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Disambiguation: This page describes the extinct mollusc, the ammonite. For the "children of Ammon" of Biblical note, see Ammonites.)
A large ammonite, 0.6 metre (2 feet) across, outside the Portland Museum, Portland, England.
Scientific classification

Ammonites are an extinct group of marine animals (Order Ammonoidea) in the phylum Mollusca and class Cephalopoda. Their closest living relative is probably the modern nautilus, whom they resemble. Their fossil shells have the form of flat spirals (though there are some other rarer forms, called heteromorphs), and are responsible for the animals' name as they somewhat resemble a tightly coiled ram's horn (the god Ammon was commonly depicted as a man with ram's horns). Plinius the Elder (died 79 AD near Pompeii) called fossils of these animals ammonis cornua, "horn of Ammon." Often the name of ammonite species ends on ceras, latin for "horn" (e.g. Pleuroceras).

The ammonite shell contained a series of progressively larger chambers divided by thin walls called septa, with only the last and largest chamber occupied by the living animal. A thin living tube called a siphuncle passed through the septa, extending from the ammonite's body into the empty shell chambers. The ammonite secreted gas into the shell chambers, enabling it to control the buoyancy of the shell. As it grew, it added newer larger chambers to the open end of the coil.

Ammonites first appeared in the late Silurian, early Devonian period (~400 million years ago) and became extinct at the close of the Cretaceous along with the dinosaurs (65 million years ago). The classification of ammonites is based in part on the ornamentation and structure of their shells, which divide this order into eight known suborders. Here they are listed from most primitive to more advanced:

Ammonites were extremely abundant, especially in the Mesozoic seas. Due to their rapid evolution and widespread distribution, ammonites are useful for geologists and paleontologists for biostratigraphy. 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.