Ontogeny of the skeleton of Spea bombifrons (anura: Pelobatidae).

The developing skeleton of a pelobatid frog, Spea bombifrons, is described based on a series of 81 cleared-and-stained tadpoles and juveniles. Most limb and limb-girdle elements commence ossification premetamorphically, along with the parasphenoid, frontoparietals, exoccipitals, and vertebral column, during Gosner Stage 36. The chondrocranium undergoes dramatic restructuring at the beginning of metamorphosis (ca. Stage 40); and the nasal cartilages, prootics, premaxillae, nasals, maxillae, septomaxillae, and ischium appear. Near the end of metamorphosis (ca. Stage 44), the branchial arches are resorbed, the hyoid plate and quadrate form, and the angulosplenials, vomers, squamosals, dentaries, and pterygoids ossify. After metamorphosis (Stage 46), the laryngeal cartilages, sternum, omosternum, and plectral apparatus chondrify; and the carpals, tarsals, sphenethmoid, posteromedial hyoid processes, mentomeckelian bones, quadrates, columellae, and opercula ossify. The development of the fused sacrococcygeal articulation in S. bombifrons is described and compared to the more widespread bicondylar articulation. The presumed palatine bone and transverse processes of the coccyx are discussed, as are several seemingly paedomorphic skeletal features of Spea. Comparison with the sequence and timing of ossification in other anurans reveals that ossification of the skull and vertebral column occurs later in S. bombifrons than in other anurans; nonetheless, many aspects of the ossification sequence seem to be shared by a surprisingly wide range of anuran taxa.