Thursday, 26 January 2012

Prep News

A brief post this week and, again, featuring the continued preparation of our Oxford Clay plesiosaur. I’ve frequently mentioned the issues that have been encountered when comparing specimens from our animal with those in other institutions and the sometime extreme differences in morphologies of the same elements.

We are lucky, in one way, in as much as we have multiple associated elements from one animal and yet we have struggled to identify this specimen, although we have now almost certainly identified this plesiosaur to genus level. But with individual elements, or even parts of larger bones, you can have much tougher problems with identification. Sure, there are elements which can be assigned to, say, a plesiosaur or ichthyosaur but generic identification is problematic.
But specific isolated elements can also be equally diagnostic although I have found the identification of some marine reptile bones to be particularly challenging. For example, plesiosaurs increased the amount of phalanges in their limbs considerably over what is regarded as the standard plesiomorphic condition. This is known as hyperphalangy and is characteristic of plesiosaur limbs.
Phalanges all look alike and found in isolation are not particularly helpful although some of the bigger ones are obviously pliosaurid in origin. In contrast some other elements can often be identified to generic level and cryptoclidid humeri are particularly useful in this respect. It’s interesting that there are almost certainly odd elements in collections all over the world that not only represent missing elements from known taxa but also unknown elements from unknown taxa.  Again, Spinops comes to mind.
That aside, here are a set of proximal carpals from our plesiosaur all finished with and prepared. These are, from left to right, the ulnare, intermedium and radiale and are from the same forelimb featured in previous blog posts and I am now working on the distal carpals. After that, there are the first five metacarpals to prepare and then this particular forelimb will be finished – the remaining part of the limb has long since vanished. Shortly after I will publish a brief description and then this particular project will be temporarily shelved because of other projects but eventually the next element up for preparation is a rather impressive femur and, of course, I will publish posts along the way.

Ventral view

Dorsal view

Thursday, 19 January 2012

The Palaeoworld in Miniature

Last time, I discussed Daspletosaurus which, by any standard, was a big theropod dinosaur approaching 30 to 35 feet long – a giant in anyone’s books. Of course, sauropods were much bigger animals and the huge bones of these giants are astonishing to observe in museums all over the world. But there is another world of vertebrate palaeontology seldom seen by the public, a world hidden from view but one that is absolutely fascinating in its own right – and at the other end of the spectrum from the world of the giants.

 The collection and study of microvertebrate fossils is one of those disciplines that are currently right at the cutting edge of palaeontology – and with good reason. It reveals so much about the flora and fauna in different ecosystems that would otherwise be ignored and lost if all we concentrated on was macrovertebrate collecting.
This increased knowledge of diversity in any given palaeoenvironment also enables a more accurate estimation of climatic conditions and increases the taphonomic value of the sites. And now, because of the increase in the amount of time and effort being put into the study of these microsites, more new species are being identified every year. The most high profile example of this in recent years was Steve Sweetman’s work on the Isle of Wight which received international exposure in 2009 as he had apparently identified over 50 new species including dinosaur, mammal, bird, pterosaur, crocodile and many other taxa.
But what exactly is a microvertebrate fossil? In simple terms they are typically exactly the same fossils that you might find in any macrovertebrate bearing strata – except that you very rarely see them or, indeed, they are invisible to the naked eye. The best way to collect this type of fossil is to remove large quantities of sediment from the exposed fossiliferous beds in preparation for screen washing.
However, this is not as simple as shovelling lots of sediment into buckets and thinking it’s a job well done. On the contrary, as with any quarry, the relevant taphonomic and stratigraphic detail need to be taken into account and the site itself must be quantified before collecting since each site is likely to be different and collection methods will vary.
Screen washing, or wet sieving, has been utilised since at least 1847 by Theodore Plieninger in continental Europe and then by Charles Moore in the UK during 1858, who sorted sedimentary matrix removed from Rhaeto-Liassic beds near Holwell, Somerset. The first record of screen washing in North America is by J.L. Wortman in 1891 for the American Museum of Natural History and later followed by Barnum Brown, also of the AMNH, who, in 1906, wet sieved sediments from the Hell Creek Formation in Montana. Brown commented that “This material gives evidence of a much greater Laramie fauna than has been heretofore described” (McKenna et al 1994).
Screen washing is a relatively simple process. Initially, water is passed through the sediment via a couple of screens of different mesh sizes. The first mesh is always of a coarser grade and this traps larger debris as well larger bone and plant fossils. The second mesh is of a much finer grade and sorts out the much smaller matrix and microfossils - and there are even finer meshes if so required.
Nowadays a lot of this matrix is then processed utilising heavy liquid separation which uses chemicals to separate fossil material from the debris due to fact that the specific gravity of fossil bone is heavier than the surrounding particles it is buried with. Using the correct chemical with a specific gravity less than the fossil material would mean that the fossils would sink in the solution and can be removed whilst the unwanted sediment would float. This is a very simplified description of a somewhat complex process which, just as with any other part of preparation, has to take a multitude of factors into account before separation can begin.
This prepared concentrated matrix can then be sorted out which is intensive, time consuming and, at the same time, compulsive and beguiling. Of course, different grades of concentrated matrix will require a different approach. The coarser grades can be sifted through comfortably with no other aids other than, perhaps, a simple hand lens but when it comes to fine and ultra-fine concentrate then a binocular microscope or, getting more common these days, a good quality USB microscope is essential.
A couple of my colleagues and I have been getting to grips with these microscopes and they take a bit of getting use to mainly because of the slight time delay between actually moving material and seeing it translate onto the screen. This comes with practice as does knowing your left from your right which, although is plainly obvious on  a day-to-day basis, can be a nuisance depending on how the scope is set up as a movement left may actually register as a movement right on the screen.
The matrix is best spread thinly and not in a little pile since this makes it difficult to discern which particles have been looked at before, then after, thus avoiding wasting time by looking at them again. When you first begin sorting you tend to check every granule because sometimes they look like they may be a fossil and you want to be sure but you soon realise they are not and the vast majority of fossils are clearly what they appear to be and are seldom difficult to recognise. Indeed, so many of them are exact copies of their larger brethren that there is no doubt as to their animal origins.
When you find a fossil it is nigh on impossible to pick them up with tweezers since you may damage them or they ping out never to be found again *cough*. Far better to use a piece of tape to gently secure it and then carefully ease it into a holding receptacle – masking tape is best since this is not very sticky at all and fits the bill nicely. McKenna et al (1994) recommend a wet brush to do the same job.
I guess this appears a lot of trouble on the face of it but this is extremely important work to aid our understanding of the palaeoenvironment, as an indication of temporal constraint and dating rock units, and just how diverse life was throughout prehistory. For me, I always amazed at how something so small and barely visible can be so perfectly preserved after millions of years have passed.  
A superbly preserved jaw recently sorted from some Maastrichtian concentrate

