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tranthuongbn



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Join date : 2011-03-04

PostSubject: AmmoniteAmmonite   Fri Dec 02, 2011 12:22 pm

Ammonite, as a zoological or paleontological term, refers to any member of the Ammonoidea an extinct subclass within the Molluscan class Cephalopoda which are more closely related to living coleoids (i.e. octopuses, squid, and cuttlefish) than they are to shelled nautiloids such as living species of Nautilus .
Ammonites are excellent index fossils, and it is often possible to link the rock layer in which they are found to specific geological time periods. Their fossil shells usually take the form of planispirals, although there were some helically-spiraled and non-spiraled 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. Pliny the Elder (d. 79 AD. near Pompeii) called fossils of these animals ammonis cornua ("horns of Ammon") because

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