Wednesday, 24 August 2011

Prep News 2


Some of you may be familiar with the fact that we have some new plesiosaur material that we are currently preparing and looking at. The material was found in association in the Oxford Clay representing what appears to be a juvenile animal from an as yet unspecified taxon.

The animal is represented by multiple elements including one virtually complete forelimb, both femora and ribs. There are cervical, dorsal and caudal vertebrae represented but not many with processes and other large unidentified elements which are probably attributable to the pelvic or shoulder girdle. Some disassociated digits and other elements complete the recovered bones.

Unfortunately no skull material was recovered and we could not find a single tooth, which was disappointing. Cryptoclidus eurymerus is the most common plesiosaur recovered from this formation but there are subtle morphological differences in some bones which suggest this animal is something else. It is similar, however, to C. richardsoni and this is a very rare taxa indeed which may make this specimen quite important.

It s almost certainly not Muraenosaurus since that appears to be a much more robust animal. So the animal remains indeterminate at the moment and although I am erring on the side that it is possibly C. richardsoni, there is an outside chance that it may be a new taxon and that would be really exciting. We hope to get back to the site at some point and see if we can possibly locate any skull material and anything else that may help to identify the animal. The fact that the humerus is considerably larger than the femora is diagnostic of cryptoclidid plesiosaurs.

The material appears well preserved and heavily mineralised but it all has a proportion of matrix adhering to the surface in varying degrees of thickness. Some of it is merely compacted clay that is easily removed but other matrix is encrusted by calcite with remnants of both shell and belemnite embedded within and this has compacted and solidified to form a crust that is almost flint-like and is much tougher to remove.

Removal of this material is exacerbated by the fact that it almost blends into the bone and it is extremely difficult to take it away cleanly without damaging the fossil. The bulk of this material is removed mechanically but the remainder has to be removed very delicately, almost grain by grain to prevent damage and this is very much a time consuming process. The last specks are gradually eased out with needles but even at this stage they are extremely hard to remove and patience is paramount.

However, having said all that, some of the bone preservation is outstanding and this humerus is as good an example as any I’ve ever seen. This form of preservation is thought to be attributable to the fact that the Oxford clay sea was shallow and the organic material that the animals were buried in was exceptionally rich and fissile and virtually sealed in the fossils, thus enhancing preservation.

Unfortunately, many vertebrate remains are contained within calcite concretions and these are often well preserved but they are almost impossible to remove from the concretion without damaging the specimens. The plesiosaur discussed here also had a concretion in situ which had split and revealed a series of vertebrae running through it. Despite our best efforts, the concretion was so heavy that we were unable to extract it without mechanical help.


However, before extraction could be arranged, the site was covered over with spoil and the nodule seems to have disappeared but we are hopeful that it will eventually be relocated and we are optimistic the site will “reappear” after more spoil movement and, of course, more natural weathering and erosion.

After the humerus is finished being prepared, the radius and ulna are next up for preparation and these will clean up much quicker since they are much smaller elements to work on and do not appear to be too heavily encrusted with matrix. I’ll publish updates as we go along eventually unveiling the forelimb in its entirety once all the associated paddle elements are finished and, of course, any other news on the identity of the animal as we get it.

Footnote

We have recently been informed that the quarry has undergone extensive remodelling recently and that the layout has changed considerably which unfortunately lessens the chance of finding any more remains of this animal. This is regrettable but, until we are able to visit the site again, it is impossible to predict if we will be able to find any more of our plesiosaur.

6 comments:

Anonymous said...

A very interesting and informative post Mark. An exciting find indeed. What is the delay in getting back to the site? Would it be worth taking a wheelbarrow in the car next time, in case you refind that concretion?

regards

Paul

Mark Wildman said...

Hello Paul and thanks for your interest. It's a long drawn out saga now with regards to the site. Suffice to say it is extremely political and the ongoing dialogue with the site owners is taking time to cultivate - but we are ever hopeful of being able to obtain a proper working relationship in time. Until then, access remains heavily restricted.

We have been back to the site during the summer but the quarry where the plesiosaur was excavated has been completely covered over and there was no sign of the nodule. We normally have at least a barrow or a sledge to remove big concretions. In this case it was so heavy that we simply couldn't remove it from the ground without mechanical help.

Anonymous said...

Hello Mark

Let's hope the access issue is resolved soon.

Could you please acquaint us with the differences between the two Cryptoclidid species that you mentioned?

thanks

Paul

P.S. that concretion does look massively heavy. Was it septarian?

Mark Wildman said...

Well Muraenosaurus is larger, on average, than Cryptoclidus and, as I mentioned in the post, appears to me to be a much more robust animal. The most obvious difference between the two is the length of the neck. The cervical vertebrae number 44 in total (as opposed to 32 in Cryptoclidus) and the centra are much longer thus exponentially increasing the total neck length. The skull, however, is proportionately small compared to Cryptoclidus.

There are other significant differences, primarily in the pectoral girdle and limb bones. For me, the most obvious difference is in the shape of both the humeri and femora which are fairly uniform and don’t appear to have the vast morphological variety of Cryptoclidus. The humerus, in particular, is rather modestly expanded at the distal end. However, I must point out that I have had only limited access to fossils of Muraenosaurus but my observations seem reasonable.

The nodules are indeed septarian, formed at the bottom of the Jurassic sea and are heavily mineralised with calcite. Very tough, very heavy and virtually impossible to extract a fossil from, whether it is bone or ammonite.

Anonymous said...

Re. the septarian concretions: yes, and the formation of crystals often distorts and breaks up the bone too, does it not?

Thanks for detailing the differences between Muraenosaurus and Cryptoclidus. The ‘plications’ on the cervicals of Muraenosaurus are quite indicative too I think. What of the differences between C. eurymerus and C. richardsoni?

Are there any other currently valid cryptoclidid species from the Oxford Clay?

Paul

Mark Wildman said...

As the nodules formed and cracks appeared then, indeed, calcite (and other minerals) permeated the nodule and bone could also be damaged and/or distorted.

The only difference between C. eurymerus and C. richardsoni is in the forelimb. The humerus is less expanded antero-distally and there is diagnostic variation in both the radius and ulna.

There have been other cryptoclidid plesiosaurs suggested over the years but these have all turned out to be different ontogenetic specimens of C. eurymerus. Thus, only the two species of Cryptoclidus are representative of Cryptoclididae. However, this does not mean that there is not something else out there waiting to be discovered.

I should also point out that Muraenosaurus is a genus in Elasmosauridae, as opposed to Cryptoclididae, of which Tricleidus seeleyi is also another genus found in the Oxford Clay, although this plesiosaur appears to be extremely rare.

Sorry about the delay in responding - Blogger has been playing up again!

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