This species is one of the zonal ammonites upon which the relative age of rocks can be determined. Ammonites evolved rapidly through time and therefore where the same ammonite is found in different locations, the rocks that they are found in must be the same age (unless the ammonite has been eroded and re-deposited). So this fossil, Rasenia, gets its name from Market Rasen in Lincolnshire but this specimen was found near Ringstead, east of Weymouth. The rocks at both places are exactly the same age, dating back about 155 million years.
The zone fossils also help to tell the relative age of other fossils, such as the large marine reptiles. This is really important because it allows us to understand how they evolved through time. If we could not tell the relative age of, say, a plesiosaur found in Lincolnshire and one found in Dorset, then we would not be able to tell which came first or how they evolved. So ammonites are ‘tools of time’.
Find out more about ammonites on the molluscs pages.