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 Vaquerias 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 dagger Clyomys riograndensis, dagger Dicolpomys fossor, dagger Ctenomys viarapaensis, and dagger 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. We examined a Late Holocene sample (approximately 3,100 to 380 years before present) of the viscacha rat Octomys (Octodontidae)-an arid-adapted rodent found only in southern South America. The estimated morphological distance to the only living species of the genus, O. mimax, indicates that the sample belongs to a new species resulting from cladogenesis, so that its absence in the current fauna represents a real extinction that is added to that of other caviomorphs also extinct in the Late Holocene. Se da a conocer una muestra de mandibulas de la rata vizcacha Octomys (Octodontidae) del Holoceno tardio (ca. 3,100 a 380 anos AP) del sitio Vaquerias Gruta 1 (VQ-G1) en la precordillera austral del oeste argentino. Los analisis comparativos, filogeneticos y morfometricos, con octodontidos vivientes sustentan que la muestra de VQ-G1 esta vinculada a los especialistas de desierto Tympanoctomys y Octomys, y es hermana de O. mimax, unica especie viviente del genero Octomys. La distancia morfologica en relacion a O. mimax es mayor que la estimada entre especies congeneres e incluso mas amplia que entre algunas especies de distintos generos de octodontidos. Esto sugiere que se trata de una especie nueva, cuya corta antiguedad impide que se interprete como ancestro anagenetico de O. mimax. Si la especie nueva es producto de cladogenesis, su ausencia en la fauna actual representa una extincion real, la cual entre los caviomorfos se suma a las de Clyomys riograndensis, Dicolpomys fossor, Ctenomys viarapaensis, y Galea tixiensis, extintas tambien en el Holoceno tardio. De este modo, la muestra de VQ-G1 provee evidencia de los cambios de diversidad y distribucion experimentados por pequenos mamiferos en la porcion austral de Sudamerica durante el Holoceno. Los fosiles holocenicos podrian contribuir a dilucidar esta cuestion desde un punto de vista evolutivo y de conservacion. Esto requiere un enfoque sistematico detallado del registro holocenico para determinar si los taxa en estudio son verdaderamente unidades evolutivas independientes. Serian tambien necesarios nuevos esfuerzos de muestreo geograficamente amplios para distinguir entre los cambios que afectan la distribucion y los cambios que producen modificaciones irreversibles a la diversidad.