XENACANTHIMORPH (CHONDRICHTHYES) PHYLOGENY: A COMPARATIVE CLADISTICS APPROACH

Xenacanthimorphs were a diverse group of fossil sharks recorded from the Carboniferous-Triassic. We apply comparative cladistics approaches to assess morphological character exhaustion, taxonomic and character overlap between analyses, and create a new data set combining all prior analyses to holistically assess xenacanthimorph in-group relationships. The relationships within Xenacanthimorpha in prior studies (Bransonella and diplodoselachids serially closer to Pennsylvanian or younger xenacanthiforms) are historically stable, but prior studies tend to draw character support from a single anatomical region in each analysis.

Therefore, we built a new composite matrix. Wherever possible, characters were combined together or modified following best practices in character construction (nearly 300 characters – 34% dental, 26% neurocranial, 38% postcranial, and 2% splanchnocranial – were collapsed into 100 characters – 26% dental, 35% neurocranial, 35% postcranial, and 3% splanchnocranial). Descriptions and illustrations of character states from the literature are cited to aid scoring of additional taxa.

Our new data set includes character scorings for 13 generic level in-group terminals reflecting all previously analyzed xenacanthimorph taxa. The resulting consensus tree from analyzing this combined data set is congruent with the topology of prior published trees, despite differences in anatomical region percentages between the original data matrices and the composite data set, suggesting xenacanthimorph in-group topology draws support diversely across the entire skeleton.

Our work should form the basis of future explorations into xenacanthimorph relationships, including descriptions of new taxa.We outline steps for future work such as resolving conflicted characters for combined in-group taxa, replacing suprageneric outgroups with genus or species level terminals, and expansion of this data set to include more Paleozoic elasmobranchs.