Pre-cambrian (or Precambrian) is a term used to describe the eons before the Cambrian. It lasted from about 4.567 billion years ago to about 542 million years ago. It starts with the beginning of the Earth, about 4.567 billion years ago. When it ends, there were many hard-shelled animals around. It is named after the Roman name for Cymru / Wales, because rocks thought to be from this time were first studied there.
The supereon is subdivided into 3 eons, the Hadean (4.5 to 4 billion years ago), the Archaean (4 to 2.5 billion years) and the Proterozoic (from 2.5 billion years to 542 million years).
The immediate Precambrian period is the Ediacaran.
↑Dalrymple, G. Brent (2001). "The age of the Earth in the twentieth century: a problem (mostly) solved". Special Publications, Geological Society of London190: 205–221. doi:10.1144/GSL.SP.2001.190.01.14.
↑Manhesa, Gérard; Allègrea, Claude J.; Dupréa, Bernard; and Hamelin, Bruno (1980). "Lead isotope study of basic-ultrabasic layered complexes: Speculations about the age of the earth and primitive mantle characteristics". Earth and Planetary Science Letters, Elsevier B.V.47: 370–382. doi:10.1016/0012-821X(80)90024-2.
↑Stanley, Steven M. 1999. Earth system history. New York: W.H. Freeman and Company. ISBN 0-7167-2882-6.