Giant Fossil Soft-Shelled Turtles of North America

Axestemys byssina—a large, potentially paedomorphic soft-shelled turtle (Trionychidae)—and a number of other of giant trionychids have been described, but no rigorous analysis has been undertaken to investigate their relationships. Because these trionychids are so unusual and occupy a broad Cretaceous-Eocene temporal range (70-46 myr), their study provides new insights into broad patterns of trionychid evolution. This project sets out to use all material previously assigned to Axestemys as well as previously undescribed material with traditional “Axestemys”-like characteristics to develop a concept of Axestemys that is meaningful in both a systematic and taxonomic context. Axestemys cerevisia sp. nov. and A. montinsana sp. nov. are established as new species, A. byssina and A. splendida comb. nov. are revised as a result of the description of new material, and A. quinni is reviewed and confirmed to be a member of Axestemys. Eugenichelys robertemryi is synonymized with Axestemys byssina. Phylogenetic analysis does not support the inclusion of “Trionyx” puercensis and Conchochelys admiribalis, previously hypothesized to belong to Axestemys, within the clade Axestemys. The taxon name ‘Axestemys’ is best defined phylogenetically as a stem-based clade rather than a clade based on many of the characters traditionally ascribed to it, which are not consistently present throughout the clade. All members of Axestemys reach a gigantic size that is today found only in a few tropical trionychid species which independently evolved gigantism. The presence of Axestemys in North America indicates both a warmer climate and a higher trionychid diversity and disparity comparable to those of Recent species confined to tropical climates today.