Oryzomys dimidiatus

The first known specimen was obtained by W.G. Palmer in 1904 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. 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. J. Hernández-Camacho described a second species of Micronectomys, Oryzomys (Micronectomys) borreroi, from Colombia in 1957. 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. Nevertheless, he did not do anything to rectify the situation, and Micronectomys remains a nomen nudum. 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). 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. Later workers affirmed the relationship between O. dimidiatus, O. palustris and associated species like O. couesi. 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. Oryzomys dimidiatus is now recognized as one of eight species in the genus Oryzomys. O. dimidiatus is further part of the O. couesi section, which is centered on the widespread Central American O. couesi and also includes six other species with more limited and peripheral distributions. O. couesi occurs with O. dimidiatus in southeastern Nicaragua. 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. Oryzomys is classified in the tribe Oryzomyini ("rice rats"), a diverse assemblage of American rodents of over a hundred species, and on higher taxonomic levels in the subfamily Sigmodontinae of family Cricetidae, along with hundreds of other species of mainly small rodents. Oryzomys dimidiatus or "divided in the middle rice rat" derives respectively from the Greek oryza "rice", mys "mouse, rat", and the Latin dimidiatus "divided in the middle", from dimidio "halve, divide in two equal parts". ==Description==

(Oryzomys palustris) is similar to O. dimidiatus. Compared to that of Nectomys, the skull is lightly built and has narrow nasals and a broad, round braincase without conspicuous ridges on it. The zygomatic plate is broad. The incisive foramina (perforations of the front part of the palate) extend between the first molars and are broadest in their back halves. The broad mesopterygoid fossa, the gap behind the end of the palate, is perforated by sphenopalatine vacuities. The molar crowns are not as simplified as in Nectomys species, but the front cusps of the upper (anterocone) and lower first molar (anteroconid) are not divided in two. In addition to the main roots, the upper and lower first molars have smaller additional roots. Measurements for the first two known examples are as follows (in each case, the first measurement given is from the holotype, taken in 1904, the second, from the specimen taken in 1966): head and body length 125 and 118 mm (4.9 and 4.6 in), tail length 115 and 110 mm (4.5 and 4.3 in), hindfoot length 27 and 28 mm (1.1 and 1.1 in), ear length 13 and 15 mm (0.51 and 0.59 in), skull length 29.8 and 29.0 mm (1.17 and 1.14 in). The 1966 specimen weighed 46.0 g (1.62 oz) and had testes 11 mm (0.43 in) long; these measurements were not recorded in the 1904 specimen. Reid, who mentioned the third specimen, reported a maximum head and body length of 128 mm (5.0 in), tail length of 150 mm (5.9 in), hindfoot length of 31 mm (1.2 in), and ear length of 19 mm (0.75 in). ==Distribution, ecology, and behavior==

Oryzomys dimidiatus is known from three specimens collected in the lowlands of the South Caribbean Coast Autonomous Region in southeastern Nicaragua. The first, an old male, was collected on November 5, 1904, in a banana plantation with very moist red clay on the Río Escondido near El Rama. The third was caught at a stream near Bluefields. Reid suggested that the species is semiaquatic, spending some time in the water, The 2019 IUCN Red List assessed the conservation status of Oryzomys dimidiatus as "Data Deficient", noting that very little is currently known about the species. ==References==