Adaptations and significance of the cranial feeding apparatus of the sunbeam snake (Xenopeltis unicolor): Part I. Anatomy of the skull.

The skull of the sunbeam snake Xenopeltis unicolor is described and illustrated. In most aspects the skull features are relatable to primitive alethinophidian snakes (henophidian vs. caenophidian snakes of some systems). There are skull characters in common with those of anilioids and others that are shared among primitive snakes generally and lizards. Some of the important characteristics are: the premaxilla bears teeth and the snout complex is articulated with the dorsal parts of the frontals; there is no "boa-like" ascending process of the premaxilla in Xenopeltis, which also is absent in most pythons and in varanid lizards; the braincase is swollen posteriorly and the supratemporal bones are partially embedded in the braincase surface; and the parietals extend anteriorly around the frontals to form a significant part of the upper orbital margins. In addition, the stapes' footplates are very large and the quadrates relatively short. For the most part the bones are comparable in shape and attachments to anilioid and pythonine snakes, although the skull is distinctive in some ways. Special cranial features possessed by Xenopeltis include the contact between premaxilla and maxillae that is mainly sutural, although less firmly braced than indicated for some uropeltids; the long and flat maxillaries; the thinness of the internal palatine processes; the firm connections of the palatines with the vomers and septomaxillae; the low (nearly absent) basipterygoid processes; presence of a small "intercalary" bone on the columellar stylus; absence of both supraorbitals and postorbitals; the relatively free dentary bones and their connections only to the fronts of the compound bones; and the absence of coronoids in the mandibles. This description of the skull is the basis for current studies on the function and evolution of the jaw apparatus in Xenopeltis. J. Morphol. 239:27-43, 1999. © 1999 Wiley-Liss, Inc.