Cambrian

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 488.3 million years BP 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. This radiation of animal phyla is referred to as the Cambrian explosion.

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.

Related Topics:
Neoproterozoic - Ordovician

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The faunal stages from youngest to oldest are:

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• 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 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.

Related Topics:
500 million years - Arthropod - Trilobite - Reef - Archeocyathids - Extinction event - Fossil - Radioactive dating - 20th century

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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.

Related Topics:
New Brunswick - 511 million years

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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.

Related Topics:
542 million years - Palaeological - Deductive reasoning - Carbon-13 - Palaeontologist - Stratigraphic sequences - Oman - Carbon - Isotope - Volcanic ash - Zircon - Uranium - Lead - Siberia - Namibia

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