Oryzomyini. Described as a subgenus of Oryzomys and usually recognized as such (Ellerman, 1941; Hall, 1981; Reig, 1984; Tate, 1932e) or as a genus (Contreras and Berry, 1983; Gyldenstolpe, 1932), with Microryzomys as a full synonym (Gyldenstolpe, 1932; Tate, 1932) or not (Cabrera, 1961). Diagnosis emended at the generic level and morphology contrasted with Microryzomys and Oryzomys sensu stricto by Carleton and Musser (1989). Monophyly of the genus supported by electrophoretic (Dickerman and Yates, 1995) and mitochondrial and nuclear gene-sequence data (Myers et al., 1995; Smith and Patton, 1999; Weksler, 2003), in a clade also containing Microryzomys and Neacomys. Attempts to define species groups have so far produced conflicting results and should be viewed as preliminary forays to stimulate critical phylogenetic study (e.g., see Andrades-Miranda et al., 2001a; Bonvicino and Weksler, 1998; Carleton and Musser, 1989; Dickerman and Yates, 1995; Myers and Carleton, 1981; Myers et al., 1995).
Species-level revisions and range documentation much needed: estimates range from one (Hershkovitz, 1966c) to 30 (Tate, 1932e), usually around 12 (Cabrera, 1961). Regional studies have typically recorded two to four species in sympatry or parapatry (Bonvicino and Weksler, 1998; Carleton and Musser, 1995; Contreras and Berry, 1983; Massoia, 1973; Myers and Carleton, 1981; Olds and Anderson, 1987). Chromosomes of many species described and compared in Andrades-Miranda et al. (2001a), Bonvicino and Weksler (1998), Espinosa and Reig (1991), Gallardo and Patterson (1985), Gardner and Patton (1976), and Myers and Carleton (1981); published karyotypes summarized for all South American species by Andrades-Miranda et al. (2001a). The 18 species recognized here generally observe the preliminary review of Carleton and Musser (1989), as amplified by later descriptions. A tally of 25-30 species remains probable as extrapolated from the following observations: the Andean complex has yet to be critically addressed; definition of O. fulvescens sensu stricto and its valid synonyms is still lacking; investigations continue to list many Oligoryzomys as species indeterminate (e.g., Anderson, 1997; Andrades-Miranda et al., 2001a; Carleton and Musser, 1989). Inclusion of vouchered material from type localities will prove crucial to advancing comprehension of specific diversity and stabilizing nomenclatural usage within the genus.