Lemmini. Many taxonomic characters associate Synaptomys with the true lemmings (Lemmus and Myopus) in a clade, usually regarded as Lemmini and believed to represent an early line of arvicoline evolution (Carleton, 1981; Chaline and Graf, 1988; Conroy and Cook, 1999; Graf, 1982; Gromov and Polyakov, 1977; Hinton, 1926a; Hooper and Hart, 1962; Koenigswald, 1980). Fossil history reviewed by Koenigswald and L. D. Martin (1984), Abramson (1993), Fejfar and Repenning (1998), Kowalski (2001), and Martin et al. (2003). The last authors segregated Synaptomys (and Mictomys as genus), together with a European fossil (Tobienia), as Synaptomyini, apart from Lemmini (Lemmus, Myopus, and Plioctomys), both groups thought to be descendants from an ancestral Mimomys stock in the early Pliocene.
Although described as a genus, Miller (1896) arranged Mictomys as a subgenus of Synaptomys, as conventionally recognized by neontologists in the 1900s (Hall, 1981; Honacki et al., 1982; Howell, 1927; Musser and Carleton, 1993), albeit not uniformly (Jarrell and Fredga, 1993). Paleontologists, on the other hand, have emphasized the dental contrasts of Synaptomys and Mictomys at the generic level (Fejfar and Repenning, 1998; Koenigswald and L. D. Martin, 1984; Kretzoi, 1969; Repenning and Grady, 1988), albeit not uniformly (Abramson, 1993). While certain early Pliocene European taxa have been linked with the origin of Synaptomys sensu lato (see Fejfar and Repenning, 1998, Chaline et al., 1999, and Martin et al., 2003 for reviews), the divergence of borealis and cooperi is thought to have occurred in North America, whether dated to a common ancestor in the late Pliocene (Repenning and Grady, 1988) or one in the middle Pleistocene (Fejfar and Repenning, 1998). As understood, the cladistic dichotomy and its recency seem equivocal for treating Mictomys and Synaptomys as separate genera; that possibility should be explored using other information and a broad sampling of New and Old World lemmings, one that includes other archaic forms outside of Lemmini, such as Arvicola, Dinaromys, Phenacomys, and Prometheomys. Current specific and subspecific classification (e.g., Hall, 1981) essentially framed by Howell (1927).