A new species of Three-striped Opossum Monodelphis ( Microdelphys ) with an updated hypothesis for its diversification in the Brazilian Atlantic Forest

Abstract The Brazilian Atlantic Forest (BAF) is one of the most threatened, extensively studied, and biogeographically significant biomes in South America, and new species of mammals are still being described for this biodiversity hotspot. This study revises the diversity and taxonomy of BAF three-striped short-tailed mouse opossums formerly identified as Monodelphis iheringi and explores the biogeographic history of the group in the Atlantic Forest. An integrative approach—combining multiloci molecular phylogenetics and morphological comparisons—revealed a new species in Rio de Janeiro state, one of the most densely populated and well-studied regions of Southeastern Brazil. Compared to related three-striped mouse opossums (M. iheringi and M. americana), the new species is unique in having 3 longitudinal dorsal black stripes that extend from the tail to the eye region, short ventral hairs near the base of the tail that do not cover it, and an expanded anterior cingulid relative to the hypoconid. The species likely originated in the Pleistocene, coinciding with the dates of other mammal species endemic to the Coastal Lowlands of Rio de Janeiro. These findings emphasize the urgent need to protect forest fragments critical for biodiversity and underscore the importance of integrative taxonomy for understanding evolution and conservation.