Oryzomys nelsoni, also known as the Nelson’s rice rat, 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. O. 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
O. 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 O. nelsoni, which were deposited in the United States National Museum and remain there. He named it as a species of the genus Oryzomys, O. nelsoni; the specific name honors Nelson. Investigators have generally retained it as a species distinct from other Oryzomys,
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. c. 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 O. 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.
O. 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). 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. 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. 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 40 species from the genus. All are classified in the tribe Oryzomyini ("rice rats"), a diverse assemblage of American rodents of over 100 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, Nelson's rice rat, Nelson's oryzomys, and Tres Marias Island rice rat.]]
O. nelsoni was a large and long-tailed Oryzomys; In O. albiventer, the rostrum and incisors were not as massive, but the molars are larger.
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, which Álvarez-Castañeda and Méndez converted to 550 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.