The Cambrian is a geologic period and system that began about 542 ± 0.3 Ma (million years ago) at the end of the Proterozoic eon and ended about 488.3 ± 1.7 Ma with the beginning of the Ordovician period. It is the first period of the Paleozoic era of the Phanerozoic eon. The Cambrian takes its name from Cambria, the classical name for Wales, the area where rocks from this time period were first studied.
The Cambrian is the earliest period in whose rocks are found numerous large, distinctly fossilizable multicellular organisms. This sudden appearance of hard body fossils is referred to as the Cambrian explosion. Despite the long recognition of its distinction from younger Ordovician rocks and older Precambrian rocks it was not until 1994 that this time period was internationally ratified. The base of the Cambrian is defined on a complex assemblage of trace fossils known as the Trichophycus pedum assemblage. This assemblage is distinct from anything in the Precambrian as it has ecologically tiered vertical burrows which are absent from the Precambrian.
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