Notes on cranial ontogeny and delayed metamorphosis in the hynobiid salamander Ranodon sibiricus Kessler, 1866 (Urodela).

The skull of larvae, juveniles and adults of the rare and primitive hynobiid salamander Ranodon sibiricus was re-examined using transparencies and illustrated by new graphics. The earliest larva available for investigations already had the dominant bones. The maxillary, however, was still lacking. Previous descriptions regarding the appearance and growth of bones could be largely confirmed. The vomer, first seen as a relatively small obliquely arranged dentate bar in the 3.8 cm long larva, became larger during ontogeny, but did not change its position remarkably. The vomerine pars dentalis with only a single tooth line was straight in larvae and juveniles, but was slightly curved in adults allowing for distinction of an outer and inner portion. This feature is typical and more pronounced in most other hynobiids. The significance of the vomer and vomerine dentition for systematic and phylogenetic purposes and its changes during metamorphosis are briefly discussed. Two of the specimens examined showed delayed metamorphosis very likely caused by low temperatures. Here the temporal course of transformation was "stretched" and therefore some alterations, e.g. regression of the palatinal portion of the palatopterygoid, were shown more clearly. Continuous growth of some skull elements in these individuals suggested a relative independence from metamorphosis perhaps due to variable thyroid activity and/or independent changes in individual tissue sensitivities. It is suggested that remodelling of the mouth roof could be used for staging urodele ontogeny.