At the back of my mind, a little problem has been niggling me for some time without me realising it. Let me explain. I’ve looked at oyster shells a lot: both modern and ancient. By ancient I mean up to two or three thousand years old. All these shells of the bivalve mollusc called the European Flat Oyster, Ostrea edulis Linnaeus have in common that one valve (or shell) is flat and one valve is sort of saucer- or cup-shaped. The space between the right and left, or upper and lower, shells in the living animal is occupied by the body of the creature.
The shape and size of shells in all species of oyster are extremely variable. This has a lot to do with the specific local environment in which they are growing and the growth stage. Sometimes the oyster has a large cavity and a lot of meat and sometimes a small cavity which would have contained very little meat. However, they always have a cavity contained between the two valves – otherwise where would the animal be?
Now here’s the problem. The fossil oyster valves (Deltoidium or Liostrea delta) at Ringstead Bay are always both perfectly flat. Right and left valves, upper or lower shells, whatever you want to call them, they are both the same shape. It might be argued that the shells picked up on the beach are all right flat valves from different individuals. But, no. Paired valves often protrude from the clay and are easily extracted. They are definitely the two shells belonging to the same individual oyster. The outline shapes of oysters are so variable that no two oysters would be an absolutely identical shape. So, two valves found stuck together in situ in the original burial sediments, with the identical outline shape and size have to be from the same individual animal.
Am I going on a bit? I know it’s a bit nerdy – but I’m like that.
A couple of weeks ago at Ringstead I found a complete oyster fossil, intact with both valves, embedded in the clay sediments at the head of the beach. There was something special about this particular fossil oyster. I gently pulled it out of the clay and used a knife to carefully separate the two halves of the shell. This would have been the first time in over 145 million years that the light of day had touched the inside of that shell. There was something inside it!
The top photograph in this post shows the opened fossil oyster laid out on the beach pebbles. As I took that shot I had no idea what I was really looking at. The shell was dirty. It looked as if the roughly rounded object inside was embedded in the inner surface of the shell – that the shell of both valves had grown around it. I wildly thought that this object might even be some sort of giant pearl!
The picture above shows the pair of oyster shells together, one on top of the other. The species has a typically almost triangular outline. I took the shell home with me and washed it. I could then see the details more clearly. I am still not certain what had been going on with this fossil but it looks to me as though a rounded pebble became trapped between the two gaping valves of the oyster prior to burial in the Jurassic period. This could have happened in the living oyster but is more likely to have happened after death. Today, oyster fishermen call these empty oyster shells, that are still joined together by the ligament, “clocks”.
The top and bottom surfaces of the “pebble” are flattened but the sides are still curved and round. Of course, this is evidence for what I should already have guessed – that the oyster shells and the pebble within them had been subjected to enormous pressures from the overlying strata. Being embeded in soft clay rather than hard rock offered no protection to the oyster. The shell itself does not appear to have been hardened by mineralisation which would have enabled some resilience under pressure. The weight of the rocks above had compressed the relatively thin shells to a greater extent than the pebble within. The shells were therefore moulded around the pebble and both valves flattened into a uniform shape eliminating the central cavity.
Looking again at the fossil, it is possible to see on the outer surface the numerous cracks and fractures caused by the pressure it withstood. The impression of the enclosed pebble is also visible externally. Of course, an expert might be able to tell me something different ….and it’s a pity the mystery object isn’t a giant pearl!
Fossil oysters from Ringstead were previously discussed in the blog posting on 26th March 2009.
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Jessica's Nature Blog 
Hi Jessica,
You have a really interesting blog, with lots of beautiful photos. I was led here by your Jurassic oyster pictures – I’ve been puzzled and intrigued by Deltoideum delta ever since I first visited Ringstead as an undergrad in the late 1990s. The flat, closely fitting valves are strange, and you raise a very interesting point about the animal – how thin was it originally?
