Preparing and conserving an important six-foot long Plesiosaur skeleton for Somerset Museum
A significant project that Nigel has recently worked on for Somerset Museum in Taunton is the conservation and preparation of a scientifically important
plesiosaur skeleton found in Bridgwater Bay on the coast of Somerset. This fossil marine reptile -
from the age of the dinosaurs about 185 million years ago - is six feet long and thought to be the most complete plesiosaur specimen
found in the UK for over a hundred years. In addition, the experts who are
now undertaking research on this skeleton -
Mark Evans (Leicester Museum) and Richard Forrest (of
think that it might well be the skeleton of a juvenile, making it even rarer -
and possibly even a new species.
Nigel is no stranger to working on fossilised marine reptiles. When working at The Natural History Museum in London he
spent three years working specifically on a project conserving and redisplaying the museum's
world-famous collection of ichthyosaurs,
pliosaurs, mosasaurs and plesiosaurs.
However, the specialists wanting to study this new important specimen did not have their
curiosity satisfied overnight. The skeleton was encased in thin, hard, brittle sheets
of fossilised seabed mud and it took many months of painstaking labour to free the bones of the animal. The general size and shape of the animal could already be seen when it was discovered, but the actual bones needed to be uncovered for the specialists to undertake their research.
This project has now been written-up and published:
The virtual and physical preparation of the Collard plesiosaur from Bridgwater Bay, Somerset, UK.
By Nigel Larkin, Sonia O'Connor and Dennis Parsons. 2010. The Geological Curator 9 (3): 107 - 116.
For a shorter account, here is the interim report
Initial work to ready the specimen for preparation:
As the four blocks of finely laminated marine sediment that contained
the plesiosaur skeleton were beginning to delaminate and split when delivered (possibly due to being on open display at Taunton Museum and experiencing diurnal fluctuations in relative humidity), the most important issue to deal with first was to arrest this process.
This meant that the lids of the wooden boxes containing the specimen were left screwed down for several weeks as the contents adjusted slowly to the new conditions. Then, conservation-grade stable and long-lasting but reversible adhesive (the methacrylate co-polymer Paraloid B72) was applied liberally around the sides of the blocks, so that any current or future changes in surrounding air humidity would not affect the fine layers of sediment from the side of the blocks. It would also physically deter the fine layers from separating.
As the next job was to remove the underburden from each of the blocks to make
them lighter and therefore less prone to mishandling and subsequent damage, the specimens had to be turned over. To ensure the least risk of damage during this process, silicone Wacker rubber (Elastosil-M, the same used in the Palaeontology Conservation Unit at the Natural History Museum in London) was applied to the top surfaces of each block after a protective layer of thick consolidant had set (Paraloid B72 at 25% in acetone, weight:volume) and after water-soluble putty had been applied to the vertical cracks in the sediment so that no rubber would penetrate into the specimen. Jesmonite acrylic resin and glass fibre cloth was then applied to the upper surface of the rubber to provide a rigid support to the protective rubber jacket (one gelcoat followed by three layers of resin and glass fibre each separated by a gelcoat). Once this resin had set, the block was turned over very carefully to lay on this comfortable and perfectly shaped rubber and acrylic resin jacket. These rubber jackets have also taken a perfect mould of the shape of the specimen as found, a record of what the outline of the skeleton looked like enclosed by the layers of sediment. A cast of this, and subsequent replicas, can be reproduced from these moulds if need be at a later date.
Once each block was turned over, the underburden had to be removed. In most cases, there was a very obvious point at which the block should be split as a very large ammonite had been preserved about halfway through the sediment and provided a major point of weakness in the blocks horizontally. Thin sculpting blades were inserted to encourage the remaining area to be separated, and the freed block of sediment was simply lifted away.
The sections of the large ammonite on the underburdens were consolidated where necessary and all the underburden was kept for future research and display purposes. The ammonite material was quite fragile, and had been preserved as a very thin specimen having undergone considerable compaction. Impressions of the ammonite remain on the undersides of the main specimen blocks. Wooden boxes lined with inert archival Plastazote foam were made for the blocks of underburden to be stored and transported in.
