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Abstract: The molluscan content from three “marine terraces” (TM) preserved along 250 km of the Santa Cruz province provided 49 molluscan taxa (33 Gastropoda and 16 Bivalvia) and the first Quaternary records for Ataxocerithium pullum, Crepidula protea, C. aculeata, Adelomelon ferrusacii, Brachidontes rodriguezi, Aequipecten tehuelchus, Ostrea tehuelcha, and Solen tehuelchus. The mollusca assemblages suggest typical marine nearshore conditions similar to the adjacent modern littoral (Southwestern Atlantic), with mostly hard substrates, shallow and cold waters. The TMVI (+5–15 m a.s.l., ca. 8–1.2 ka A.P.) is mainly characterized by dominance of Brachidontes spp., Nacella spp. and Mytilus edulis, associated with Siphonaria lessoni. No evidence has been found for the TMVI of clear biotic responses or strong palaeobiogeographic effects during the time span of the Climatic Optimum (“Hypsithermal”, mid-Holocene) which has been documented for other coastal areas northward in Argentina, down to ca. 44–45° S. Scarce differences distinguish the TMV (+15–30 m a.s.l., ca. 108–137 ka A.P., MIS5?) and TMIV (+20–30 m a.s.l., ca. 242 ka A.P.) regarding the Holocene and the modern littoral. Within the TMV, Clausinella, Eurhomalea, Solen tehuelchus and Brachidontes cf. darwinianus (typically in warmer waters) are exclusive; in the TMIV, Trophon, Buccinanops, Brachidontes and Pectinidae are characteristic. Predominance of Trophon spp. (typically in cold Magallanic waters) in modern samples vs. scarcity or absence in the TM suggest slightly lower modern SST. Several taxonomic similitudes with the Pacific Sudamerican coast (e.g., Fissurella, Nacella, Tegula, Crepidula, Trochita, and Acanthina) could respond to cold currents helping cold-water immigrants to join southern Southamerican Atlantic after the opening of the Drake Passage. [Copyright &y& Elsevier]

Abstract: Our analysis of palaeobiodiversity and palaeobiogeography of Quaternary molluscan assemblages provide information on the last coastal scenarios in the SW Atlantic. We present patterns of species richness, areas of endemism and between areas relationships based on records (721 taxa) from Argentina and northwards in South America (Surinam, Brazil, Uruguay), including Pleistocene, Holocene and modern sites (48 localities). In Argentina, our data since the Mid-Late Pleistocene (MIS11-1; ca. 400ka B.P. to present) suggest environmental changes linked to sea-level and climatic fluctuations, large enough to alter benthic associations (compositional, geographical variations, including a few extinctions). The southwards decreasing diversity trend for modern bivalves (mostly infaunal taxa) and gastropods (mostly epifaunal taxa), from Surinam to Tierra del Fuego, responds mainly to latitudinal SST, salinity barriers, hydrodynamic forces (rivers and shallow currents): minima peaks fit with local estuarine and marginal marine sites influenced by the Río de la Plata plume; highest peaks with fully marine, warmer waters influenced by the Brazilian Current (Bonaerensian sector); a similar trend applies for the Holocene; Pleistocene maxima peaks in Patagonia belong to MIS11 and MIS5 highstands (warmer than today). Multivariate (Cluster Analysis, CA; Minimum Spanning Trees superimposed to Principal Coordinates Analysis, PCO) and cladistic (Parsimony Analysis of Endemicity, PAE) methods showed congruent relationships between localities, discriminating marginal from open marine environments, but PCO combined with PAE was most advantageous for our area and dataset (382 gastropods, 339 bivalves; 29 Pleistocene, 28 Holocene and 35 modern sites) allowing objective, quick comparisons. Shifts of areas of endemism during the Holocene and Pleistocene relate with SST and salinity changes; a more homogeneous Mid-Holocene (ca. 7–5ka B.P.) scenario, of regionally increased salinity and higher (ca. 2°C) SST, defined the Bonaerensian and northern Patagonian sectors, facilitating southwards shifts of bivalve and gastropod taxa; for Patagonia a colder Pleistocene scenario south of SAO is documented by Tegula atra (exclusive Pleistocene cold water species extinct in the SWA), probably a signal of post-LGM palaeoceanographical changes in the Mar Argentino. [Copyright &y& Elsevier]