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AMMONITES
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 million years ago) along with the dinosaurs.
Ammonites can be composed of agate, galena, or iridescent minerals. Some ammonites occurring in the Jurassic period reach a size exceeding 23 centimetres (9 inches) in diameter. A variety called titanites from the Portland Stone of southern England are often 53 centimetres (2 feet) in diameter. One specimen dating to the Cretaceous period in Germany, which is one of the largest known ammonites, has been know to reach 2 metres (6.5 feet) in diameter. (Continued below...)
Ammonites fall into the class Cephalopoda, phylum Mollusca. They closely resemble the chambered nautilus (see pictures, at right), but their closest living relative is probably not the modern nautilus, but rather the subclass Coleoidea, which includes octopus, squid, and cuttlefish.
Ammonites' fossil shells usually take the form of planispirals. 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 for "horn" (for instance, Pleuroceras).
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 Modern chambered nautilus
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