This is the second in a series of Postings about fossil oysters. In this Post there are detailed photographs, taken from all angles, of a fossil oyster found with the two halves still in position, on the beach at Ringstead Bay, Dorset, UK. It’s a specimen of Deltoideum delta, and in common with many species of present day oysters such as Ostrea edulis, the shells of this extinct species can exhibit a wide variety of variations – in shape, size, and many other characteristics. The most noticeable feature of this particular specimen is the attachment of six other oyster shells – four to the outer surface of the left valve and two to the exterior of the right valve.
The left or lower valve would have been the original surface of attachment for the spat oyster as it settled after a brief larval life. Old oyster shells are a preferred settlement substrate. It is not possible to say whether the oyster shells on which it settled were empty or still occupied by the living animals. Eventually the oyster would have overgrown and incorporated the other oyster shells as it developed.
On the other hand, the oyster shells attached to the outer surface of the right or uppermost valve would have settled on it only when the oyster had itself reached maturity.
Additionally, the ligament – which is the horny proteinaceous (conchyiolin) structure that connects the two halves of the oyster at the dorsal margins of the shells in life – seems to have been fossilised along with the calcareous shell valves. The shells have a brittle texture and an almost porcelain-like appearance. There are multiple cracks and fractures, as is common with the oyster fossils from this locality, and a certain flattening of the valves seems to have occurred so that there is now very little space indeed between the the two halves of the oyster.
[At the bottom of this posting is a slide show of the oyster shells photographed with a reference scale to indicate size. These pictures have a minimum of labelling. If anyone should want more information about these fossil oysters, please do not hesitate to get in touch.]
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