This ammonite is from the Junction Bed which is known as a ‘condensed sequence’ because the sediments formed slowly and at times were even eroded away. As a result, the fossils tend to be quite poorly-preserved but they remain identifiable. This ammonite is Hildoceras bifrons, the classic ammonite known from Whitby, and named after St Hilda.
The other photo is the same ammonite from Whitby but it is in a completely different type of rock. That is because during the Upper Lias, the sea in what is now Dorset was shallow and very little sediment was deposited, whereas the sea in Yorkshire was deep and a huge column of dark clay was deposited. We know that ammonites evolved slowly through time so this ammonite tells us that these two very different rocks formed at the same time but under different conditions.
Find out more about ammonites on the molluscs pages.