Thomasomyini. Tribe (1996) explained why the genus-group name Rhipidomys is available from Tschudi (1845), not 1844 as conventionally cited (e.g., Cabrera, 1961; Ellerman, 1941). External and cranial morphology characterized and contrasted with superficially similar species of Oecomys by Tribe (1996), Patton et al. (2000), and Voss et al. (2001). Sister genus to the Chilomys Thomasomys clade based on phylogenetic evaluations of mitochondrial DNA sequences from 2-6 species surveyed (D’Elía et al., 2003; Smith and Patton, 1999). As noted by Tribe (1996), monophyly of the associated species within a single genus has yet to be convincingly demonstrated; relationships among the distinctive morphologies identifiable within the taxon may involve other thomasomyines and eventual rearrangement of generic boundaries. Tribe designated those morphologies as three formal "sections": Fulviventer (R. caucensis, R. fulviventer, R. venustus, R. wetzeli); Leucodactylus (R. austrinus, R. couesi, R. emiliae, R. gardneri, R. latimanus, R. leucodactylus; R. macrurus, R. mastacalis, R. modicus, R. nitela, R. ochrogaster, R. venezuelae); and Macconnelli (R. macconnelli).
No published revisionary standard is available. Species and synonyms acknowledged herein in general observe our earlier listing (1993), as based on examination of holotypes and series in AMNH, BMNH, and USNM, and the unpublished thesis of Tribe (1996), which is the best synthesis of valid species and their distributional limits based on exhaustive examination of museum specimens. Nonetheless, the distributions of many species remain inadequately documented, and the continued listing of Rhipidomys as species indeterminate or undescribed species (e.g., Anderson, 1997; Costa, 2003; Pereira et al., 2001; Tribe, 1996) concedes our incomplete knowledge of their alpha-level systematics. Volobouev and Catzeflis (2000) summarized karyotypic data for the genus and discussed mechanisms of cytogenetic change; Andrades-Miranda (2002) provided additional karyotypic records and also summarized reports for the genus; Costa (2003) analyzed phylogeographic patterns among select species distributed in the Amazon Basin and Atlantic Forest.