A review on deep molluscan phylogeny: old markers, integrative approaches, persistent problems

Molluscs are diverse and important enough to receive considerable scientific attention. We herein question recent opinions that the molluscan origin, deep inner relationships, and early evolution have been largely resolved. The traditional Testaria concept implied progressive evolution from worm-like aplacophoran to polyplacophoran and then conchiferan body plans; sole evidence for this came from morphocladistic analyses, which we conclude were confounded by homoplasy. The recently preferred Aculifera-Conchifera concept is supported by some but not all analyses using massive sequence data on rather small and uneven taxon sets. Recent results from mitogenomics indicate that gene-rearrangement events could cause sequence biases overriding potential phylogenetic signal. We discuss recent progress regarding multilocus marker analyses, particularly refining the neglected Serialia hypothesis with Monoplacophora sister to Polyplacophora. This third hypothesis for molluscan relationships is supported by an integrative interpretation and is roughly compatible with available fossil evidence if the first molluscs were small and had a true shell rather than a chiton-like body organization. Despite some claims of consensus, there are substantial discrepancies among recent molecular studies regarding class-level topologies. These may be symptomatic of a plethora of factors and evolutionary processes – obvious or more hypothetical – that might hinder successful reconstruction of early molluscan diversification.