The status of Mimomys in North America revisited

Dental characters of the type species of Mimomys, M. pliocaenicus FORSYTH MAJOR, 1902, are compared with those from the earliest North American arvicolids. Mimomys on this continent is limited to the early Pleistocene (early Irvingtonian) M. virginianus REPENNING & GRADY 1988 and M. dakotaensis MARTIN 1989. The crucial characters for Mimomys are well-developed negative enamel differentiation and a Mimomys-type schmelzmuster (microscopic enamel banding pattern), in concert with rooted molars (to separate Mimomys from early Microtus and Arvicola). Because mosaic character evolution often dominates in the early expression of character states in a highly speciose clade, it seems appropriate to use unique character combinations to define higher level taxa, even if the characters are individually homoplasic or symplesiomorphic within the clade.