An ammonite to catch a Pliosaur

Aulacostephanus eudoxus is the ammonite that defines the Eudoxus Zone in the Kimmeridge Clay. Ammonites evolved rapidly through time and so some species, such as this one, can be used to define periods of time and determine the relative age of the rocks in which they are found. Why is that important? Well in the case of this specimen it is really helpful in understanding the evolution of another, much bigger animal; the Pliosaur. The Osmington Pliosaur is from the base of the Cymodoce Zone which places it three ammonite zones lower, and older, than the next nearest known beasts, the Westbury Pliosaurs. So we know that the Osmington Pliosaur is older than the Westbury Pliosaur – essential if we want to understand the evolution of these apex predators. 

Find out more about ammonites on the molluscs pages.

Common name


Scientific name

Aulacostephanus cf eudoxus


Molluscs > Cephalopods


Kimmeridge Clay Formation

Time period

Upper Jurassic


152 million years

Where found


Found by



Dorset County Museum, Dorchester

Accession number