Eremoryzomys
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. Discovered in 1912 and first described in 1913 by Wilfred Osgood, it was originally placed in Oryzomys and named Oryzomys polius. 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. The Brazilian genus Drymoreomys, named in 2011, is probably the closest relative of 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. There are well-developed ungual tufts of hair on the hindfeet. Females have eight mammae. The rostrum (front part of the skull) is long and robust and the braincase is rounded. The bony palate is relatively short. The IUCN assesses the conservation status of the species as "Data Deficient"; it is poorly known but may be threatened by habitat destruction.
The first two specimens of Eremoryzomys polius were collected by Wilfred Osgood and M.P. Anderson in 1912. The next year, Osgood described these animals as a new species in the genus Oryzomys, Oryzomys polius.[3] Osgood wrote that he was unable to find any species closely related to O. polius and compared it with O. xanthaeolus (currently Aegialomys xanthaeolus) "for convenience".[6] Its relationships remained obscure ever afterward and it was never assigned to any of the several groups of species recognized within Oryzomys.[7]
In 2006, Marcelo Weksler published a large-scale cladistic analysis of Oryzomyini ("rice rats"), the tribe to which O. polius belongs. He used both morphological data and molecular characters from the IRBP gene. 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. Two other molecular synapomorphies supported the clade of all members of clade D except O. polius, coupled with three morphological traits—in these species, but not in O. polius, the first upper molar has an additional small root at the outer (labial) side; the first lower molar has additional small roots; and the second upper molar has the mesoflexus (one of the valleys between the cusps and crests) divided in two.[9]
