Tres Marías Rice Rat

O. 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. O. nelsoni was distinctive in its large skull with broad, well-developed incisors and a strong front part (rostrum) that is strongly curved downwards. 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 282 to 344 mm (11.1 to 13.5 in), averaging 322 mm (12.7 in); head and body length is 122 to 153 mm (4.8 to 6.0 in), averaging 140.5 mm (5.53 in); tail length is 160 to 191 mm (6.3 to 7.5 in), averaging 181.5 mm (7.15 in); and hindfoot length is 35 to 39 mm (1.4 to 1.5 in), averaging 37.3 mm (1.47 in).

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, 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. 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". Another Islas Marías endemic, the deermouse Peromyscus madrensis, still occurred on María Madre in 1976. O. 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.