Phyllotini. Generally viewed as a primitive clade relative to other phyllotine genera (Hershkovitz, 1962; Pearson and Patton, 1976; Reig, 1986), a viewpoint corroborated by phylogenetic analyses of trait data, both morphological (Braun, 1993; Steppan, 1993; 1995) and molecular (Smith and Patton, 1999). Those morphological studies differ in disclosing the basal position of Calomys spp. as monophyletic (Braun, 1993) or paraphyletic (Steppan, 1993); gene-sequence data convincingly ratify the taxon’s monophyly (Salazar-Bravo et al., 2001, 2002a). Salazar-Bravo et al. (2001) dated the initial cladogenesis among Calomys species to ca. 9 million years ago, possibly spurred by the early development of South American grasslands, and reviewed hypotheses and data that concern the Great American Interchange and areas of origination. Known from Pleistocene of Argentina (e.g., Pardiñas, 1999). Status of North American Miocene-Pliocene form Bensonomys as a subgenus of Calomys (Baskin, 1978, 1986; Czaplewski, 1987; Korth, 1994; Lindsay and Jacobs, 1985) or as a merely dentally similar neotomine (Reig, 1980, 1984) needs decisive resolution, as it bears importantly on larger issues of New World rodent evolution and biogeography. McKenna and Bell (1997) assigned Bensonomys to Neotomini, within Sigmodontinae sensu lato.
Generic revision by Hershkovitz (1962), who reduced the previously recognized 10-15 species (e.g., Cabrera, 1961; Ellerman, 1941) to only four, two of which, laucha and callosus, have been revealed as species complexes (Bonvicino and Almeida, 2000; Corti et al., 1987; Massoia et al., 1968; Olds, 1988; Pearson and Patton, 1976; Reig, 1986; Williams and Mares, 1978). See Bonvicino and Almeida (2000) and Bonvicino et al. (2003c) for compilation of the prolific karyological literature on the genus and correction of misidentified voucher specimens. Interspecific genealogies and species groups inferred from morphology (Olds, 1988), banded chromosomes (Espinosa et al., 1997; Vitullo et al., 1990), and cytochrome b sequences (Salazar-Bravo et al., 2001) are substantially incongruent. The following recognition of species attempts to integrate recent morphological, karyotypic, and molecular studies (principally, Bonvicino and Almeida, 2000; Olds, 1988; Salazar-Bravo et al., 2001). Nonetheless, the continued listing of Calomys specimens as species indeterminate (e.g., Anderson, 1997; Salazar-Bravo et al., 2001) and reports of new karyotypic variants (Espinosa et al., 1997; Lima and Kasahara, 2001) caution that considerable alpha taxonomic investigation is yet required.