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In this study investigates the predatory activity on a Holocene gastropod from the Argentinean Malacological Province. Buccinanops deformis (King, 1832), endemic in the southern hemisphere, is a typical taxon of cold-waters and high latitudes. We observed direct evidence of durophagy as lethal and sublethal damage. Sublethal damage was studied using traces of repaired shell as an indicator of activity by durophagous predators. Three parameters were used to evaluate durophagous predation: frequency of predation, repair scar frequency and prey effectiveness. A predatory-prey interaction was identified between Danielethus crenulatus (A. Milne-Edwards, 1879) and B. deformis during the Holocene on the Argentinean littoral coast. The low value observed for shell repair frequency (0.06), the low frequency of predation (0.16), and the intermediate value for prey effectiveness (0.37) may be interpreted because of a low but effective predatory activity. [ABSTRACT FROM AUTHOR]

On the north shore of the Bahia Blanca estuary, Buenos Aires, Argentina, sand shell ridges composed by a high proportion of boulders of sedimentary origin are present. Many of these clasts show significant signals of bieorosion. The procedence of this material is linked to the extensive abrasion platforms present to the east of the estuary (coasts of Pehuén Co). Two ichnofacies were established: Trypanites ichnofacies comprising Gastrochaenolites, Maeandropolydora, Trypanites, Gnathichnus and Radulichnus, typical of hard substrates and Skolithos ichnofacies represented by Ophiomorpha galleries developed in soft substrates. Both characterize a shallow coastal environment. During the Holocene transgression, coastal substrates were affected by the advancing sea and eroded fragments were transported by littoral drift into the estuary. Finally, storm events deposited material with signs of bioerosion in the littoral ridges. Today, many of these boulders, which exhibit gradations in size into the estuary, are in transit across the beaches by east-west littoral drift. [ABSTRACT FROM AUTHOR]

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]

Ichnological investigations were carried out on late Quaternary shells of the intertidal deep infaunal bivalve Tagelus plebeius ( Lightfoot, 1786 ) found along the southwestern Atlantic, between Uruguay and the southernmost Buenos Aires Province in Argentina. Analyses reveal distinctive marks that are spread on the outer shell surface only. The marks are regular-unbranched-elongate, perpendicular to the outer shell growth lines, with deflections on the margins, never interconnected, without bifurcations, conforming bottom-up constructions. They occur in hundreds of specimens from many samples taken from sediments ranging in age from the late Pleistocene to the Recent. These marks have never been reported or described for this species and their origin and formation remain elusive. We describe these traces thoroughly and we propose an explanation for their preservation on about half the shells examined. Potential destructive boring structures (excavated from outside-in) or bioerosion activities by other macro- or micro-organisms are dismissed. These antimarginal asymmetric traces point instead to a process of constructive bioclaustrations (grown from the bottom-up) produced in situ during the life of the bivalve by unknown symbiont organisms. Additionally, the regular pattern observed for the marks exclude host growth as a consequence of abiotic/extrinsic causes. From a palaeoecological perspective, these structures suggest a biotic interaction that was hitherto undescribed neither for bivalves nor for the late Quaternary of the southwestern Atlantic. [ABSTRACT FROM AUTHOR]

The last two Quaternary transgressive-regressive events (Late Pleistocene and Holocene) left abundant marine deposits along coastal areasin various regions of the world, including the Bahia Blanca Estuary in Argentina. In this work, the sedimentation paleoenvironmental conditions prevailing during the marine transgressions are established, on the basis of sedimentological and taphonomic characteristics. Analysis of the two deposits, which are located well inland of the currentlittoral zone and are separated by an unconformity, shows that Pleistocene materials are distinguished by a higher degree of cementation,specially in the uppermost part of deposit. Sedimentological and taphonomic features reflect differences in the dominant sedimentation regime. The Pleistocene deposit, with sediment finer grained and less well sorted, were formed under lower energy conditions with an important role of stream flows carrying lithic material coming from coastal formations. The Holocene transgressive maximum is proposed to have given rise to a coastal geography more exposed to the action of storm waves (concentrated fossils in beach ridges), with sediments mobilizedby coastal drift. The diversity difference of the marine fauna (Pleistocene:17 species, Holocene: 106 species) suggest a Pleistocene ecosystem poor for the development of organisms. The disappearance of species between the two transgressive events would be indicative of a fall in the sea water temperature. [ABSTRACT FROM AUTHOR]

Abstract: Quaternary records of unique skeletal deposits studied at two coastal areas of the Bonaerensean littoral, Bahía Samborombón in the north (at Puente de Pascua and Canal 15 area) and Bahía Blanca southwards, are thick (4–5 m) and exceptionally rich in molluscan assemblages (ca. 75–90% bivalve shells), indicating different high sea level episodes. Through taphonomical aspects of target bivalves (Mactra, Brachidontes) which differ in shell structure, life habit and habitat, abundantly present in the marine Late Quaternary from both areas, the aims were: 1) to establish quantitative grades of taphonomic alteration of Mactra from four lithostratigraphical units at Bahía Samborombón and identify which signature/s better discriminates palaeoenvironments; 2) test whether Mactra shows different alteration than does Brachidontes in response to differences in intrinsic features and ecological requirements; 3) compare isotaphonomic assemblages complementing independent geological and palaeoenvironmental interpretations. Attributes (articulation, fragmentation, abrasion, colour, encrustation, bioerosion) were classified for about 5400 shells from 11 samples. Bar plots, ternary taphograms and Q-Mode Cluster analysis discriminate beach, beach ridges and tidal flat environments and two taphofacies: Late Pleistocene and Holocene high energy units; and low energy Holocene units. Only abrasion and colour allow discrimination between high and low energy conditions. Rules cannot be applied to the processes of deposition of shells, at least for the Late Quaternary marine deposits along the Buenos Aires Province coastal area. Despite the harder (Mactra) or more fragile (Brachidontes) shell architecture, in softer (Mactra, muddy substrates, infaunal) or harder (Brachidontes, varied sandy-rocky bottoms, epibyssate) and deeper (Mactra, infralittoral) or shallower waters (Brachidontes, intertidal to supratidal), the taphonomic grades of alteration are similar, a product mainly of long after-death exposure along the Taphonomically Active Zone in high to moderate wave energy conditions. [Copyright &y& Elsevier]

