What is an ammonite from the Yorkshire coast near Whitby doing on our fossil finder? Compare it with the ammonite from Eype; the same ammonite in a different rock found 300 miles apart.
Ammonites evolved through time and some individual species only lived for short periods of geological time. If the same ammonite is found in rocks from different places, then the rocks are the same age (unless the ammonite has been eroded and re-deposited), and that even applies if the rocks look completely different, as they do in this case. The Upper Lias Junction Bed, now called the Beacon Limestone Formation, is exactly the same age as the dark clay rocks found on the Yorkshire coast near Whitby, because they contain the same ammonite species. At that time, in what we now call ‘Dorset’ the Jurassic sea was shallow and pale limestone sediments were deposited but to the north the sea was much deeper and a thick sequence of black shale was deposited.
Find out more about ammonites on the molluscs pages.