Biostratigraphic significance of new fossil species of the diatom genera Stephanodiscus and Cyclotella from Upper Cenozoic deposits of Lake Baikal, Siberia

Three new extinct taxa of the genus Stephanodiscus Ehrenberg (S. williamsii sp. nov., S. princeps sp. nov., S. yukonensis var. antiquus var. nov.) and four new extinct species and two new extinct varieties of the genus Cyclot ella (Kutzing) Brebisson (C. iris var. insucta var. nov., C. tempereiformic a sp. nov., C. distincta sp. nov., C. comtaeformica sp. nov., C. comtaeform ica var. spinata var. nov. and C. praeminuta sp. nov.) are described from U pper Cenozoic lacustrine sediments of Lake Baikal (boreholes BDP-96-1 and B DP-96-2). The narrow biostratigraphic ranges of the new taxa warrant their use as index fossil species for defining a diatom zonation, for accurate co rrelation and age control of Lake Baikal sediments. The comparative morphol ogical analysis of the newly described Cyclotella taxa allowed us to trace the evolution of morphological features in these diatoms through time. The short time intervals during which the Stephanodiscus taxa existed in ancien t Baikal imply that these species were unable to adapt to rapidly changing paleoecological and paleolimnological conditions caused by abrupt Pleistoce ne climatic fluctuations.