CHARACTERISTICS OF SKELETAL SYSTEM AND SKELETON DEVELOPMENT SEQUENCE IN RANODON SIBIRICUS

In order to adapt to the transition from aquatic to terrestrial, the skeletal system of amphibians has undergone tremendous changes before and after metamorphosis. The characteristics of the skeleton development sequence in amphibians reflect their evolutionary adaptation to lifestyle and are also ideal materials for studying vertebrate skeleton formation, growth, and remodeling. In order to reveal the characteristics of the amphibian skeletal system and the sequence of skeletal development, the process of skeletal system development of amphibians at different ages was observed and compared by the double staining of cartilage and bone procedure. The results showed that the skeletal system of Ranodon sibiricus was divided into the skull, vertebra, and appendicular skeleton, with a total of (217±1) bones, including 53 skull bones, (48±1) vertebra bones, and 116 appendage bones. Furthermore, the skeletal system of Ranodon sibiricus has some significant morphological features that are different from other species of Hynobiidae. The contour of the skull appears broad and flat, and the skull fragments are thin and weak. In addition to the fontanelle between the frontals in the dorsal center of the skull, there is also a premaxillary fontanelle in the dorsal front of the skull, and the ratio of the length of the premaxillary fontanel to the length of the nasal bone is up to or more than 1/2. The two rows of vomerine dentition are positioned between the nostrils and separated. The arrangement pattern of the vomerine pars dentition changed from ‘/ \’ to accompanied by the metamorphosis from larva to adult of Ranodon sibiricus. The anterior of the pterygoid is very close to the posterior of the maxilla, even linked by the bone bridge at the larval stage until the bone bridge breaks after metamorphosis. The shape of the maxillary arch is arched, and the lower jaw is thin and weak. Squamosal is ‘T’ shaped and narrower. The first branchial arch is completely cartilage. The cornua of the basibranchial cartilage show two cylindrical branches gradually tapering anterolaterally to a pointed tip. The process of prearticular is not developed, with small articular. The caudal vertebrae are flat. The first three caudal vertebrae still have ribs, and the haemal arch appears from the fourth caudal vertebra. With 4 fingers and 5 toes, it usually has 8 carpals and 10 tarsals, respectively. At the age of 6 months, the morphology and structure of the skull of Ranodon sibiricus began to change significantly compared with the previous development stage. For example, the maxilla, lacrimale, and prefrontal were born. The outline of the premaxillary fontanelle is gradually clear. The palatine degenerates, the pterygoid retracts backward, the squamosal extends outward, and the angle formed by the outer edge of the left and right squamosal increases further. At the same time, the prepubis was born in the pelvic girdle. These massive newborn or remodeling bones at the skull and the pelvic girdle suggest that Ranodon sibiricus has entered the metamorphosis stage to adapt to the transition from aquatic to terrestrial. The second significant change in the skeletal number and morphology of Ranodon sibiricus occurs at the age of 2 years. Although most bones only showed the enhancement of ossification degree from 6 months to 2 years old, there were significant changes in the apparatus hyoideus. The third and fourth branchial arches degenerate or disappear, meaning the outer gills disappear. The disappearance of the outer gills also suggests that the development of metamorphosis is nearing its end. Based on the comparative analysis of the skeletal system of Ranodon sibiricus at different ages, it is speculated that the metamorphosis of Ranodon sibiricus occurred from 6 months and lasted until the age of 2 years. During the development of metamorphosis, the newborn, degeneration, and reorganization of bones are mainly to adapt to the changes in feeding, breathing, and movement before and after metamorphosis. The results of this study can provide a theoretical basis for studying the skeletal functional development of Ranodon sibiricus and its phylogenetic classification in Hynobiidae species.