Ammonoids are a group of extinct marine mollusc animals in the subclass Ammonoidea of the class Cephalopoda. These molluscs, commonly referred to as ammonites, are more closely related to living coleoids (i.e., octopuses, squid, & cuttlefish) than they are to shelled nautiloids such as the living Nautilus species.[1] The earliest ammonites appeared during the Devonian, & the last species vanished in the Cretaceous-Paleogene extinction event or shortly after during the Danian epoch of the Paleocene. Ammonites are excellent index fossils, & linking the rock layer in which a particular species or genus is found to specific geologic time periods is often possible. Their fossil shells usually take the form of planispirals, although some helically spiraled & nonspiraled forms (known as heteromorphs) have been found. The name ammonite, from which the scientific term is derived, was inspired by the spiral shape of their fossilized shells, which somewhat resemble tightly coiled rams’ horns. Pliny the Elder (d. 79 AD 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.[2] Often, the name of an ammonite genus ends in -ceras, which is Greek () for “horn”." Rare Ammonite Fossil 5 X 4.1 Includes Metal Stand."
