Resumen
Middle Miocene fluvo-lacustrine strata of the Río Pedregoso Member of the Cura-Mallín Formation are broadly exposed to the east and south of the town of Lonquimay, Chile. These strata accumulated in the extensional Cura-Mallín Basin, a basin which was subsequently inverted. The original depocenter is interpreted as a double half-graben, diachronic sub-basin system. A partial left mandible was collected from the Cura-Mallín Formation near the base of Cerro Tallón along the west bank of the Río Biobío. Morphological and morphometric comparisons permit referral of the specimen to the toxodontid notoungulate Nesodon conspurcatus. This represents the first report of Nesodon conspurcatus outside of Patagonia and one of the few well-documented occurrences of Nesodon outside of high latitude Argentina. The widespread geographic range of Nesodon, combined with its restricted temporal range, suggest it may be one of the most useful indicator taxa for Santacrucian age faunas. The absence of Nesodon in the type Friasian assemblage at Río Frías/Cisnes in Chilean Patagonia strengthens the case for a faunal distinction between the Santacrucian and Friasian SALMAs.