Clarifying gastropod phylogenies: Exploring deep relationships among vetigastropoda and related taxa

This dissertation explores deep evolutionary splits and major evolutionary patterns among the molluscan class Gastropoda, especially those once placed in the group Archaeogastropoda---Patellogastropoda, Vetigastropoda, Neritimorpha, and Neomphalina---through phylogenetic analyses using morphological and molecular data. Gastropods are molluscs with a single shell and an operculum, at least in the larval stage, that have undergone torsion through ontogeny. They have radiated enormously in comparison to other molluscan classes and exhibit great variety in external form, anatomy, behavior, and physiology. Early speciation events and high amounts of character homoplasy have made it difficult to obtain a robust phylogenetic hypothesis for gastropod evolution. Relationships among major gastropod clades, therefore, differ significantly among analyses based on morphological and molecular data sets. As a result, this dissertation increases data collection and sampling amongst gastropod taxa in order to further test relationships among main clades such as Patellogastropoda, Vetigastropoda, Neritimorpha, Neomphalina, and Apogastropoda. Chapter One, the first phylogenetic analysis of Gastropoda incorporating both morphological and molecular data, reveals a monophyletic Gastropoda composed of two main clades: ((Vetigastropoda + Patellogastropoda) + (Cocculinida + Neomphalina)) and (Neritimorpha + Apogastropoda). This study demonstrates that neither morphology nor molecules alone resolved the deepest gastropod nodes. Molecular phylogenetic analyses of Vetigastropoda and other 'archaeogastropod' clades in Chapters Two and Three recover Neomphalina, Cocculinoidea, and Pleurotomariidae separate from Vetigastropoda sensu stricto, indicating a need to recognize these clades as independent of vetigastropods. Molecular clock dating further suggests that modern gastropod higher taxa may have originated in the Mesozoic and that Paleozoic fauna actually represent stem group gastropods. Chapter Four, the first molecular phylogeny of Fissurelloidea, reveals a monophyletic Diodorinae and Fissurellinae, but also indicates a polyphyletic or paraphyletic Emarginulinae recovered as two main groups. These results contradict traditional fissurellid classifications as well as theories about the evolution of shell characters by suggesting that the presence of an anterior foramen or notch is plesiomorphic, and the multiple evolution of an anterior notch or slit in Fissurellidae. Overall, this dissertation advances alternative theories regarding the evolution of Gastropoda as well as the origin of modern gastropod clades.