A decharming metamorphosis: The larval and adult morphology of the common spadefoot toad, Pelobates fuscus

Abstract

Anurans are a highly diverse amphibian order with a biphasic lifestyle. The larva, better known as tadpole, and the adult frog of one species typically occupy different habitats and exploit different resources. Therefore, both are subject of largely independent selection and are able to evolve stage-specific adaptations. Among others, the musculoskeletal features of the larval and adult head of anurans and the description of their specific traits are important to gain a deeper understanding of the evolution and the phylogeny of this highly diverse group. Here we present an overview of the larval and adult cranial morphology of the common spadefoot toad Pelobates fuscus. We provide a detailed description of the cranial musculoskeletal structures based on micro-computed tomography and following 3D reconstruction of two long-term stored specimens of P. fuscus and refer to striking adaptations of this species. P. fuscus inhabits a wide spectrum of habitats including mixed forests, steppes, fields and sandy areas with soft soils in close proximity to permanent water bodies. Adults have a semi-fossorial lifestyle and they are able to dig themselves in using their hindlimbs whereas the tadpoles are the largest of all European anurans. Derived larval features are the presence of an adrostral cartilage within the lower jaw, the presence of three portions of the M. subarcualis rectus I, the presence of the M. diaphragmatopraecordialis and a bipartite suprarostral cartilage. The presence of an enlarged prehallux and the co-ossification of the frontoparietal, as well as a pterygoid that borders the orbit ventrally and rostrally are striking adult features and adaptation to the semi-fossorial lifestyle. With the described morphological features, we want to underline their importance for several adaptations which may have facilitated and stimulated the evolution of anurans and provide baseline data for further investigations.