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Brief Description: Molluscs appear in the fossils archive about 545 million years ago, namely in the Cambrian Period, but it has not been discovered yet how they occurred and which was their early development. Until the Late Cambrian (about 520-505 million years ago), most of the contemporary groups of molluscs can be located in some primitive fossilized form in the sea deposits. During the Ordovician (about 505-438 million years ago), an important development was recorded, with thousands of species of molluscs appearing in the fossils archive of that period. During the Mesozoic (about 245-65 million years ago) the ammonites flourished, a relative species of the contemporary nautilus, and they constituted an important part of the fossils archive, but became extinct at the end of the Cretaceous period at the same time as the dinosaurs, as well as several other types of molluscs and snails. The extinction of these maritime animals freed locations and habitations that were occupied by a new evolution of all types of animals in the beginning of the Cainozoic century. The Cainozoic century (about 65 million years ago) is distinguished, among other things, by the evolution of contemporary species of molluscs. During the last million years, the terrestrial and the freshwater molluscs evolved fast, occupying the terrestrial habitations to such an extent that has never been recorded in their fossils archive. 
Bibliography: Γεωργιάδου-Δικαιούλια, Ε., Συμεωνίδης, Ν., Θεοδώρου, Γ. (2005) Παλαιο-ντολογία (μέρος Γ?), Εκδόσεις Γ. Γκελμπέση, Αθήνα.