Cambrian
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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 490 million years BP with the beginning of the Ordovician period. 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", including almost all the basic body plans of modern animals, emerged suddenly, in most cases without evident precursors. This radiation of animal phyla is referred to as the Cambrian explosion.
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Origin
Cambria is the Latin name for north Wales, a place of extensive 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.
Cambrian subdivisions
The Cambrian period follows after the Neoproterozoic and is followed by the Ordovician period. The Cambrian is classically divided into three stages -- a lower (Caerfai or Waucoban), middle (St Davids or Albertian) and upper (Merioneth or Croixan) Cambrian. 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 dating
The time range for the Cambrian has classically been thought to have been from about 500 million years BP to about 570 million years Before present. 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 some question. Date inconsistencies as large as 20 million years are common between authors. Framing dates of ca. (approximately) 545 to 490 million years ago 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 million years. This leaves 21 million years for the other two stages of the Cambrian.
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. 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 million years ago (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 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".
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.
External links and references
- Amthor, J. E., et al.. 2003. Extinction of Cloudinia and Namacalathus at the Precambrian-Cambrian boundary in Oman. Geology, 31: pp 431-434.
- Report on the web on Amthor et al. from Geology vol. 31 (http://www.earth-pages.com/archive/geobiology.asp).
- Paleomap Project (http://www.scotese.com/).
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