| Summary: | Small spiny marine palynomorphs have been the focus of recent palynological studies since their high proportions in Antarctic Late Cretaceous and Cenozoic assemblages were noted. These palynomorphs were assigned to the dinoflagellate cyst Impletosphaeridium clavus and they were believed to have had an affinity with some modern round brown spiny cysts (RBSCs). Our study aims to analyse there together with potential modern analogues by comparing Late Cretaceous and Cenozoic Antarctic specimens with some RBSCs recorded from the Holocene of southern Argentina. We confirm several features in common between these specimens, although differences in processes and the cyst wall are observed. The species could have been produced by dinoflagellates similar to those that generate some RBSCs. Their abundance in the Late Cretaceous may have occurred in response to short term cooling pulses without development of sea-ice cover. Cenozoic records are considered to be reworked, restricting the stratigraphical range of Impletosphaeridium clavus. Fil: Amenabar, Cecilia R. Ministerio de Relaciones Exteriores, Comercio Internacional y Culto. Direccion Nacional del Antártico. Instituto Antártico Argentino. Instituto Antártico Argentino - Sede Cricyt (Mendoza); Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Estudios Andinos; Argentina Fil: Candel, Maria Soledad. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Bahía Blanca. Instituto Geológico del Sur; Argentina Fil: Guerstein, Gladys Raquel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Bahía Blanca. Instituto Geológico del Sur; Argentina
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