A detailed study of the con odont distribution within the condensed carbonate beds around the boundary of Billingen and Volkhov Stages (Lower Ordovician, Arenig) in northwestern Russia confirms the presence of a continuous succession of conodont biownes recognized in contemporaneous sequences in Scandinavia. The Baltoniodus triangularis and Microzarkodina sp. A zones are reported for the first time from East Baltica,where carbonates of this interval underwent extensive bioturbation and are characterized by a number of closely spaced and distinctive glauconitic hardground surfaces. It has been revealed that the distribution of conodont elements around the hardground surfaces is strongly affected by bioturbation.
The Ordovician Billingen/Volkhov boundrary interval (Arenig) at Lava Rive, northwestern Russia