‘Ammonoids are a group of extinct marine mollusc animals in the subclass Ammonoidea of the class Cephalopoda. Commonly referred to as ammonites, they are more closely related to living coleoids (i.e., octopuses, squid, and cuttlefish) than they are to shelled nautiloids such as the living Nautilus species.

Ammonites are excellent index fossils, and linking the rock layer in which a particular species or genus is found to specific geologic time periods is often possible.

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 the Egyptian god Ammon (Amun) was typically depicted wearing ram’s horns.’ [Source: Wikipedia]

40 cm diameter.

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