Some Bivalvia Of The Topola Formation (Middle Sarmatian), Northeastern Bulgaria

The fossil bivalvian fauna established in the Topola Formation, the Zelenka section, Northeastern Bulgaria was studied. Six taxa from the species group are described: Obsoletiforma cf. pseudosemisulcata (Andrussov, 1902), Obsoletiforma cf. balcicense (Gillet, 1938), Obsoletiforma cf. centopleura (Andrussov, 1902), Obsoletiforma sp. (aff. obsoleta Eichwald, 1830), Obsoletiforma sp. (aff. lucino-idea Paramonova, 1977), and Inaequicostata sp. (aff. barboti R. Hoernes, 1874). Two of them - O.centopleura, and O. lucinoidea are announced for the first time in Bulgaria. One taxon is determined at generic level - Obsoletiforma sp. The stratigraphic position of the section in the upper part of the middle (Bessarabian) substage of the Sarmatian (s.l.) stage was determined as the upper part of the interval zone Cryptomactra pseudotellina-Cryptomactra pesanseris and/or lowerest part of the local taxon-range zone Obsoletiforma balcicense. The factors of the abiotic environments and their influence on the presence of the bivalvian fauna in the section are clarified.