Cambrian

 This period is part of the
Paleozoic era.
 Cambrian
 Ordovician
 Silurian
 Devonian
 Carboniferous
 Permian

The Cambrian is a geologic period that began around 542 million years ago (see below) and ended about 490 million years ago. The Cambrian Period is the earliest period in whose rocks numerous large, distinctly-fossilizable multicellular organisms more complex than sponges or medusoids are found. 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.

Table of contents
1 'Cambrian' naming
2 Cambrian dating
3 Cambrian subdivisions
4 Cambrian palaeogeography
5 Cambrian fauna

'Cambrian' naming

'Cambria' is the Roman name for Wales, which has areas of Cambrian-age rocks investigated by Adam Sedgwick in the 1830s. Eventually as the stratigraphic series was filled out, the youngest "Cambrian" came to overlap the oldest parts of the 'Silurian' sequence of strata,that had been identified by Sir Roderick Murchison. In 1879, Charles Lapworth defined an 'Ordovician' period that included the overlapping beds.

Thus 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.

Cambrian dating

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. Framing dates of ca 545 to 490 mya were proposed by the International Subcommission on Global Stratigraphy as recently as 2002.

A more precise date 542 million years (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. J. E. Amthor et al., 'Extinction of Cloudinia and Namacalathus at the Precambrian-Cambrian boundary in Oman.' (in Geology v. 31 (2003), pp 431-434) describe the evidence from Oman that the carbon-isotope excursion relates to a mass extinction: the disappearance of distinctive fossils from the latest pre-Cambrian coincides exactly with the carbon anomaly. Luckily, in the Oman sequence, so too does a volcanic ash horizon, from which zircons provide a very precise age of 5420.3 mya, calculated on the decay rate of uranium to lead. This newly 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.

Cambrian subdivisions

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)

Cambrian palaeogeography

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 verbally. 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/

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 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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