Stratigraphy of the Cambrian

The International Commission on Stratigraphy−ICS scheme has set a stratotype section for the base of the Cambrian, dated quite accurately to 538.8 ± 0.6 million years ago.

[4] This combination of factors marks the end of the slow-moving, extinction-proof Precambrian period, and the start of a more complex food web with hugely different dynamics.

This assemblage has been thought to appear and diversify rapidly at the base of the Cambrian, but re-dating of some Russian samples has placed them long before 539 million years ago, and shown that they are stratigraphically below beds containing 'Ediacaran' fossils.

[6]: 226  More detailed analysis of the fossils suggest that they might have been formed by extensive under-mat mining, rather than true vertical burrowing, which may go some way to explain this - they may not represent a fundamentally new biological innovation.

Chemical signals in rocks can be used to identify biological changes such as extinction or diversification, so they probably provide the most reliable indicator of the dawn of the Cambrian.

[9] Their limited geographical extent restricts their utility on other continents; a good stratum in each geological region is required to allow accurate cross-continental correlation.

[9] Even biostratigraphic markers fall foul of bioprovinciality, and are of limited utility in correlating rocks on opposite sides of the globe.