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We describe a new sigmodontine fossil sequence ranging from 8,800 ± 40 to 3,730 ± 60 years BP retrieved from the archaeological site RS-S-327:Sangão (Rio Grande do Sul, Brazil). The studied material includes 2,683 craniomandibular remains totalizing about 20 sigmodontine species. The assemblage encompasses a variety of taxa associated to both open and forest environments. Among the former, we recorded species today disappeared in southern Brazil such as the rare 'giant' rats Gyldenstolpia and Kunsia, but also the coney rat Reithrodon and the akodontine Necromys obscurus. Conversely, an important assemblage of sylvan species, including the genera Delomys, Oecomys, and Wilfredomys, reflects forested environments. Several of the recorded sigmodontines, such as Deltamys or Nectomys, constitute first mentions for the southern Brazil Quaternary. One of the most remarkable features of the studied sequence is its noteworthy specific richness, probably due to a combination of local environmental heterogeneity in a regional tendency of changing climatic conditions. The evidence of Sangão plus the previously studied samples from Garivaldino and Pilger sites exposes faunal changes during the Holocene in southern Brazil. In this context, the impoverishment of recent sigmodontine assemblages seems a natural result from the progressive disappearance of extensive open environments since Middle Holocene. [ABSTRACT FROM AUTHOR]