New data on acritarchs from the Upper Ordovician of the Tungus basin, Siberian Platform

Distinctive late Ordovician acritarch assemblages have been discovered for the first time from about a 100 m sedimentary succession exposed along the Bol¢shaya Nirunda River in Siberia. The studied stratigraphic interval includes the uppermost Baksian, Dolborian, Nirundian and Burian regional stages, which correspond to the Katian–?lowermost Hirnantian global stages. Acritarch assemblages from the Dolbor Regional Stage are exceptionally diverse and include aside from the long-ranging taxa several unique (endemic) morphotypes and a number of distinctive stratigraphically valuable species, well known outside Siberia. The occurrence of the acritarchs widespread outside Siberia is potentially important for interregional bio­stratigraphic correlations and might also play a significant role in biogeographic reconstructions. Having in mind that the Siberian palaeocontinent was located in a low-latitude tropical area during the entire Ordovician, the presence of taxa typical of cold-water settings along the Peri-Gondwana margin can be regarded as an additional evidence for penetration of cool-water currents into the epicontinental Tungus basin in the Upper Ordovician.