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Abstract: In the Bahía Blanca Estuary (Argentina), Holocene deposits are fossiliferous sand ridges originated by the last transgressive–regressive marine event. By means of ternary taphograms applied on valves of Brachidontes rodriguezi a taphonomic analysis was carried out in two ridges, both in the external and internal estuary area. Six taphonomic attributes were considered. The disarticulation and the fragmentation are higher in the external ridge, whereas the bioerosion and incrustation, both barely manifested, are similar in the two environmental settings. In the outer deposit, as much the abrasions as the dissolution of the shells are more intense. Genetically, it is considered that these shell concentrations were originated by multiepisodic storm events. Based on the taphonomic analysis, it arises that in the generation of the internal ridge waves of moderate energy with different sorting degrees prevailed, whereas in the formation of the external deposits waves of greater energy with smaller sorting capacity prevailed. [Copyright &y& Elsevier]

Most research on bivalves from the south-western Atlantic used morphological (shell) characters for taxonomic discriminations. Dominant Veneroids from Argentinian Quaternary coastal deposits exhibit wide morphological variation – often making objective discriminations difficult/impossible, which could be objectively described and compared through geometric morphometrics techniques. This work focuses on comparison of geometric morphometrics methods applied to fossil and modern shells, to assess inter- and intra-generic variations. Three approaches were considered: landmarks (L), semi-landmarks (SL) and outlines. Shell shape analyses for different time spans (Pleistocene, fossil Holocene and modern) and areas (Patagonia and Bonaerensian) showed that Elliptic Fourier analysis (EFA), Landmarks and Landmarks plus Semilandmarks (L+SL) can discriminate at generic levels:Mactra, Mulinia(Mactridae) vs.Pitar, Protothaca, Eurhomalea, Clausinella(Veneridae). L and L+SL are powerful for inter/intraspecific distinctions ofMactra. Variability ofMactra isabelleanaincludes the remaining nominal ‘species’ (transitional morphs). Causal environmental factors of (phenotype) variation could be addressed for modern environments (substrate, salinity and energy). Subtrigonal-inflated shells predominate in muddy, quieter, shallow mixo-polyhaline waters; ovate-elongate-compressed in sandy, poly-euhaline, deeper habitats. Differential spatial distribution (and abundance) across time responds to Late Quaternary high sea-level stands: transgressive maxima allowed higher salinity in marginal-marine areas and optimal conditions forMactra isabelleanacontrasting with scarcer records in the Mar Argentino today. [ABSTRACT FROM AUTHOR]

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]