Position of the Precambrian-Cambrian Boundary in the Sections of Western Mongolia according to the Malacological Data

The study of mollusk fauna from the Vendian-Cambrian sections of the Western Mongolia revealed common species of mollusks between the Bayangol Formation of Mongolia and the Nemakit-Daldynian and Tommotian stages of the Siberian Platform. We show that the Cambrian lower boundary (in terms of the General Stratigraphic Chart of Russia) in the Zavkhan structural-facies zone of the Western Mongolia can be recognized in the sections by the first appearance of the following Tommotian species of mollusks, i.e. Watsonella crosbyi Grabau, Bemella jacutica (Missarzhevsky), Aldanella sibirica Parkhaev et Karlova, Merismoconcha tommotica (Zhegallo), Asiapatella undulata (Yu), Pseudoscenella taishirica (Zhegallo), Purella tenuis Zhegallo, and Protoconus orolgainicus (Zhegallo). These species compose the faunal assemblage of the Watsonella crosbyi-Merismoconcha tommotica Zone, which is traced in the middle and upper parts of the Bayangol Formation. Thus, in contrast to previous studies that placed the base of the Cambrian near the base of the Bayangol Formation, we significantly raised the lower boundary of the Cambrian system up to the level of the middle to upper part of the Bayangol Formation.