Disentangling implications of changes in morozovellids coiling direction at the (EECO, ca 53-49 Ma) (Pacific, Atlantic and Indian Oceans).

A major change in planktic foraminiferal populations occurred at the start of the Early Eocene Climatic Optimum (EECO), the interval of peak Cenozoic warmth from between ~53–49 million years ago (Ma). The symbiont-bearing genus Morozovella suffered an abrupt and permanent decline in abundance and taxonomic diversity after dominating tropical subtropical early Paleogene assemblages. Conversely, the genus Acarinina markedly increased in abundance and diversity. Moreover, at ocean drilling sites in the tropical Pacific (1209-1210), and the Indian Ocean (762) morozovellids display a switch from dominantly dextral coiling preceding the EECO, to sinistral coiling within the EECO, as previously recorded in Atlantic Ocean sites (1051, 1258, 1263). This happens close to the carbon isotope excursion known as K/X or ETM-3 (~52.8 Ma), which also provides a new biostratigraphic tool for correlation. To interpret the observed changes, we measured the δ13C composition of dextral and sinistral morozovellid and acarininid morphotypes spanning the start of EECO. Carbon isotope data reveal that sinistral morphotypes belonging to the same morphospecies typically have lower δ13C values. The dominance of sinistral morphotypes, at the expense of dextral forms within the EECO, coupled with the lower δ13C signatures of the former, suggests that the sinistral forms were less dependent on their photosymbiotic partnerships, possibly moving slightly down in the mixed-layer, and thus able to adapt more readily to paleoceanographic change at the EECO. Remarkably, the genus Acarinina does not display coiling preferences throughout and its δ13C data suggest greater flexibility giving evidence of major resilience to the EECO perturbance. Whether sinistral and dextral morozovellids were the result of cryptic speciation, our record implies an evolutionary selection favouring sinistral forms. Alternatively, whether the coiling changes were exclusively environmentally controlled, the different species were able to preferentially adopt sinistral coiling as a result of changed conditions in the mixed-layer during the EECO.