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.

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