page_title: Stratigraphy of the Cambrian
section_title: Biological changes at the base of the Cambrian
hierarchical_section_title: Stratigraphy of the Cambrian / Biological changes at the base of the Cambrian
caption_attribution_description: English: Treptichnus pedum fossil marking the Cambrian-Ediacaran GSSP
context_page_description: The 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. 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.
context_section_description: 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.
  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.  However, since there are a large number of inter-related biological factors, defining a single stratum to define the base of this period is impossible - the change did not happen over-night, but over a prolonged period.  The sequence of events leading to the Cambrian revolution in fact begun in the Ediacaran period, and culminated in the middle of the Cambrian; defining a sharp base to a gradual biological process is impossible, but it is necessary to have a globally recognised base to the period to allow scientists across the globe to know what each other are referring to and to converse meaningfully.
caption: The first occurrence of Treptichnus pedum marks the Cambrian-Ediacaran boundary in Newfoundland (sort of, there are a few occurrences a couple of metres below!)
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