Incisor enamel microstructure places New and Old World Eomyidae outside Geomorpha (Rodentia, Mammalia)

The lower incisor enamel microstructure of the fossil rodent family Eomyidae was believed to be three-layered and highly derived but rather uniform throughout clade. Here, we describe a new four-layered schmelzmuster in consisting three-fold portio interna with longitudinal oriented, uniserial Hunter-Schreger bands one-fold externa, accounting for unique character combination Rodentia. This type has developed early eomyid evolution is detectable already late Eocene (Chadronian) North America. In European eomyids, it first occurs Miocene (MN 3), implying that this not present all members restricted species included Eomyini some genera currently considered incertae sedis within Eomyidae. Additionally, our analysis recognizes three taxa divergent from other eomyids. Incisor does advocate close phylogenetic relationship either or extant Heteromyidae Geomyidae, nor Heliscomyidae Florentiamyidae. Our results support view are placed outside Geomorpha.