Towards a molecular phylogeny of Mollusks: bivalves' early evolution as revealed by mitochondrial genes.

Despite huge fossil, morphological and molecular data, bivalves' early evolutionary history is still a matter of debate: recently, established phylogeny has been mostly challenged by DNA studies, and little agreement has been reached in literature, because of a substantial lack of widely-accepted methodological approaches to retrieve and analyze bivalves' molecular data. Here we present a molecular phylogeny of the class based on four mitochondrial genes (12s, 16s, cox1, cytb) and a methodological pipeline that proved to be useful to obtain robust results. Actually, best-performing taxon sampling and alignment strategies were tested, and several data partitioning and molecular evolution models were analyzed, thus demonstrating the utility of Bayesian inference and the importance of molding and implementing non-trivial evolutionary models. Therefore, our analysis allowed to target many taxonomic questions of Bivalvia, and to obtain a complete time calibration of the tree depicting bivalves' earlier natural history main events, which mostly dated in the late Cambrian.