Insights on climate change-driven geographical shift of Tegula atra (Lesson, 1830) (Archaeogastropoda) from Patragonia (SW Atlantic); Late Pleistocene biostratigraphical tool and palaeoceanographical signal

AGUIRRE M.L; RCHIANO S.; FARINATI E.

Huelva

Congreso; V RCANS Congress; 2013

International Union of Geological Sciences - Subcomission on Negene Stratigraphy

In Late Quaternary (MIS11-1) coastal deposits from Argentina (Southwestern Atlantic, SWA), molluscs are predominant. Among gastropods, Tegula atra (Lesson, 1830) offers a particularly interesting case study from biostratigraphical/palaeoclimate standpoints: excellently preserved along ca. 1000 km, abundant with all ontogenetic stages, autochthonous element of the original nearshore associations, curiously absent nowadays in the SWA, living in the Southeastern Pacific (SEP) typically as an intertidal, free-epifaunal herbivorous species along the cold Humboldt system. We aimed at defining its taxonomy, distribution (space and time), regional palaeobiogeographical context in South America, to assess plausible explanations for its extinction in the SW Atlantic. A systematic review, field observations, own collections, museum collections, records from oceanographic expeditions along the SWA, bibliographic compilations, morphometric analysis (Relative Warps) of fossil (SWA) and modern (SEP) specimens from several geographical areas confirmed: no shape differences between fossil Patagonian and modern SEP specimens, its value as Late Pleistocene biostratigraphical tool exclusive for Patagonia and for Marine Isotope Stages (MIS) 9, 7 and 5. Multivariate (cluster analysis; Principal Coordinate analysis) and cladistic (Parsimony analysis of endemicity) methods applied to a large dataset of 382 gastropod records between Surinam and Tierra del Fuego (49 Pleistocene, 28 fossil Holocene, 35 modern sites) allowed to define areas of endemism (PAE) and patterns between localities through time with similar results. PAE more objectively and clearly showed Patagonia always as an AE. During MIS9-5 the dominance of T. atra associated with other cold taxa, northern sites linked to southern Patagonian localities, altogether suggested predominantly colder nearshore scenarios with enhanced Malvinas (Falkland) current. Its absence during MIS1 represents a biotic response to atmospheric-circulation changes post-Pleistocene/Holocene transition in the SWA: intensified thermal/salinity fronts and Brazilian current, lower wind velocities, high drainage pattern, less nutrient availability, altering growing and floating capabilities of D.antarctica (kelps, macroalgae) responsible for dispersal by rafting of T. atra along the Argentine Sea.