Cambrian Period 542–488.3 million years ago PreЄ Є O S D C P T J K Pg N
Mean atmospheric O2 content over period duration	ca. 12.5 Vol %[1] (63 % of modern level)
Mean atmospheric CO2 content over period duration	ca. 4500 ppm[2] (16 times pre-industrial level)
Mean surface temperature over period duration	ca. 21 °C[3] (7 °C above modern level)
Sea level (above present day)	Rising steadily from 30m to 90m[4]

The Cambrian is the first geological period of the Paleozoic Era, lasting from 542 ± 0.3 to 488.3 ± 1.7 million years ago (mya) (ICS, 2004^[5]); it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's Cambrian rocks are best exposed.^[6] The Cambrian is unique in its unusually high proportion of lagerstätten. These are sites of exceptional preservation, where 'soft' parts of organisms are preserved as well as their more resistant shells. This means that our understanding of the Cambrian biology surpasses that of some later periods.^[7]

The Cambrian Period marked a profound change in life on Earth; prior to the Cambrian, living organisms on the whole were small, unicellular and simple. Complex, multicellular organisms gradually became more common in the millions of years immediately preceding the Cambrian, but it was not until this period that mineralised – hence readily fossilised – organisms became common.^[8] The rapid diversification of lifeforms in the Cambrian, known as the Cambrian explosion, produced the first representatives of many modern phyla, representing the evolutionary stems of modern groups of species, such as the arthropods. While diverse life forms prospered in the oceans, the land was comparatively barren – with nothing more complex than a microbial soil crust. Most of the continents probably resembled deserts.^{[citation needed]} Shallow seas flanked the margins of several continents created during the breakup of the supercontinent Pannotia.The seas were relatively warm, and polar ice was absent for much of the period.

The United States Federal Geographic Data Committee uses a "barred capital C" character similar to the capital letter Ukrainian Ye ‹Є› to represent the Cambrian Period.^[9] The proper^[10] glyph is in the Unicode standard at code point A792.^[11]

Stratigraphy[link]

Despite the long recognition of its distinction from younger Ordovician rocks and older Precambrian rocks, it was not until 1994 that this time period was internationally ratified. The base of the Cambrian is defined on a complex assemblage of trace fossils known as the Treptichnus pedum assemblage.^[12] Nevertheless, the usage of Treptichnus pedum, a reference ichnofossil for the lower boundary of the Cambrian, for the stratigraphic detection of this boundary is always risky because of occurrence of very similar trace fossils belonging to the Treptichnids group well below the T. pedum in Namibia, Spain and Newfoundland, and possibly, in the western USA. The stratigraphic range of T. pedum overlaps the range of the Ediacaran fossils in Namibia, and probably in Spain.^[13][14]

[edit] Subdivisions

The Cambrian period follows the Ediacaran and is followed by the Ordovician period. The Cambrian is divided into four epochs or series and ten ages or stages. Currently only two series and four stages are named and have a GSSP.

Because the international stratigraphic subdivision is not yet complete, many local subdivisions are still widely used. In some of these subdivisions the Cambrian is divided into three epochs with locally differing names – the Early Cambrian (Caerfai or Waucoban, 542 ± 0.3 to 513 ± 1.7 mya), Middle Cambrian (St Davids or Albertian, 513 ± 0.3 to 499 ± 1.7 mya) and Furongian (499 ± 0.3 to 488.3 ± 1.7 mya; also known as Late Cambrian, Merioneth or Croixan). Rocks of these epochs are referred to as belonging to the Lower, Middle, or Upper Cambrian.

Trilobite zones allow biostratigraphic correlation in the Cambrian.

Each of the local epochs is divided into several stages. The Cambrian is divided into several regional faunal stages of which the Russian-Kazakhian system is most used in international parlance:

*In Russian tradition the lower boundary of the Cambrian is suggested to be defined at the base of the Tommotian Stage which is characterized by diversification and global distribution of organisms with mineral skeletons and the appearance of the first Archaeocyath bioherms.^[15][16][17]

Cambrian dating[link]

Archeocyathids from the Poleta formation in the Death Valley area

The time range for the Cambrian has classically been thought to have been from about 542 mya to about 488 mya. The lower boundary of the Cambrian was traditionally set at the earliest appearance of trilobites and also unusual forms known as archeocyathids (literally 'ancient cup') that are thought to be the earliest sponges and also the first non-microbial reef builders.

The end of the period was eventually set at a fairly definite faunal change now identified as an extinction event. Fossil discoveries and radiometric 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. 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 Lower Cambrian around 511 mya. This leaves 21 mya for the other two series/epochs 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.^[18] 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 Precambrian 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 mya (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.

