Abstract: The Ordovician was the most significant Period in bivalve diversification. From a small Cambrianstock of palaeotaxodonts, the most fundamental radiation occurred in the early Ordovician. An intrinsic factor was the most significant, involving the evolution of the feeding gill within the palaeotaxodonts. This fundamental morphological change allowed development of more effective feeding strategies, permitting bivalves to take advantage of the increased Ordovician nutrient supply. This allowed rapid size increase and diversification, and colonization of a variety of habitats from entirely infaunal into semi-infaunal and epifaunal. By the end of the early Ordovician, seventeen families belonging to six subclasses have hitherto been recognised. Early Ordovician bivalves are restricted to peri-Gondwanan shelves and favoured inshore clastic sedimentary environments; it was not until the mid Ordovician that bivalves were able to migrate to other continents, but initially remained largely confined to nearshore clastic facies. The succeeding late Ordovician radiation was largely driven by extrinsic factors, as the development of low-latitude carbonate platforms promoted the evolution of many epifaunal groups, particularly the modiomorphoids and the pteriomorphians — the latter often characterized by a calcitic outer shell layer.