Abstract
Few detailed accounts of the developmental morphology of caecilian amphibians exist and recent studies have highlighted problems concerning the homology of some skull elements. We investigated the embryonic and post-hatching development of the skeleton and musculature of Idiocranium russeli, a possibly miniaturized caecilian endemic to Cameroon. Idiocranium has been suggested to be direct developing; our results strongly support this hypothesis. The external morphology of different embryonic stages, the ossification sequence, and the configuration of the cranial muscles of I. russeli indicate heterochronic shifts of adult traits into embryonic development, as well as the loss or absence of various larval and metamorphic traits. For example, the tentacle, which plesiomorphically develops during metamorphosis, is already fully developed in late embryos. The maxilla and the palatine, which fuse to form the maxillopalatine (during metamorphosis in most biphasic species), fuse well before hatching. Muscles exclusive to the larva, such as the m. interhyoideus and m. hyomandibularis, are absent during development, whereas adult muscles including the m. genioglossus and m. cephalodorsosubpharyngeus, form during embryonic development. A larval ceratobranchial IV is present and fuses to the ceratobranchial III very early in ontogeny. In its near complete absence of larval traits during development, I. russeli resembles the Indian indotyphlid Gegeneophis ramaswamii; this similarity complicates a straightforward explanation for the re-evolution of free-living larvae in Seychelles indotyphlid caecilians.
