This ammonite, called Harpoceras, is very common in the thick, black shales near Whitby on the Yorkshire coast. Yet here it is, in a completely different rock type from the Dorset coast. As ammonites evolved quickly through time, this species only lived for a short time. Therefore we know that the black shales at Whitby and the Junction Bed (Beacon Limestone Formation) near Seatown are exactly the same age, even though the rocks look completely different. The second photo is of the same ammonite but from Yorkshire.
Find out more about ammonites on the molluscs pages.