Phylogenetic Implications of Glires (Eurymylidae, Mimotonidae, Rodentia, Lagomorpha) Incisor Enamel Microstructure

Abstract

Incisor enamel schmelzmuster of Rodentia is characterized by the presence of Hunter-Schreger bands (HSB). A two-layered schmelzmuster with HSB in the portio interna (PI) and radial enamel in the portio externa (PE) apparently represent the grundplan of the Rodentia. During later rodent history in some groups new enamel layers were added (e.g. Muroidea) or the PE was lost in a few taxa, but pauciserial, multiserial, or uniserial HSB always remain a typical element of rodent incisor enamel. Within Lagomorpha the differences in the incisor schmelzmuster are greater. Leporidae is characterized by a singlelayered schmelzmuster with thick multilayered HSB resembling the multiserial HSB of Rodentia. Ochotonidae lacks HSB in upper incisors, and in lower incisors only a thin layer of weakly developed HSB is intercalated between radial and tangential enamel. In Paleocene Eurymylidae a single-layered schmelzmuster with plesiomorph radial enamel (cf. Eomylus and “Sinomylus”) is present which closely resembles the schmelzmuster of Tribosphenomys. In Eocene Eurymylidae a double-layered schmelzmuster with multilayered HSB (cf. Rhombomylus. probably congeneric with Gomphos sp. indet.) occurs, and in Rhombomylus sp. indet. (small) from China a single-layered schmelzmuster with HSB was found. The Mimotonidae studied exhibit a double-layered schmelzmuster with multilayered HSB resembling the pauciserial condition (Mimotonidae gen. et sp. indet.). The eurymylid Decipomys mongoliensis has a double-layered schmelzmuster with tangential enamel in the PI and radial enamel in the PE. For Glires, a single-layered schmelzmuster with radial enamel is plesiomorphic. From this stage a doublelayered schmelzmuster with an inner zone of HSB and an outer zone of retained radial enamel evolved. In Leporidae, the outer zone with radial enamel was reduced, resulting in a single-layered schmelzmuster with HSB. In Rodentia, a two-layered schmelzmuster was retained. Understanding of the ochotonid schmelzmuster requires a more extensive survey of fossil ochotonids; possibly Ochotonidae split off much earlier than hitherto believed.