Ontogenetic development of additional bone elements of new Brachycephalus from Serra do Brigadeiro, southeastern Brazil

Species of Brachycephalus genus, popularly known as pumpkin toadlets, lack some bones of the skull (e.g. neopalatine and columella), phalanges, fingers and toes. Moreover, the skull and post-cranial region might present degrees of hyperossification that range from absent to the formation of dorsal plates: parotic, spinal and paravertebral plates. Brachycephalus sp. nov. from Serra do Brigadeiro mountain range, Minas Gerais state, Brazil, presents hyperossification and have well developed skull and post-cranial plates. Here we describe the ontogenetic development of these additional bone elements of this new species, using the diaphonization technique. The parotic plate, in early juvenile, is a small pair of convex disc on top of the optic ramus of squamosal, lacking ornamentation. During the development of the individual, their plate grows medially toward the frontoparietals, attaching with it, and develop dorsal ornamentations. In early juveniles, the spinal plates are as a espinal process extention, individual and not fused with each other. During the development of the specimen, the spinal plates grow and I-II and IV-VIII fuses with each other, also evolves dorsal ornamentation. The paravertebral plates, in early juveniles, are isolated from the vertebrae and vertebral processes, and, during the development of the individual, the paravertebral plates make contact with the transverse process of vertebrae IV and V, fusing with them ventrally and dorsally with the lateral edges of the IV-VII spinal plates in adults. These additional bone elements are homologous within the Brachycephalus genus, and evidence the presence of at least two clades on this genus: 1) with and 2) without plates. However, despite some hypothesis – water balance – the functionality of these extra bone elements is not clear, and the elucidation of the ontogenetic development may help find answers.