Ammonites varied hugely in their shape, form, ribbing and decoration. This specimen is what is known as a ‘cadicone’ form in which the outer whorls almost completely enclosed the inner whorls as the animal grew. The last chamber wall can be seen on the right-hand side of the specimen, while the ‘top’ has been broken away to show the body chamber, filled with sediment. The whole ammonite is enclosed in a septarian nodule which is a hard limestone that cracked apart after formation. The cracks were then filled with calcite crystal and they now form the radial walls or ‘septa’ in the rock surrounding the specimen.
Find out more about ammonites on the molluscs pages.