Cambrian

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The Cambrian Period is the earliest period in whose rocks large numbers of fossils 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 arthropoda 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.

Current dates for the start of the Cambrian hover around 544 million years. A radiodate from New Brunswick puts the end of the first stage of the Cambrian around 511 million years. This leaves only 11 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 anamolously 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. 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. Fairly extensive preCambrian Ediacarian faunas have been identified in the past 50 years, but their relationships to Cambrian forms are quite obscure.