Oryzomys Nelsoni

''Oryzomys nelsoni'' is an extinct rodent of María Madre Island, Nayarit, Mexico. Within the genus ''Oryzomys'' of the family Cricetidae, it may have been most closely related to the mainland species '' O. albiventer''. Since its first description in 1898, most authors have regarded it as a distinct species, but it has also been classified as a mere subspecies of the marsh rice rat (''O. palustris'').

After its discovery in 1897, it has never been recorded again and it is now considered extinct; the presence of introduced black rats on María Madre may have contributed to its extinction. ''Oryzomys nelsoni'' was a large species, distinguished in particular by its long tail, robust skull, and large incisors. It was reddish to yellowish above and mostly white below. Its diet may have included plant material and small animals.

Taxonomy

''Oryzomys nelsoni'' was collected by Edward William Nelson and Edward Goldman in May 1897 and never found again. Their visit for the Biological Survey of the United States Department of Agriculture was one of the first scientific exploration of the islands. Clinton Hart Merriam identified the mammals they obtained, including four specimens of ''Oryzomys nelsoni'', which were deposited in the United States National Museum and remain there. He named it as a species of the genus ''Oryzomys'', ''Oryzomys nelsoni''; the specific name honors Nelson. Investigators have generally retained it as a species distinct from other ''Oryzomys'', but in 1971 Hershkovitz listed it as one of many subspecies of ''Oryzomys palustris'', which he envisaged as a wide-ranging species encompassing what is now the marsh rice rat (''O. palustris'') of the southern and eastern United States, '' O. couesi'' of Central America, and several other species with more limited distributions.

In his 1918 revision of North American ''Oryzomys'', Goldman considered ''O. nelsoni'' to be most closely related to the nearest mainland subspecies of ''O. couesi'', ''O. couesi mexicanus''. In 2009, Michael Carleton and Joaquin Arroyo-Cabrales revised the ''Oryzomys'' of western Mexico and confirmed that ''O. nelsoni'' is a very distinct species. Their morphometrical analysis found some resemblance between the species and ''Oryzomys albiventer'' of interior mainland Mexico, and they suggested that although ''O. nelsoni'' likely represents an old, distinctive lineage, it may have derived from a common ancestor with ''O. albiventer''.Carleton and Arroyo-Cabrales, 2009, p. 110

''Oryzomys nelsoni'' is one of about eight species in the genus ''Oryzomys'', which occurs from the eastern United States (''O. palustris'') into northwestern South America ('' O. gorgasi'').Carleton and Arroyo-Cabrales, 2009, p. 106 ''O. nelsoni'' is further 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 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'' previously included many other species, which were progressively removed in various studies culminating in a contribution by Marcelo Weksler and coworkers in 2006 that removed more than forty species from the genus. All are 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.

Common names proposed for this species include Nelson rice rat,Goldman, 1918, p. 46 Nelson's rice rat,Álvarez-Castañeda and Méndez, 2003, p. 1 Nelson's oryzomys,Musser and Carleton, 2005, p. 1152 and Tres Marias Island rice rat.

Description

''Oryzomys nelsoni'' was a large and long-tailed ''Oryzomys''; its tail was longer than that of any other western Mexican ''Oryzomys''. The upperparts were ochraceous to buff, most richly so on the rump, and paler further to the front and low on the flanks. On the head and the back, blackish hairs somewhat darkened the overall color. The underparts were white, with lead-colored underfur that was visible in some places. The ears were covered on both sides with scanty grayish hairs. The large hindfeet were sparsely covered with pale hairs. The tail was largely dark, but the underside of the basal one third to one half was light yellow.

''Oryzomys nelsoni'' was distinctive in its large skull with broad, well-developed incisors and a strong front part (rostrum) that is strongly curved downwards.Carleton and Arroyo-Cabrales, 2009, p. 121 In ''O. albiventer'', the rostrum and incisors were not as massive, but the molars are larger. The interparietal bone, part of the roof of the braincase, was broad and the incisive foramina, which perforated the palate between the incisors and the molars, were relatively short.

