Unveiling hidden diversity of <i>Oecomys</i> (Rodentia: Cricetidae) from Brazilian Central Amazonia: description of a new species and new lineages

The arboreal rice rat of the genus <i>Oecomys</i> Thomas, 1906 is one of the most speciose genera of the subfamily Sigmodontinae, with 19 species currently recognized and occurring from eastern Panama to northern Argentina, Paraguay, and in northern, central and eastern Brazil. Herein we describe a new species using an integrative approach based on molecular, morphological, and morphometric data. We used in our assessment recently collected specimens from the states of Pará and Rondônia, one of the most deforested regions in Brazil. We examined 51 specimens of <i>Oecomys</i> from museum collections including name-bearing types from most of the distributional range of the genus. We also sequenced 32 specimens of <i>Oecomys</i>, and for the molecular analyses, we used the mitochondrial marker Cytochrome <i>b</i> and the nuclear marker intron 7 of β-fibrinogen. Our mitochondrial marker results recovered a strongly supported clade composed of two divergent clades (3.78%), one including lineages of <i>O. bicolor</i> and <i>O. cleberi</i>, and the other clade representing the new species. The topology of concatenated mitochondrial and nuclear data also recovered <i>Oecomys</i> sp. nov. as a sister lineage of the <i>O. bicolor</i> and <i>O. cleberi</i> clade. Also, both markers recovered new lineages from the <i>O. bicolor</i> and <i>O. cleberi</i> species group. The new species can be discriminated from other <i>Oecomys</i> species by pelage colour and craniodental characters, such as absent or small mastoid fenestra, and the presence of alisphenoid strut, small subsquamosal fenestra, presence of sphenopalatine vacuities, and presence of accessory loph of M1 and M2 paracones. The new species occurs exclusively in the Rondônia centre of endemism, delimited by the rivers Amazon to the north, Tapajós to the east, and Madeira to the west. The description of this new <i>Oecomys</i> increases the diversity, and also contributes to elevate Amazonian Sigmodontinae species richness and endemism in this still poorly known biome. http://zoobank.org/urn:lsid:zoobank.org:7477D37A-4D02-443F-B0FB-149379BEDE92