How ammonites tell the time
The display of ammonites in Beaminster Museum was collected by Bob Chandler, who has been studying the ammonite zonation of the Inferior Oolite for some 30 years and still has not fully figured it out!
Horn Park Quarry is famous for the profusion of ammonites that have been found there over the years. If you imagine that the rock layers are the pages of a book then the ammonites are like the changing page numbers because they evolved and changed, through time. If the same ammonite is found in two different places, then the rocks must be the same age (unless the ammonite has been eroded and re-deposited later).
Researchers like Bob have spent years painstakingly recording where in the rock sequence each type of ammonite occurs. They use the best ones, the ones that lived for the shortest periods of time and are common to different areas, to define that period of time (or zone). In that way geologists can determine the relative age of rocks, even if they are thousands of miles apart.
Very roughly speaking, ammonite zones identify periods of time lasting about 250,000 years. Subzones can define considerably less time, or offer finer resolution when trying to figure out the history of time as it is recorded in the rock layers.