We studied the mouth roof of some larval stages of the Alpine newt Ichthyosaura alpestris apuanus, collected in summer 1977 in the field, using total preparations stained with Alizarinred, X-ray computed microtomography, and scanning electron microscopy. In all specimens we found traits indicating a delayed metamorphosis, i.e. a temporarily slowed down or even halted development of traits dependent on thyroid hormones (TH), and a slight further growth of probably TH-independent traits. Especially noteworthy is the temporary partial fusion of vomer and palatine forming a vomeropterygopalatinum. Presence of vomeropterygopalatina was recently shown in overwintering larvae of the related species Lissotriton vulgaris. On the basis of these findings we suggest that vomeropterygopalatina appear to be a quite frequent side effect of delayed metamorphosis. Further, we hypothesize that large parts of the larval vomer become incorporated into the definite (transformed) vomer. Three larvae in the series showed developmental stages of the vomerine bar, a posterior outgrowth of the vomer, typical for salamandrids, which is formed around metamorphosis. Development of the vomerine bar begins on the front end of the larval vomer and not (as generally assumed) on its posterior inner edge. These findings support our previous view that the formation of the vomerine bar in L. vulgaris and I. a. apuanus is more complex than literature assumes. However, due to some shortcomings regarding number and preservation of the available larvae a deeper study is needed using standardized conditions and a more complete developmental series of these (protected) or related species to substantiate the observations reported here.

Delayed metamorphosis, vomeropterygopalatinum, vomerine bar, salamandrids.