a fossil shark Triodus sessilis. This mistake was corrected and identified as Xenacanthus by Mr. Schnur. Classification The teeth of Orthacanthus texensis and Orthacanthus platypternus from bonebeds from the Lower Permian of Texas, and the teeth of Orthacanthus compressus from the Upper Pennsylvanian of Nebraska and Dunkard Basin of the central Appalachians were used to determine the origin of O. texensis and O. platypternus. It has been proposed that both O. texensis and O. platypternus could be derived from O. compressus, where juvenile features of O. compressus are retained in the adult teeth of O. platypternus via paedomorphosis, and the juvenile features of O. compressus teeth are observed in the adult teeth of O. texensis. Taxon Mimia sits outside of a clade that contains two monophyletic sister groups. The first monophyletic sister group defines a clade that includes the stem-group chondrichthyans which is visualized as a sister group of Doliodus and a large clade that comprises the cladodont sharks and Orthacanthus. The “cladodont sharks” plus Orthacanthus comprise two monophyletic sister groups: Orthacanthus and the Cladodoides and Tamiobatis on one side, and the Symmoriiformes on the other side. The second monophyletic sister group characterizes crown chondrichthyans which contains two monophyletic sister groups (Euchondrocephali on one side and Euselachii on the other side). Cladistic Analysis The following cladogram follows a 2011 analysis by Alan Pradel and colleagues. Paleobiology A 2013 analysis of oxygen and strontium isotope composition of the teeth and spines of Late Carboniferous and Early Permian shark taxa was performed to infer the hydrochemistry of their ambient water, thus contributing to the controversy between an obligate freshwater or euryhaline diadromous lifestyle. Facies interpretations in the Permian of North America suggested that salinity tolerance of xenacanthids was restricted to near marine environments while only Orthacanthus can tolerate brackish water environments. A study covering the morphology and histology of dorsal spines of Orthacanthus platypternus also reported that "The comparative analyses of the ontogenetic stages of the recorded specimens of O. platypternus and their distribution along different facies and localities indicate that this species was euryhaline, diadromous with a catadromous life-cycle which was strongly regulated by the semi-arid, seasonally dry tropical climate affecting western Pangaea during the Early Permian." The 2013 analysis provided evidence leaning towards an obligate freshwater lifestyle of the sharks from the studied Variscan European basins, and nonmarine ratios suggested teeth formation was influenced by meteoric waves enriched by evaporation.