Cambrian

Cambrian

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The Cambrian is the first geological period of the Paleozoic
Paleozoic
The Paleozoic era is the earliest of three geologic eras of the Phanerozoic eon, spanning from roughly...

 Era, lasting from Mya ; it is succeeded by the Ordovician
Ordovician
The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...

. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick
Adam Sedgwick
Adam Sedgwick was one of the founders of modern geology. He proposed the Devonian period of the geological timescale...

, who named it after Cambria
Cambria
Cambria is the classical name for Wales, being the Latinised form of the Welsh name Cymru . The etymology of Cymry "the Welsh", Cimbri, and Cwmry "Cumbria", improbably connected to the Biblical Gomer and the "Cimmerians" by 17th-century celticists, is now known to come from Old Welsh combrog...

, the Latin
Latin
Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...

 name for Wales
Wales
Wales is a country that is part of the United Kingdom and the island of Great Britain, bordered by England to its east and the Atlantic Ocean and Irish Sea to its west. It has a population of three million, and a total area of 20,779 km²...

, where Britain's Cambrian rocks are best exposed. It should not be confused with the Cambrian supereon, the current supereon of the planet.

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.

The Cambrian Period marked a profound change in life on Earth
Life on Earth
Life on Earth: A Natural History by David Attenborough is a television natural history series made by the BBC in association with Warner Bros. and Reiner Moritz Productions...

; prior to the Cambrian era, life was on the whole was small and simple. Complex 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. This diversification of lifeforms was relatively rapid, and is termed the Cambrian explosion
Cambrian explosion
The Cambrian explosion or Cambrian radiation was the relatively rapid appearance, around , of most major phyla, as demonstrated in the fossil record, accompanied by major diversification of other organisms, including animals, phytoplankton, and calcimicrobes...

. This vast increase in diverse forms of life produced the first representatives of many modern phyla, representing the evolutionary stems of modern groups of species. While life prospered in the oceans, the land was barren – with nothing more than a microbial 'crud' known as soil crust
Soil crust
A soil crust is a layer of soil whose particles cohere because of organic material including live organisms and what they produce.- External references :* by the USGS...

 covering the land. Apart from some tentative evidence suggesting that a few animals foundered around on land, most of the continent
Continent
A continent is one of several very large landmasses on Earth. They are generally identified by convention rather than any strict criteria, with seven regions commonly regarded as continents—they are : Asia, Africa, North America, South America, Antarctica, Europe, and Australia.Plate tectonics is...

s resembled deserts spanning from horizon to horizon. Shallow seas flanked the margins of several continents created during the breakup of the supercontinent
Supercontinent
In geology, a supercontinent is a landmass comprising more than one continental core, or craton. The assembly of cratons and accreted terranes that form Eurasia qualifies as a supercontinent today.-History:...

 Pannotia
Pannotia
Pannotia, first described by Ian W. D. Dalziel in 1997, is a hypothetical supercontinent that existed from the Pan-African orogeny about six hundred million years ago to the end of the Precambrian about five hundred and fifty million years ago. It is also known as the Vendian supercontinent...

. The seas were relatively warm, and polar ice was absent for much of the period.

The United States Federal Geographic Data Committee
Federal Geographic Data Committee
The Federal Geographic Data Committee is a United States government committee which promotes the coordinated development, use, sharing, and dissemination of geospatial data on a national basis. Its 19 members are representatives from the Executive Office of the President, and Cabinet level and...

 uses a "barred capital C" character similar to the capital letter Ukrainian Ye
Ukrainian Ye
Ukrainian Ye is a character of the Cyrillic script. It is considered as an individual letter of modern Ukrainian alphabet and as a variant form of Ye in modern Church Slavonic language...

 ‹Є› to represent the Cambrian Period.
The proper glyph
Glyph
A glyph is an element of writing: an individual mark on a written medium that contributes to the meaning of what is written. A glyph is made up of one or more graphemes....

 is a proposed addition to the Unicode
Unicode
Unicode is a computing industry standard for the consistent encoding, representation and handling of text expressed in most of the world's writing systems...

 standard at code point
Code point
In character encoding terminology, a code point or code position is any of the numerical values that make up the code space . For example, ASCII comprises 128 code points in the range 0hex to 7Fhex, Extended ASCII comprises 256 code points in the range 0hex to FFhex, and Unicode comprises 1,114,112...

 A792.

Stratigraphy


Despite the long recognition of its distinction from younger Ordovician
Ordovician
The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...

 rocks and older Precambrian
Precambrian
The Precambrian is the name which describes the large span of time in Earth's history before the current Phanerozoic Eon, and is a Supereon divided into several eons of the geologic time scale...

