Middle Miocene Pholadid Borings at the Base of the Isidro Formation, Arroyo Mezquital, Baja California Sur, Mexico

Abxxnddinl sediment-infilled clavate (club-shaped) borings, assignable to ichnogenus Gastrochaenolites, are reported from the base of the Miocene Isidro Formation, Baja California Sur, Mexico. The infilled borings were made by pho- ladid bivalves that lived in a nearshore-marine environment on a semi-consoli- dated substrate ( \u27Glossifungites\u27\u27 ichnofacies). Some borings are connected by horizontal burrows that are sculptured (Spongeliomorpha) or unsculptured (Tha- lassinoides). The localized pholadid community was short lived and abruptly terminated by an influx of sediment