This ammonite is an example of Harpocers from the Junction Bed near Seatown. Below is another Harpoceras from the Yorkshire coast near Whitby. As ammonites evolved rapidly, each species only existed for a relatively brief period of time. Therefore the Junction Bed on the Dorset coast and the dark shales near Whitby must be the same age – even though they are completely different types of rock that formed in different environments 180 million years ago. This is what enables geologists to make geological maps of the same layers or strata of rock. Look at the geological map of England and you will see that the rocks on the Dorset coast run through the country and come out on the Yorkshire coast.
Find out more about ammonites on the molluscs pages.