Ammonitina

Ammonitina is an extinct group of marine animals, commonly called Ammonites, of the order Ammonitida in the class Cephalopoda, phylum Mollusca. They are excellent index fossils, and it is often possible to link the rock layer in which they are found to specific geological time periods.

Ammonites' closest living relative is probably not the modern Nautilus (which they outwardly resemble), but rather the subclass Coleoidea (octopus, squid, and cuttlefish).

Their fossil shells usually take the form of planispirals, although there were some helically-spiraled and non-spiraled forms (known as "heteromorphs"). Their name came from their spiral shape as their fossilized shells somewhat resemble tightly-coiled rams' horns. Pliny the Elder (d. 79 A.D. near Pompeii) called fossils of these animals ammonis cornua ("horns of Ammon") because the Egyptian god Ammon (Amun) was typically depicted wearing ram's horns. Often the name of an ammonite genus ends in -ceras, which is Greek (κέρας) for "horn" (for instance, Pleuroceras).

Terminological note
The words "ammonite" and "ammonoid" are both used quite loosely in common parlance to refer to any member of subclass Ammonoidea. However, in stricter usage the term "ammonite" is reserved for members of suborder Ammonitina (or sometimes even order Ammonitida).