 Reference

McKenna, M.C., Bleefield, A.R. & Mellett, J.S. 1994. Microvertebrate collecting: Large-scale wet sieving for fossil microvertebrates in the field; pp. 93 – 111 in P. Leiggi and P. May (eds), Vertebrate Paleontological Techniques, volume 1. Cambridge University Press, Cambridge, UK.

Thursday, 12 January 2012

Daspletosaurus is the Key


Southern Alberta, 77 million years ago and beside a large tract of marshland, feeding on the carcass of an adult Brachylophosaurus, are three adult daspletosaurs. All three tyrannosaurs consume everything they can rip away from the carcass with their powerful jaws and great swathes of meat, tendon and bone are swallowed whole – nothing is disregarded.

There are frequent disputes over dominance of the carcass and although the snarling displays of aggression rarely amount to much, they do, on occasion, snap at each other’s head and bite down hard. These clashes are only brief – instinct tells the tyrannosaur that this form of interaction can cause severe damage and infection if prolonged.
Suddenly, all three tyrannosaurs become aware that they are not alone and each one, in turn, notices movement in the undergrowth and in the surrounding treeline. This merely stimulates them to eat faster and now copious amounts of hadrosaur disappear down their gullets with hardly any thought. All that matters is to eat as much as you can as fast as you can.
A troodontid, almost casually, steps into view but although the daspletosaurs are already aware of him, they give him short  shrift and carry on feeding – they know they still have some minutes remaining. Eventually other troodontids appear and then another, then another until there several of them surrounding the feeding carnivores. A sudden movement behind them causes one of the tyrannosaurs to look behind him and he sees other predators approaching – this time they are dromaeosaurs. He knows it is nearly time.
The troodontids slowly approach the carcass and, as they do, the daspletosaurs raise their heads, snap their jaws and issue a very deep low frequency growl from within. The intruders both front and rear stop and take a step back but it does not put them off for long and this game is played out again and again over the coming minutes and still the daspletosaurs continue to feed as much as possible.
Eventually the intruders almost reach the carcass and then one of them takes a quick bite out of the carcass. This time one of the daspletosaurs quickly reacts and issues a large roar and takes a step forward toward the impudent troodontid. Again this temporarily halts the advance but not for long. Soon other troodontids, as well as the dromaeosaurs, begin nipping at the remnants of the hadrosaur and now, as their stomachs become full, the tyrannosaurs eventually begin to give way and allow the smaller predators to keep darting in and out, taking scraps of flesh.
All of a sudden the harassment becomes too much for one daspletosaur and he tears off a huge haunch of hadrosaur and carries it away into some trees. The remaining two daspletosaurs continue to feed but, in the end, they too break away from the feast and move off leaving what is left of the carcass for their tormentors and the carcass almost disappears from sight as the seething mass of small predators’ attack what’s left with abandon.
The daspletosaurs go their own ways for this was not a structured, coordinated kill – rather it was mutually advantageous to bring the hadrosaur down as a threesome as opposed to an individual attacking on their own. Each will return several times to the spot of the kill in the days ahead to see if anything remains and what can be scavenged. If they meet up with each other again, they will give their opposites a wide berth since they do not usually interact unless it is time to mate or if the opportunity to join a kill arises again.

