ALBERT

Description: Bivalves have a wide distribution in the Lower and to a lesser extent Middle Cambrian rocks, but they have not yet been certainly identified in the Upper Cambrian. Recent discoveries have significantly increased our knowledge of Lower Ordovician bivalve faunas and their explosive radiation from the Early Ordovician apparently coincides with the evolution of the feeding gill. Early Ordovician faunas were confined to the siliciclastic facies of Gondwanan shelf seas; most genera were clearly latitudinally constrained, but others apparently migrated over wide latitudes. By the Mid-Ordovician, bivalves had begun to escape the confines of Gondwana and marked latitudinal differences in the composition of the faunas became apparent, with pteriomorphians showing clear preference for low latitudes, whilst heteroconchs clearly preferred median to high latitudes; surprisingly, nuculoids were both most diverse in terms of species and most abundant as a percentage of individuals within the bivalve populations at low latitudes. It was in the Late Ordovician that bivalves colonized the low-latitude carbonate platforms of Laurentia and Baltica, leading to a second major diversification particularly within the pteriomorphian bivalves, which developed semi-infaunal and epifaunal habits; they became the dominant low-latitude bivalve group. The latest-Ordovician eustatic regression which exposed the low-latitude carbonate platforms resulted in a major reduction in the epifaunal and semi-infaunal bivalves involving extinction of many genera, including the only Ordovician boring bivalves.