Gastropoda
The first gastropods were exclusively marine. The first of the group appeared in the Upper Cambrian (Chippewaella, Strepsodiscus). By the Ordovician period the gastropods were a varied group present in a few aquatic habitats. Commonly, fossil gastropods from the rocks of the early Palaeozoic era are too poorly preserved for accurate identification. Still, the Silurian genus Poleumita contains fifteen identified species. Fossil gastropods are less common during the Palaeozoic era than bivalves.