been piled up as a big spoil heap that probably ran for about fifty yards. This too had been receiving attention and there was lots more material recovered.
However, many people were so engrossed in the spoil that they neglected to look under their feet but we did not and began to recover yet more scales. But only five yards from where we had recovered the fish spine we started to find much bigger scales from a lepidotid fish and they appeared to be in a much tighter area – most people had simply walked over them.
They appeared to be weathering out from a large mound of clay situated just before the previously mentioned spoil heap began. A careful search of the mound revealed yet more scales but no bone was forthcoming. The scales were large and uniform and it seemed likely that there was more to be found of this fish in this proximity but try as we may we could find no more.
Having exhausted this area we decided to spread out and try more places but there was not too much more forthcoming and people began to drift away and the site got quieter. Except, that is, for the NHM crew who were still at it. We had to hand it to them – they had really put a lot of effort in today and would deserve their forthcoming finds. Our colleagues were suffering though as Mark was starting to suffer some back issues and Simon was starting to run on empty.
Some of the NHM crew hard at it.
 
Eventually the day was drawing to a close but even then it was still possible to find the odd scale glistening in the sun and you could not resist them. I don’t think I’ve had so much fun collecting fossils for ages. Other notable finds reported include some insect and plant remains form the siltstones including one nice cockroach with legs attached, an odd reptilian vertebrae, a small limb bone and, I imagine, the find of the day was an unfortunately water abraded theropod tooth which Steve Sweetman believes to be velociraptorine.
But it was the fish remains that ensured that everyone had an enjoyable day and even the weather was about spot on as well. As we slowly drifted away from the quarry, we looked back and the NHM crew had actually finished excavating and were in the process of transporting the bags of matrix to the vehicles ready for shipping out. We later found out that they had managed to ship eighty one sacks all told – an excellent effort. I hope they are rewarded with a few excellent finds – especially mammal teeth which I know they are quite eager to find.
It was with a tinge of sadness that we left Keymer’s in the realisation that our first visit was likely to be our last but at least there had been lots of material recovered and it was great to see familiar faces. It’s just such a shame that another classic quarry is earmarked for closure – our palaeontological heritage is shrinking fast.

Footnote
Those of you who have followed this blog for some time will know that I tend to use pseudonyms for most localities to protect them from illegal collecting but, in this case, since this is meant to be the last visit before the site is developed, it seemed right to give the quarry its correct title.
It was interesting that, when examined under the microscope, a few of the scales turned out to be actually teeth and there are a few sections of minute jaws with teeth in as well – which is interesting indeed. The fish spine is missing substantial parts and there was only two parts that fit together but this also looks good under the microscope. And we have also heard that, if all goes well, there may yet be another “final” trip to the quarry and, if there is, you can count me in for a return visit.
Here are some of the fossils recovered:
 
 

 

Tiny jaw section
 

Part of the fish spine 
 

Teeth
 
 
References
Batten, D.J. & Austen, P.A. 2011. The Wealden of south-east England. 15 - 51. in Batten, D.J. (ed). English Wealden Fossils. Palaeontological Association, London, Field Guides to Fossils, 14, ix + 769 pp.
Cook, E. & Ross, A.J. 1996. The stratigraphy, sedimentology and palaeontology of the Lower Weald Clay (Hauterivian) at Keymer Tileworks, West Sussex, southern England. Proceedings of the Geologists' Association, 107, 231 - 239.