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 spiral shape begot their name, as their fossilized shells somewhat resemble tightly-coiled
rams' horns.
Plinius the Elder (died 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).
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AMMONITI. SPIRULE
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