CONSTRAINTS ON THE EARLY CAMBRIAN RADIATION AND CORRELATION OF THE TOMMOTIAN AND NEMAKIT-DALDYNIAN REGIONAL STAGES OF EASTERN SIBERIA

Acritarchs of early Cambrian age occur in strata attributed to the Mid dle Tommotian in northeastern Siberia. A diagnostic assemblage from a part of the Tyusersk Formation referred to the Dokidocyathus regularis Zone (Middle Tommotian) at Chekurovka, and a much poorer assemblage f rom Ulakhan-Aldyarkhay, indicate time-equivalence with the Heliosphaer idium dissimilare-Skiagia ciliosa acritarch Zone and the correlative H olmia kjerulfi trilobite Zone in Baltica. The bulk of the species turn over which is part of the Cambrian explosion, including archaeocyathan s, small shelly fossils, trilobites and acritarchs, is inferred to occ ur in the Skiagia ornata-Fimbriaglomerella membranacea (=Schmidtiellus /Rusophycus faunal zone) and H. dissimilare-S. ciliosa (=Holmia kjerul fi trilobite Zone) acritarch zones.