Dinoflagellate cysts and microforaminifera of the Lower Cretaceous Yatria River section, Subarctic Ural, NW Siberia (Russia). Biostratigraphy, palaeoenvironmental and palaeogeographic discussion

Rich assemblages of phytoplankton and microforaminifera (foraminifera linin gs) are found in the marine Lower Cretaceous section from the Yatria River (Subaretic Ural). The Lower Cretaceous (Upper Volgian to Lower Hauterivian) dinocyst and microforaminifera biostratigraphy is calibrated against the a mmonite zones. The dinocyst stratigraphy differs from Arctic Canada zonatio n, except for the Berriasian Paragonyaulacysta? borealis zone, and shows si gnificant similarity with the zonal subdivision of Boreal regions of Europe . The distribution of phytoplankton and microforaminifera is related to relat ive sea level variations. Consecutive changes of phytoplankton associations (2 in the Berriasian. 2 in the Valanginian-Early Hauterivian, 4 in the Ear ly Hauterivian), lateral zonations of microforaminifera (2 in the Berriasia n, 2 in the Valanginian) and their relation to environmental changes in the Lower Cretaceous seas of the Subarctic Ural Basin are established.