This is a deformed example of the ammonite, Asteroceras. Look closely at the ribs and you’ll see that they are not all arranged in the same way. These deformities are common and might be caused by damage to the shell, perhaps from a predator or from animals like tubeworms or oysters trying to attach themselves. In doing so, they damage the way in which the ammonite shell grows, resulting in the deformity.
Find out more about ammonites on the molluscs pages.