Development of the skull of Dermophis mexicanus (amphibia: Gymnophiona), with comments on skull kinesis and amphibian relationships

The development of the skull of Dermophis mexicanus (Caeciliidae) is described and compared to that of other caecilians. The chondrocranium is well developed in embryos of 25 mm total length (TL); ossification begins in the quadrate and the articular element of the lower jaw at about 30 mm TL. The occipital arch then ossifies, followed by the dorsal and lateral dermal bones, then the ventral endochondral and dermal elements. The stapes ossifies at 55 mm TL. The amount of skull roofing increases during ontogeny, although the anterior rims of the nasal capsules, the anterior part of the mesethmoid, and the hyoid apparatus remain cartilaginous throughout life. Dermophis mexicanus lacks many primary embryonic ossification centers reported by Marcus et al. ('35) for Hypogeophis ; presence of these ossification centers has been presumed to be indicative of a primitive skull morphology derived with little modification from archaic amphibians (“stegocephalians”). The fetal skull is highly kinetic, and some kinesis is retained in adults. We suggest that fetal skull kinesis and early ossification of jaw suspension elements are functionally related to the intraoviducal feeding mode of this viviparous species. Based on this evidence, together with the observed ossification pattern and bone homologies, we conclude that stegokrotaphy (complete skull roofing) in caecilians is a derived condition, correlated with fossoriality, and does not indicate a direct relationship of caecilians to any known early amphibian taxon.