Deep gastropod relationships resolved

Gastropod mollusks are arguably the most diverse and abundant animals in the oceans, and are successful colonizers of terrestrial and freshwater environments. Here we resolve deep relationships between the five major gastropod lineages - Caenogastropoda, Heterobranchia, Neritimorpha, Patellogastropoda and Vetigastropoda - with highly congruent and supported phylogenomic analyses. We expand taxon sampling for underrepresented lineages with new transcriptomes, and conduct analyses accounting for the most pervasive sources of systematic errors in large datasets, namely compositional heterogeneity, site heterogeneity, heterotachy, variation in evolutionary rates among genes, matrix completeness and gene tree conflict. We find that vetigastropods and patellogastropods are sister taxa, and that neritimorphs are the sister group to caenogastropods and heterobranchs. With this topology, we reject the traditional Archaeogastropoda, which united neritimorphs, vetigastropods and patellogastropods, and is still used in the organization of collections of many natural history museums. Several traits related to development and life history support our molecular results. Importantly, the time of differentiation of the embryonic 4d cell (mesentoblast, responsible for mesoderm formation), differs between the two major clades, highlighting the degree of conservation and significance of development in the evolution of gastropods, as it is also known for spiralian animals more broadly.