Total length in the four known specimens is , averaging ; head and body length is , averaging ; tail length is , averaging ; and hindfoot length is , averaging .Carleton and Arroyo-Cabrales, 2009, table 2

Ecology and extinction

Nelson and Goldman found the species only in a damp, herbaceous site now known as the "Sacatal" near a spring high on María Madre Island, the largest of the Islas Marías off the coast of Nayarit, western Mexico, and Nelson wrote that it was rare. He gave the elevation of this place as 1800 ft,Nelson, 1899b, p. 16 which Álvarez-Castañeda and Méndez converted to 550 m, but in his 1918 paper, Goldman gave 800 ft instead, which Carleton and Arroyo-Cabrales in 2009 converted to 245 m.Carleton and Arroyo-Cabrales, 2009, p. 114 The next survey of small mammals on the island took place in March 1976 by a team led by Don E. Wilson. They failed to collect ''O. nelsoni'' and instead found only the introduced black rat (''Rattus rattus'') at the locality where Nelson and Goldman had collected ''O. nelsoni''; this species may have contributed to the decline of the indigenous rodent.

The species is now considered extinct, although as late as 2002 the Mexican government listed it as "threatened".Álvarez-Castañeda and Méndez, 2003, p. 2 Another Islas Marías endemic, the deermouse '' Peromyscus madrensis'', still occurred on María Madre in 1976.Musser and Carleton, 2005, p. 1071; Carleton and Arroyo-Cabrales, 2009, p. 114 ''Oryzomys nelsoni'' is thought to have fed on plant material such as weeds, fruit, and seeds, and more rarely on animals such as fish and invertebrates.

References

Literature cited

*Álvarez-Castañeda, S.T. and Méndez, L. 2003
Oryzomys nelsoni
''Mammalian Species'' 735:1–2.
*Carleton, M.D. and Arroyo-Cabrales, J. 2009
Review of the ''Oryzomys couesi'' complex (Rodentia: Cricetidae: Sigmodontinae) in western Mexico
''Bulletin of the American Museum of Natural History'' 331:94–127.
*Goldman, E.A. 1918
The rice rats of North America
''North American Fauna'' 43:1–100.
*Merriam, C.H. 1898
Mammals from the Tres Marias Islands, off western Mexico
''Proceedings of the Biological Society of Washington'' 12:13–19.
*Musser, G.G. and Carleton, M.D. 2005. Superfamily Muroidea. Pp. 894–1531 in Wilson, D.E. and Reeder, D.M. (eds.)
''Mammal Species of the World: a taxonomic and geographic reference''. 3rd ed
Baltimore: The Johns Hopkins University Press, 2 vols., 2142 pp. 
*Nelson, E.W. 1899a
General description of the Tres Marias Islands, Mexico
''North American Fauna'' 14:7–14.
*Nelson, E.W. 1899b
Mammals of the Tres Marias Islands
''North American Fauna'' 14:15–20.
*Weksler, M. 2006
Phylogenetic relationships of oryzomyine rodents (Muroidea: Sigmodontinae): separate and combined analyses of morphological and molecular data
''Bulletin of the American Museum of Natural History'' 296:1–149.
*Weksler, M., Percequillo, A.R. and Voss, R.S. 2006
Ten new genera of oryzomyine rodents (Cricetidae: Sigmodontinae)
''American Museum Novitates'' 3537:1–29.
*Wilson, D.E. 1991
Mammals of the Tres Marías Islands
''Bulletin of the American Museum of Natural History'' 206:214–250.

{{Taxonbar, from=Q1767964

Oryzomys
Endemic mammals of Mexico
Extinct rodents
Extinct animals of Mexico
Fauna of Islas Marías
Jalisco dry forests
Rodents of North America
Rodent extinctions since 1500
Taxa named by Clinton Hart Merriam
Mammals described in 1898