As for the pebble you found inside the shell, I wonder if it is a mineral nodule that grew within the cavity after the animal died. I’ve not seen any large pebbles in the Ringstead Waxy Clay sediment, so I’m not sure where such clasts might come from in what was probably a low-energy environment. Whatever its origin, though, it does raise more questions about the size of the animal inside.
I look forward to more fossil oysters in the future!
Best wishes,
Liam Herringshaw
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Hello, Liam.
Thank you for commenting on my fossil oyster posts and making suggestions to account for the presence of the ‘pebble’. It’s all a bit of an enigma.
I couldn’t help being curious about these fossil shells. I have been researching variations in modern Flat Oyster shells as a means to making archaeological interpretations regarding sources of seafood, the role in the diet, economy and trade for a long time. Mostly I have been examining shells from excavations in the UK dating from the last two thousand years. This has entailed a detailed look at recent oyster shells and oyster fishing practices as well.
You may have picked up from some of the posts on the blog that I am keenly interested in the relationship between natural processes in the marine/littoral environment and the visible evidence of their impact on objects such as shells and much more. Understanding the effects of (relatively small scale) present day environmental processes and conditions on shells may lead to a greater identification of the ecological conditions that ancient shells were subjected to. Hence, the focus on size, shape, colouration, epibiont encrustation, wear, infestation damage, and relationships between different molluscan species in naturally-occurring shell death assemblages. Similarly, the detailed interest in decomposition processes and agents of change on the beach. Qualitative and quantitative information is all useful to me.
I was interested to read of your own research pursuits in the university biography.
Thank you for getting in touch. I will be posting another item on the fossil oysters from Ringstead in due course.
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Hi,While hunting for Indian artifacts in south-central Pennsylvania,in a valley in the Appalachian mountains where I live. I picked up what looks like a fossilized oyster or clam shell.Is there anyone who can help me identify it or can point me in the right direction?
Thanks Bill
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Hello, Bill. I am not sure I can really help you because I am based in the UK and I am not familiar with fossils in the US. May I suggest that you get in touch with the nearest museum to the location in which you found the fossils. Perhaps you can find a directory of museums on-line. Even if they do not have fossils in their collection, and they lack the expertise to identify your specimens themselves, they should be able to recommend someone who can help you. Good luck with your search.
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Nice blog! 🙂 Read this on DD’s mode of life: app.pan.pl/archive/published/app43/app43-609.pdf
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Hello, Dawid. Thank you so much for that reference. It looks really interesting and relevant and I shall enjoy reading it this evening.
Thank you
Jessica
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Dear Jessica,
in the last days I tried to find out some details about Deltoideum and today I found this Blog via Google. I read it with interest and may add a few comments. The pebble inside the Deltoideum looks for me after a small concretion build in the past during the 2 adductor muscle scars of the left and right valve. This circle round impressions were conected during lifetime with the adductor muscles and are responsible to hold the shells closed. My suggestion is, that after the death of this oyster the “rotten” soft tissue of the muscle was the seed for a small growing carbonate or may be somewhat phosphatic small concretion inside the shell. I hope carbonate concretions are not unheard within the english Kimmeridge Clay, so I think it´s be a possibility.
There is a bulk of literature about Deltoideum, but only few is accessible as pdf like this one here.
Click to access app43-609.pdf
In Poland Deltoideum is also known together with several more jurassic oysters. Hope you enjoy this paper and get some ideas where to find more about that;-) See especially in Machalsky 1989 and the is a interestings paper from Todd:
http://www.repository.naturalis.nl/document/148615
By the way, Deltoideum is a very flat oyster, so your specimen looking pretty fine to me, not crushed or with lesser inflation than usual. Oyster shells are really compact in the fossil record, the calicitic foliate shell is stable against dissolution and moderate pressure.
Greetings from Germany, ehm, actually I´m in Switzerland.
So, please ignore any miswrites or typo-errors;-)
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Dear Jens
Thank you so much for all the useful information about my fossil oyster. All very welcome. I will look up the references you have included. Your suggestion about the calcareous concretion based on the decomposing adductor muscle makes absolute sense to me. Thank you.
