Often, the name of an ammonite genus ends in - ceras, which is from κέρας ( kéras) meaning "horn".Īmmonites (subclass Ammonoidea) can be distinguished by their septa, the dividing walls that separate the chambers in the phragmocone, by the nature of their sutures where the septa join the outer shell wall, and in general by their siphuncles.Īmmonoid septa characteristically have bulges and indentations and are to varying degrees convex when seen from the front, distinguishing them from nautiloid septa, which are typically simple concave, dish-shaped structures. 79 AD near Pompeii) called fossils of these animals ammonis cornua (" horns of Ammon") because the Egyptian god Ammon ( Amun) was typically depicted wearing rams' horns. 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. Their fossil shells usually take the form of planispirals, although some helically spiraled and nonspiraled forms (known as heteromorphs) have been found.
The earliest ammonites appeared during the Devonian, with the last species vanishing during the Cretaceous–Paleogene extinction event.Īmmonites are excellent index fossils, and linking the rock layer in which a particular species or genus is found to specific geologic time periods is often possible.
These molluscs, commonly referred to as ammonites, are more closely related to living coleoids (i.e., octopuses, squid and cuttlefish) than they are to shelled nautiloids such as the living Nautilus species. Ammonoids are a group of extinct marine mollusc animals in the subclass Ammonoidea of the class Cephalopoda.
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