Morphological and molecular evidence supports the position of the Patellogastropoda as the most basal gastropod clade. Although the clade contains members that are mostly intertidal, it also has representatives that live in deeper water habitats, including hot vents and cold seeps. Different workers have argued different patterns for the distribution of taxa in the deep sea. Some have argued that presence of taxa at vents and seeps is the result of immigration and colonization from shallow water habitats. Others have argued that deep taxa are "relicts" and thus potentially basal groups that have not accumulated autapomorphies. Although either model may be correct, neither has been tested by examining patterns of cladogenesis. Instead, workers have tested the model by studying fossil occurrences of taxonomically similar but possibly unrelated groups or by weighting character sets that suggest relictual status in modern deep sea taxa.

I have gathered a dataset of morphological and molecular characters and taxa in order to assess the phylogeny (i.e., patterns of cladogenesis) of the patellogastropods. The analysis includes seventeen ingroup taxa (five of which are deep sea) and four outgroup taxa. Eighty-two morphological characters and sequence data from the mitochondrial gene 16S are used for optimization. All morphological characters have been coded from primary data, representing histological information, shell microstructure data, dissection, and observations of external anatomy. Distributional depth data, as a continuous character, was mapped onto the phylogeny. The phylogenetic hypothesis I generate does not support an onshore-offshore model or the relictual model as they now stand. Instead, the pattern of cladogenesis suggests that taxa in one major clade in the Patellogastropods have migrated from the offshore to the onshore while the other clade is exclusively intertidal. The state at the Patellogastropod node remains ambiguous. Stratigraphic distribution of the patellogastropod lineages indicates that anoxic events may be correlated with recolonization of onshore habitats during the Cretaceous.

75/125 YEARS