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The morphology of archaeocyaths from the Oldynda Formation of the Uda–Vitim lithofacial zone and the Aikta Formation of the Birami–Yanguda Zone of West Transbaikalia are described. The age of these formations is refined for the first time on the basis of C and Sr isotope chemostratigraphy. Three Archaeocyatha horizons (Uldzuitui of the Atdabanian Stage and Sukhaya Rechka and Khuludyi of the Botomian Stage) are described in the Oldynda Formation instead of the four previously described horizons, and the Kachin Horizon of the Toionian Stage is described in the Aikta Formation. The morphogenesis of archaeocyaths is analyzed: from the forms with simple porosity (Uldzuitui period) to cups with scales, rings, and canals of walls (Sukhaya Rechka–Khuludyi period) to cups with an additional microporous sheath (Kachin period). All stages reflect the blossom and extinction of the archaeocyatha community of the Sayany–Baikal mountainous area. The 87Sr/86Sr ratio and δ13C values of limestones of three horizons of the Oldynda (0.70828–0.70847 and 0.2–2.7‰ PDB, respectively) and Aikta (0.70866–0.70877 and –2.4 to 0.5‰ PDB, respectively) formations coincide with the C and Sr isotopic variations of the Early Cambrian seawater of the Atdabanian, Botomian, and Toionian periods. The C and Sr chemostratigraphic data are consistent with paleontological findings and correspond to the ideas of the formation of the Uda–Vitim island arc system and relation to an epicontinental marine basin of the Siberian Platform in the Early Cambrian. [ABSTRACT FROM AUTHOR]