![]() The classification of ammonoids is based in part on the ornamentation and structure of the septa comprising their shells' gas chambers by these and other characteristics we can divide subclass Ammonoidea into three orders and eight known suborders. Originating from within the bactritoid nautiloids, the ammonoid cephalopods first appeared in the Late Silurian to Early Devonian (circa 400 million years ago) and became extinct at the close of the Cretaceous (65 m.y.a.) along with the dinosaurs. ![]() Often the name of an ammonite genus ends in ceras, which is Greek (κέρας) for "horn" (for instance, Pleuroceras). 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. Their spiral shape begot their name, as their fossilized shells somewhat resemble tightly-coiled rams' horns. Their fossil shells usually take the form of planispirals, although there were some helically-spiraled and non-spiraled forms (known as "heteromorphs"). Ammonites' closest living relative is probably not the modern Nautilus (which they outwardly resemble), but rather the subclass Coleoidea ( octopus, squid, and cuttlefish). 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. Artist's reconstruction of a live ammonite.Īmmonites are an extinct group of marine animals of the subclass Ammonoidea in the class Cephalopoda, phylum Mollusca.
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