Genus Stensioina (benthic foraminifera) from Turonian-Santonian deposits of the Eastern European Platform: Species distribution, taxonomy and morphological evolution

We have studied the morphology and stratigraphic distribution of Turonian- Santonian Stensioina species from various outcrops and boreholes of the Eastern European platform. Our results indicate the occurrence of two periods of species evolution within the studied area. The first period, from the middle Turonian to middle Coniacian, includes the development of S. praexsculpta - S. granulata and S. praexsculpta - S. emsherica lineages and represents evolutionary change in the test spiral surface morphology. The second period, from the middle Coniacian to later Santonian, reflects the evolution of the test umbilical area within the S.exsculpta - S.incondita - S.pommerana lineage. The general patterns in early Stensioina evolutionary paleobiogeography can be traced based on the stratigraphic distribution of species, early Turonian endemic species like S. pokornyi suggest that Stensioina first evolved in Western Europe and subsequently, around middle-later Turonian, appeared in the eastern parts of the European paleobiogeographical realm (Koch, 1977). During Coniacian time Stensioina complexes from Western Europe show predominance of members from S. praexsculpta - S. granulata lineage (Bailey, 2005) whereas foraminiferal assemblages from Eastern Europe are strongly dominated by species from the S. praexsculpta - S. emsherica lineage. The last member of this lineage reached the Caspian region only at middle Coniacian time. These differences in species development in Eastern and Western regions probably reflect patterns of geographical isolation in the early evolution of Stensioina. Despite the complex evolution of these benthic foraminifera, they stand to enhance present biostratigraphic schemes for the Upper Cretaceous.