Oryzomys is one of about thirty genera within the tribe Oryzomyini, a diverse group of well over a hundred species, many of which were formerly also included in Oryzomys. Oryzomyini is one of several tribes within the subfamily Sigmodontinae of the family Cricetidae, which includes hundreds of other species of mainly small rodents, distributed mainly in the Americas and Eurasia.
Within Oryzomyini, a 2006 phylogenetic analysis by Marcelo Weksler which used both morphological and DNA sequence data found some evidence that Oryzomys is most closely related to a group including Holochilus, Lundomys, and Pseudoryzomys. Although analyses based on morphological and combined data supported this relationship, sequences of the Rbp3 gene alone instead placed Oryzomys among a group that included Nectomys, Sigmodontomys, and a few other genera. In all analyses, Oryzomys appeared within clade D of Oryzomyini. The relationship between Oryzomys and the Holochilus group was supported by five synapomorphies (shared derived characters)—absence or reduction of both the hypothenar and interdigital pads; reduction of ungual tufts of hairs surrounding the claws; having the back margin of the zygomatic plate of the skull at the same level as the front of the first upper molar; and the anterocone (front cusp) of the first upper molar divided by an anteromedian fossette. The first three are adaptations to the semiaquatic lifestyle that Oryzomys and the members of the Holochilus group share, and may thus be examples of convergent evolution.
The name Oryzomys was introduced in 1857 by Spencer Fullerton Baird for the marsh rice rat (now Oryzomys palustris) of the eastern United States, which had been first described twenty years earlier by Richard Harlan. The name combines the Greek oryza "rice" and mys "mouse" and refers to the feeding habits of the marsh rice rat. Baird placed Oryzomys as a subgenus of the now-defunct genus Hesperomys and included only the marsh rice rat in it, a classification which was followed by Elliott Coues in 1874 and 1877. In 1890, Oryzomys was raised to generic rank, and in subsequent years numerous additional species were ascribed to it, many of which were soon moved to separate genera. In the 1898 Catalogus Mammalium, Édouard Louis Trouessart listed 67 species of Oryzomys, including some that are now placed in Calomys, Necromys, Thomasomys, and other genera unrelated to Oryzomys. Some of the new genera proposed were soon subsumed in Oryzomys again, and in The Families and Genera of Living Rodents (1941), John Ellerman listed Microryzomys, Oligoryzomys, Melanomys, Nesoryzomys, and Oecomys as synonyms of Oryzomys and included about 127 species in it. In 1948, Philip Hershkovitz suggested that other oryzomyines like Nectomys and Megalomys could as well be included in Oryzomys, and
Clayton Ray followed this suggestion in 1962.
Hershkovitz and Ray's classification was never widely followed, and from 1976 on authors started to reinstate some of the other groups lumped in Oryzomys as separate genera. The genus was reduced to 43 species (out of 110 in Oryzomyini) in the third edition (2005) of Mammal Species of the World, but it was still not a natural, monophyletic group; rather, it mostly united those oryzomyines that lacked the conspicuous specializations of other genera. In 2006, Marcelo Weksler's comprehensive phylogenetic analysis produced further evidence that the genus was polyphyletic, as species of Oryzomys were dispersed all over the oryzomyine tree. He proposed that eleven new genera should be created to accommodate those species that were not closely related to the type species of Oryzomys, the marsh rice rat; he considered other options that would require fewer new genera, but argued that that would result in less meaningful genus-level groups in Oryzomyini. Later in the same year, Weksler, Percequillo, and Voss created ten new genera—Aegialomys, Cerradomys, Eremoryzomys, Euryoryzomys, Hylaeamys, Mindomys, Nephelomys, Oreoryzomys, Sooretamys, and Transandinomys—for species formerly placed in Oryzomys and placed six more species related to "Oryzomys" alfaroi in Handleyomys pending the description of more new genera for them. They left only five species in Oryzomys, which was now finally a natural, monophyletic group. Because of subsequent taxonomic work, the number of species has since increased to at least eight.
Some problems remain: ?Oryzomys pliocaenicus, a Miocene fossil from Kansas, is of uncertain identity but may belong in
Bensonomys, and fossils from the Miocene of Oregon and Pliocene of New Mexico have also been ascribed to Oryzomys, but probably incorrectly. A possible Oryzomys has been recorded from the Irvingtonian (Pleistocene) of Saskatchewan.
The current concept of Oryzomys derives from the palustris-mexicanus group recognized within a much larger genus Oryzomys by Merriam (1901) and the palustris group proposed by Goldman (1918). Merriam recognized 21 species within his group, but Goldman consolidated them into eight—the marsh rice rat in the United States, O. couesi in far southern Texas, Mexico, and Central America, and six others with small distributions. In 1960,
Raymond Hall united O. couesi and the marsh rice rat into a single species, Oryzomys palustris, and thereafter, other localized forms were also included in O. palustris. Hershkovitz described another species in the group, O. gorgasi from Colombia, in 1970 and the next year he noted that O. dimidiatus, previously classified as a Nectomys, was similar to O. palustris. After 1979, the marsh rice rat and O. couesi were again regarded as separate as a result of further work in Texas, where their ranges meet. While reviewing O. gorgasi in 2001, J. Sánchez H. and colleagues redefined and characterized the O. palustris group and listed O. couesi, O. dimidiatus, O. gorgasi, and the marsh rice rat as its members; Guy Musser and
Michael Carleton in the 2005 third edition of Mammal Species of the World additionally listed O. nelsoni from María Madre Island in western Mexico.
