TAXONOMY AND BIOSTRATIGRAPHY OF EXQUISITELY PRESERVED CENOMANIAN-SANTONIAN BISERIAL PLANKTIC FORAMINIFERA FROM TANZANIA DRILLING PROJECT BOREHOLES

The taxonomy of mid-Cretaceous (Cenomanian- Santonian) biserial planktic foraminifera has undergone major revision based on detailed SEM and x-ray study. However the proposed phylogenetic hypotheses and biostratigraphic inferences have not been tested in more than one stratigraphic section and taxonomic distinctions have not been established based on biometric comparison of species populations. We present a biostratigraphic and biometric study of remarkably well preserved late Cenomanian-early Santonian biserial planktic foraminifera recovered from a series of boreholes drilled during the Tanzania Drilling Project. In this study we combine detailed SEM observations with measurements obtained from high-resolution x-ray images using primary type specimens (where possible) and populations from multiple stratigraphic levels in order to more reliably (1) characterize intraspecies variability, (2) delineate interspecies differences, and (3) test the new phylogenetic hypotheses. The x-ray images provide the best means of discriminating chamber outlines for the entire ontogenetic growth trajectory in well preserved biserial tests and they enable rapid collection of a wide array of shape and size measurements. We provide comparative biometric data for populations of Heterohelix globulosa (Ehrenberg), Hx. reussi (Cushman), Globoheterohelix paraglobulosa Georgescu and Huber, Planoheterohelix moremani (Cushman), Pl. postmoremani Georgescu and Huber, Huberella praehuberi Georgescu, and Hu. huberi Georgescu. We demonstrate that (1) Pa. paraglobulosa and Hx. globulosa show distinct differences in their ontogenetic growth profiles, (2) Hx. reussi and Hx. globulosa are indistinguishable, (3) Pl. moremani and Pl. postmoremani are indistinguishable, and (4) the evolutionary transition from Hu. praehuberi to Hu. huberi consistently occurs in the late Turonian and is marked by easily recognizable changes in the external test morphology. Our approach of using SEM observations and measurements together with biometric data obtained from x-ray images enables development of a more reliable taxonomic framework for comparison of biserial planktic foraminiferal species populations.