Taxonomy
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.
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