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:
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