Osteology and skeletal development of Discoglossus sardus (Anura:Discoglossidae).

Abstract

Although frogs in the archaeobatrachian family Discoglossidae are reasonably well known, descriptions of their larval skeletons and osteogenesis are almost nonexistent. Skeletogenesis, chondrocranial development, and the adult skeleton of Discoglossus sardus are described on the basis of cleared and stained, dry, and radiographed specimens. In D. sardus, the first elements to ossify are the parasphenoid, frontoparietals, exoccipitals, neural arches, ischium, long bones, and dermal elements of the pectoral girdle (Gosner Stage 36). Major reconstruction of the chondrocranium begins at the onset of metamorphosis (Stage 41), contemporaneous with the ossification of the premaxillae, maxillae, vomers, and septomaxillae. Several cranial (e.g, pterygoid, mentomeckelian, sphenethmoid) and postcranial (e.g., carpals, hyoid) elements do not commence ossification until metamorphosis (Stage 46). Discoglossids are characterized by the presence of a facial foramen in the lateral wall of the chondrocranium, a rod-like epipubis developing from two primordia, and the lack of a neopalatine bone. Adult male Discoglossus possess an enlarged, crested metacarpal II and a broad prepollical element. This detailed description serves as a model to compare the development of other discoglossid frogs and provides detailed descriptions of several enigmatic structures.