Size patterns of the coccolith Watznaueria barnesiae in the lower Cretaceous: Biotic versus abiotic forcing

Body size is a major trait allowing the assessment of the respective control of evolution vs. (palaeo)environment on unicellular micro-organisms. Size patterns can be retraced for fossilisable micro-organisms, especially for coccolithophores, over geological time spans. Here, we present coccolith size patterns of Watznaueria barnesiae, which was a dominant and ubiquitous taxon in the Mesozoic. Measurements were conducted at two sites spanning the Upper Berriasian to Lower Hauterivian, namely the Vergol-La Charce section in the epicontinental Vocontian Basin, and the ODP Leg 185 Hole 1149B located in the Nadezhda Basin, in the Pacific Ocean. The obtained results were treated using multivariate analysis in order to detect significant trends of the measured parameters. Size values of coccoliths from both sites are remarkably similar and display comparable patterns, namely a statistically significant increase in the Lower Valanginian until maximum sizes recorded in the aftermath of the Weissert event (Upper Valanginian). Although W. barnesiae coccolith-size fluctuations, in the short term, might be related to environmental changes, the observed long-term size patterns are interpreted as the result of directional selection, in which a single phenotype is favored through time. Comparison with data available in the literature shows a return to a smaller size after the Turonian.