Paleogeography[link]

Plate reconstructions suggest that a global supercontinent, Pannotia, was in the process of breaking up early in the period,^[19][20] with Laurentia (North America), Baltica, and Siberia having separated from the main supercontinent of Gondwana to form isolated landmasses.^[21] Most continental land was clustered in the southern hemisphere at this time, but was gradually drifting north.^[21] Large, high-velocity rotational movement of Gondwana appears to have occurred in the Early Cambrian.^[22]

With a lack of sea ice – the great glaciers of the Marinoan Snowball Earth were long melted^[23] – the sea level was high, which led to large areas of the continents being flooded in warm, shallow seas ideal for thriving life. The sea levels fluctuated somewhat, suggesting that there were 'ice ages', associated with pulses of expansion and contraction of a south polar ice cap.^[24]

Climate[link]

The Earth was generally cold during the early Cambrian, probably due to the ancient continent of Gondwana covering the South Pole and cutting off polar ocean currents. There were likely polar ice caps and a series of glaciations, as the planet was still recovering from an earlier Snowball Earth. It became warmer towards the end of the period; the glaciers receded and eventually disappeared, and sea levels rose dramatically. This trend would continue into the Ordovician period.

Fauna[link]

All animal life during the Cambrian was aquatic. The period marked a steep change in the diversity and composition of Earth's biosphere. The incumbent Ediacaran biota suffered a mass extinction at the base of the period, which corresponds to an increase in the abundance and complexity of burrowing behaviour. This behaviour had a profound and irreversible effect on the substrate which transformed the seabed ecosystems. Before Cambrian, the sea floor was covered by microbial mats. By the end of the period, burrowing animals had destroyed the mats through bioturbation, and gradually turned the seabeds into what they are today. As a consequence, many of those organisms who were dependent on the mats went extinct, while the other species adapted to the changed environment who now offered new ecological niches.^[25] Around the same time there was a seemingly rapid appearance of representatives of all the mineralized phyla.^[26] However, many of these phyla were represented only by stem-group forms; and since mineralized phyla generally have a benthic origin, they may not be a good proxy for (more abundant) non-mineralized phyla.^[27]

There are also suggestions that some Cambrian organisms ventured onto land, producing the trace fossils Protichnites and Climactichnites.

In contrast to later periods, the Cambrian fauna was somewhat restricted; free-floating organisms were rare, with the majority living on or close to the sea floor;^[28] and mineralizing animals were rarer than in future periods, in part due to the unfavourable ocean chemistry.^[28] (Most Cambrian carbonates were formed by microbial or non-biological processes.)^[28]

Many modes of preservation are unique to the Cambrian, resulting in an abundance of lagerstätte.

Flora[link]

Although there were a variety of macroscopic marine plants, it is generally accepted that there were no land plants at this time, although it is likely that a microbial "scum" comprising fungi, algae, and possibly lichens covered the land.^[29]

See also[link]

Part of a series on
The Cambrian explosion
Fossil localities Burgess shale Chengjiang Sirius Passet Doushantuo
Key organisms Ediacara biota Kimberella Vernanimalcula Burgess-type Marrella Anomalocaridids Halwaxiids Opabinia Odontogriphus Small shelly fauna Helcionellids
Evolutionary concepts Trends Cambrian substrate revolution Themes Cladistics Convergent evolution Stem and crown groups
v t e • taskforce

• End Botomian extinction event—circa 517 mya
• Dresbachian extinction event—circa 502 mya
• Cambro-Ordovician extinction event – circa 488 mya
• List of fossil sites (with link directory)
• Type locality (geology), the locality where a particular rock type, stratigraphic unit, fossil or mineral species is first identified

v t e Modes of preservation in the Cambrian Exceptional Phosphate Orsten type · Doushantuo type Silica Bitter Springs type Carbonaceous film Burgess Shale type Pyrite Beecher's Trilobite Bed type Casts and moulds Ediacaran type Legged trilobite fossil Conventional Shelly Small shelly fossils · Conventional fossils Organic Acritarchs · Organic-walled microfossils Traces Microbialites · Trace fossils

References[link]

Further reading[link]

• Amthor, J. E.; Grotzinger, John P.; Schröder, Stefan; Bowring, Samuel A.; Ramezani, Jahandar; Martin, Mark W.; Matter, Albert (2003). "Extinction of Cloudina and Namacalathus at the Precambrian-Cambrian boundary in Oman". Geology 31 (5): 431–434. Bibcode 2003Geo....31..431A. DOI:10.1130/0091-7613(2003)031<0431:EOCANA>2.0.CO;2. 
• Ogg, Jim; June, 2004, Overview of Global Boundary Stratotype Sections and Points (GSSPs) http://www.stratigraphy.org/gssp.htm Accessed April 30, 2006.
• Gould, Stephen Jay; Wonderful Life: the Burgess Shale and the Nature of Life (New York: Norton, 1989)

External links[link]

Wikimedia Commons has media related to: Cambrian

• Cambrian period on In Our Time at the BBC. (listen now)
• Biostratigraphy – includes information on Cambrian trilobite biostratigraphy
• Dr. Sam Gon's trilobite pages (contains numerous Cambrian trilobites)
• Examples of Cambrian Fossils
• Paleomap Project
• Report on the web on Amthor and others from Geology vol. 31
• Weird Life on the Mats