Ammonoids are extinct, typically coiled-shelled cephalopods composing the subclass Ammonoidea. They are more closely related to living octopuses, squid, and cuttlefish (which compose the clade Coleoidea) than they are to nautiluses (family Nautilidae), which they resemble. The earliest ammonoids appeared during the Emsian stage of the Early Devonian, around 410-408 million years ago, with the last species vanishing during or soon after the Cretaceous–Paleogene extinction event approximately 66 million years ago. They are often called ammonites, which is most frequently used for members of the order Ammonitida, the only remaining group of ammonoids from the Jurassic up until their extinction.

Ammonoids exhibited considerable diversity over their evolutionary history, with over 10,000 species having been described. Ammonoids are excellent index fossils, and they have been frequently used to link rock layers in which a particular species or genus is found to specific geologic time periods. Their fossil shells usually take the form of planispirals, although some helically spiraled and nonspiraled forms (known as heteromorphs) have been found, primarily during the Cretaceous period. Because ammonites and their close relatives are extinct, little is known about their way of life. Their soft body parts are very rarely preserved in any detail. Nonetheless, much has been worked out by examining ammonoid shells and by using models of these shells in water tanks.

...