Bivalve xenomorph of ammonite
An ammonite on an oyster?
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 in its own shell – known as xenomorphism.
Find out more about bivalves on the molluscs pages.