Gastropod phylogeny, biogeography and shell shape evolution

Abstract

Gastropods are the most speciose invertebrates in modern oceans, with impressive morphological disparity and fossil diversity. These snails, limpets and slugs represent an old animal radiation, with more than 450 million years. This dissertation combines different sources of data into an integrative research program on gastropod evolution. Despite numerous previous studies, ancient divergences in the gastropod tree remain contentious and largely unresolved. In Chapter 1, I resolve deep relationships between the main gastropod lineages by expanding taxon sampling with new transcriptomes, and by using several inference methods to account for sources of error in phylogenomic datasets. I find one congruent and well supported topology, in which vetigastropods and patellogastropods are sister taxa, and neritimorphs are the sister group to apogastropods. In Chapter 2, I use the same strategy to target old divergences between superfamilies and families of Vetigastropoda (abalones, keyhole limpets, turban snails). With the first comprehensive phylogenomic tree for the group, I discuss how previous studies seem to have been greatly affected by methodological artifacts. In Chapter 3, I study the evolution of shell morphospace with hundreds of vetigastropod fossils from the Mesozoic and early Cenozoic eras. Intensification of predation at this time is hypothesized as a driving force for the evolution of putatively better defended, low-spired shells, but long-term quantitative data is scarce. I find that the most highly spired shells disappear by the Cretaceous, while shorter shells become more common. I further observe a large morphospace expansion in the Paleogene occupied by keyhole limpets with wider openings and flat shells. Trends by family indicate that such changes were not driven by a turnover of families with different shapes, but by the independent evolution of shell shape in different families. Chapter 4 focuses on the family Fissurellidae, leading to changes in classification for these globally distributed keyhole limpets. I reconstruct a calibrated phylogeny, estimating that crown fissurellids originated in the Jurassic around 175 mya. With a biogeographical reconstruction, I then infer an Indo-West Pacific origin for the family, with a westward colonization of new oceans via the Tethys Seaway upon the breakup of the supercontinent Pangea.