Series 2 has not been formally defined by the International Commission on Stratigraphy, lacking a precise lower boundary and subdivision into stages.

The proposed lower boundary is the first appearance of trilobites which is estimated to be around 521 million years ago.

[3] The Siberian nomenclature distinguishes three stages (lowest first): Atdabanian, Botomian and Toyonian.

Correlating this event on different continents has proven difficult and resolving this is essential for the definition of the lower boundary of this series.

[6] The end of the 2nd series of the Cambrian is marked by the first major biotic extinction of the Paleozoic.