Morphometry of the Eocene nannofossil Reticulofenestra umbilicus lineage and its biochronological consequences

Morphometruc measurements of Eocene and early Oligocene reticulofenestrids from DSDP Site 523 in the South Atlantic show that a 90% size increased led to the appearance of Reticulofenestra umbilicus. This increase occurred between about 45.2 Ma and 42.9 Ma. The attempts made to recognize R. umbilicus during this period of change as a distinct subpopulation among the reticulofenestrid assemblages did not yield any tenable conclusions. However, using a lower size limit of 14 μm for recognition of the R. umbilicus morphovariant resulted in an easily identified first appearance event which occurred during Chron 19R, at an age of about 44.4–44.5 Ma. From a morphometric point of view, the two morphovariants R. hillae and R. umbilicus could not be distinguished as two distinct subpopulations. The Oligocene assemblages show markedly smaller (10%) central opening/placolith length ratios as compared to the Eocene forms, but for several reasons it seems difficult to use this decrease biostratigraphically.