Contribution to the biodiversity of the lower Frankfurt Formation (Miocene) in the Hanau Basin (Germany): a new Hymenomonas (calcareous nannoplankton) and a new Scrippsiella (dinoflagellate) species

Fine-grained calcareous sediments often contain mass occurrences of well-preserved micro- and nannofossils, providing a wide range for biodiversity studies and enable to establish index-fossils. In our research, we depicted very tiny nannofossils which might be easily overlooked. Two new species Hymenomonas kelleri (calcareous nannoplankton) and Scrippsiella francofurtana (dinoflagellate, calcareous resting stage) are described, representing ecologically important forms. Their stratigraphic occurrence is documented from the lower part of the Frankfurt Formation (Miocene, Upper Hydrobia Beds). The collected sediment samples were taken from the temporary excavation site “WestendDuo” tower in the city of Frankfurt/Main, Germany. Both extremely small species show exceptionally fine structures and only slight signs of diagenesis. Their association with remains of further dinoflagellates and other species of algae ( S . francofurtana together with a mass occurrence of the calcareous nannoplankton species Coccolithus pelagicus ) as well as stromatoliths, shells of gastropods ( Hydrobia species) and skeletons of fishes ( Gobius species) gives hints to a highly diversed former biocoenosis and to a characteristic Miocene thanatocoenosis. Furthermore, a new mass occurrence of nannoplankton named layer +21 is described.