The paper examines trilobite assemblages found in the Botomian–Toyonian (Lower Cambrian) boundary stratotype section in the middle reaches of the Lena River, Siberian Platform. It has been found that the trilobite assemblage from the Bergeroniellus ketemensis Zone (bottom of the Toyonian) is very much the same as that from the underlying Bergeroniaspis ornata Zone (terminal Botomian zone). It has been suggested to combine these zones into one with the index species Bergeroniaspis ornata. The greatest changes in the trilobite assemblages are observed at a higher stratigraphic level, at the bottom of the Lermontovia grandis Zone (second Toyonian zone). This suggests that the lower boundary of the Toyonian should be placed at the bottom of this zone. This new boundary shows substantial changes in the trilobite abundance and diversity and is well-pronounced, not only in the stratotype sections of the Lena River, but also elsewhere in the Siberian Platform (the northeast and northwest of the region, the Anabar area, and others).

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