The time range for the Cambrian has classically been thought to have been from about 570 Ma to about 500 Ma. The lower boundary of the Cambrian was traditionally set at the earliest appearance of 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 Ma were proposed by the International Subcommission on Global Stratigraphy as recently as 2002.
A radiometric date from New Brunswick puts the end of the Lower Cambrian around 511 Ma. This leaves 21 Ma for the other two series/epochs of the Cambrian.
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