Resumen

We describe a new species in the didelphid marsupial genus Marmosa based on a single specimen collected at 2664 m on the eastern side of the Andes in San Martín department, Peru. The new species is closely related to Marmosa lepida (Thomas, 1888) and M. andersoni Pine, 1972, in the subgenus Stegomarmosa, but differs from these and other congeneric taxa by mitochondrial and nuclear DNA sequences and by unique external and cranial characters. Diagnostic morphological traits of the new species include gray-based buffy-reddish ventral fur without a midventral zone of self-whitish fur; a remarkably narrow and long rostrum with a very long premaxillary process; absence of postorbital processes; and presence of diastemata among the canines, first premolars, and second premolars in both upper and lower dentitions. Marmosa chachapoya, sp. nov., is one of several recently discovered new species from the Parque Nacional del Río Abiseo, a protected area with remarkably high mammalian diversity