Evolutionary Lineages of Peridinioid Dinoflagellate Cysts (Class Dinophyceae Pascher, 1914) in the Middle Eocene of the Lower Volga Region

The sequential evolutionary appearance of closely related species of peridinioid dinoflagellate cysts has been established in the Middle Eocene deposits of the Lower Volga region (Ravninnaya R-43 borehole, Volgograd region, Russia). Two new species of dinoflagellate cysts, Wilsonidium imperfectum sp. nov. and Wilsonidium michouxi sp. nov., are described from the Keresta Formation in the Volga region. Evolutionary morphological changes of species from the genera Phthanoperidinium Drugg et Loeblich Jr., 1967 and Wilsonidium Lentin et Williams, 1976 are considered. Analysis of the stratigraphic and geographical distribution of taxa indicates that the species studied have a high correlation potential on the Russian Platform.