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''Oryzomys dimidiatus'', also known as the Nicaraguan oryzomys,〔Musser and Carleton, 2005, p. 1148〕 Thomas's rice rat,〔 or Nicaraguan rice rat,〔Duff and Lawson, 2004, p. 54; Jones and Engstrom, 1986, p. 12〕 is a rodent in the genus ''Oryzomys'' of the family Cricetidae. It is known from only three specimens, all collected in southeastern Nicaragua since 1904. Placed in ''Nectomys'' upon its discovery, it was later classified in its own subgenus of ''Oryzomys'' and finally recognized as closely related to other species now placed in ''Oryzomys'', including the marsh rice rat and ''Oryzomys couesi'', which occurs in the same region. With a head and body length of 110 to 128 mm (4.3 to 5.0 in), ''Oryzomys dimidiatus'' is a medium-sized rice rat. The upperparts are gray-brown and the underparts are grayish, not buffy as in ''O. couesi''. The tail is only slightly darker above than below. All three specimens were caught near water and the species may be semiaquatic, spending some time in the water. Its conservation status is currently assessed as "Least Concern". ==Taxonomy== The first known specimen was obtained by W.G. Palmer in 1904〔Thomas, 1905, p. 587〕 and the next year, Oldfield Thomas of the British Museum of Natural History described this animal as the holotype of a new species he named ''Nectomys dimidiatus''.〔 He placed it in the genus ''Nectomys'', commenting that it was much smaller than but otherwise similar to previously known members of that genus.〔 The species was listed as a ''Nectomys'' in taxonomic overviews in the next decades, including a 1944 review of the genus by Philip Hershkovitz.〔Hershkovitz, 1944, p. 80〕 After examining the holotype in London, Hershkovitz instead placed the species in the genus ''Oryzomys'' in 1948. He remarked that it was an especially distinctive member of that genus, and hence classified it in its own subgenus ''Micronectomys''.〔Hershkovitz, 1948, pp. 54–55〕 J. Hernández-Camacho described a second species of ''Micronectomys'', ''Oryzomys (Micronectomys) borreroi'', from Colombia in 1957.〔Hershkovitz, 1970, p. 792〕 In 1970, Hershkovitz treated ''O. dimidiatus'' in another publication and conceded that his name ''Micronectomys'' was a ''nomen nudum'' ("naked name") because he had not explicitly mentioned characters differentiating it from other taxa in his 1948 publication.〔Hershkovitz, 1970, p. 791〕 Nevertheless, he did not do anything to rectify the situation, and ''Micronectomys'' remains a ''nomen nudum''.〔Musser and Carleton, 2005, p. 1144〕 Hershkovitz also noted that while ''O. dimidiatus'' resembles a juvenile ''Nectomys'' in external anatomy, it is otherwise similar to the marsh rice rat (''Oryzomys palustris'').〔 He accepted ''O. borreroi'' as an ''Oryzomys'', but did not think it closely related to ''O. dimidiatus''.〔 Six years later, Alfred Gardner and James Patton instead suggested that ''O. borreroi'' was a ''Zygodontomys'', and in his 1991 review of that genus Robert Voss confirmed that it is the same as ''Zygodontomys brunneus''.〔Voss, 1991, p. 46〕 A second specimen was obtained in 1966 and the find was published in 1971 by Hugh Genoways and Knox Jones, who noted that the species is closely similar to ''O. palustris''.〔Genoways and Jones, 1971, p. 433〕 Later workers affirmed the relationship between ''O. dimidiatus'', ''O. palustris'' and associated species like ''O. couesi''.〔 Fiona Reid reported in 1997 that a third specimen had been found.〔Reid, 1997, p. 204〕 In 2006, Marcelo Weksler and coworkers removed most of the species formerly placed in ''Oryzomys'' from the genus, because they are not closely related to the type species ''O. palustris'', but kept ''O. dimidiatus'' as an ''Oryzomys''.〔Weksler et al., 2006, table 1, footnote e〕 ''Oryzomys dimidiatus'' is now recognized as one of eight species in the genus ''Oryzomys''.〔Carleton and Arroyo-Cabrales, 2009, p. 116〕 ''O. dimidiatus'' is further part of the ''O. couesi'' section, which is centered around the widespread Central American ''O. couesi'' and also includes six other species with more limited and peripheral distributions.〔Carleton and Arroyo-Cabrales, 2009, p. 117〕 ''O. couesi'' occurs with ''O. dimidiatus'' in southeastern Nicaragua.〔Jones and Engstrom, 1986, p. 12〕 Many aspects of the systematics of the ''O. couesi'' section remain unclear and it is likely that the current classification underestimates the true diversity of the group.〔Carleton and Arroyo-Cabrales, 2009, p. 107〕 ''Oryzomys'' is classified in the tribe Oryzomyini ("rice rats"), a diverse assemblage of American rodents of over a hundred species,〔Weksler, 2006, p. 3〕 and on higher taxonomic levels in the subfamily Sigmodontinae of family Cricetidae, along with hundreds of other species of mainly small rodents.〔Musser and Carleton, 2005〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Oryzomys dimidiatus」の詳細全文を読む スポンサード リンク
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