Many ammonite species evolved rapidly but only survived for about 250,000 years before evolving into something different. So rock layers can be dated based on the appearance and disappearance of different ammonites. These time periods are known as ammonite zones. This fossil, Quenstedoceras lamberti, defines the Lamberti Zone at the very top of the Corallian rock sequence.
Find out more about ammonites on the molluscs pages.