Panboreal correlation of the Upper Volgian and Lower Ryazanian based upon some well-traced ammonite immigrational events which provide well-constructed fremework of the zonal resolution
There are no Tethyan "guests" inhabited Boreal and Subboreal regions at the Late Volgian time 
Key ammonite events:
1) Base of the Upper Volgian. FAD of Kachpurites fulgens which penetrates eastwards up to Lena basin. At the same time first true large Craspedites of the okensis group scattered through Boreal and Subboreal seas, from Northern Siberia to Svalbard and SE of the Russian Platform.
2) Rare Subcraspedites (preplicomphalus group) invaded Middle Volga area.
3) Craspedites with well-developed periumbilical nodes widely spreads through the Arctic. Siberian species C. pseudonodiger mentioned from Kashpir section, while C.plicomphalus known from East England
4) Volgidiscus event
5) Hectoroceras instantly scattered through the Arctic
6) first appearance of Submediterranean ammonites in the Russian Sea, after invasion of Hectoroceras
7) second immigrational event
