The group of extinct cephalopods known for coiled shells and used as index fossils is ammonites.
Ammonites belong to the extinct cephalopod subclass Ammonoidea. Most had planispiral shells, meaning the coils grew in a flat, symmetrical plane. Their shells were divided into chambers, while the living animal occupied the outermost chamber and controlled buoyancy through a tube-like siphuncle.
Ammonites are especially valuable as index fossils because they evolved rapidly, were widely distributed in marine environments, and many species existed for relatively short intervals. Finding a particular ammonite species can therefore help geologists match rock layers from distant locations to the same period.
A common mix-up is with nautiloids: modern nautiluses also have coiled chambered shells, but they belong to a different cephalopod lineage and survived to the present. Goniatites are an older, more primitive ammonoid group, while belemnites were mostly bullet-shaped relatives with internal shells. Ammonites disappeared during or shortly after the end-Cretaceous mass extinction about 66 million years ago.