Tools of time
This ammonite, Asteroceras obtusum, comes from the Obtusum ammonite zone within the Lower Jurassic Black Ven Marls (now known as the Charmouth Mudstone Formation). Because ammonites evolved rapidly through time, the rocks that contain a particular species must be the same age (unless the fossil has been eroded and re-deposited in a younger rock). Asteroceras obtusum is found at Charmouth and also in Robin Hood’s Bay on the Yorkshire coast. Therefore geologists know that the rocks in both places are exactly the same age, or rather, they were deposited at exactly the same time.
Find out more about ammonites as tools of time on the molluscs pages.