Late Quaternary marine molluscan skeletal concentrations from Argentina constitute a remarkable record of variations in palaeoceanographical conditions during interglacial times (mainly ca. 125 ka to present). Particularly, the Golfo San Jorge coastal area represents an extraordinary geographical zone to target from different points of view, mainly due to its linkage between northern and southern Patagonia, characterized by particular and contrasting physico-chemical conditions with direct consequences for littoral marine communities, determining their composition and structure. Among varied biological activities controlled by different environmental factors (i.e., substrate nature, sedimentation rates, water depth, sea surface temperature, salinity, nutrients-productivity), bioerosion traces can provide palaeoenvironmental evidence with important implications for palaeoclimate interpretations. In addition, the application of bioerosion patterns regionally and through time is a recent valuable worthy palaeoenvironmental tool not as yet developed for Patagonia. We attempted to characterize, qualitatively/semiquantitatively, the ichnotaxonomic composition of the coastal area of northern Golfo San Jorge since the Late Pleistocene; to compare results with those obtained for other geographical areas along Patagonia and the Bonaerensian coastal sectors; lastly, to evaluate its palaeoenvironmental/palaeoclimatic significance in a clue area in terms of circulation patterns near the Southern Ocean climatic pump. At Bustamante (Northern Patagonia Frontal System) Domichnia traces were dominant during the Late Pleistocene while Praedichnia in the mid-Holocene. Bustamante exhibits the highest ichnodiversity for the whole Argentinean coastal area. Ichnodiversity is not strongly different between Late Pleistocene and mid-Holocene interglacials and compared to present; however, the relative abundance of some ichnotaxa (e.g., Oichnus, Iramena, Pennatichnus , at Camarones; Oichnus , Iramena, Pinaceocladichnus, at Bustamante) differs across time. These variations, particularly the highest abundance in the Late Pleistocene (mainly Last Interglacial) of traces made by bryozoans- associated at present with modern enhanced productivity levels and coastal fronts in the Argentine continental shelf- point to higher productivity and more intensified northern Patagonia Front, as a result of a different palaeocirculation pattern, reinforcing previous independent sources of evidence based on molluscan palaeobiogeographical analyses. [ABSTRACT FROM AUTHOR]

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: Results of the systematic review and regional palaeobiogeographical context of Tegula atra, including morphometric, multivariate and cladistic analyses, show that it is a keystone species in the marine Quaternary of Argentina that can be used as Pleistocene biostratigraphical tool and paleoclimate-palaeoceanographical signal. While it was absent in warmer than present high sea-level episodes during the Miocene (“Entrerriense” transgression, ca. 10 Ma) and Pleistocene (MIS11), it exhibits an excellent and abundant fossil record within dominantly cool coastal settings exclusively during the Late Pleistocene (MIS9, 7 and 5) between Río Negro and southern Santa Cruz provinces (Patagonia). It first appeared in the SEP during the late Pliocene (cooling trend), dispersed during the Late Pleistocene into the SWA presumably by rafting on macroalgae along the Cabo de Hornos and Malvinas (Falkland) currents, but became extinct in the Mar Argentino (Magellan Malacological province) during the Holocene (amelioration trend). Its absence at present represents a climate change-driven range shift and independent evidence of palaeoceanographical changes after the LGM and at the Pleistocene/Holocene transition: changes in SST (ca. 2 °C higher), wind velocities (less), light (less), nutrient availability (less), extension and intensity of cold (less) and warm (increased) shallow water currents altering water masses and biogeographical boundaries. Altogether, these changes and the Holocene scenario were disadvantageous, causing direct effects on its physiology and survival, in turn preventing the occurrence of the associated macroalgae (Durvillaea antarctica) and its successful dispersal in the SWA or retraction to the cold Humboldt System waters. This study reinforces the importance of dispersalist models to explain the origin of key taxa, adding for a better understanding of molluscan taxonomic differences along the SWA and SEP margins of South America, with implications for future coastal scenarios. The distribution of T. atra across time is a new example of the strong linkage between earth history-climatic cycles-atmospheric and oceanic circulation and the late Quaternary biotic responses, showing a possible consequence of future climate change on nearshore communities. [Copyright &y& Elsevier]

Abstract: The decisive influence of Late Quaternary sea level changes on the geological evolution of the coastal plain and adjacent continental shelf around the world has long been recognized. Coastal environments evolve actively during transgressive–regressive cycles whose development depends on sea level and sediment supply variations. The interaction of these variables was key to the current morphological and sedimentological configuration of coastal regions. Particularly, the estuarine system of Bahía Blanca (Argentina) presents various types of deposits and marine fossil accumulations, such as paleochannels in the subbottom, sand-shell ridges and extensive layers with fossils in life position. These features are important geological indicators, because its analysis allows us to define different paleoenvironmental conditions that prevailed during the coastal evolutionary process. [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]