Are an extinct group of marine mollusc animals in the subclass Ammonoidea of the class Cephalopoda. These molluscs 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. The earliest ammonites appear during the Devonian, and the last species died out during the Cretaceous–Paleogene extinction event.
Ammonites are excellent index fossils, and it is often possible to link the rock layer in which a particular species or genus is found to specific geologic time periods. Their fossilshells usually take the form of planispirals, although there were some helically spiraled and nonspiraled forms (known as heteromorphs).
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.
Goniatite or Goniatids
Informally Goniatites, are ammonoid cephalopods that form the Order Goniatiida, derived from the more primitive Anarcestida during the Middle Devonian some 390 million years ago. Goniatites (goniatitida) survived the Late Devonian extinction to flourish during the Carboniferous and Permian only to become extinct at the end of the Permian some 139 million years later.