Belemnites in the Jurassic-Cretaceous boundary interval of the Mauryn’ya and Yatriya River sections, Western Siberia: Biostratigraphic significance and dynamics of taxonomic diversity

The stratigraphic distribution and taxonomic diversity of belemnites from the Volgian and Ryazanian deposits exposed in the eastern foothills of the North and Subpolar Urals, are studied and analysed. Studies of new collections from the Mauryn’ya and Yatriya rivers have revealed within the Laugeites groenlandicus-basal Surites analogus ammonite zones, the East Siberian Lagonibelus napaensis, Cylindroteuthis knoxvillensis, and Liobelus russiensis belemnite zones (the latter in the beds’ rank), the Lagonibelus gustomesovi and Arctoteuthis porrectiformis Beds, and local beds with Boreioteuthis explorata and Simobelus compactus. Since in the boundary beds of the Volgian and Ryazanian stages a great number of belemnite species known from the Tordenskjoldberget Member, Kong Karls Land, Svalbard, were found, it is inferred that the accumulation of the member started not in the Valanginian as many researchers believe but immediately at the beginning of the Cretaceous. The dynamics of belemnite species diversity in the northwestern margin of the West Siberian marine basin correlates well with climatic events. An increase of species number in the terminal Volgian-beginning of the Ryazanian corresponds to a temperature elevation in the Siberian paleoseas, and the subsequent reduction of species diversity in the second half of the Ryazanian is correlated with a gradual cooling. The peak of belemnite taxonomic diversity falls at the beginning of the Cretaceous when up to 15 species belonging to eight Cylindroteuthididae genera occurred concurrently. The new species Cylindroteuthis ornata sp. nov., Acroteuthis pseudoconoides sp. nov., Pachyteuthis eximia sp. nov., and Simobelus compactus sp. nov. are described.