Tools of time
This is a quite rare ammonite in the Dorset lias, and these specimens are very crushed. However, if they can be identified, then they can be used to date the relative age of the rocks in which they are found. Many ammonites evolved rapidly through time, then they became buried in the rocks as the sediments settled on the sea bed. So the rock layers are a bit like the pages of a book and the ammonites are the changing page numbers. If the same ammonite is found in two different places, then the rocks that they are found in must be the same age.
Find out more about ammonites on the molluscs pages.