Giant fossil soft-shelled turtles of North America

Abstract

Axestemys byssina, a large, potentially paedomorphic soft-shelled turtle (Trionychidae), and a number of other of giant trionychids were described over a century ago, but no rigorous analysis has been undertaken to investigate their relationships. These trionychids are unusual and are known from a broad temporal range spanning from the Cretaceous to the Eocene (70-46 m.y.a.). I examined all material previously assigned to Axestemys as well as previously undescribed material with traditional "Axestemys"- like characteristics and provided a concept of Axestemys that is meaningful in both systematic and taxonomic contexts. Two new species are established. Axestemys byssina and Axestemys splendida comb. nov. are revised as a result of the description of new material, and Axestemys quinni is confirmed to be a member of Axestemys. Eugenichelys robertemryi Chkhikvadze, 2008 is synonymized with Axestemys byssina. Phylogenetic analysis and description of new material does not support the inclusion within the clade of "Axestemys" puercensis and Conchochelys admiribalis, previously hypothesized to belong to Axestemys. The taxon name 'Axestemys' is best defined phylogenetically as a stem-based clade rather than defined based on many of the characters traditionally ascribed to it, which are not consistently present throughout all species of Axestemys. All members of Axestemys reach a gigantic size that is today found only in a few tropical species of trionychid that independently evolved gigantism. The presence of Axestemys reflects both a warmer climate in the past in North America. Additionally, it reflects a high trionychid diversity and disparity comparable to those of Recent species confined to tropical climates today. © Society of Vertebrate Paleontology March 2012.