Searching for cells: the potential of fossil coccospheres in coccolithophore research

The majority of fossil coccolithophore studies are based on disaggregated coccoliths but preserved complete coccospheres provide documentation of true cellular traits (cell size, numbers of coccoliths per cell and calcite quotas) and represent a relatively untapped archive of palaeobiological information. Coccospheres are more frequently present when coccolith preservation is good or exceptional. Their preservation is dependent on a variety of taphonomic factors that combine to minimize the destructive effects of grazing and sinking, sea floor bioturbation and, finally, carbonate diagenesis. Clay-rich hemipelagic sediments that have not been deeply buried are particularly productive. Herein, we highlight the occurrences and potential of these coccosphere fossils and illustrate the range of morphologies we have so far encountered, predominantly from the Paleogene interval. Our observations show that coccosphere morphology is generally a conservative, long-lived character, with fossil representatives of living taxa (e.g., Braarudosphaera, Coccolithus, Helicosphaera, Reticulofenestra), all showing very similar coccosphere style back through their evolutionary history. In addition, we have also observed a variety of sphere shapes (ovoid, ellipsoidal, cylindrical) and significant coccolith polymorphism and varimorphism across a number of different families and in taxa for which fossil coccospheres have not previously been known. Fossil Acanthoica and Calciosolenia spheres, for example, indicate that polymorphism is a long-lived characteristic of the Syracosphaerales group. Lastly, by combining morphometric data from both fossil and living populations we show that coccosphere geometries (coccosphere size, number of coccoliths per coccosphere and coccolith size) show systematic trends (e.g., larger cells have larger coccoliths) but there is significant divergence from this trend that reflects intrinsic, taxon-specific factors (e.g., coccolith shapes and packing) and growth phase of the population. Although our preliminary coccosphere studies have uncovered new, surprising and useful observations we are still far from understanding the fundamental controls on cell size, coccosphere geometry and architecture.