CLIMATIC CONTROL ON THE MORPHOLOGY OF COCCOLITHOPHORES (RETICULOFENESTRA) FROM THE EOCENE—OLIGOCENE TRANSITION

It is well established that coccolithophores responded strongly to paleoceanographic and paleoclimatic events, either through extinction or abundance changes. Although little documented, morphologic changes within genera and species are also associated with such events. To analyze them we have undertaken a study of changes in shape that coccoliths of the genus Reticulofenestra (Order Isochrysidales) underwent across late Middle Eocene to Early Oligocene (~40-30 Ma), a time of major cooling which resulted in Antarctic glaciation. This genus was abundant and varied in size from around 3 μm to 20 μm. To quantify these changes, we have measured the fossil Reticulofenestra in 63 samples from the South Atlantic and the Indian Ocean. For each sample, the length and width of the distal shield, proximal shield, and the central opening of 100 randomly selected specimens were measured. Distinct morphologic changes were observed, including 1) a trend towards increasing size beginning around 35 Ma within the larger clade (>8 μm), this large Reticulofenestra, becoming extinct just after the size maximum at around 32 Ma. 2) The ratio of central opening size/placolith size decreased from around 0.45 to 0.35 in all size groups at around 35 Ma. 3) A gradual trend towards enhanced circularity of the placoliths throughout the study interval and 4) the ratio of the proximal/distal shield diameters remained relatively steady at around 0.8 in all groups. Published morphometric studies of extant Gephyrocapsa, which is a descendant of Reticulofenestra, indicate that the morphotypes with a narrower central area are more likely to thrive in eutrophic sea water or in deeper layers of the photic zone. By analogy, the decrease in size of the central opening observed in Eocene-Oligocene Reticulofenestra is possibly the result of strengthened stratification and Atlantic meridional overturning circulation since the Late Eocene, which fertilized the lower photic zone and favored the abundance of deeper water dwellers.