MOLLUSCAN PALAEOBIODIVERSITY AND PALAEOBIOGEOGRAPHY: PLEISTOCENE AND HOLOCENE INTERGLACIAL ASSEMBLAGES FROM BONAERENSIAN AND PATAGONIAN LITTORAL (SW ATLANTIC)

Londres, Inglaterra

Congreso; International Palaeontological Congress; 2010

Quaternary littoral deposits along the SW Atlantic (Surinam, Brazil, Uruguay, Argentina) contain rich molluscan assemblages. In Argentina qualitative and quantitative variations are recorded between the Río de La Plata and Tierra del Fuego where several sea-level highstands (MIS11/9, 7, 5, 1) are characterized by abundant, exceptionally well preserved shell concentrations (mainly bivalve and gastropod benthic taxa). From a total of 197 taxa (112 gastropods, 85 bivalves; 1 escaphopod, 7 polyplacophors), 89% occur in the Holocene (20% gastropods and 25% bivalves are exclusive), 40% gastropods and 17% bivalves since the Pleistocene, and ca. 40% since the Late Miocene. Molluscs have modern counterparts in the Argentine and Magellanean zoogeographical provinces, determined by the extension and seasonal intensification of the cold Malvinas (Falkland) and warm Brazilian currents. Results from quantitative comparisons (cluster analysis, principal component analysis, parsimony analysis of endemism techniques applied to 35 localities) show that, during the Pleistocene (ca. 400 Ka) and the mid-Holocene (ca. 7 Ka), interglacials, taxonomic composition and distribution discriminate Patagonian and Bonaerensian deposits. Extinctions (Tegula atra, Ostrea tehuelcha), latitudinal shifts (Triphora, Urosalpinx rushi, Anachis avara, Thais haemastoma; Noetia bisulcata, Crassostrea rhizophorae, Anomalocardia brasiliana, Venericardia procera) and changes in relative abundance for dominant individual taxa (i.e., Littoridina, Nacella spp., Fissurella spp; Mactra spp., Brachidontes spp., Aulacomya atra, Protothaca antiqua, Mulinia edulis,Tagelus plebeius, Ensis macha) suggest that salinity, SST and substrate nature are the main controlling factors, reinforcing the conclusion that these variations are biotic responses to environmental and rapid climate changes.