Mantellid frogs present an extensive adaptive radiation endemic to Madagascar and Comoros, being the subfamily Mantellinae the most morphologically and ecologically diverse. The Mantellinae present key innovative evolutionary traits linked to their unique reproductive behavior, including the presence of femoral glands and a derived vomeronasal organ. In addition, previous studies pointed to size differentiation in playing an important role in species’ dispersal capacities and shaping of their geographic ranges. Despite the high phenotypic variation observed in this clade, to date an exhaustive morphological analysis of their anatomy has still not been performed, much less in relation to internal structures. Here, we present a comprehensive skeletal description of a mantellid species, Blommersia transmarina, from the island of Mayotte in the Indian Ocean, which has potentially undergone a process of moderate gigantism compared to other Blommersia species. We describe its intraspecific skeletal variation utilizing non-destructive volume renderings from μCT-scans, and characterize the presence of sexual dimorphism and size covariation in skeletal structures. Notably, we found numerous signs of hyperossification, a novel structure for mantellids: the clavicular process, and the presence of several appendicular sesamoids. Our findings suggest that skeletal phenotypic variation in this genus may be linked to biomechanical function for reproduction and locomotion.

Mantellids are a hyperdiverse group of frogs endemic to Madagascar, showing a large variation in morphology and ecological specialization that parallels their phylogenetic diversification. Among them, the genus Blommersia comprises several small species of small to miniature frogs, two of which occur in syntopy on the Comorian archipelago, which was colonized by their ancestor several millions of years ago in an event of transoceanic dispersal from Madagascar. They have evolved in isolation, which has supposed an evolutionary experiment of morphological and genetic divergence without competition. In mantellids, as well as in other amphibians, body size is correlated with dispersal capabilities and these may have allowed the colonization of new habitats in parallel to morphological divergence. Consequently, we hypothesized that variation in the Blommersia musculoskeletal anatomy implicated in locomotor performance would reflect potential differences between species in accordance to their diversification into different ecological niches and lifestyles. To tackle this question we focused on one of the main components of the anuran locomotor system: the pelvis. We compared the pelvic shape of seven Blommersia species, with special emphasis on the Comorian sister species pair and their Malagasy sister taxon. In addition, we mapped for the first time the insertion sites of the musculature on the Blommersia pelvis by means of a customized contrast staining technique prior to CT-scanning. Morphological distances resulting from the pelvic shape comparison between species correlated to a high extent with their 16S phylogenetic distances. In accordance, we discuss the observed patterns of pelvic shape variation and their relationship with size variation in the context of pelvic musculoskeletal architecture evolution in anurans and its potential interactions with locomotor function in the Blommersia genus.