Article: The early phylogeny of the class Bivalvia

Differences between zoologists and palaeontologists over high-level bivalve taxonomy reflect the hitherto poorly understood early phylogeny of the class Bivalvia. Recent description of a diverse early Arenig fauna and recognition that explosive radiation of early Ordovician bivalves can be correlated with evolution of the filibranch gill within the palaeotaxodonts, together with parsimonious analysis of early bivalve shell characters, has allowed the production of a modified scheme of high-level taxonomy reflecting phylogeny. Shell microstructure has proved of use in detecting phylogenetic links. The subclass Palaeotaxodonta, primitively with prismato-nacreous shells, includes the earliest bivalves and gave rise to the subclass Lipodonta. A new family of palaeotaxodonts, Cardiolariidae, is proposed to include palaeotaxodonts with a modified hinge and probably with filibranch gills; the subclass Palaeoheterodonta, also with prismato-nacreous shells, was derived from these. Radiation from the palaeoheterodonts produced the subclasses Anomalodesmata and Neotaxodonta by the early Ordovician. The Neotaxodonta probably had a shell microstructure with both crossed-lamellar and complex crossed-lamellar elements, and gave rise to the Pteriomorphia, characterized by a calcitic outer shell layer, in early Ordovician times. The shell microstructure of the Neotaxodonta suggests that the Heterodonta evolved from this group, probably by mid or late Ordovician times, but their early fossil record is very poor. A new neotaxodont, Alytodonta gibbosa gen. et sp. nov., is described from the lowermost Silurian of Girvan, Scotland. The genus Babinka is recorded for the first time from Britain, from the Arenig of South Wales, and the new genus Homilodonta is proposed for some mid and late Ordovician palaeotaxodonts from Ireland and North America.