The marginal-marine, low-energy, carbonate ramp deposits of the La Tosca Member (Huitrín Formation, Barremian) in southern Mendoza Province, Argentina, bear an abundant though low-diversity macrobenthic invertebrate fauna that is scarce or absent in the outcrops of the unit in the Neuquén Province. Here we describe new taxa from the Quebrada del Gastrópodo and Ranquil 1 sections and provide taxonomic revisions for other key faunal elements. The species discussed herein are the gastropods Paleoanculosa macrochilinoides (Doello-Jurado), Provalvata minor sp. nov., and Provalvata maior sp. nov., the bivalve Rutitrigonia cintarojensis sp. nov., and the serpulid Parsimonia aff. laevis (Goldfuβ). Additionally, in order to analyze the observed changes in faunal content along the studied section, a paleoecological analysis was performed combining estimation of diversity indices and the paleoautoecology of the fauna recorded in Quebrada del Gastrópodo. Three macrobenthic assemblages were qualitatively recognized in the field; from base to top: 1) Paleoanculosa macrochilinoides, 2) serpulid-gastropod, and 3) bivalve-serpulid assemblages. Species richness and equitability increase upwards. This trend suggests a reduction in environmental stress from bottom to top. The first two assemblages indicate settings with fluctuating environmental parameters, whilst the third points to higher environmental stability. Euryhaline taxa dominate, whilst stenohaline species are scarce or absent in the lower two thirds of the succession. The nature of the stress is interpreted herein to be mainly related to high-frequency salinity variations, especially in the first and second assemblages in which temporal deviations from euhaline to meso-brachyhaline and brachyhaline conditions are hypothesized.