A single layer of radial enamel in the incisors represents the plesiomorphic condition for stem-lineage representatives of Glires (Lagomorpha and Rodentia). A single layer of radial enamel is detected in the incisors of the zalambdalestid Barunlestes butleri and the pseudictopid Pseudictops lophiodon as well as the eurymylid Eurymylus laticeps. The eurymylid Matutinia nitidulus exhibits a more derived, double-layered schmelzmuster with Hunter-Schreger bands (HSB) in the portio interna (PI) and radial enamel in the portio externa (PE). Mimotonids are characterized by a schmelzmuster with HSB: Anatolimys rozhdestvenskii has double-layered enamel (PI with HSB, PE with radial enamel) and two undetermined mimotonids from Mongolia exhibit a single-layered enamel. An incisor of an undetermined leporid (Strenulagus or Aktashmys) from the early Eocene of Kyrgyzstan, one of the geologically oldest studied, has double-layered schmelzmuster with HSB in the PI and radial enamel in the PE, whereas all other leporids have single-layered enamel with HSB. This is evidence that the single-layered leporid incisor schmelzmuster is derived from a double-layered condition. The presence of multilayered schmelzmuster in upper and lower incisors of Ochotonidae with modified radial enamel, HSB, and tangential enamel is corroborated. All three species of the supposed ochotonid Desmatolagus (D. gobiensis, D. robustus, and D. vetustus) exhibit a single-layered schmelzmuster with HSB and therefore are transferred to the Leporidae. Ochotonid schmelzmuster was derived by a different pathway than leporid schmelzmuster and possibly originated from that of yet poorly known early Paleogene Gliriformes such as Decipomys mongoliensis. This indicates that the ochotonid-leporid divergence is much older than previously believed.
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Vol. 24 • No. 2