Oyster copy cat

Ammonites in the Kimmeridge Clay shale are nearly always completely crushed. However, they can still be recognised as different species and are therefore important for determining the relative age of the rocks. This specimen also has oysters growing over it; they need a hard shell on which to establish themselves. Look really carefully and you can see that the oyster is mimicking the decoration of the ammonite shell almost perfectly. This is a feature known as xenomorphism and there are several more spectacular examples in our collections.

Find out more about ammonites on the molluscs pages.

Common name


Scientific name

Subdichotomoceras websteri


Molluscs > Cephalopods


Kimmeridge Clay Formation

Time period

Upper Jurassic


152 million years

Where found

Brandy Bay

Found by

C P Van der Veyer


Dorset County Museum, Dorchester

Accession number