What’s a nodule?
This ammonite, Asteroceras stellare, lies within a massive Stellare nodule. The rock is known as a septarian nodule, that is a hard, compact mass of sedimentary rock with angular cracks running through it. They are also called concretions. The nodule formed around the ammonite, preserving it from being crushed as more layers of sediment settled. But at a later stage, it shrank causing the radial cracks to form and shattering the ammonite in the process. Later still, calcium-rich waters filled the cracks and calcite crystal grew in them.
Find out more about ammonites on the molluscs pages.