Fossil oysters with ammonite impressions

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An impression of an ammonite on a fossil oyster shell found on the pebble beach at Ringstead Bay, Dorset, UK on the Jurassic Coast (1) 

This fossil is one of my most interesting beach-combing finds. The two valves of the oyster shell were still in position together. The shells had washed out of the clay and were just lying on the flint pebbles at the base of the low cliff at Ringstead Bay in Dorset. The species is probably Liostrea (Deltoideum) delta and dates from over 135 million years ago in the Upper Jurassic period.

The shells of recent oysters such as Ostrea edulis Linnaeus exhibit a great deal of variation. The same appears to be true for this type of fossil oyster shell. Some of the variability results from the way that the shell shape reflects the contours of the hard substrate or object on which the larva orginally settled. The young oyster is always attached by the left valve [see the earlier post Natural objects on which Flat Oysters settle].

The oyster will remain on and continue to grow with the left valve attached to the substrate. It is the left valve that usually has objects attached to it and mimics the shape of  the cultch or settlement substrate. It seems as if the fossil oyster illustrated here attached itself to an empty ammonite shell. 

The left valve is often lowermost and the oyster cannot change position when the settlement substrate is a permanently fixed rock. On the other hand, if the oyster larva settles on a loose object, like a shell or stone, then stormy waters can potentially tip it over and turn the oyster upside down. This could have happened with this fossil oyster and would account for ammonite impressions  on both the right and the left valves.

The two impressions are made in different ways. The one on the left valve shows the concave inner surface of the ammonite (a negative of the shape) with some fossilised shell remants adhering. This indicates that the shell may still have been attached when the oyster flipped over. On the right oyster valve the impression is a positive ‘cast’ made from a mould in the underlying clay that had been previously occupied by an ammonite shell – possibly even the actual one attached to the left valve. This specimen therefore provides an intriguing insight into the conditions prevailing in the seabed palaeo-environment in which the oysters lived and died.

For more information in Jessica’s Nature Blog about this subject see:

Fossil oysters from Ringstead 

More about fossil oysters from Ringstead Bay

Oyster Variations 

P1220143Blog2 Impression of an ammonite on the outer surface of the left valve shell of a Jurassic fossil oyster from Ringstead Bay, Dorset, UK - part of the Jurassic Coast (2) 

Inner surface of the left valve shell of a Jurassic fossil oyster (bearing the ammonite impression) from Ringstead Bay, Dorset, UK - part of the Jurassic Coast (3)

Close-up of the spiral shape of the ammonite impression on a fossil oyster shell from Ringstead Bay, Dorset, UK - part of the Jurassic Coast (4)

Another impression of an ammonite on the external surface of the right valve (matching the left valve with the same feature) of a fossil oyster shell from Ringstead Bay, Dorset, UK - part of the Jurassic Coast (5)

The inner surface of the right valve of a fossil oyster shell, on the outer surface of which is an ammonite impression, from Ringstead Bay, Dorset, UK - part of the Jurassic Coast (6)

Detail of the impression of an ammonite on the outer surface of the right valve of a fossil oyster (which also had an ammonite impression on the matching left valve of the pair) from Ringstead, Dorset, UK - part of the Jurassic Coast (7)

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14 Replies to “Fossil oysters with ammonite impressions”

  1. I moved to Florida from Boston in 2007 to small fishing/oyster community, Apalachicola. I discovered a pile of fossilized oyster shells, apparently dug from a mine in the middle of Florida, used to seed the existing beds here. Digging through the pile (it must me 40 feet tall and 100 long) I found what appears to be a fragment of a tusk. I of course imagine it to be a walrus’ tusk. It is scarred with grooves, is about 3 inches long. May I send you pictures? I’m only just getting interested, I’m really an Architect in my other guise. the old oysters are individually old too…some 50-70 years old if I count the hinge rings correctly.

    Thanks Jessica

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  2. Hello, George. It’s interesting to hear what you say. I am fascinated by the thought of fossil oysters being use to catch spat of modern oysters. Do you think there are published references for this, the history of the activity, the type of fossil and the source from which they were mined? It would be a good topic to investigate and write up. I would be interested in seeing pictures of the oyster shells if possible.

    I once helped to excavate and research a huge midden of oyster shells which was metres deep and maybe about a 100 metres long. It was hidden beneath the waterfront in Poole, Dorset. We had the shells radio-carbon dated and discovered they were ‘only’ about 1200 years old. They were the result of shucking oysters. Not too much further along the coast there are similar looking extensive beds of closely compacted fossils oysters embedded in clay – like the one with the ammonite impression at Ringstead Bay.

    The tusk sounds intriguing, too. I would like to see a photo of it but I am not sure I am the right person to confirm the identification for you – I’m not a palaeontologist, just an interested amateur when it comes to fossils and rocks. If you want to me to look at photographs of the tusk and the oyster shells, you can e-mail them to winderjssc@aol.com.
    Thanks for getting in touch.

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  3. Just imagine the thrill when you pull a fossil from the ground, or open a fossil shell, seeing something that no other human eye has ever seen, and that has lain hidden for millions of years.

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  4. Thanks, James

    Great picture. Where did you find these fossil oysters? They are a lot smaller species than the ones I find at Ringstead Bay and the valves are more rounded in outline with deeper cupped shape.
    Jessica

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