In 2006, Weksler and colleagues followed the 2001 definition by Sánchez and others for the restricted genus Oryzomys, but added O. antillarum from Jamaica as a species. Carleton and Joaquin Arroyo-Cabrales reviewed Oryzomys from western Mexico in 2009 and in this context provided an extended diagnosis of Oryzomys. They recognized eight species: the six previously mentioned plus O. albiventer and O. peninsulae. Also in 2009, Robert Voss and Weksler identified the subfossil Oryzomys curasoae from Curaçao as an island population of O. gorgasi. The next year, Delton Hanson and colleagues published a study using DNA sequence data from the cytochrome b, interphotoreceptor retinoid-binding protein, and alcohol dehydrogenase 1 genes to assess relationships within Oryzomys. They recommended that the marsh rice rat be split into two species and that O. couesi be split into four species on the basis of the observed sequence divergence and other data.
Merriam divided his palustris-mexicanus group in two "series" according to the color of the underparts (white or fulvous). Goldman divided his palustris group in two "sections"—a couesi section with O. couesi and six related species, and a palustris section with O. palustris only. He noted that the latter differed from the former in the generally darker, more brownish, longer fur, and larger sphenopalatine vacuities (openings in the
mesopterygoid fossa, the gap behind the end of the palate). As Weksler's 2006 analysis included only O. couesi and the marsh rice rat among species of Oryzomys in the strict sense, he could not test those groups. Carleton and Arroyo-Cabrales concurred with Goldman's division, listing additional characters, and noted that the palustris group may be more semiaquatically adapted than the members of the couesi group are. In the latter, the fur is usually reddish-brown, as opposed to grayish-brown in the palustris group. Members of the couesi group have smaller sphenopalatine vacuities and a smaller sphenopalatine foramen, a foramen (opening) in the side of the skull above the molars, and a more highly developed anterolabial cingulum on the third lower molar (a crest at the front of the tooth). The hypothenar pad of the hindfoot, located on the sole far from the fingers, is present in the couesi group, but absent in the palustris group. Interdigital webbing may be more highly developed in the palustris group. Using morphological data, Voss and Weksler found a closer relationship between O. couesi and O. gorgasi to the exclusion of O. palustris, but with low confidence. The DNA sequence data of Hanson and colleagues supported a deep separation between the palustris and couesi groups, but a Costa Rican sample (assigned to O. couesi) was about as distant from the two groups as they were from each other.
The genus currently includes the following species:
||Michoacán, Guanajuato, Jalisco (inland western Mexico)
||Large, long tail, robust skull
||Described in 1901 as a species; subspecies of O. couesi in 1918; reinstated as a species in 2009
||Long nasal bones, short incisive foramina (perforations in the palate), robust zygomatic arches (cheekbones)
||Described in 1898 as a species; subspecies of O. couesi in 1966; reinstated as a species in 1993/2006
||Southern Texas and coastal Sonora (northwestern Mexico) to northwestern Colombia
||Upperparts buff to reddish, underparts white to buff, sphenopalatine vacuities small to absent
||Described in 1877 as a species; subspecies of marsh rice rat in 1960; reinstated as a species in 1979; various other species split from and lumped into it from time to time. Genetic data suggest four species may be present: one along the Pacific coast from Sonora to El Salvador; one from Texas to Nicaragua; one in Costa Rica; and one in Panama.
||Gray underparts, brown feet, tail nearly the same color above as below
||Described in 1905 as a species of Nectomys; reclassified in Oryzomys in 1948
||Northwestern Colombia, northwestern Venezuela; extinct on Curaçao
||Robust rostrum (front part of skull); incisive foramina tapering at the back; sphenopalatine vacuities absent; subsquamosal fenestra (opening at the back of the skull) small
||Described in 1970; Oryzomys curasoae described in 2001; both synonymized in 2009
||María Madre Island, Nayarit, western Mexico (extinct)
||Large, long tail, rostrum heavy and bent downward, incisors large and wide
||Described in 1898 as a species; subspecies of O. palustris in 1971; otherwise regarded as a distinct species
|Marsh rice rat (Oryzomys palustris)
||In the eastern United States from New Jersey and Kansas south to Florida and Texas, and into Tamaulipas, Mexico; previously further north to Iowa and southwestern Pennsylvania
||Compared to sympatric O. couesi where the two meet: short tail, white underparts, sphenopalatine foramen large
||Described in 1827; specific status of Florida Keys form (argentatus; first described in 1978) disputed; genetic data suggest populations west of Alabama may be a separate species
||Southern tip of Baja California Sur (perhaps extinct)
||Moderately large, gray on head and forequarters, broad, squared zygomatic arches, long, broad incisive foramina, upper incisor nearly orthodont
||Described in 1899 as a species; subspecies of O. palustris in 1971 and of O. couesi in 1994; reinstated as a species in 2009