Bivalve xenomorph of ammonite
In very muddy seas, oysters need a hard surface to attach to when they start life, otherwise they are simply smothered by the mud. In the Oxford Clay sea, dead ammonites lying on the sea bed provided a good starting point. As the oyster grew, it made an exact copy of the original ammonite shell in its own shell and this is known as xenomorphism. Other animals have in turn grown on the oyster, in this case tube worms – you can see them on the right-hand side and top of the photo.
Find out more about bivalves on the molluscs pages.