The layer of sediment now exposed uppermost (on the original undersides of the blocks containing the skeletal material) was consolidated and a layer of silicone wacker rubber was applied, followed by Jesmonite acrylic resin as described above. This was to provide a permanent, or at least long-term, supportive jacket for the skeletal blocks to be stored on and transported in. Now all the blocks containing the specimen were lighter and easier to handle and had supportive jackets made for their new undersides, so they could be turned over to lie the right way up again. The protective rubber and Jesmonite jackets for their top surfaces were removed and kept for future use. The water-soluble putty that had been applied to the vertical cracks in the sediment and the bones on the top surface was removed with scalpels.
Because the bones of this specimen seemed to be well fossilised and possibly had
been at least partly replaced with dense minerals such as pyrite, yet lay in a finely
laminated sediment of mud that had not been similarly replaced but was in fact still soft
and easily damaged, the potential for taking useful X-ray images of the specimen seemed
high. As Nigel knew an expert in X-raying difficult and unusual items - Dr Sonia O'Connor, of Bradford University's Department of Archaeological Sciences - it seemed reasonable to explore this possibility. The smaller block was selected for experimentation as this would be easier to handle, would be less likely to suffer any damage during transportation and would be more easily fitted into or under any relevant equipment. This block contained the skull and the neck vertebrae.
The specimen was taken to Argos Inspection Co. Ltd, Washington, Tyne & Wear, where it was put under an industrial X-ray machine and was X-rayed at a variety of exposures. This yielded immediate and astonishingly clear results. The skull appeared on the screens in very fine detail, with many separate teeth visible and possibly even blood vessel channels and other tiny details. Individual vertebrae in the neck were shown clearly, detailing their internal structure. This investigative work was followed the next day by CT Scanning at the Department of X-radiography at Bradford Royal Infirmary, with what seemed to be similarly useful results.
The resulting images were shown as soon as possible to Dennis Parsons
(Curator at Somerset Museum) and plesiosaur specialists Mark Evans, Richard Forrest
and Arthur Cruickshank. Everyone agreed that these images alone were extremely important
and would be very useful in understanding plesiosaur morphology and growth as well as help
identify the species of plesiosaur concerned. It was soon agreed unanimously that all
the blocks should be both X-rayed and CT scanned before preparation of the skeleton ensued,
and that everyone involved in the project should ideally be present.
On a date that suited everyone involved, all the blocks were taken first to Argos Inspection
in Tyne and Wear and then to Bradford Royal Infirmary for X-raying and CT scanning respectively. They were transported in their custom-made wooden boxes packed with Plastazote foam, lying on foam mattresses in the back of carefully-driven transit van.
The X-rays and CT scans yielded excellent results and helped to inform
subsequent preparation of the specimen. The specimen did not suffer any damage a
s the result of these investigative processes or the journeys they necessitated
thanks to the preventive measures employed at each stage.
Preparation of the specimen:
Before preparation commenced, experiments were undertaken with some of the underburden of the small tail block. This was to ascertain which tools to use and whether consolidants could be employed and if so at what consistencies. All but the thickest consolidants were found to make the individual layers of sediment swell and split apart, achieving the opposite effect of that desired. Even the thickest consolidants caused this to an extent and could only be used for areas of sediment that were to be removed later on. Using a scalpel blade to tease the layers of sediment away from one another proved to be more effective (and gentler) than mechanical preparation using airabrasives and percussion tools.
The block of sediment containing the tip of the tail had delaminated so badly when on display and looked so vulnerable in the badly split block that it was decided to remove the portion of the block containing the tail bones. The smaller the amount of sediment surrounding these bones, the fewer problems it would experience in terms of splitting. A thin, high speed dental wheel was used to cut out a small rectangular block containing these bones. The vertebrae were then prepared with a scalpel. They were found to be poorly preserved with little surface detail. In fact there seemed to be a gap between the last layer of sediment and the bone itself, where the surface of the bone might once have been but had since deteriorated. Luckily this did not prove to be true of the whole of the skeleton, just parts of it.
