The chondrocranium of the tadpole of Hymenochirus boettgeri (Anura: Pipidae) and the sequence and timing of its development

The evolutionary development of the vertebrate head has played a critical role in shaping the diversity of body plans and ecological roles within vertebrate species. This evolutionary journey is characterised by the emergence of the head compartment during the Cambrian radiation, the evolution of jaws during the Silurian, and the subsequent diversification of vertebrate groups. The cranium, consisting of the neurocranium, dermatocranium, and viscerocranium, plays a central role in craniofacial support. Research on the development of the cranium has been carried out in several vertebrate species, providing valuable insights into their unique patterns of cartilage development. While most vertebrates show typical sequences of chondrocranial development, anurans divert from these patterns. Our study focused on the chondrocranial development of Congo dwarf frog Hymenochirus boettgeri (Anura: Pipidae) tadpoles, providing a detailed sequential analysis from mesenchymal elements to the premetamorphic chondrocranium. The chondrocranium of these tadpoles showed several reductions as a consequence of their miniaturization and their unique suction feeding lifestyle. We found a mosaic-like development of anuran mandibular and hyoid arches, and we also discussed the intricate patterns of suprarostral cartilage development and deviations in branchial basket development compared to other anuran species. Our study also contrasts anuran neurocranial development with that of other osteichthyan species, highlighting unique features such as the similar direction of development of anterior and posterior neurocranial components. Overall, our research contributes to our understanding of chondrocranial development in anurans, highlighting both conserved and unique features in H. boettgeri tadpoles. Our findings enrich our knowledge of vertebrate cranial development and provide insights into the evolutionary history of anuran chondrocranial features.