Most molluscs are marine. Molluscs are very ancient organisms believed to have evolved from a flatworm like ancestor during the Precambrium about 650 million years ago. Because many species secrete a shell of some sort the fossil record is good. Different classes of molluscs have been predominant in the past and the Ammonites represent a group of Cephalopods which were extremely abundant for millions of years before they became extinct. There close relatives the Nautiloid cephalopods were also once very successful but are now only represented in the world by one species, Nautilus.
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