Stratigraphy of the Cambrian

Stratigraphy of the Cambrian period currently has several schemes used for ordering geologic formations from the period. The International Commission on Stratigraphy−ICS scheme has set a stratotype section for the base of the Cambrian, dated quite accurately to 541 ± 1.0 million years ago. Russian and Chinese scientists have developed a different scheme.

Stratigraphy relates to the order of rock units, without referring to their absolute ages (which is chronology). However, because fossils — which are traditionally the cornerstone of stratigraphy — are relatively rare in the Cambrian period, chronology has a significant part to play. If an absolute age can be obtained with a high degree of accuracy for different strata, their relative age can also be established.

The base of the Cambrian is officially defined as the first appearance of a certain trace fossil, Treptichnus pedum. However, this fossil also appears in older rocks than the locality which officially marks the start of the period. However, there is a period of biological change which makes this time period a good one to demarkate the Cambrian from the earlier Phanerozoic eon's periods and the Precambrian supereon. Consensus holds that fossils of the Ediacara biota disappear here, as do some shelly fossils and acritarchs; and a new small shelly fossil biota emerges.

The opening of the Cambrian period is marked by a number of biological changes, including the extinction of the Ediacara biota, the preponderance of armoured organisms (e.g. the small shelly fossils), and a "widening of the behavioural repertoire" indicated primarily by an increase in vertical burrowing, first for protection and later for feeding - the Cambrian substrate revolution.

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Wikipedia