Bivalve xenomorph of ammonite
An ammonite in an oyster? Impossible! Well, actually not so. 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. The copy is so good that we can confidently identify the ammonite as Cardioceras – see below for an image.
Find out more about bivalves on the molluscs pages.