The Lower Triassic is widespread over the Russian Platform and is represented by thick, mostly continental deposits containing few fossils. In

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some areas the Lower Triassic deposits have been thoroughly studied but the exact positions of the upper and lower boundaries still remain questionable. Paleontological data strongly suggest the presence of a disconformity separating the Upper Permian from the Lower Triassic.

In the Petschora Basin and in the northern part of the Fore-Urals, red Triassic deposits of continental origin disconformably overlie red and grey strata of Tatarian age. The stratigraphic break is marked by an abrupt change in the conchostracan and plant assemblages, with the appearance of Lioestheria ex. gr. gutta, Pseudoestheria aequale, Glossophyllum sp., and Pseudoaraucarites gorskii. In anticlinal fold regions, such as the Petschora Range and the Seduyakhin Wall, Triassic deposits overlie Lower Permian, Carboniferous and Devonian rocks.

In the Kama and Volga Basins, it can be demonstrated that vertebrates, crustaceans and molluscs from the uppermost Vyatsky Beds of the Tatarian are quite different from those of the Ryabinsky Beds (lower part of Vetlugian Series) at the base of the Triassic. The Ryabinsky Beds contain numerous new species of Conchostraca, as well as several relict species from the Upper Permian.

Triassic deposits continue southwards from the Urals to the Kuibyshev area of the Volga Basin. Here the Vetlugian Series is divided into three sedimentary cycles, each with characteristic assemblages of terrestrial vertebrates, according to Shishkin and Otshev. In central and western areas of the southern Fore-Urals the lowermost sedimentary cycle lies at the base of the Triassic. However, in the eastern area, and also northeast of the Buzuluk Basin, the Triassic is represented only by the second or third cycle.

In the Pre-Caspian Basin typical Lower Triassic assemblages of Ostracoda and Charophyta occur, including several new forms belonging to the family Darwinulidae. The thickest and most complete Triassic sections are present in the central part of the basin and along its northern, western and southwestern sides. In the eastern and southeastern parts of the basin and in the Inder Lake district, conglomeratic sandstones of the Vetlugian Series overlap lower horizons of Tatarian age. The oldest zone of the Vetlugian Series (Marginella Zone) occurs in the central part of the basin (Aralsor District), where it directly overlies the Tatarian. South of the Volga and Ural rivers, rocks of Kazanian age are overlain by younger deposits represented by the Sphaerachara karpinskyi and S. globosa zones. Similarly, in the middle Emba Valley, different parts of the Sorkul Formation (Vetlugian Series) overlie the Upper Permian.

In the southern part of the Russian Platform, Lower Triassic deposits overlie Paleozoic deposits of the Pre-Volga monocline, the Pre-Donetz Basin, the northwest margin of the Donetz folded belt, and the Dneiper-Donetz and Pripyat Basins. The Early Triassic age of these strata is established by the presence of Ostracoda, Conchostraca and Charophyta.

In the Pre-Donetz Basin the Vetlugian once again transgressively overlies different horizons in the Permian (from Kazanian in the east to Asselian in the west) and Carboniferous. The Vetlugian Series, represented by the Dronovskaya, Korenevskaya and possibly by the lower part of the Serebrjanskaya Formations, can be traced from the northwestern margin of the Donetz folded belt through the Dneiper-Donetz Basin to the Pripyat Basin. The exact age of the Dronovskaya Formation has not yet

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been established, for it contains a mixture of Lower Triassic and Upper Permian (Tatarian) Ostracoda and Charophyta. While the formation overlies the various horizons of the Lower Permian, Carboniferous and Pre-Cambrian with considerable angular unconformity in the marginal areas of the basin, some authorities, such as Lapchik, assign both the Dronovskaya and Korenevskaya formations to the Upper Permian.

Triassic deposits in the Polish-Lithuanian syncline are represented by the Neman Formation (argillo-arenaceous and marly rocks of the Bunt-sandstein), which transgressively overlies the Zechstein carbonate sequence (lowermost Verra to lower part of Leine cycles).

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