Eremoryzomys polius, also known as the gray rice rat[1] or the Marañon oryzomys,[5] is a rodent species in the tribe Oryzomyini of the family Cricetidae.

In 2006, a cladistic analysis found that it was not closely related to Oryzomys in the strict sense or to any other oryzomyine then known, so that it is now placed in its own genus, Eremoryzomys.

Eremoryzomys has a limited distribution in the dry upper valley of the Marañón River in central Peru, but may yet contain more than one species.

A large, long-tailed rice rat, with head and body length of 138 to 164 mm (5.4 to 6.5 in), E. polius has gray fur and short ears.

The IUCN assesses the conservation status of the species as "Data Deficient"; it is poorly known but may be threatened by habitat destruction.

[7] In 2006, Marcelo Weksler published a large-scale cladistic analysis of Oryzomyini ("rice rats"), the tribe to which O. polius belongs.

In all of his analyses, O. polius was found to be part of clade D, one of four large groups within Oryzomyini, as the sister group to a clade containing all the other species of clade D.[8] Clade D was supported by two shared derived (synapomorphic) molecular characters and by seven morphological synapomorphies—the tail has a different color above and below; the parietal bone extends to the side of the skull; the incisive foramina (openings in the palate) extend back between the first molars; the posterolateral palatal pits (perforations of the palate near the third molars) are complex; the sphenopalatine vacuities (openings in the mesopterygoid fossa, the gap behind the end of the palate) are large; the pattern of the arterial circulation in the head is derived; and the posteroloph (a crest at the back) is present on the third upper molar.

[1] In 2011, a new oryzomyine, Drymoreomys albimaculatus, was described from southeastern Brazil, and phylogenetic analysis of morphological and molecular data suggested that this animal is the closest known relative of Eremoryzomys.

Sigmodontinae itself is the largest subfamily of the family Cricetidae, other members of which include voles, lemmings, hamsters, and deermice, all mainly from Eurasia and North America.

[14] Eremoryzomys polius is a large, long-tailed rice rat that in color resembles some North American woodrats (Neotoma).

[12] The hindfeet have well-developed ungual tufts (patches of hair) along the plantar margins and between all of the digits, a character shared only with Sooretamys angouya among oryzomyines.

[23] The squamosal lacks a suspensory process that contacts the tegmen tympani, the roof of the tympanic cavity, a defining character of oryzomyines.

[29] As far as now known, Eremoryzomys polius is confined to a small area in central Peru, at an altitude of 760 to 2,100 m (2,490 to 6,890 ft),[30] but the species may range more widely.

It occurs in forest in the dry lowlands of the upper parts of the basin of the Marañón River, east of the main mountain range of the Andes.