A new extinct desert rodent from the Holocene of South America and its bearing on the diversity of Octodontidae (Hystricognathi)

A Late Holocene (ca. 3,100 to 380 BP) sample of the viscacha rat genus Octomys (Octodontidae) from the Vaquerías Gruta 1 site (VQ-G1) in western Argentina is reported. Phylogenetic and morphometric comparative analyses with living octodontids support that the VQ-G1 sample is related to the desert specialists Tympanoctomys and Octomys, and is sister to the only living species of the latter, O. mimax. The estimated morphological distance to O. mimax is greater than that between pairs of congeneric octodontid species, and even greater than that between some species belonging to different genera. This suggests that the sample represents a new species, whose young age prevents interpreting it as an anagenetic ancestor of O. mimax. If the new species is the result of cladogenesis, its absence in the current fauna represents actual extinction, which among caviomorphs is added to those of †Clyomys riograndensis, †Dicolpomys fossor, †Ctenomys viarapaensis, and †Galea tixiensis—also extinct in the Late Holocene. Thus, the VQ-G1 sample provides evidence of changes in diversity and distribution undergone by small mammals in southern South America during that time. The potential contribution of the Holocene record may be key to elucidate this issue from both an evolutionary and a conservation perspective. This requires a detailed systematic approach to determine whether the taxa under study are truly independent evolutionary units, as well as geographically broad sampling efforts to distinguish the changes affecting distribution from those producing irreversible changes in diversity.