The cranial, mandibular, and hyoid anatomy of softshell turtles ( <i>Trionychidae</i> ): A revised character list for phylogenetic analysis

Softshell turtles (Pan-Trionychidae) are an early branching clade of hidden-necked turtles (Cryptodira) with a rich fossil record extending back to the Early Cretaceous. The evolutionary history of softshell turtles is still unresolved because of their conservative morphology combined with high levels of polymorphism related to morphological plasticity and ontogeny. We present a revised list of characters focused on the skull and hyoid of extant trionychids. To better assess the polymorphism present in extant softshell turtles, we assembled a set of photographs and μCT scans that document the morphology of 312 specimens, individually scored to better document all polymorphism. Several matrices were then assembled that variously reduce observed polymorphism. To explore which matrix performs best under which setting, we carried out numerous parsimony analyses and established phylogenetic accuracy by comparing results to the emerging molecular consensus. The matrix excluding low-frequency polymorphism and ontogenetically related polymorphism is shown to utilize the greatest amount of character information and performs best with ordered characters and moderate implied weighting, but many relationships remain incorrectly resolved. Use of early fossil trionychids or Carettochelys insculpta as outgroups affects the polarity of the trees but leaves the topology largely unchanged. Although our revised list of characters is an improvement upon previous ones, we strongly advise the use of a molecular backbone or total evidence analysis, ideally within a Bayesian framework. A succinct description is provided for the skull and hyoid of all extant clades and species of softshell turtles.