Comparative osteology of paradoxical frogs (Hylidae: Pseudae) with comments on diagnostic features, evolutionary trends and potential aquatic adaptations

Paradoxical frogs (Pseudae) are mostly aquatic anurans thriving in tropical and subtropical South America that have long attracted attention because of their gigantic tadpoles, but also by their disputed evolutionary relationships. The latter appear to have reached a reasonable consensus; yet morphological diagnoses of its constituent taxa remain poorly defined. While osteology could help resolving this issue, most of their skeletal morphology remains undocumented. Here we provide a detailed comparative description of the entire adult skeleton of paradoxical frogs, their sister group Scarthyla and other hylids. Our survey supports the distinctiveness of Lysapsus and Pseudis, proposing several osteological synapomorphies and diagnoses for different groups. We also find remarkable osteological diversity within Pseudis, even if only some isolated bones are considered, providing valuable data for paleontological and zooarcheological studies. Parsimony optimization also uncovers some distinct evolutionary trends in different aspects of the skeleton of paradoxical frogs likely related to the acquisition of adaptations to an aquatic lifestyle, showing some remarkable convergences with the highly aquatic pipids.