Size variations of coccoliths in Cretaceous oceans : a result of preservation, genetics and ecology?

Biometric studies of coccoliths, the remains of coccolithophores, offer the opportunity to survey single species instead of entire assemblages. We obtained and analyzed size data of three common species (Biscutum constans, Zeugrhabdotus erectus and Watznaueria barnesiae) in a stratigraphically very well-defined interval of early Aptian age (~126 Ma; Cretaceous). Material is derived from four sites (Lower Saxony Basin, North Sea, western Tethys, Mid-Pacific) covering nearshore to open-oceanic paleosettings. Length and width measurements of 1986 specimens were evaluated. The recorded size patterns show a larger data spread for B. constans and W. barnesiae in the western Tethys and the Mid-Pacific than in the North Sea and the Lower Saxony Basin. The latter two sites are dominated by small coccoliths of B. constans while coccoliths of W. barnesiae show similar sizes at all four sites. Solely small specimens of Z. erectus characterize the samples from the North Sea and the Lower Saxony Basin while only large ones are present in the samples of the western Tethys and Mid-Pacific. For explaining the recorded size patterns, three theories are discussed in detail; these include (1) preservation of nannofossils, (2) genetics and (3) palecology. (1) Intense dissolution or overgrowth of the nannofossils may have altered the original coccolith sizes particularly when biometric data from different sites with potentially varying states of preservation are compared.Due to its delicate morphology, Z. erectus appears most prone to dissolution, probably explaining its size pattern. (2) If the recorded size data of the remaining two species represent original patterns, these can be interpreted by new findings in recent coccolithophore genetics. It has been shown that size variations within a single cryptic species are only minor. Shifts of coccolith sizes, both in recent and fossil taxa, can be related to genotypic variation. The varying size ranges recorded for B. constans and W. barnesiae may therefore reflect diversity changes of cryptic species at the different sites. (3) These cryptic species may have preferred different depth habitats depending on geographically-controlled factors such as sea-surface temperatures, light availability or trophic load.