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Oryzomys nelsoni, also known as the Nelson’s Rice Rat, is an extinction 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 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 . It was reddish to yellowish above and mostly white below. Its diet may have included plant material and small animals.
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 morphometrics 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 ( Oryzomys 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.Weksler et al., 2006, table 1 All 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
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.
Oryzomys nelsoni was distinctive in its large skull with broad, well-developed 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 foramen, 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
The species is now considered extinct,Álvarez-Castañeda and Méndez, 2003, p. 1; Musser and Carleton, 2005, p. 1152; Timm et al., 2008; Carleton and Arroyo-Cabrales, 2009, p. 114 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.
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 to , 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.
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,Álvarez-Castañeda and Méndez, 2003, p. 1; Carleton and Arroyo-Cabrales, 2009, p. 114 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.Wilson, 1991, p. 239; Carleton and Arroyo-Cabrales, 2009, p. 114
Literature cited
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