The plan is for its lower boundary to correspond approximately to the first appearance of trilobites, about 521 million years ago,[2] though the globally asynchronous appearance of trilobites warrants the use of a separate, globally synchronous marker to define the base.
[3] The upper boundary and beginning of Cambrian Stage 4 is informally defined as the first appearance of the trilobite genera Olenellus or Redlichia around 514.5 million years ago.
[4] The International Commission on Stratigraphy has not officially named the 3rd stage of the Cambrian.
The stage approximately corresponds to the "Atdabanian", which is used by geologists working in Siberia.
[6] The Cambrian radiation of animal phyla ends here, after the diversification and origination of arthropods, molluscs, lophophorates, chordates (including vertebrates) among others.