Using Larger Foraminifers in High Resolution Biostratigraphy: An Example from the Eocene of the Gulf of Mexico and Northern Caribbean

Dating by planktic microfossils is commonly difficult in very shallow marine paleoenvironments. On the other hand most groups of larger foraminifers thrived under such conditions. Although many larger foraminifers are regarded as being of limited use in studies involving high resolution biostratigraphy, exceptions should occur in cases where a new group is evolving. One such group, the Lepidocyclinidae, evolved in the early middle Eocene of the Americas from a coiled, Amphistegina-like ancestor via intermediate forms, such as Eulinderina and Polylepidina. Morphological modifications include shortening of the nepionic chamber spire and change in the position of the first chamber with a retrovert aperture. The precise nature and timing of these changes have remained obscure. Using samples from Alabama, Jamaica, and St. Bartholomew it is shown that progressive reduction in length of the nepionic spire of Eulinderina up-section occurred through planktic foraminiferal zones P11 and P12 (calcareous nannoplankton zones NP15 and NP16). Assemblages corresponding to Polylepidina appear at the NP16/CP14a level. A similar stage of development appears to be reached in populations of similar age from widely separated localities, based on available planktic microfossil dating. In shallow marine paleoenvironments, the biostratigraphic control provided by the evolutionary state of the Eulinderina-Polylepidina lineage is potentially superior to that providable by conventional planktic microfossils. A model correlating observed clustered values for two parameters of the nepionic spire with the TA 3.3 to TA 3.5 third order middle Eocene sequences of Haq and others is presented.