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Free Content Morphology of Actinocyclus and Lobodiscus species (Bacillariophyta) from the Miocene deposits of the Vitim Plateau, Russia

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Background and aims – In 2006, during a field expedition in the Amalat (Southern) paleovalley of the Vitim Plateau, 85 samples were taken from sediments of a new key hole 7236. The composition and distribution of siliceous microfossils were studied with light microscope (Usoltseva et al. 2008). The prevailence of species of the planktonic genera of Actinocyclus Ehr. and Aulacoseira Thwaites was recorded. The aims of this paper were electron-microscopic investigations of fossil freshwater species Actinocyclus and Lobodiscus from the Miocene lacustrine sediments of a new key hole 7236, and the comparison of our morphological data with formerly published descriptions of known freshwater taxa of these genera.

Methods – Samples from the section 7236 were examined using LM and SEM. Abundance was calculated based on the LM slides.

Key results – Valve ultrastructure of dominant biostratigraphically valuable species of Actinocyclus and Lobodiscus from the Miocene freshwater sediments of the Vitim Plateau was for the first time studied with SEM. The comparison of results of carried out investigations with formerly published descriptions of known freshwater taxa of Actinocyclus and Lobodiscus permits to belong the studied species to Actinocyclus gorbunovii (Sheshukova) Moisseeva & Sheshukova and A. krasskei (Krasske) Bradbury & Krebs, to expand their certain morphometric data, as well as to describe A. vitimicus Usoltseva & Khursevich sp. nov., A. intermedius Usoltseva & Khursevich sp. nov. and Lobodiscus peculiaris Usoltseva & Khursevich sp. nov., with definite diagnostic features.

Conclusion – Freshwater extinct species of Actinocyclus can be used as stratigraphic markers. Thus, occurred planktonic taxa of Actinocyclus, including the new species of A. vitimicus and A. intermedius, described above, as well as Lobodiscus peculiaris together with dominant accompanying planktonic representatives of Aulacoseira represent the characteristic diatom complex in lacustrine sediments of the Upper Dzhilinda subformation recovered from key hole 7236 within the Amalat paleovalley (the Vitim Plateau). The Middle-Late Miocene age of studied lacustrine deposits agrees with K-Ar dating of lavas in this part of section.

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Document Type: Regular Paper

Publication date: 01 November 2010

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