Ammonite

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Ammonoids are a group of ancient marine molluscs that lived in the subclass Ammonoidea of the class Cephalopoda. These cephalopods, often called ammonites, are more closely linked to living coleoids like octopuses and squid than they are to shelled nautiloids such as the Nautilus species. The earliest ammonite fossils appear during the Devonian period, and the last species disappear in the Cretaceous-Paleogene extinction event. Ammonites serve as excellent index fossils, making it often possible to link a rock layer containing a particular species or genus to specific geologic time periods. Typically, their fossil shells take the form of planispirals, although some were helically spiraled and nonspiraled in shape (referred to as heteromorphs).