Biometric analysis of Pliensbachian-Toarcian (Lower Jurassic) coccoliths of the family Biscutaceae: intra- and interspecific variability versus palaeoenvironmental influence

Abstract

Biometric analyses were applied to nine Lower Jurassic species of the genera Biscutum and Similiscutum in order to test whether these species can be typified by their morphological characters, and to evaluate the influence of the palaeoenvironment on coccolith morphology. This study was carried out on samples coming from different localities of the western Tethys, situated at different palaeolatitudes. The studied areas are: offshore Morocco (DSDP 547B), western Sicily, central Italy, and SW Germany. The analysed samples are of Pliensbachian and Toarcian ages. Length and width of the coccolith, and of its central area, were measured on digitally captured images of 1489 specimens, using a semi-automated method. Biometric analyses do not allow differentiation between all the considered species, with the exception of Biscutum grande, Biscutum dubium, and Biscutum intermedium that have peculiar morphological characters. Some other species form a cluster and cannot be significantly differentiated based on the measured parameters. These are: Similiscutum cruciulus, Similiscutum orbiculus and Similiscutum avitum, which likely represent different ecophenotypes of the same species. The biometric parameters analysed in this study (coccolith size, ellipticity, and central area dimensions) may be subject to palaeoenvironmental control. An increase in coccolith size may be related to high surface-water temperatures and/or to high nutrient concentration. An influence of critical conditions during the Early Toarcian anoxic event on coccolith size is speculated in the present work.