An extinct marine animal with a coiled shell, related to modern squid and octopuses, that lived from about 240 million to 66 million years ago.
From ammon- (from the Egyptian god Ammon, whose curved horn resembled the spiral shells) + -oid (Greek -oides, meaning 'resembling'). The coiled shells were thought to resemble the ram horns of Ammon, hence the name, which was then formalized in scientific nomenclature.
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