WHAT ARE CALCISPHERES? – PRISTINE SPECIMENS FROM THE TANZANIA DRILLING PROJECT PROVIDE UNPRECEDENTED INSIGHT INTO AN ENIGMATIC CRETACEOUS MICROFOSSIL GROUP

Calcispheres constitute the second-most abundant calcareous microfossils of many Late Cretaceous rocks and show significant spatial and temporal blooms, the most important of which is the Late Cenomanian - Early Turonian global calcisphere bioevent associated with OAE2. Because of their common appearance calcispheres could give substantial paleo-ecological information if their taxonomic affinities would be better known. However, a lack of morphological features limits our understanding of what these microfossils actually are. The most common calcispheres belong to the genus Pithonella. They are 20 to 180 micrometers in diameter and have been interpreted as calcareous cysts of dinoflagellates. However, the dinoflagellate cyst nature of Pithonelloideae has been controversial due to their lack of dinoflagellate-specific features. Unusually well-preserved Pithonelloideae are abundant in the Turonian from Tanzania. The pristine specimens reveal a number of shell-morphological features that have never been described before. Our study aims at revising the taxonomy of this complex microfossil group based on these new morphological findings. We combine light-optical imaging, SEM-imaging, cross-section study and microprobe analysis on single specimens. Most crucial to the determination of their taxonomic affinity with dinoflagellates is our discovery of an operculum-like structure that can be associated with the apical 3’ plate known from the peridinialean tabulation pattern of dinoflagellates. Furthermore, comparison between Pithonella sphaerica and the known calcareous dinoflagellate cyst Pirumella krasheninnikovii reveals striking biomineralogical similarities in wall-crystal architecture and chemistry. Such similarity hints at a common biomineralisation principle in both species. Stable oxygen isotope values of Pithonella plot with the values of planktic foraminifera suggesting a habitat of Pithonella near the water surface.