Evolution of the Cretaceous Calcareous Nannofossil Genus Eiffellithus and Its Biostratigraphic Significance

Jamie L. Shamrock and David K. Watkins
Geosciences, University of Nebraska, Lincoln, NE

The calcareous nannofossil genus Eiffellithus is one of the most important groups for chronostratigraphical and paleoecological characterization in the mid to Upper Cretaceous deep-sea sediments. The species definitions for these marker taxa have been both broadly interpreted and variably applied by nannofossil workers. This is particularly true for the Eiffellithus eximius species plexus. While the mid-Cretaceous taxonomy recently has been well-defined, the remaining 35 m.y. history of the genus has not been closely examined. Our investigation of sediments from the Western Interior Seaway and the Gulf of Mexico, spanning the Cenomanian to the Maastrichtian, has indicated the presence of at least six new species of Eiffellithus that can be reliably differentiated. The currently used biostratigraphical markers (E. turriseffelii and E. eximius) have been redefined in a more restricted sense to increase their utility. These refinements in taxonomy have revealed an obvious shift in dominance both within the genus and within the nannofossil assemblage as a whole through the Late Cretaceous. In the Cenomanian and Maastrichtian the genus is composed exclusively of coccoliths bearing an X-shaped central cross, such as E. turriseiffelii, while in the Coniacian through Campanian axial-cross forms such as E. eximius comprise up to 65% of the genus. Within the nannofossil assemblage the genus has low abundances in the Cenomanian but increases to ~15% of the assemblage in well preserved samples in the Santonian and Campanian. In addition, the pattern of diversification of this genus, whereby a diagonal cross repeatedly gives rise to an axial cross by rotation about the central axis, is an excellent example of iterative evolution that may be related to repetitive shifts in Late Cretaceous climatic and paleoceanographic regimes.

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas