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Cambrian

2007 Schools Wikipedia Selection. Related subjects: Geology and geophysics

   The Cambrian is a major division of the geologic timescale that begins
   about 542 mya (million years ago) at the end of the Proterozoic eon and
   ended about 488.3 mya 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 (Gould, 1989). This
   radiation of animal phyla is referred to as the Cambrian explosion.
                         Paleozoic era
   Cambrian Ordovician Silurian Devonian Carboniferous Permian

Cambrian subdivisions

   The Cambrian period follows after the Neoproterozoic and is followed by
   the Ordovician period. The Cambrian is divided into three epochs — the
   Early Cambrian (Caerfai or Waucoban), Middle Cambrian (St Davids or
   Albertian) and Furongian (also known as Late Cambrian, Merioneth or
   Croixan). Rocks of these epochs are referred to as belonging to the
   Lower, Middle, or Upper Cambrian.

   Each of the epochs are divided into two faunal stages. Only one, the
   Paibian, has been recognized by the International Commission on
   Stratigraphy, and others are still unnamed. However, the Cambrian is
   divided into several regional faunal stages:
   Chinese North American Russian-Kazakhian Australian Regional
   Furongian Ibexian (part) Ayusokkanian Idamean Dolgellian
   Sunwaptan Sakian Mindyallan Festiniogian
   Steptoan Aksayan Payntonian Maentwrogian
   Marjuman Batyrbayan
   Middle Cambrian Maozhangian Mayan Boomerangian
   Zuzhuangian Delamaran Amgan Undillian
   Zhungxian Florian
   Templetonian
     Dyeran Ordian
   Early Cambrian Longwangmioan Toyonian Lenian
   Changlangpuan Montezuman Botomian
   Qungzusian Atdabanian
   Meishuchuan Tommotian
   Nemakit-Daldynian

Cambrian dating

   The time range for the Cambrian has classically been thought to have
   been from about 500 mya to about 570 mya. 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 question. Date
   inconsistencies as large as 20 Ma are common between authors. Framing
   dates of ca. (approximately) 545 to 490 mya 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 mya. This leaves 21 Ma for the other two
   stages of the Cambrian.

   A more precise date of 542 ± 0.3 mya 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
   paleological deductive reasoning. Exactly at the Cambrian boundary
   there is a marked fall in the abundance of carbon-13, a "reverse spike"
   that paleontologists 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 Ma (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 paleogeography

   Cambrian continents are thought to have resulted from the breakup of a
   Neoproterozoic supercontinent called Pannotia. 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. Laurentia, Baltica and Siberia remained
   independent continents since the break-up of the supercontinent of
   Pannotia. Gondwana started to drift towards the South Pole. Panthalassa
   covered most of the northern hemisphere, and minor oceans included the
   Proto-Tethys Ocean, Iapetus Ocean, and Khanty Ocean, all of which
   expanded by this time.

Cambrian fauna

   Fossil trilobite Redlichia chinensis from the Cambrian of China
   Enlarge
   Fossil trilobite Redlichia chinensis from the Cambrian of China

   Aside from a few enigmatic forms that may or may not represent animals,
   all modern animal phyla with any fossil record to speak of (except
   bryozoans) appear to have representatives in the Cambrian, and of these
   most except sponges seem to have originated just after or just before
   the start of the period. However, several modern phyla, primarily those
   with small and/or soft bodies, have no fossil record, in the Cambrian
   or otherwise. 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 the 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 the People's Republic of China's Yunnan Province
   (see Maotianshan shales). Fairly extensive pre-Cambrian Ediacaran
   faunas have been identified in the past 50 years, but their
   relationships to Cambrian forms are quite obscure.

Ordovician flora

   There are no land plants at this time. The land at this time was a
   barren land of desert and badlands. Marine green algae probably
   appeared at this time, and they will eventually evolve into land
   plants, in the Latest Ordovician.

Fungal life

   There were also no land fungi at this time. Marine fungi were probably
   common in the oceans.

Name

   The Cambrian is named for Cambria, the classical name for Wales, the
   area where rocks from this time period were first studied.
   Retrieved from " http://en.wikipedia.org/wiki/Cambrian"
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