Mollusks are the most morphologically disparate living animal phylum, they have diversified into all habitats, and have a deep fossil record. Monophyly and identity of their eight living classes is undisputed, but relationships between these groups and patterns of their early radiation have remained elusive. Arguments about traditional morphological phylogeny focus on a small number of topological concepts but often without regard to proximity of the individual classes. In contrast, molecular studies have proposed a number of radically different, inherently contradictory, and controversial sister relationships. Here, we assembled a dataset of 42 unique published trees describing molluscan interrelationships. We used these data to ask several questions about the state of resolution of molluscan phylogeny compared to a null model of the variation possible in random trees constructed from a monophyletic assemblage of eight terminals. Although 27 different unique trees have been proposed from morphological inference, the majority of these are not statistically different from each other. Within the available molecular topologies, only four studies to date have included the deep-sea class Monoplacophora; but 36.4% of all trees are not significantly different. We also present supertrees derived from 2 data partitions and 3 methods, including all available molecular molluscan phylogenies, which will form the basis for future hypothesis testing. The supertrees presented here were not constructed to provide yet another hypothesis of molluscan relationships, but rather to algorithmically evaluate the relationships present in the disparate published topologies. Based on the totality of available evidence, certain patterns of relatedness among constituent taxa become clear. The internodal distance is consistently short between a few taxon pairs, particularly supporting the relatedness of Monoplacophora and the chitons, Polyplacophora. Other taxon pairs are rarely or never found in close proximity, such as the vermiform Caudofoveata and Bivalvia. Our results have specific utility for guiding constructive research planning in order to better test relationships in Mollusca as well as other problematic groups. Taxa with consistently proximate relationships should be the focus of a combined approach in a concerted assessment of potential genetic and anatomical homology, while unequivocally distant taxa will make the most constructive choices for exemplar selection in higher-level phylogenomic analyses.

Supplementary Table 1Tree file containing all trees used in analyses.SL_SuppTable_s1_treefile.nexSupplementary Figure 1Percentage of trees wherein a given taxon pair are separated by two nodes or fewer (e.g. sister-taxa are separated by 1 node). Zero-values indicate that a given pair are always more than 2 nodes apart, in our dataset of 42 trees including morphological (blue, n=27) and molecular (green, n=15) datasets.suppl-figure1_nodedistances.epsSupplementary Figure 2Multidimensional scaling plot of 5000 randomly-generated trees including 8 molluscan classes, and our dataset of 42 trees from the literature (morphological, blue circles; molecular, red circles). Trees plotted outside of the randomly generated trees represent source trees with fewer than 8 classes in their topology.suppl-figure2_TreViz_SourceTrees.epsSupplementary Tables 2-4TABLE S2. Tree notation, scores, permutation tail probability tests, source tree fit, and related data for each of the molecular supertrees analyses. TABLE S3. Tree notation, scores, permutation tail probability tests, source tree fit, and related data for each of the morphological supertrees analyses. TABLE S4. Symmetric distances between supertrees. Partition: Morph = morphology, Molec = molecular.SL_SuppTables_S2-S4.docx