Many ammonites evolved rapidly through time and therefore certain species can be used to identify very specific periods of time which are known as zones. This specimen, Echioceras raricostatum, is the highest zonal ammonite in the Black Ven Marls. It marks the Raricostatum Zone which is also the youngest zone in the Sinemurian Stage.
So if someone talks about rocks, anywhere in the world, being within the Raricostatum Zone, they know that they are the same age as those at Charmouth.
Find out more about ammonites on the molluscs pages.