The skull of Daspletosaurus
Image by AStrangerintheAlps

Of course, all the above is purely conjecture on my part but there is some palaeontology in there as well. The age, terrain and fauna are all correct as are some other details such as face biting and the crushing and swallowing of bone. But it is really my way of discussing what I consider to be, not only the most fascinating, but also looks like turning out to be probably one of the most important tyrannosaurids in the next few years – Daspletosaurus.
Daspletosaurus torosus was named in 1970 by Dale Russell. The holotype CMN 8506 essentially consisted of the skull and skeleton but there were no hind limbs recovered save for one femur. The specimen was recovered by Charles Sternberg in 1921 from the Late Cretaceous Oldman Formation in Alberta, Canada. When first collected, Sternberg initially thought the skeleton represented Gorgosaurus or indeed, as it turned out, to be a new species.
Daspletosaurus has a number of things going for it which make it such an intriguing object of study. Firstly, it is a tyrannosaurine and is closely related to Tyrannosaurus although its actual systematic relationship within tyrannosaurinae remains unclear for now.  Secondly, there are multiple species of Daspletosaurus with the majority of these still to be diagnosed, formally described and named. Just what this will mean for tyrannosaurine systematics and synonymy is unclear.
Another fascinating issue is the fact that, in the Dinosaur Park Formation in Alberta, Daspletosaurus sp. actually co-existed with Gorgosaurus which has been taken as suggestive of niche partitioning between large theropods, since Daspletosaurus is a much more robust animal than the lightly built Gorgosaurus. This was considered a somewhat unusual occurrence at first but now more formations around the world are demonstrating similar co-occurrences of large theropods co-existing and this further blurs the issue.
There is even a little legend surrounding Daspletosaurus and that is that, not only did it have proportionately the biggest forelimbs of any tyrannosaurid, but it also had the biggest teeth, proportionately, of any tyrannosaur – including T.rex. But I have never been able to find a reference for the statement about the teeth and if you are aware of the origin of this “fact” then I would very grateful if you would let me know.  
Daspletosaurs are also known from bone beds and a site in the Two Medicine Formation in Teton County, Montana has revealed the remains of over three individuals, including a juvenile, and may be suggestive of gregarious tyrannosaurs similar to the albertosaurs of the Dry Island bone bed. This is still a matter of conjecture, however, and the Teton County site also contains multiple remains of hadrosaurs.
Daspletosaurus, as a species, is undoubtedly of crucial importance in our understanding of faunal endemism and isolation in Late Cretaceous North America, the origins and dispersal of tyrannosaurines and our overall understanding of tyrannosaurid systematics. Essentially, Daspletosaurs are very cool animals and are, for me, the quintessential tyrannosaurid – they are uber-cool.
Incidentally, Anthony Maltese, over at the Rocky Mountain Dinosaur Resource Center (RMDRC), has published a number of times on his blog about preparation of two daspletosaurs known as Sir William and Pete 3. Sir William has finished being prepped now  but Pete 3 is undergoing preparation right now and, with the main jacket still to come,  I would expect more posts in the future about the project. A lot of the material is truly awful and they are performing miracles in restoring this important specimen. Head over there now and take a look if you have not already done so.
Daspletosaurus in the Field Museum
Image by brianbrarian 

Reference

Russell, D.A. 1970. Tyrannosaurs from the Late Cretaceous of Western Canada. National Museum of Natural Sciences Publications in Palaeontology 1:1-30.

