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We describe the morphological species boundaries and geographic distributions of 10 Neotropical Oryzomys based on analyses of museum specimens (skins and skulls, examples preserved in fluid, chromosomal spreads, and information about collection sites from skin tags, field catalogs, and other sources). These species have been regarded as members of an Oryzomys capito complex and for a long time were consolidated into a single entity identified as O. capito.
Our study documents the following:
1. Defining the limits of species within the O. capito complex first requires a comprehensive review and rigorous definition of O. capito itself. We consider Fischer's (1814) Mus megacephalus to be valid and available, designate a neotype to bear the name, and reinstate it as a senior synonym of capito Olfers (1818). We then provide a working definition of O. megacephalus and its close relative, O. laticeps, derived from analyses of morphometric variation, estimates of geographic distributions, and evaluations of synonyms. In our view, O. megacephalus occurs in Amazonia but also extends into eastern Paraguay; its synonyms are capito Olfers (1818), cephalotes Desmarest (1819), velutinus Allen and Chapman (1893), goeldi Thomas (1897), modestus Allen (1899), and perenensis Allen (1901). Oryzomys laticeps Lund (1840) occurs in the Atlantic Forest region of eastern Brazil. We designate a lectotype for laticeps and allocate the names saltator Winge (1887) and oniscus Thomas (1904) as synonyms.
2. We provide the first comprehensive taxonomic revision of Oryzomys yunganus Thomas (1902). Its range covers tropical evergreen rainforest formations in the Guiana Region and the Amazon Basin where, as documented by voucher specimens, it has been collected at the same localities as O. megacephalus, O. nitidus, and O. macconnelli. Specimens of O. yunganus can be distinguished from those of the other three by a combination of body size, pelage texture and coloration, pattern of carotid arterial circulation, occlusal patterns of second upper and lower molars, cranial proportions, and chromosomal features.
Appreciable intraspecific geographic variation occurs in diploid number of chromosomes and frequency of occurrence of the hypothenar plantar pad, but sampling inadequacies obscure the significance of this variation. Large body size is characteristic of populations in the western Amazon Basin and in the tepui region of eastern Venezuela; smaller size characterizes populations in the Guianas and along the eastern margin of the Amazon Basin. No other scientific name has been correctly associated with the species. Samples from Mirador, Palmera, and Mera in the western Andean foothills of central Ecuador possess a combination of pelage, cranial, and dental traits that distinguish them from all samples of O. yunganus. These specimens are the basis for a new species we describe here, one that is more closely related to O. yunganus than to any other member of the former O. 'capito' complex.
3. We redescribe Oryzomys bolivaris (reviewed by Pine, 1971, under the name O. bombycinus), amplify its geographic range, and contrast it with O. talamancae and O. alfaroi, two sympatric congeners with which it is often confused. A distinctive set of morphological traits allows unambiguous identification of specimens belonging to O. bolivaris. It is a trans-Andean species recorded from very wet tropical evergreen rain forests extending from eastern Honduras and Nicaragua through Costa Rica and Panamá to western Colombia and Ecuador. Allen's (1901) bolivaris is the oldest name for this species; castaneus Allen (1901), rivularis Allen (1901), bombycinus Goldman (1912), alleni Goldman (1915), and orinus Pearson (1939) are synonyms.
4. We revise the definition of Oryzomys talamancae Allen (1891) provided by Musser and Williams (1985), document additional specimens, describe karyotypes from Ecuadoran and Venezuelan samples, and contrast its morphology, chromosomes, and distribution with those of O. alfaroi and O. megacephalus. The geographic distribution of O. talamancae is also trans-Andean, but it inhabits a wider variety of habitats than does O. bolivaris. We also provide a new synonymy and identify the following scientific names as synonyms of O. talamancae: mollipilosus Allen (1899), magdalenae Allen (1899), villosus Allen (1899), sylvaticus Thomas (1900), panamensis Thomas (1901), medius Robinson and Lyon (1901), and carrikeri Allen (1908).
5. We present hypotheses of species boundaries of four morphologically similar species that we identify as members of the Oryzomys nitidus group: O. nitidus Thomas (1884), O. macconnelli Thomas (1910), O. russatus Wagner (1848), and a species described as new. We recognize the four species by morphological and chromosomal traits, and contrast characteristics of each species with one another. One synonym, boliviae Thomas (1901), is associated with O. nitidus, and two scientific names, incertus Allen (1913) and mureliae Allen (1915), are allocated to O. macconnelli. Synonyms of O. russatus are physodes Brants (1827), intermedia Leche (1886), coronatus Winge (1887), lamia Thomas (1901), legatus Thomas (1925), kelloggi Ávila-Pires (1959), and moojeni Ávila-Pires (1959). We designate lectotypes for russatus and intermedia and identify the holotype of coronatus. Based on voucher specimens, the geographic distribution of O. nitidus is mainly along the Andean foothills and adjacent lowlands in Perú, Bolivia, and nearby western Brazil, but scattered records document its eastward extension through south-central Brazil to Paraguay and northeastern Argentina. Oryzomys macconnelli inhabits the tropical evergreen rain forests of Amazonia. Its distribution partially overlaps that of O. nitidus in western Amazonia, where the two species have been collected together at one locality in Perú, and it is sympatric with the new species, which is recorded only from the lower regions of rios Xingu and Tocantins in northern Pará, Brazil. The distribution of O. russatus is documented by specimens from southeastern and south-central Brazil, southern Bolivia, and northern Argentina; its range is allopatric to those of O. macconnelli, the new species, and O. nitidus except in southern Bolivia, where the latter was collected at the same site with O. russatus. We also examined types and descriptions of taxa associated with Oryzomys subflavus and O. ratticeps to determine if any of those names actually reference members of the O. nitidus group. Although the original description of subflavus Wagner (1842) is vague, the holotype clearly represents an example of that very distinctive species; vulpinus Lund (1840), for which we designate a lectotype, and vulpinoides Schinz (1845) are synonyms of O. subflavus. The oldest name for the species currently known as Oryzomys ratticeps is Mus angouya Fischer (1814), a name not based on a specimen but on Azara's (1801) description of 'Rat troisième, ou Rat Angouya.' Azara's account is so general that it could also apply to individuals of O. subflavus, O. nitidus, or O. russatus. To stabilize the nomenclature of these species, we designate a neotype for Mus angouya Fischer (1814) and treat the following scientific names as synonyms: buccinatus Olfers (1818), leucogaster Wagner (1845), ratticeps Hensel (1872), rex Winge (1887), tropicius Thomas (1924), and paraganus Thomas (1924). We also designate lectotypes for leucogaster and ratticeps. We have not analyzed phylogenetic relationships among the species in the former O. 'capito' complex discussed here. Documenting morphological and distributional boundaries of other biological species now grouped in the genus Oryzomys (alfaroi and its close relatives, for example) must precede, in our view, attempts at phylogenetic reconstruction.
