Comparisons of a species' modern and historic geographic distributions can yield important insights into patterns of biodiversity, including faunal responses to environmental change. Using morphological and genetic analyses of museum specimens and samples recently collected in the field, we document the historical occurrence of a population of colonial tuco-tucos (Ctenomys sociabilis) from Laguna Nahuelquir in Chubut Province, Argentina. This locality represents a significant increase in the geographic distribution of this species, which had been thought to be endemic to the Sierra de Cuyín Manzano located ∼130 km to the north in Neuquén Province. Field surveys of the Laguna Nahuelquir locality failed to reveal extant populations of C. sociabilis; although the habitat at this location is generally consistent with areas occupied by C. sociabilis in the Cuyín Manzano, the Laguna Nahuelquir site is now occupied by the Patagonian tuco-tuco, C. haigi. This apparent replacement of ctenomyid species is consistent with patterns of faunal turnover revealed by analyses of late Pleistocene–early Holocene cave deposits near the Sierra de Cuyín Manzano and supports the hypothesis that changes in local environmental conditions have contributed to the loss of C. sociabilis from these localities. Thus, comparisons of historical and modern specimens of ctenomyids provide evidence of range retractions by colonial tuco-tucos over multiple time scales and add to a growing body of evidence regarding the apparent effects of environmental change on the small mammal fauna of northern Patagonia.