Changes in coiling direction of Cibicidoides pseudoacutus (Nakkady) across the Cretaceous–Tertiary boundary of Tunisia: palaeoecological and biostratigraphic implications

The analysis of coiling direction preference in the benthic foraminifera Cibicidoides pseudoacutus (Nakkady) has been carried out across the Cretaceous–Tertiary boundary (K–T boundary) from four Tunisian sections representing a palaeobathymetric transect from a middle–outer neritic to lower upper bathyal depositional setting. Our study reveals that C. pseudoacutus developed a preference for sinistral coiling in a short time period during the lowermost Danian. mparison of the coiling ratio (number of sinistral vs. dextral individuals) record with isotope data and dinoflagellate cyst assemblage distribution suggests that the development of sinistrally coiled populations of Cibicidoides pseudoacutus might be related to a short-term cooling of both surface and bottom waters which occurred at the K–T boundary and lasted for some 7 kyr. ntinuous occurrence and the high abundance of Cibicidoides pseudoacutus through the K–T boundary in neritic to upper bathyal depositional environments around southern Tethys make identification of the shift in its coiling ratio relatively easy. The development of a sinistrally coiled population in this benthic foraminiferal species is regarded as a potential marker for the base of planktonic foraminiferal Zone P0 in Tethyan shallow water settings.