Ostracod assemblages and palaeoenvironmental reconstruction of a Lower pliocene shallow marine system in the East Atlantic (Bonares-Casa del Pino, Guadalquivir Basin, Spain)

The ostracod assemblages from the Bonares-Casa del Pino section in the Lower Guadalquivir Basin, part of the lower Pliocene Arenas de Huelva Formation, have been analysed for palaeoenvironmental purposes. These assemblages form part of a subtropical/tropical biota, consistent with findings from earlier studies on fossil remains within the Zanclean deposits in the Southwestern Iberian Peninsula. The present study marks the first application of the Specific Population Species Index Method, originally developed to distinguish autochthonous from allochthonous species in Recent assemblages, to fossil ostracods. Through both quantitative and statistical analyses, the autochthonous nature of both dominant and characteristic species was recognized. The succession was deposited in the northernmost sector of the East Atlantic lower Pliocene tropical palaeo-ecoregion (also known as Atlantic Andalusian Pliocene Mollusc Unit 1), within shallow waters. The attribution to the offshore transition zone, close to the boundary between infralittoral and circalittoral zones, prompted clarification of vertical zonation terminology for both the Atlantic and Mediterranean, in Recent and Pliocene marine environments. • First application of the Specific Population Species (SPS) index method to fossil ostracod assemblages. • Analysis of ostracods from the Lower Pliocene Arenas de Huelva Formation in the Guadalquivir Basin. • Assemblages indicate a shallow marine subtropical/tropical palaeoenvironment during the Zanclean. • Autochthonous nature of dominant and characteristic species confirmed through quantitative analysis. • Results contribute to refining vertical zonation frameworks for Atlantic and Mediterranean marine systems.