What are ammonites?
Ammonites are an extinct group of molluscs that flourished in the seas of the Mesozoic Era (from around 245-64 million years ago). They secreted coiled shells made of the mineral aragonite and the soft parts of the animal lived within the shell, much like a snail.
In terms of shell shape and probably also the ecology of the living animal, the closest modern analogue of ammonites is the pearly nautilus. This cephalopod mollusc inhabits the Pacific and Indian Oceans, swimming at depths of up to 500 metres and feeding on fishes and crustaceans.
Nautilus belongs to a cephalopod group - the nautiloids - more ancient than ammonites. Nautiloids co-existed in the seas with ammonites for hundreds of millions of years. However, whereas the last species of ammonites disappeared during the mass extinction event at the end of the Cretaceous Period, nautiloids survived and have persisted as rare animals to the present day.