Bivalves are a major component of present day aquatic faunas, especially in the marine realm, and they have a rich fossil record in the later Palaeozoic, Mesozoic and Cenozoic. However, in Lower Palaeozoic their record is far more scant, especially in rocks of Ordovician age, where keys to the inter-relationships of the major bivalve groups may be expected to be found. The discovery of a particularly rich and diverse bivalve fauna in early Ordovician (Arenig) strata in Carmarthenshire has proved critical in revealing the early history of several lineages. Extension of this work into Ordovician rocks in Asia (Turkey and Iran), and Australia is providing extensive new data which are complementing the results obtained from the faunas of South Wales. Together, this is revolutionizing our understanding of the early evolution of the bivalves, and is beginning to resolve questions of higher taxonomic levels within the group.