Cambrian

The Cambrian is a major division of the geologic timescale that begins about 542 million years before the present (BP) at the end of the Proterozoic eon and ended about 488.3 million years BP 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.

Cambrian fauna

Aside from a few enigmatic forms that may or may not represent animals, all modern animal phyla except bryozoa appear to have representatives in the Cambrian, and most except sponges seem to have originated just after or just before the start of the period. 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".

Related Topics:
Animal - Bryozoa - Sponges - Extinct - Cambrian Explosion

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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.

Related Topics:
Burgess shale - British Columbia - Shale - China - Yunnan - Maotianshan shales - Ediacarian

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