Cambrian

The Cambrian is a major division of the geologic timescale that begins about 542 Ma (million years ago) at the end of the Proterozoic eon and ended about 488.3 Ma with the beginning of the Ordovician period ( ICS, 2004). It is the first period of the Paleozoic era of the Phanerozoic eon. The Cambrian is the earliest period in whose rocks are found numerous large, distinctly-fossilizable multicellular organisms that are more complex than sponges or medusoids. During this time, roughly fifty separate major groups of organisms or " phyla" (a phylum defines the basic body plan of some group of modern or extinct animals) emerged suddenly, in most cases without evident precursors. This radiation of animal phyla is referred to as the Cambrian explosion.

Paleozoic era
Cambrian	Ordovician	Silurian	Devonian	Carboniferous	Permian

Cambrian subdivisions

The Cambrian period follows after the Neoproterozoic and is followed by the Ordovician period. The Cambrian is divided into three epochs — the Early Cambrian (Lower Cambrian, Caerfai or Waucoban), Middle Cambrian (St Davids or Albertian) and Furongian (a.k.a. Late/Upper Cambrian, Merioneth or Croixan).

Each of the epochs are divided into two faunal stages. Only one, Paibian has been recognized by the International Commission on Stratigraphy. Others are still unnamed. However, the Cambrian is divided into several regional faunal stages:

	Chinese	North American	Russian-Kazakhian	Australian	Regional
Furongian		Ibexian (part)	Ayusokkanian	Idamean	Dolgellian
		Sunwaptan	Sakian	Mindyallan	Festiniogian
		Steptoan	Aksayan	Payntonian	Maentwrogian
		Marjuman	Batyrbayan
Middle Cambrian	Maozhangian		Mayan	Boomerangian
	Zuzhuangian	Delamaran	Amgan	Undillian
	Zhungxian			Florian
				Templetonian
		Dyeran		Ordian
Early Cambrian	Longwangmioan		Toyonian		Lenian
	Changlangpuan	Montezuman	Botomian
	Qungzusian		Atdabanian
	Meishuchuan		Tommotian
			Nemakit-Daldynian

Cambrian dating

The time range for the Cambrian has classically been thought to have been from about 500 Ma to about 570 Ma. The lower boundary of the Cambrian was traditionally set at the earliest appearance of early arthropods known as trilobites and of primitive reef-forming animals known as archeocyathids. The end of the period was eventually set at a fairly definite faunal change now identified as an extinction event. Fossil discoveries and radioactive dating in the last quarter of the 20th century have called these dates into question. Date inconsistencies as large as 20 million years are common between authors. Framing dates of ca. (approximately) 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 first stage of the Cambrian around 511 Ma. This leaves 21 million years for the other two stages of the Cambrian.

A more precise date 542 Ma (plus or minus 300,000 years) for the extinction event at the beginning of the Cambrian has recently been submitted. The rationale for this precise dating is interesting in itself as an example of palaeological deductive reasoning. Exactly at the Cambrian boundary there is a marked fall in the abundance of carbon-13, a 'reverse spike' that palaeontologists call an excursion. It is so widespread that it is the best indicator of the position of the Precambrian-Cambrian boundary in stratigraphic sequences of roughly this age. One of the places that this well-established carbon-13 excursion occurs is in Oman. Amthor (2003) describes evidence from Oman that indicates the carbon- isotope excursion relates to a mass extinction: the disappearance of distinctive fossils from the pre-Cambrian coincides exactly with the carbon-13 anomaly. Fortunately, in the Oman sequence, so too does a volcanic ash horizon from which zircons provide a very precise age of 542 ± 0.3 Ma (calculated on the decay rate of uranium to lead). This new and precise date tallies with the less precise dates for the carbon-13 anomaly, derived from sequences in Siberia and Namibia. It is presented here as likely to become accepted as the definitive age for the start of the Phanerozoic eon, and thus the start of the Palaeozoic era and the Cambrian period.

Cambrian palaeogeography

Cambrian continents are thought to have resulted from the breakup of a neoproterozoic supercontinent called Rodinia. The waters of the Cambrian period appear to have been widespread and shallow. It is thought that Cambrian climates were significantly warmer than those of preceding times that experienced extensive ice ages discussed as the Varanger glaciation. Continental drift rates in the Cambrian may have been anomalously high. Because of their complexity, it is difficult to describe continental motions in text. Time-sequenced maps of paleo-continents and other major geologic features are called paleomaps and are available at several Internet sites (see below).

Cambrian fauna

Aside from a few enigmatic forms that may or may not represent animals, all modern animal phyla with any fossil record to speak of except bryozoa appear to have representatives in the Cambrian, and of these most except sponges seem to have originated just after or just before the start of the period. However, several modern phyla, primarily those with small and/or soft bodies, have no fossil record, in the Cambrian or otherwise. Many extinct phyla and odd animals that have unclear relationships to other animals also appear in the Cambrian. The apparent "sudden" appearance of very diverse faunas over a period of no more than a few tens of millions of years is referred to as the " Cambrian Explosion".

The best studied sites where soft parts of organisms have fossilized are in the Burgess shale of British Columbia. They represent strata from the middle Cambrian and provide us with a wealth of information on early animal diversity. Similar faunas have subsequently been found in a number of other places -- most importantly in very early Cambrian shales in China's Yunnan Province (see Maotianshan shales). Fairly extensive pre-Cambrian Ediacarian faunas have been identified in the past 50 years, but their relationships to Cambrian forms are quite obscure.