Wednesday, 25 August 2010

A Cryptoclidoid Rib from the Oxford Clay

A little while ago I finished preparation of an excellently preserved plesiosaur rib, recovered from Quarry 4 during May 2009. As you are probably aware from my previous blog entries regarding this quarry, the rib was located, in situ, in Bed 10 of the Lower Oxford Clay (Peterborough Group). It is Mid- Jurassic in age (Callovian), about 165 million years old.

Unusually for these ribs from the clay it hadn’t suffered any significant compression or distortion and had survived more or less intact and the rib retains its diagnostic plesiosaurian shape . The rib was stabilised in a few places and then removed in matrix since there were several fractures along its length and I wanted to be sure we weren’t missing any pieces.

The process of removing the bone from the clay was fairly straightforward and when the main pieces were removed, I sifted through the clay matrix to be absolutely certain that nothing had been missed. The rib was fairly easy to clean and the fine striated surface was cleared using a pin vice.

Then the process of putting the pieces back together began. Throughout the whole procedure, I was very careful to piece the rib back together as it had been fossilised, and the pieces joined together extremely well. At this point it became apparent that a tiny section of rib had obviously gone astray during fossilisation and prevented two main sections from joining together. Or so I thought.

Because I was in the process of finishing off the rib, I started to get the preparation record up to date on the database and the backup Excel spreadsheet. As I started to group the images of the rib together, I noticed that the images of the bone in situ quite clearly showed a complete rib and yet I could not piece the two sections together.

Closer inspection revealed that there was a definite section missing and yet we had been very thorough when we had removed the bone from the quarry. The answer, it seems, was that when I removed the bone from the matrix I had inadvertently placed the missing section in the wrong grouping and it was residing with a few fish bones from the same bed!

I soon had the last piece prepped and before long I had a complete rib – and very nice it is too. I gave the rib a light dressing of B76 to finish the job.
So this is a plesiosaur rib from what is almost certainly Cryptoclidus eurymerus (MWB-029.0509), since this is the most common taxon at this level. The images show the rib in situ and after preparation. Enjoy!


Post a Comment