CELL PHYSIOLOGY TO SPECIES SUCCESS: UNIQUE INSIGHTS INTO COCCOLITHOPHORE COMMUNITIES FROM EXQUISITE FOSSIL COCCOSPHERES AND MODERN GROWTH PROXIES

Intact fossil coccolithophore ‘cells’ from well-preserved clay-rich sediments have great potential for novel paleobiological and paleoecological research. The body size, coccolith length and number of coccoliths forming each fossil cell exoskeleton captures a snapshot of growth, whilst integrated populations of individuals reveal species-level responses of communities to environmental changes on multiple timescales.

Here we illustrate the diversity of fossil coccosphere morphologies that we have observed, spanning more than 40 predominantly Paleogene species, many of which have never before been imaged as intact coccospheres. Relationships between coccosphere and cell size, coccolith length, and number of coccoliths per cell measured from more than 4,000 individual fossils identify within- and between-species differences in coccosphere geometry. We can draw direct comparisons between these measurements and the coccosphere geometry of modern descendent species under different growth conditions to develop a growth ‘proxy’ that can be observed across a range of culture experiments, natural field populations and fossil material.

This exciting new approach, from the perspective of physiological fitness, allows us to explore the relative success of species within a community experiencing changes through both long-term and rapid climate perturbations. For instance, reconstructed community cell size profiles through the Eocene reveal a trend towards larger mean sizes due to the ecological replacement of the small, cosmopolitan coccolithophore genus Towieus by Reticulofenestra, which has high morphological diversity. Variability within long-term records of coccosphere geometry, such as for the ‘living fossil’ Coccolithus, identify intervals of more and less favorable environmental conditions for growth. Smaller cells with fewer coccoliths indicate the ability to maintain higher division rates and larger cells with more coccoliths suggest extended periods of sub-optimal environmental conditions. The ecological success of different evolutionary lineages is therefore preserved in the coccosphere geometry of individual cells. This invaluable information can be decoded by uniting both modern and paleontological biological approaches.