Comparative osteology of the fossorial frogs of the genus Synapturanus (Anura, Microhylidae) with the description of three new species from the Eastern Guiana Shield

The genus Synapturanus includes three nominal species of fossorial Amazonian frogs. A previous study combining molecular, morphological and acoustic data suggested that there may be six times more species than currently recognized. Herein we describe and name three of these new species and compare their osteology. Synapturanus zombie sp. nov. occurs in French Guiana and Amapá (Brazil), Synapturanus mesomorphus sp. nov. in Guyana and adjacent Venezuela, and Synapturanus ajuricaba sp. nov. in the northern part of the Brazilian states of Amazonas and Pará. These species are readily differentiated from congeners by a combination of external morphological characters such as body size, development of fringes on fingers and coloration, by advertisement call variables, and by osteological traits. Along with osteological reinforcement of the skull, atlas and scapular region, the reduction of the size of phalanges, more developed fringes on fingers, smaller eyes and larger body size, altogether suggest an overall increase of the fossorial habits in the easternmost species. In contrast, the relatively conserved morphology of the posterior part of the body across the genus suggests that fossoriality mostly involves the anterior part. Furthermore, the fusion of tarsal bones in the species of the western clade may indicate locomotory adaptation to more epigean habits.