Cranial skeletogenesis in the Japanese giant salamander, Andrias japonicus. Cranial bones and cartilages are shown for eight of 12 developmental phases. Scale bars: 1 mm. Abbreviations: cl, columella; exo, exoccipital; fr, frontal; ma, maxilla; na, nasal; oc, otic capsule; osph, orbitosphenoid; pa, parietal; pfr, prefrontal; pma, premaxilla; psph, parasphenoid; pt, pterygoid; q, quadrate; seth, sphenothmoid; sq, squamosal; vo, vomer.
There are approximately 750 extant species of Caudata (salamanders), an order of amphibians. Their living environments range from underwater to terrestrial to arboreal. In this study, Associate Professor Masayoshi Tokita and his colleagues at the Faculty of Science of Toho University prepared transparent skeletal specimens of four species from four families of salamanders, including the Japanese giant salamander, and qualitatively described the formation process of the skull. Furthermore, by using the geometric morphometric method, which has been actively used in recent years in comparative morphology research, the group quantitatively compared the skull formation process of the four species.
As a result of the study, significant differences among the species were observed in the onset of skull ossification, the direction of ossification, and the patterns of morphological changes in the hyoid and gill bones. In the Japanese giant salamander, ossification of each bony element of the jaw occurs very early in the growth process, and as the jaw grows, the maxilla elongates posteriorly and the squamosal slope backward.
These features of skull formation may be related to the unique foraging method known only to the family Cryptobranchidae. They forage by suction feeding, using the water current generated by both the submergence of the hyoid bone and by the negative pressure when the jaw is opened up and down, in addition to asymmetrical occlusion, in which the left and right sides of the jaw move independently to catch prey.
This study suggests that the process of skull formation in salamanders may be closely related to the life history and feeding style of each species.