Sexual dimorphism in two Ordovician ostracods from the Baltoscandian region and their phylogenetic significance

For the first time, sexual dimorphism can be proved for Tvaerenella modesta and Oepikium novum, two ostracod species established by Sarv (1959) from the middle Kunda stage (B3β), Middle Ordovician, Estonia. In Oepikium novum it is developed as dolonate botulate antral dimorphism, in Tvaerenella modesta as dolonate loculate antral dimorphism. The latter species is the oldest known palaeocope with loculate dimorphism. Because it belongs to Loculibolbina rather than to Tvaerenella, the stratigraphical range of Loculibolbina, which was known only from the Late Ordovician can now be extended to the Middle Ordovician. Particularly, L. modesta proved to have great significance for phylogenetic reconstructions.