Each of the blocks was tackled in turn with a succession of fresh scalpel blades, teasing the layers of sediment off one another to find the bones within. Body outlines and gut contents etc were looked out for, but none observed. The preservation of the bone surfaces varied, and it was noted that some of the bones of the paddles were extremely porous and friable, with no decent surface, whereas other limb bones exhibited fine surfaces in the middles portions but were particularly poorly preserved at their proximal and distal edges.
Once a bone had been exposed as best as it could with scalpel blades, it was swabbed gently clean with cotton buds dipped briefly in acetone and a small amount of water to get as much of the remaining sediment off as possible. Acetone helped the water to evaporate more quickly and therefore penetrate the specimen less.
The main block containing the body before, during and after preparation.
Cleaned bones were treated with two or three layers of Paraloid B72 in acetone at 5% weight to volume (usually with a very small brush, sometimes with a pipette), to consolidate them as they were often riddled with tiny cracks as well as occasionally being friable. When appropriate, Paraloid B72 adhesive was applied straight from the tube if the area was particularly friable or if a break needed repairing.
The individual pieces that were broken off during the excavation were re-adhered to the main specimen blocks.
Physical condition of the specimen:
During the preparation of the specimen, the following points were noted about its preservation:
The skeleton is almost totally complete. The only bones missing are some of the phalanges in the middle of the rear right paddle - possibly due to predation that may have caused the death of the specimen.
The upper parts (in death position) of some of the bones (particularly the vertebrae, symphysis of the pelvic girdle and the skull) were often less well preserved. These areas were possibly more prone to damage during the burial process as they would be the last parts of the body to be covered by sediment and therefore would be potentially subject to more decay and scavenging.
The gastralia and some of the vertebra in the main body area were poorly preserved and very brittle. This area possibly suffered more chemical/biological damage than other areas due to the gut and stomach contents acting upon the bone in the early stages of burial.
When the specimen was found, it was clear that with each tide stones had been rolling back and forth over it, damaging the tops of some of the bones that had become exposed (particularly the pectoral girdle and skull). Also, cracks had developed along the spine and salt water had penetrated the specimen. This will not have done it any good, especially as the matrix and the specimen would then partly dry out during the low tides.
Many, if not all, of the bones are highly fractured and have little or no integrity. If removal of individual bones were attempted, they would simply crumble.
A very hard but ultimately crumbly (and mostly whitish) crystalline infill was found between the vertebrae. Not all this was removed and the remainder might be worth investigating with X-ray diffraction techniques as part of any investigation into diagenetic processes.
The fine-grained sediment was composed of very regular thin layers rather than a homogenous mass. Could this regularity be indicative of tidal conditions?
Lots of small ammonites and other crustaceans, as well as the occasional fish scale and a very large ammonite were found preserved in the sediment. Most of the sediment was saved for further analysis or handling/teaching sessions.
The skeleton appears to be laying upside-down. The skull is upside down, and is at a slight angle. Part of the mandible (left side) was broken and folded over the rest of the underside of the skull and mandible during the burial process. This is unfortunate but it does allow us to see more teeth than we might otherwise have done.
I was hoping that more of the front teeth would be available for preparation but the matrix and the teeth seem to present a uniform mass that is difficult to disassociate and prepare. If more preparation time was available, this would be a good area to target.
The skull before preparation, the X-ray and after preparation.
Future considerations regarding storage, display and handling:
Now that the underburden has been removed and the bones exposed, the specimen is much more vulnerable than before. It must be stored in a stable environment and protected from fluctuations in humidity, and in particular kept away from high humidities. It must never go on open display as this would expose it to high humidities that change on a daily basis. It should be exhibited in a sealed glass case with the right amount of pre-conditioned Artsorb, or at least silica gel, and lit with fibre-optics. I would recommend that the X-rays and CT scans are used as part of any display, along with any relevant 3D stereolithigraphic reproductions that can be made from the digital data of the CT scans. A replica could also be made of what the specimen looked like when it was first found, using the rubber moulds, and the large ammonite preserved in the underburden could also be displayed.
If the specimen requires further transportation, a good mattress must be placed underneath it to protect it from bumps and potholes in the road and continuous vibration.
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