Don’t go with the flow: cranial adaptations of stream tadpoles in the Afrobatrachian family Arthroleptidae

Arthroleptidae are an ecologically diverse group of sub-Saharan frogs. Arthroleptid tadpoles predominately occur in slow flowing to torrent waters. Their musculoskeletal system and the relationship between tadpole morphology and lifestyle are poorly understood. Here, we investigated the cranial morphology of four arthroleptid tadpoles occurring in different microhabitats: Leptopelis parkeri, Astylosternus occidentalis, Trichobatrachus robustus, and Nyctibates corrugatus. The crania of the tadpoles of A. occidentalis, N. corrugatus, and T. robustus are highly modified relative to L. parkeri, with wide and partly or completely fused cornua trabeculae, a fused cartilago labialis superior, as well as several modifications of the palatoquadrate, such as a broad commissura quadratocranialis and a processus ventralis which expands the surface for the origin of the large m. orbitohyoideus. The processus hyoquadratis is an evolutionary novelty, forming a functional unit together with the ceratohyale in the cranium of N. corrugatus. Finally, we compared the cranial morphology of these arthroleptids to other lotic and non-lotic species. Although the processus ventralis is not a generalized feature of the cranium in lotic species, it is not present in lentic species.