CONICET | Buscador de Institutos y Recursos Humanos

The Huitrín Formation in west-central Argentina records the final connection stage of the Neuquén Basin to the Pacific Ocean. It comprises a variety of continental to marginal-marine deposits developed behind an Andean volcanic arc and under warm and arid paleoclimatic conditions. Here we focus on a largely neglected bivalve fauna from carbonate ramp deposits within the Barremian La Tosca Member. This fauna is very abundant and widely distributed, but surprisingly, it has never been studied in detail previously and its paleoecological implications remained equivocal. Freshwater, brackish, and marine affinities have all been claimed. We studied the faunas taxonomy, paleoecology and associated paleoenvironments based on more than 500 specimens collected at ten fossil localities in combination with new field observations. The bivalve assemblage was recorded from middle to outer carbonate ramp deposits and is composed of five taxa of marine affinity: Phelopteria huitriniana n. sp., Isognomon cf. I. nanus (Behrendsen), Placunopsis? pichi n. sp., Anthonya jarai n. sp., and Argenticyprina mulensis n. gen. and n. sp.; the first three may be regarded as eurytopic and/or opportunistic. Reduced diversity, low evenness, overall small size (L<4cm), thin shell thickness, eurytopic/opportunistic life strategies, and high endemism point to a restricted marine setting for La Tosca Member. The most important limiting factors were low food supply and fluctuations in temperature and salinity. High evaporation rates combined with low continental runoff and reduced rainfall regime may have induced deviations from euhaline towards hypersaline conditions and pronounced changes in water temperature. Thick evaporite deposits below and above La Tosca Member and thin intercalated gypsum beds support the hypersaline interpretation.