Enamel microstructure ofTribosphenomys (Mammalia,glires): Character analysis and systematic implications

Enamel distribution on the upper and lower incisors ofTribosphenomys minutus (from Late Paleocene-Early Eocene of Inner Mongolia of China) is typically rodent-like, i.e., primarily confined to the anterior surface throughout these transversely compressed, evergrowing teeth. AlthoughTribosphenomys incisor enamel is differentiated into two layers, it does not possess Hunter-Schreger bands (HSB). The incisor and molar enamels are radial in type, a condition regarded as either an autapomorph or a primtive retention forTribosphenomys. Character polarities concerning enamel thickness, enamel layer number, HSB, enamel types, and functional and phylogenetic implications of the enamel structures are discussed. Overall, enamel microstructural evolution at high taxonomic levels within Glires displays considerably more homoplasy than generally appreciated. A phylogenetic definition of Rodentia is proposed.Tribosphenomys is the sister-group of a taxon here named Rodentia, and thus is not itself a member of the order, from a systematic viewpoint.