Systematics of the Enigmatic Neotropical Rodent Genus <scp><i>Microakodontomys</i></scp> (Cricetidae: Sigmodontinae): New Evidence Supporting Its Distinctiveness and Endangered Status

ABSTRACT The diversification of Neotropical sigmodontine rodents represents one of the most remarkable evolutionary events among mammals, encompassing numerous endemic, rare, and endangered genera and species. One of the most emblematic examples is Microakodontomys transitorius Hershkovitz 1993, originally described from a single specimen based on morphological traits, and considered a “transitional” taxon between the tribes Akodontini and Oryzomyini, due to its unique combination of characters. However, the lack of additional specimens and its morphological resemblance to small species of Oligoryzomys led to its taxonomic synonymization with that genus. Here, we report new karyological, morphological, and natural history information on this taxon, and present results of total evidence phylogenetic analyses of the tribe Oryzomyini. Cytogenetic analyses revealed two distinct karyotypes: a female with 2n = 38, FNa = 46, and a male with a mosaic 2n = 36–37, FNa = 44–45. Phylogenetic analyses integrating morphological, mitochondrial ( cytochrome b ), and nuclear ( IRBP ) data support the placement of M. transitorius within the Oryzomyini, but distinct from Oligoryzomys , and as a sister taxon to Lundomys molitor . We redescribe the taxon, discuss its phylogenetic relationships, and the implications of its diversification within the Cerrado biome.