It is commonly believed that very large basalt volumes produced by Early Triassic magmatism had effused synchronously in both the Siberian Platform and Western Siberia in a very short time interval 249.4 +/- 0.5 Ma ago ([15] and others). This magmatism and the climate changes it induced are considered as the main reason for the most catastrophic (Permian-Triassic) extinction event in the Earth's history. However, these conclusions are based on incomplete and unrepresentative data on Western Siberia. Based on analysis of the pyroxene monofraction from kainotypic basalts of the Guslinskaya P-430 well, we obtained an Ar-Ar age of 268.4 +/- 7.5 Ma. The respective age for the Taurovskaya 503 well is 268.1 +/- 7.5 Ma. Hence, volcanism in the axial rift zones of the West Siberian Plate basement began earlier than was thought and significantly earlier than that in the Siberian Platform.