Morphometric data of the late middle Eocene to early Oligocene (~40 - 31 Ma) Coccolithophore Reticulofenestra (Order Isochrysidales)

The first size reduction (FSR) in the
Reticulofenestra-Gephyrocapsa-Emiliania (RGE) lineage (Order
Isochrysidales) which occurred in the early Oligocene (~ 32 Ma), is of
great significance for understanding the lilliput effect that has affected
the coccolithophore communities from late Eocene to this day. We conducted
a morphologic analysis on the coccoliths of Reticulofenestra species that
lived during the late middle Eocene to early Oligocene (~40-31 Ma), using
marine sediments from the South Atlantic Ocean. Our data show increasing
size and decreasing abundance of the large species during the late Eocene,
leading to their disappearance at the FSR, and a concurrent decrease in
the size variability of the small-medium-sized coccoliths whose diameter
of the central opening had become very reduced. Although the cosmopolitan
late Paleogene through Neogene size decrease in coccolithophores has been
linked to the concomitant long-term decline in global pCO2, we suggest
here that the FSR was the result of environmental destabilization caused
by the expansion of eutrophic environments following the late Eocene
establishment of overturning circulation associated with ice build-up on
Antarctica. This study also leads us to propose a hypothetical model that
links coccolith morphology of species of the RGE lineage and trophic
resources in the upper ocean: the small- to medium-sized, r-selected
coccolithophores with smaller coccolith central opening live in
nutrient-rich waters where they rely mostly on photosynthesis and little
on mixotrophy; whereas the larger, K-selected species with larger
coccolith central opening live in oligotrophic waters where they are more
dependent on mixotrophy.