For: Amatol

Ammonites are an extinct group of marine animals of the subclass Ammonoidea 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).

Classification

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. 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. While nearly all nautiloids show gently curving sutures, the ammonoid suture line (the intersection of the septum with the outer shell) was folded, forming saddles (or peaks) and lobes (or valleys).

Suture patterns

Three major types of suture patterns in Ammonoidea have been noted:

• Goniatitic - numerous undivided lobes and saddles; typically 8 lobes around the conch. This pattern is characteristic of the Paleozoic ammonoids.
• Ceratitic - lobes have subdivided tips, giving them a saw-toothed appearance, and rounded undivided saddles. This suture pattern is characteristic of Triassic ammonoids and appears again in the Cretaceous "pseudoceratites".
• Ammonitic - lobes and saddles are much subdivided (fluted); subdivisions are usually rounded instead of saw-toothed. Ammonoids of this type are the most important species from a biostratigraphical point of view. This suture type is characteristic of Jurassic and Cretaceous ammonoids but extends back all the way to the Permian.

Orders and suborders

• Goniatitida (Devonian to Permian) have round saddles, pointed lobes
• Anarcestina (Devonian only)
• Clymeniina (upper Upper Devonian only)
• Goniatitina (Devonian to Upper Permian) includes the true goniatites
• Ceratitida (Carboniferous to Triassic) have round saddles, serrated lobes
• Prolecanitina (Upper Devonian to Upper Triassic)
• Ceratitina (Permian to Triassic) includes the true ceratites
• Ammonitida (Permian to Cretaceous) have folded saddles and lobes, fractal patterns
• Phylloceratina (Lower Triassic to Upper Cretaceous)
• Ammonitina (Lower Jurassic to Upper Cretaceous) includes the true ammonites
• Lytoceratina (Lower Jurassic to Upper Cretaceous)
• Ancyloceratina (Upper Jurassic to Upper Cretaceous) the heteromorph ammonites