Skeletal development of the Mexican spadefoot, Spea multiplicata (Anura: Pelobatidae).

The larval chondrocranium of Spea multiplicata is described, as is the development and adult morphology of the skeleton. There are major modifications to the larval chondrocranium throughout development, including the presence of embryonic trabeculae in young tadpoles and significant reorganization of cartilaginous structures at metamorphosis. The first bone to ossify is the parasphenoid (Stage 35), followed by the presacral neural arches, ilium, and femur (Stage 36). By Stage 39, most of the postcranial elements have begun to ossify. Metamorphic climax is accomplished over three Gosner stages (39-41) and involves major modifications to the chondrocranium, as well as the appearance of three cranial elements (septomaxilla, nasal, and premaxilla). After metamorphosis, the exoccipital, vomer, dentary, angulosplenial, squamosal, pterygoid, sphenethmoid, ischium, and hyoid begin to ossify. The stapes, mentomeckelian, operculum, carpals, and tarsals do not appear until juvenile and adult stages. The development of the hyoid and cartilaginous condensations of the carpals and tarsals are described. In addition, phenotypic plasticity within the genus and the absence of a palatine (= neopalatine) bone are discussed.