Synthesis of molecular data on Mactroidea (Bivalvia): phylogenetic implications and open questions

Molecular phylogenetics have clarified high-level evolutionary relationships across the tree of life, but sampling is highly uneven at lower levels, particularly for marine organisms. Here, we synthesize all publicly available Sanger sequencing data for the diverse yet under-sampled marine bivalve superfamily Mactroidea to provide a unified molecular and paleontological framework, and to guide targeted sampling for a more complete understanding of the group's evolutionary history. We infer two complementary phylogenetic estimates, one using a comprehensive eight-gene alignment that maximizes taxon sampling, and a second high-coverage three-gene alignment that maximizes molecular coverage. Both phylogenies are largely concordant with morphology-based classifications at the genus and subgenus levels, but reveal that the recognized mactroid families with sequence data, as currently described, and the subfamilies of Mactridae are not reciprocally monophyletic. Among the genera, evidence suggests that Spisulona and Taria, often synonymized to various extant taxa, may be valid genera or subgenera, although further molecular data are needed to confirm these taxonomic implications. Budding-informed time calibration using 18 fossil constraints reveals several conflicts between fossil ages and the inferred molecular topology, likely owing to incomplete fossil and molecular sampling and ambiguity of certain fossil assignments. By tabulating conflicting or missing data, we identify priorities for future molecular sampling, and show that gaps in the molecular data are unevenly distributed geographically, with concentrations in the tropical East Pacific and tropical West Pacific. Combined phylogenetic, biogeographic, and paleontological inventories, supported by voucher-based verification of specimens, can guide targeted sampling to efficiently improve understanding of within-clade relationships and evolutionary dynamics in Mactroidea.