The Middle-Upper Jurassic Boundary in the North of Siberia: Problems of the Evidence

The grounds for identifying the Eboraciceras subordinarium Zone in the Upper Callovian of Siberia are critically discussed. The key sections of this zone and the ammonite assemblage that characterizes it are examined in detail. It is concluded that it is impossible to recognize the Eboraciceras subordinarium Zone and its further use in stratigraphic schemes is not suggested. North Siberian taxa, previously identified as Late Callovian Eboraciceras, are mostly assigned to Rondiceras and indicate a Middle Callovian age of the host rocks. Beds with Cadoquenstedtoceras begichevi have been proposed as an approximate equivalent of the European Q. lamberti Zone in the Upper Callovian of Siberia.