Toads of the genus Melanophryniscus are known since the mid-late 19th Century, and the first skeletal description was made early in 1875. However, it was not until the 1970s that osteological variation was discussed in a more inclusive taxonomic scenario. Derived from this, the first morphological synapomorphies proposed for the genus represented skeletal traits extracted from the few species considered in those studies. In this work, we examined the skeletons of 25 of the 29 currently recognized species of Melanophryniscus, plus three species under description, to examine their osteological variation and discuss the validity of those synapomorphic characters proposed, not only by analyzing their distribution within the genus but also by comparing them with the skeletal data available in the bibliography for other early branching bufonids. Our results show that main variations within the genus are related to changes in absolute body size and some proportions of postcranial elements. Except for M. setiba, an early diverging species that exhibits a number of autapomorphies, most skeletal traits are quite conserved throughout the genus. Members of the Melanophryniscus tumifrons group are distinctive by their nasal region, which tends to be taller than in other species, dome-shaped, and strongly exostosed. Most features considered diagnostic of the genus occur in other early branching bufonids, and are highly polymorphic, and this challenges earlier discussions on putative synapomorphies. For instance, exostosed frontoparietals only occur in M. setiba and M. klappenbachi, and the condition was also recorded for Osornophryne. The frontoparietal fontanelle, if present, may show different shapes and sizes (often similar to those in Osornophryne and some species of Atelopus). The zygomatic ramus of the squamosal may be absent or present as a small process, as described for some species of Atelopus, Osornophryne, and Truebella. Finally, posterolateral processes of the hyoid were observed in some early diverging species and hence we propose an alternative interpretation on its presence and distribution in the genus.

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