CENTRIC DIATOMS FROM THE MIOCENE DEPOSITS IN THE BAIKAL RIFT ZONE (Tunka Valley) Available to Purchase

A few new species of centric diatom algae of the Cyclotella Kütz. and Stephanodiscus Ehr. genera, and a variety of Aulacoseira Thw. genus from Miocene sediments of the Baikal Rift Zone (Tunka Valley, BH-73) are described. Cyclotella tuncaica sp. nov. has strongly tangentially waved frustules that have diameters of 14–40 μm. The species is similar to C. distinguenda Hust., but, unlike the latter one, has an oval central part, which is strongly tangentially waved. It also differs in the frequency of marginal fultoportulae and in the structure of the rimoportula. Scanning electron microscopy revealed that the species is dissimilar to the endemic species of Lake Baikal Cyclotella minuta (Skv.) Antipova, but can be mis-identified as the latter when using light microscopy. Stephanodiscus tuncaensis sp. nov. has frustules of diameters equal to 15–24 μm, and is similar to S. bellus var. minor Churs. et Log. in having a single central fultoportula with three satellite pores but differs from it in strongly concentrically waved face, in the pattern of areolae over the central area, and in the absence of expressed radial rows. The species is dissimilar from fossil Baikalian S. grandis Churs. et Log., S. carconeiformis Churs. et Log., S. flabellatus Churs. et Log. The new variety Aulacoseira praegranulata var. tuncaica has frustules of diameters equal to 5–7 μm, heights equal to 4–17 μm, and falls, according to many characters, into the diagnosis of the widely variable species A. praegranulata (Jouse) Simonsen, which has a few varieties. However, it differs from A. praegranulata (Jouse) Simonsen var. praegranulata and var. praeislandica (Jouse) Moiss. in the form of linking spines, and in the form and structure of areolae. The paper also presents descriptions of the frustules of a few specimens belonging to the Aulacoseira genus that demonstrate diversity of these algae in the sediments that were studied. Some of these specimens are similar to A. baicalensis, the extant endemic species of Lake Baikal.