Thursday, 5 January 2012

Comprehending Time & Morphology


As we enter 2012, it is always fascinating to realise that our comprehension of time, on a planetary scale, is somewhat vacuous. We live our lives by the calendar and by the clock as the year becomes months, weeks, hours, minutes and seconds – we can even split seconds into thousandths and beyond if we wanted to. But our very existence on this planet is as fleeting as smoke in the wind.
As palaeontologists, we are used to dealing with a much larger time scale – mostly the years quoted are in the million, only daring to drop into the mere hundreds of thousands of years when radiometric dating establishes a timeline, for example, of 75.3 million years. And yet despite our understanding of the vastness of past time we all truly struggle to comprehend it and little wonder.
This often crosses my mind and is sometimes brought home when you least expect it. In the vaults of museums around the world there are many plaster jackets securing what must be hundreds of tons of fossil bones collected over the years. For example, during my recent visit to the Natural History Museum (NHM) in London, I was able to observe sauropod material undergoing preparation for the first time since it was collected back in the eighties. Around thirty years have passed until the material was selected for preparation.
Thirty years, by today’s standards, is a long time in the modern world but would not register in the geologic sense. Indeed, the new centrosaurine Spinops was originally excavated in 1916 by the Sternbergs on behalf of the NHM but was only begun to be prepared in 2008 – a gap of 92 years. I’ve seen older though and witnessed hadrosaur casts dated 1912, also from Alberta, under preparation a couple of years back but the oldest specimen that was (and still is) undergoing preparation is the holotype of Hylaeosaurus armatus. Named by Mantell in 1833, the bones are encased in an unforgiving hard matrix that only yields after substantial man hours and all this 179 years after the animal was named.
179 years! It seems astonishing that such an important specimen is still being prepared but, of course, there are always other considerations such as money, priorities, resources and even a couple of world wars that get in the way of such things. But 179 years is still as nothing compared to world of the Early Cretaceous of around 135 million years ago when Hylaeosaurus was alive.
But is the fact that these bones, and the jackets they are encased in, are left unprepared for many years actually of any consequence? Probably not. They were lying undiscovered for millions of years before being removed from their stony graves anyway and whether they are prepared now or in a further 200 years’ time makes no difference time wise – unless, of course, there is a pyritisation problem but even this is being addressed now and methods are being developed that can stabilise this form of decay to a degree.

One specimen that is being prepared is our plesiosaur from the Oxford Clay and here are the latest bones to be prepared and represent an articulating ulna and radius. There is a clear foramen between the two which has manoeuvred us into yet another area of investigation and may lead to something that is going to, perhaps, make me eat my words!

We took some of our “diagnostic” plesiosaur material to the NHM just before Christmas and were very fortunate to be granted access to some of the cabinets that holds the Leeds Collection – a huge amount of marine reptile material that had been amassed from the Oxford Clay many years ago. We examined specimens of both Cryptoclidus and Muraenosaurus, including the specimens described by Andrews (1910) in his Catalogue of Marine Reptiles of the Oxford Clay.
A few things of note. Some of these plesiosaurs are huge animals and some examples of the forelimbs are well over a metre in length. The humeri are massive and clearly dictate that our specimen is very much a juvenile. The other thing of note, and so striking, is that a great many of the bones are crushed and/or distorted whilst our animal is not and, as a result, even more interesting. The radii, ulnae and carpals in the vast majority of specimens we examined were crushed and direct comparison always had to be tempered with a degree of caution because of the amount of pressure induced variance on the bones.
So what did we find out? The humeri of the specimens we examined were all well preserved with only a little distortion and yet there was not one example that we could say for sure was a match. The proximal end, in particular, is problematic in our example and there was nothing like it that we could find. The other compared elements fared little better as we might find one bone extremely similar and yet all the others would be considerably different - and so it went on.
There were multiple examples examined, including both juveniles and adults, but in the end it was obvious there would be no definitive answer for us that day. If I now had to guess an identity from the known taxa in the clay I would, believe it or not, err on the side of it being a juvenile Muraenosaurus and, if Chris Traxon is reading this, then well done you since you pointed that out to me some months ago. I said at the same time that I thought Muraenosaurus to be a much more robust animal than this specimen but I have also underestimated the astonishing amount of changes in bone morphology throughout ontogeny.
However, there are still too many variables in the overall morphology of the bones that still suggest it is something different and we still have to check out Cryptoclidus richardsoni which is, without a doubt, the most similar example we’ve seen – especially the specimen (GLAHM V1809) at the Hunterian. It is apparent that further comparison will be required and if we are still dissatisfied with the outcome then the specimen will go to London at some point for further study.
Incidentally, the Leeds collection is absolutely astonishing and we were granted access to only part of it and it is truly a wonderful contribution to our understanding of the Callovian Sea and is, of course, of international importance.  It would be nice to see the collection displayed one day, perhaps in a special exhibition – it is truly a unique collection.

Reference

Andrews, C. W. 1910. A descriptive catalogue of the marine reptiles of the Oxford Clay, based on the Leeds Collection in the British Museum (Natural History), London, Part I. British Museum (Natural History), London.