Dextral to sinistral coiling switch in planktic foraminifer Morozovella during the Early Eocene Climatic Optimum

Abstract Coiling direction is a basic characteristic of trochospiral planktic foraminifera. Modifications in the coiling direction within ancient planktic foraminiferal populations may reflect important changes in evolution or environment, yet they remain scarcely discussed. Here we investigate fluctuations in the coiling direction within Morozovella assemblages from sections that span the interval of peak Cenozoic warmth, the Early Eocene Climatic Optimum (EECO; ~53–49 million years ago, Ma), at Atlantic Ocean Drilling Program (ODP) sites 1051, 1258 and 1263. The surface-dwelling genus Morozovella is of particular interest because it dominated tropical-subtropical early Paleogene assemblages then suffered an abrupt and permanent decline in abundance and taxonomic diversity at the start of the EECO. At all ODP sites, morozovellids display a dominant dextral coiling preference during the interval preceding the EECO. However, all the Morozovella species at all sites modify their coiling from preferentially dextral to sinistral coiling within the EECO,