A new and unusually well preserved bivalve fauna of Late Ordovician age was recovered from pebbles within the Late Ashgill (Hirnantian) glacigenic diamictite of the Don Braulio Formation, in the Argentine Precordillera. According to the associated brachiopods and the presence of graptolites of the bicornis Zone in the matrix of the diamictite, an early Caradoc age is accepted for the bivalve fauna. The clasts contain bivalve molluscs representing nine genera, of which seven are new. Identified taxa are the following: Praenucula sp., Cuyopsis symmetricus new genus and species. Villicumia canteraensis new genus and species. Trigonoconcha acuta new genus and species. Concavodonta ovalis new species. Hemiconcavodonta minuta new genus and species. Hemiconcavodonta sp., Emiliania cuerdae new genus and species, and Concavoleda braulense new genus and species. Two new subfamilies are proposed: Praenuculinae and Concavodontinae. The genera Praenucula Pfab, Cuyopsis new genus. Villicumia new genus and Trigonoconcha new genus are included in the new subfamily Praenuculinae. The genera Concavodonta Babin and Melou, Hemiconcavodonta new genus and Emiliania new genus are included in the new subfamily Concavodontinae.