The morphology of the pareiasaurian vomer

An isolated vomer of a pareiasaur from the Upper Permian Tropidostoma Assem blage Zone (Beaufort Group) of South Africa allows detailed description of this poorly known palatal element. It is characterised by a single, median prepalatal foramen, a thin, horizontal alar flange, a bipartite suture with the premaxilla, and a large medial dorsal lamella which when in articulati on with its mate formed a tall bony septum in the nasal chamber. Comparison s with the vomers of other early reptiles, particularly with the basal turt le Proganochelys, suggest that most of these features are apomorphic for pa reiasaurs. If turtles are nested deeply within Pareiasauria, as recently pr oposed, then reversals for two of these characters and modification of a th ird would be required to account for the comparatively primitive constructi on of the testudinate vomer.