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

Coiling direction is a basic characteristic of trochospiral planktic foraminifera. Modifications in the coiling within ancient foraminiferal populations may reflect important changes evolution or environment, yet they remain scarcely discussed. Here we investigate fluctuations Morozovella assemblages from sections that span interval peak Cenozoic warmth, 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 particular interest because it dominated tropical-subtropical early Paleogene then suffered an abrupt permanent decline abundance taxonomic diversity start EECO. At all ODP sites, morozovellids display dominant dextral preference during preceding However, species modify their preferentially to sinistral EECO, <200 kyr after K/X event (~52.8 providing new biostratigraphic tool for correlation. We also document before major shift morozovellid coiling, transient excursions higher abundances tests occurred conjunction with negative carbon isotope excursions. Significantly, data reveal morphotypes belonging same morphospecies typically have lower δ 13 C values. dominance morphotypes, expense forms coupled signatures former, suggests were less dependent on photosymbiotic partnerships thus able adapt more readily paleoceanographic change observed can be genetically heritable lies cryptic speciation across multiple morphologically defined species. Alternatively exclusively ecophenotypic responses whereby different adopt reaction changed conditions mixed-layer Previous interpretations flips foraminifera Eocene, especially including morozovellids, favoured genetic explanation rather than ecological response. Our present cannot validate disprove this idea, but should stimulate renewed thought matter. • are presented three widely separated sites. switch Optimum. Changes distinct Carbon isotopes indicate differ ecology. Sinistral photosymbiosis change.