The Cambrian is a geologic period and system that began about Ma (million years ago) at the end of the Proterozoic eon and ended about Ma with the beginning of the Ordovician period . It was 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.

Cambrian subdivisions

The Cambrian period follows the Ediacaran and is followed by the Ordovician period. The Cambrian is divided into three epochs — the Early Cambrian (Caerfai or Waucoban), Middle Cambrian (St Davids or Albertian) and Furongian (also known as Late Cambrian, Merioneth or Croixan). Rocks of these epochs are referred to as belonging to the Lower, Middle, or Upper Cambrian.

Each of the epochs are divided into several stages. Only one, the Paibian, has been recognized by the International Commission on Stratigraphy, and others are still unnamed. However, the Cambrian is divided into several regional faunal stages of which the Russian-Kazakhian system is most used in international parlance:

Cambrian dating

The time range for the Cambrian has classically been thought to have been from about 500 mya to about 570 mya. The lower boundary of the Cambrian was traditionally set at the earliest appearance of early arthropods known as trilobites and also unusual forms known as archeocyathids (literally 'ancient cup') that are thought to be the earliest sponges and also the first non-microbial reef builders.

The end of the period was eventually set at a fairly definite faunal change now identified as an extinction event. Fossil discoveries and radiometric dating in the last quarter of the 20th century have called these dates into question. Date inconsistencies as large as 20 Ma are common between authors. Framing dates of ca. () 545 to 490 mya were proposed by the International Subcommission on Global Stratigraphy as recently as 2002.

A radiometric date from New Brunswick puts the end of the first stage of the Cambrian around 511 mya. This leaves 21 Ma for the other two stages of the Cambrian.