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Cambrian

The Cambrian is a geologic period that began around 545 million years ago and ended about 490 million years ago. During this time, roughly fifty separate major groups of organisms or "phyla" (including almost all the basic body plans of modern animals) emerged suddenly without evident precursors. This radiation of animal phyla is referred to as the Cambrian explosion.

The Cambrian Period is the earliest period in whose rocks large numbers of fossils of organisms more complex than algae are found. Cambria is the Roman name for Wales which has areas of Cambrian age rocks investigated by Adam Sedgwick in the 1830s. Eventually as the series was filled out, the youngest "Cambrian" came to overlap the oldest parts of Murchison's Silurian. In 1879, Charles Lapworth defined an Ordovician period that included the overlapping beds. The Cambrian Period follows the Neoproterozoic and is followed by the Ordovician Period. The Cambrian is classically divided into three stages -- the Lower, Middle, and Upper Cambrian. 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 Cambrian period was eventually set at a fairly definite faunal change now identified as an extinction event. The time range has classically been thought to have been from about 500 million years before the present to about 570 million years before the present. Fossil discoveries and radioactive dating in the last quarter of the 20th Century have called these dates into some question. Inconsistencies in dates of as much 20 million years between authors are common. The dates used here are hopefully consistent with those proposed by the International Subcommission on Global Stratigraphy as of 2002

Current dates for the start of the Cambrian hover around 545 million years. A radiometric date from New Brunswick puts the end of the first stage of the Cambrian around 511 million years. This leaves only 21 million years for the other two periods of the Cambrian.

The Cambrian is usually broken into Lower (Caerfai or Waucoban), Middle (St Davids or ALbertian) and Upper (Merioneth or Croixan) subdivisions. The faunal stages from youngest to oldest are:

  • Franconian/Trempealeauan/Dolgellian (Upper)
  • Dresbachian/Maentwrogian (Upper)
  • Solvan (Middle), Menevian (Middle)
  • Toyonian/Lenian/Botomian (Lower)
  • Atdabanian (Lower)
  • Tommatian (Lower)

The Cambrian continents are thought to have resulted from the breakup of a Neoproterozoic supercontinent called Rodinia. It is thought that Cambrian climates were significantly warmer than those of preceding times which experienced extensive ice ages discussed as the Varanger glaciation. Continental motion rates in the Cambrian may have been anomalously high. 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. One such site is http://www.scotese.com/

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 Cambrian. 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 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 preCambrian Ediacarian faunas have been identified in the past 50 years, but their relationships to Cambrian forms are quite obscure.



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