Parallel evolution of the sacro-pelvic complex in pipid frogs: unraveling adaptation to an aquatic lifestyle

GÓMEZ, R.O.

Diamante

Congreso; XXIX Jornadas Argentinas de Paleontología de Vertebrados; 2015

Pipids are fully aquatic frogs with a good fossil record dating back to the mid-Cretaceous. Their skeletal morphology, including the sacropelvic functional complex, has usually been considered to reflect adaptation to an aquatic lifestyle, although this has not been tested. It is noteworthy that pipid sacrourostyle and pelvis has been stereotyped and their actual variation within crown-group Pipidae understated. However, a recent study showed that there is more variation among pipid ilia than generally recognized and personal observations indicate that this also apply for sacrourostyles, whose variation has largely been unexplored in cladistic analyses. Here I explore this variation in a phylogenetic framework by optimizing 51 binary characters of the sacro-pelvic complex on a restricted topology derived from a parsimony analysis of pipoids. Additionally, in order to easily visualize evolutionary patterns of this functional complex across the phylogeny, a phylomorphospace was constructed. The latter was based on a non-metric multidimensional scaling analysis of a pair-wise dissimilarity matrix of the terminals plus their parsimony-reconstructed ancestors for the same 51 binary characters of the sacro-pelvic complex. The phylomorphospace depicts the three extant pipid lineages, namely Pipa, Xenopodinae and Hymenochirini, occupying discrete regions of a single domain separate from the rest of pipimorphs, whereas most fossil forms, including those of the crown-group Pipidae, occupy a domain somewhat intermediate between the former and the outgroup taxa. Results highlight that homoplasy definitely exists, but also point to a striking parallel pattern of evolution of the pipid sacro-pelvic complex.