This is a ‘cadicone’ form of ammonite, where the outer whorls almost completely enclose the inner whorls as the shell grows. The result is an ammonite that is almost completely round, like a football. Because each whorl is quite thin, the body chamber is much longer than in more typical forms. Normally the body chamber takes up about the last half of the outer whorl But in this example, the body chamber, which is filled with sediment, takes up the whole of the outer whorl. The hollow, chambered shell has been completely dissolved away and replaced with calcite crystal.
Find out more about ammonites on the molluscs pages.