I know this species is characteristically flattened – however, all the examples I have seen from Ringstead Bay show crazing and cracking from the compression so that they are really flat – with no space at all remaining between the paired valves. I’ll photograph some of the fossil shells to demonstrate this phenomena when I get a moment.
My apologies for the delay in responding to your comment. I have been away in the US on holiday and internet access was not easy from my particular location on the Oregon coast.
Thank you once more. The information is much appreciated.
Jessica
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Dear Jessica,
thank you for your kind response. Now, it was me who need a long time to answer, sorry for that. I thought I get a automatic reminder, when someone write a response. However.
I´m happy that you like my explanations about this interesting oyster. Yesterday evening I looked again at the oysters and there is something what I´ve overlooked before. At the first and third picture, there is a 2nd impression visible on the shell, but only one of them can be the muscle adductor scar, so whtat is the other ? Please can you send a closer view, if you find the time? It may be a juvenile oyster, which settled down after the death of the Deltoideum, but to see it better, I need a close-up.
all the best,
Jens
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The second depression is caused by the oyster valves being compressed and flattened over and around the enclosed pebble-like mystery object.
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Have you ever found an oyster fossil with the oyster intact in it? Or does that just not happen?
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Hello, Ruth-Anne,
No, I have never found an oyster fossil with the soft tissue (the meat) of the animal also fossilised within it – but then I only know a little about the fossils in my own locality. I am not certain if it is possible. Mostly I think it is only the impression of soft tissue that it preserved in the rock rather than a petrification of the meat itself. However, I understand that evidence has been unearthed fairly recently about fossilisation of soft tissues in dinosaurs – as you can see from the abstract from the Scientific American journal in this link: http://www.scientificamerican.com/article.cfm?id=blood-from-stone.
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I have a fossilised oyster with what I believe to be the meat fossilised but it could just be sand that is in there. I came on this web site because I wondered about it. It certainly looks like a complete oyster both the upper and lower shells are in place with a different material in the centre. What do you think?
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Hi, Ewan
Thanks for they enquiry. It would be difficult to say whether there was fossilised meat inside your fossilised oyster shells without examining the specimen. However, if it were fossilised meat, I think it would be unique. Where did you find your fossil? Do you know what species it is? Mostly, the paired valves of fossil oysters that I have found at Ringstead Bay in Dorset are filled with sediments, sometimes soft clays as in the surrounding matrix and so forth that can be washed away, or else consolidated into hard stone that cannot be removed. The meat of an oyster in generally very soft, watery, and easily and rapidly decomposed, offering little chance of fossilisation. I had theorised that the object which I found between two fossil oyster valves was the remnants of the muscular ligament but experts have disagreed about that theory of mine. Perhaps you could take your fossil into a local museum with a natural history department and have a specialist look at it?
Best wishes
Jessica
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Thanks Jessica, I’ll do that. As you’ve no doubt guessed I’m not an expert so I’ll take it in to the museum for their view. I found it on The island of Colonsay well up on the raised beaches they have on the wets coast of the island. I just picked it up because I thought it was a new complete oyster.
Kind regards – Ewan
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Just thought, Ewan. Oyster shells from raised beaches on Colonsay might not strictly speaking be fossil. There is a possibility I think that they are very ancient (sub-fossil) but not actually fossilised. They might even be archaeological food remains. I know that ancient shell middens have been excavated on Colonsay.
Jessica
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Ah, interesting and it would make some sense too.
Regards – Ewan
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I have what I absolutely know is a fossilized oyster and both shells that my boyfriend and I found during the Texas/Oklahoma drought.while the Red River was so low. I thought it was rare, A near perfect specimen too. Thank-you for sending me this info.
Regards, Ruth-Anne
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Just wanted to say that the two halves fit together perfectly, too. It really is an interesting fossil, isn’t it?
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