Microanatomy of Dermal Roofing Bones in the Skull of Pipoid Frogs

Previous research on bone microanatomy in amphibians suggests a correlation of microanatomical traits with both environmental and phylogenetic factors, but so far, studies had been limited to long bones. Using Pipoidea, a uniquely adapted clade of fully aquatic anurans, we investigated whether the microanatomical structures of anuran cranial bones display not only an ecological but also a clade-specific signal. Micro-CT scans of the skulls of five extant and three extinct pipoids species were compared with those of four phylogenetically distant, yet similarly aquatic anurans. We focused on the frontoparietal and maxillary bones, because they are among the largest bones in the anuran skull and are often preserved as fossils. From each of the bones the overall compactness, cross-sectional area, and thickness were extracted. Statistical analysis revealed no significant difference between the three groups in thickness and cross-sectional area, which is consistent with their shared lifestyle. Compactness, however, revealed a statistically significant difference between the Pipoidea clade and the phylogenetically distant group. Our findings suggest the presence of both a clade-specific and an environmental signal in the bone compactness of the pipoidean skull.