The unusual orbitosphenoid of the snakelike lizard Bachia bicolor.

We studied the morphology of the chondrocranial orbitotemporal region in the snakelike gymnophthalmid lizard Bachia bicolor and its relation to other structures such as the ophthalmic division of the trigeminal nerve and eye muscles, to show its particular morphology, and discuss the homology of some skeletal structures relative to other squamates. We used three-dimensional computer reconstructions from serial histological sections; additionally we studied the embryonic and postembryonic development of the orbitosphenoid bone using cleared and double-stained whole-mount skeletal material. The chondrocranial orbitotemporal morphology in B. bicolor was found to be severely reduced as described for other miniaturized snakelike squamates, but it was accompanied by extensive orbitosphenoid ossifications. Within squamates, only amphisbaenians showed an expanded orbitosphenoid, which originates from fused endochondral and membranous ossifications. In B. bicolor the orbitosphenoid was also found to be formed by endochondral and membranous ossifications, but contrary to the amphisbaenian condition the membranous ossifications were found to arise as membrane bone outgrowths from the perichondral ossification of the chondral core. Despite its derived morphologies, we argue that the orbitosphenoids in amphisbaenians and B. bicolor are homologous to the orbitosphenoids of other squamates. Thus, the expanded orbitosphenoid morphology is found to be achieved by different ontogenetic processes in amphisbaenians and B. bicolor, representing a case of independent evolution by convergence.