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Abstract
Pliocaenicus taxa living or in very recent sediments from upland lakes around Lake Baikal are examined in relation to those found in Pleistocene and Miocene deposits in Kamchatcka and elswehere.
The validity of the genus Pliocaenicus is substantiated and subdivision of P. costatus sensu lato is examined in some detail. Re-examination of material from Skabitchevsky's type locality and an LM and SEM study of conspecific and closely related taxa has been undertaken. Skabichevsky's type material is unavailable and two morphotypes have been described in the literature under this epithet. The taxonomy of P. costatus var. sibiricus (Skabitch.) Round & Håkansson is now clarified and a new variety, P. costatus var. leprindus, is established.
Comparison with fossil material shows that P. costatus taxa are very stable over time with morphologies differing only in detail of valve central area, areolar density, valve central area texture (colliculate or smooth), spacing of marginal costae, and number of central area fultoportulae. Valve shape and presence of marginal spines can be used as supporting characteristics.
The occurrence of living P. costatus var. sibiricus (Skabitch.) Round & Håkansson in one Siberian lake permitted us to examine some modern ecological and recent palaeoecological attributes of this representative of a largely fossil genus. It is the most abundant planktonic diatom in several pristine, extremely softwater wilderness lakes near to Siberian Lake Baikal. In one of these lakes (Lake Bolshoie), its frequency abundance has increased since the 19th century. The P. costatus group of taxa could have considerable potential for indicating past biogeographic and climatic changes in Siberia and elsewhere.
