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Oryzomys peninsulae
''Oryzomys peninsulae'', also known as the Lower California rice rat,〔Goldman, 1918, p. 45〕 is a species of rodent from western Mexico. Restricted to the southern tip of the Baja California peninsula, it is a member of the genus ''Oryzomys'' of family Cricetidae. Only about twenty individuals, collected around 1900, are known, and subsequent destruction of its riverine habitat may have driven the species to extinction.
Medium in size for its genus, it was first described as a separate species, but later lumped into other, widespread species until it was reinstated as separate in 2009. It is distinctive in fur color—grayish brown on the forequarters and reddish brown on the hindquarters—and in some dimensions of its skull, with a high braincase, robust zygomatic arches (cheekbones), and long incisive foramina (perforations of the palate between the incisors and the molars).
==Taxonomy==
''Oryzomys peninsulae'' was first collected in 1896 and Oldfield Thomas described it in 1897 as a full species of ''Oryzomys''.〔Thomas, 1897, p. 548〕 It was retained as a distinct species related to ''O. couesi'' and ''O. palustris'' until 1971, when Philip Hershkovitz swept it, and other outlying populations of the same species group, as subspecies under an expanded concept of ''O. palustris''.〔 Raymond Hall concurred in the second edition (1981) of ''Mammals of North America'', arguing that ''O. peninsulae'' differed less from mainland ''Oryzomys'' populations (currently classified as ''O. couesi mexicanus'') than some other forms he included in ''O. palustris'' differed from each other.〔Hall, 1981, pp. 610–611〕 After studies of the contact zone between North American ''O. palustris'' and Central American ''O. couesi'' in southern Texas and northeastern Tamaulipas (by Benson and Gehlbach in 1979 and Schmidt and Engstrom in 1994) made clear that the two are distinct from each other, ''O. peninsulae'' remained as a subspecies of ''O. couesi''.〔Carleton and Arroyo-Cabrales, 2009, p. 122; Benson and Gehlbach, 1979, p. 227; Schmidt and Engstrom, 1994, p. 914〕 In 2009, Michael Carleton and Joaquín Arroyo-Cabrales reviewed the classification of western Mexican ''Oryzomys'' and used morphological and morphometrical data to characterize four distinct ''Oryzomys'' species in the region. ''O. peninsulae'' and another isolated population, ''O. nelsoni'' from the Islas Marías, were both retained as separate species, as was ''O. albiventer'' from montane mainland Mexico. They kept the population in the coastal lowlands as a subspecies, ''O. couesi mexicanus'', of ''Oryzomys couesi''.〔Carleton and Arroyo-Cabrales, 2009, p. 94〕
The genus ''Oryzomys'' currently includes about eight species distributed from the eastern United States (''O. palustris'') into northwestern South America (''O. gorgasi'').〔Carleton and Arroyo-Cabrales, 2009, p. 106〕 ''O. peninsulae'' is part of the ''O. couesi'' section, which is centered on the widespread Central American ''O. couesi'' and also includes various other species with more limited and peripheral distributions.〔Carleton and Arroyo-Cabrales, 2009, p. 117〕 Many aspects of the systematics of this section remain unclear and it is likely that the current classification underestimates the group's true diversity.〔Carleton and Arroyo-Cabrales, 2009, p. 107〕 ''Oryzomys'' was previously a much larger genus, but most species were progressively removed in various studies, culminating in contributions by Marcelo Weksler and coworkers in 2006 that excluded more than forty species from the genus.〔Weksler et al., 2006, table 1〕 ''Oryzomys'' and many of the species removed from it are 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〕
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