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.

Morphometric measurements were performed by randomly taking photographs in bright field of 10 to 15 fields of view per smear slide at 1000x magnification. A field of view typically contained 5 to 10 Reticulofenestra placoliths. For each sample, one hundred (100) specimens were measured. To avoid distortion caused by different orientations of the coccoliths as seen in light microscopy, only specimens horizontally oriented with the microscope stage were measured.
Measurements were conducted manually using the biometric analysis software Fiji version 1.52 P by dragging ellipses over the contours of each placolith in order to measure its length and width. The long and short diameters of the central opening were likewise measured. Our procedure has resulted in a database of 6400 measurements for the whole study.
We undertook a morphometric analysis with the double objective of quantifying the morphological features of the placoliths at any stratigraphic level and analyzing their change through time. Three characters yield meaningful biological and physiological information:

1. Length of coccolith.
2. Central opening/coccolith length ratio:
Length ratio=  (length of the central opening/length of the distal shild)*100%.
3. Circularity of coccolith:
Circularity = width of coccolith/length of coccolith.
Higher circularity means a more circular shape, while lower circularity means a more elliptical shape of coccoliths. 

This file includes the raw data and supplementary descriptions that required in the research. The supplementary material includes 1. coccolith assemblage, 2. data reproducibility evaluation, 3 estimated cell and coccosphere sizes that were mentioned in the manuscript and 4. our biostratigraphic interpretation.
In the data file, we delineate coccolith morphology by two major structures, the distal shield and the central opening. Then the length and width of both are given in microns.
One hundred specimens are measured in each sample, in which the sample IDs are present in the first specimens. The ages are estimated based on our biostratigraphic interpretation, which is introduced in the supplementary text.