 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
Treptichnus pedum
Treptichnus pedum is regarded as the earliest widespread complex trace fossil...

assemblage.
Nevertheless, the usage of Treptichnus pedum, a reference ichnofossil
Trace fossil
Trace fossils, also called ichnofossils , are geological records of biological activity. Trace fossils may be impressions made on the substrate by an organism: for example, burrows, borings , urolites , footprints and feeding marks, and root cavities...

 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
Namibia
Namibia, officially the Republic of Namibia , is a country in southern Africa whose western border is the Atlantic Ocean. It shares land borders with Angola and Zambia to the north, Botswana to the east and South Africa to the south and east. It gained independence from South Africa on 21 March...

, Spain
Spain
Spain , officially the Kingdom of Spain languages]] under the European Charter for Regional or Minority Languages. In each of these, Spain's official name is as follows:;;;;;;), is a country and member state of the European Union located in southwestern Europe on the Iberian Peninsula...

 and Newfoundland
Newfoundland and Labrador
Newfoundland and Labrador is the easternmost province of Canada. Situated in the country's Atlantic region, it incorporates the island of Newfoundland and mainland Labrador with a combined area of . As of April 2011, the province's estimated population is 508,400...

, and possibly, in the western USA. The stratigraphic range of T. pedum overlaps the range of the Ediacaran
Ediacaran
The Ediacaran Period , named after the Ediacara Hills of South Australia, is the last geological period of the Neoproterozoic Era and of the Proterozoic Eon, immediately preceding the Cambrian Period, the first period of the Paleozoic Era and of the Phanerozoic Eon...

 fossils in Namibia, and probably in Spain.

Subdivisions


The Cambrian period follows the Ediacaran
Ediacaran
The Ediacaran Period , named after the Ediacara Hills of South Australia, is the last geological period of the Neoproterozoic Era and of the Proterozoic Eon, immediately preceding the Cambrian Period, the first period of the Paleozoic Era and of the Phanerozoic Eon...

 and is followed by the Ordovician
Ordovician
The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...

 period. The Cambrian is divided into four epochs
Epoch (geology)
An epoch is a subdivision of the geologic timescale based on rock layering. In order, the higher subdivisions are periods, eras and eons. We are currently living in the Holocene epoch...

 or series
Series (stratigraphy)
Series are subdivisions of rock layers made based on the age of the rock and corresponding to the dating system unit called an epoch, both being formally defined international conventions of the geological timescale. A series is therefore a sequence of rock depositions defining a...

 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), 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.

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:
Chinese North American Russian-Kazakhian Australian Regional
C
A
M
B
R
I
A
N
Furongian Ibexian (part) Ayusokkanian Datsonian Dolgellian (Trempealeauan
Trempealeauan
The Trempealeauan is the upper or latest stage of the Upper or Late Cambrian in North America, spanning about 4 million years from about 492.5 to 488.3 m.y.a., equivalent to the Fengshanian of China...

, Fengshanian)
Payntonian
Sunwaptan Sakian Iverian Festiniogian (Franconian
Franconian (stage)
The Franconian is the middle stage of the Upper or Late Cambrian in North America, equivalent to the Chinese Changshanian with a span of nearly 4.5 million years, from about 497 to 492.5 Ma...

, Changshanian)
Steptoan Aksayan Idamean Maentwrogian
Marjuman Batyrbayan Mindyallan
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
PRECAMBRIAN Nemakit-Daldynian*

*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
Archaeocyatha
The Archaeocyatha or archaeocyathids were sessile, reef-building marine organisms of warm tropical and subtropical waters that lived during the early Cambrian period. It is believed that the centre of the Archaeocyatha origin is in East Siberia, where they are first known from the beginning of...

 bioherms.

Cambrian dating



The time range for the Cambrian has classically been thought to have been from about 570 Mya to about 500 Mya. The lower boundary of the Cambrian was traditionally set at the earliest appearance of trilobite
Trilobite
Trilobites are a well-known fossil group of extinct marine arthropods that form the class Trilobita. The first appearance of trilobites in the fossil record defines the base of the Atdabanian stage of the Early Cambrian period , and they flourished throughout the lower Paleozoic era before...

s 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
Reef
In nautical terminology, a reef is a rock, sandbar, or other feature lying beneath the surface of the water ....

 builders.

The end of the period was eventually set at a fairly definite faunal change now identified as an extinction event
Extinction event
An extinction event is a sharp decrease in the diversity and abundance of macroscopic life. They occur when the rate of extinction increases with respect to the rate of speciation...

. Fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...

 discoveries and radiometric dating
Radiometric dating
Radiometric dating is a technique used to date materials such as rocks, usually based on a comparison between the observed abundance of a naturally occurring radioactive isotope and its decay products, using known decay rates...

 in the last quarter of the 20th century have called these dates into question. Date inconsistencies as large as 20 Mya 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
New Brunswick
New Brunswick is one of Canada's three Maritime provinces and is the only province in the federation that is constitutionally bilingual . The provincial capital is Fredericton and Saint John is the most populous city. Greater Moncton is the largest Census Metropolitan Area...

 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. The rationale for this precise dating is interesting in itself as an example of paleological
Paleontology
Paleontology "old, ancient", ὄν, ὀντ- "being, creature", and λόγος "speech, thought") is the study of prehistoric life. It includes the study of fossils to determine organisms' evolution and interactions with each other and their environments...

 deductive reasoning
Deductive reasoning
Deductive reasoning, also called deductive logic, is reasoning which constructs or evaluates deductive arguments. Deductive arguments are attempts to show that a conclusion necessarily follows from a set of premises or hypothesis...

. Exactly at the Cambrian boundary there is a marked fall in the abundance of carbon-13
Carbon-13
Carbon-13 is a natural, stable isotope of carbon and one of the environmental isotopes. It makes up about 1.1% of all natural carbon on Earth.- Detection by mass spectrometry :...

, 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
Oman
Oman , officially called the Sultanate of Oman , is an Arab state in southwest Asia on the southeast coast of the Arabian Peninsula. It is bordered by the United Arab Emirates to the northwest, Saudi Arabia to the west, and Yemen to the southwest. The coast is formed by the Arabian Sea on the...

. Amthor (2003) describes evidence from Oman that indicates the carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...

-isotope
Isotope
Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...

 excursion relates to a mass extinction: the disappearance of distinctive fossils from the Precambrian
Precambrian
The Precambrian is the name which describes the large span of time in Earth's history before the current Phanerozoic Eon, and is a Supereon divided into several eons of the geologic time scale...

 coincides exactly with the carbon-13 anomaly. Fortunately, in the Oman sequence, so too does a volcanic ash
Volcanic ash
Volcanic ash consists of small tephra, which are bits of pulverized rock and glass created by volcanic eruptions, less than in diameter. There are three mechanisms of volcanic ash formation: gas release under decompression causing magmatic eruptions; thermal contraction from chilling on contact...

 horizon from which zircon
Zircon
Zircon is a mineral belonging to the group of nesosilicates. Its chemical name is zirconium silicate and its corresponding chemical formula is ZrSiO4. A common empirical formula showing some of the range of substitution in zircon is 1–x4x–y...

s provide a very precise age of 542 ± 0.3 Mya (calculated on the decay rate of uranium
Uranium
Uranium is a silvery-white metallic chemical element in the actinide series of the periodic table, with atomic number 92. It is assigned the chemical symbol U. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons...

 to lead
Lead
Lead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...

). This new and precise date tallies with the less precise dates for the carbon-13 anomaly, derived from sequences in Siberia
Siberia
Siberia is an extensive region constituting almost all of Northern Asia. Comprising the central and eastern portion of the Russian Federation, it was part of the Soviet Union from its beginning, as its predecessor states, the Tsardom of Russia and the Russian Empire, conquered it during the 16th...

 and Namibia
Namibia
Namibia, officially the Republic of Namibia , is a country in southern Africa whose western border is the Atlantic Ocean. It shares land borders with Angola and Zambia to the north, Botswana to the east and South Africa to the south and east. It gained independence from South Africa on 21 March...

. 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 Paleozoic era and the Cambrian period.

Geography


Reconstructions of Cambrian geography contain relatively large sources of error. They suggest that a global supercontinent, Pannotia
Pannotia
Pannotia, first described by Ian W. D. Dalziel in 1997, is a hypothetical supercontinent that existed from the Pan-African orogeny about six hundred million years ago to the end of the Precambrian about five hundred and fifty million years ago. It is also known as the Vendian supercontinent...

, was in the process of breaking up, with Laurentia
Laurentia
Laurentia is a large area of continental craton, which forms the ancient geological core of the North American continent...

 (North America) and Siberia
Siberia
Siberia is an extensive region constituting almost all of Northern Asia. Comprising the central and eastern portion of the Russian Federation, it was part of the Soviet Union from its beginning, as its predecessor states, the Tsardom of Russia and the Russian Empire, conquered it during the 16th...

 having separated from the main mass of the Gondwana
Gondwana
In paleogeography, Gondwana , originally Gondwanaland, was the southernmost of two supercontinents that later became parts of the Pangaea supercontinent. It existed from approximately 510 to 180 million years ago . Gondwana is believed to have sutured between ca. 570 and 510 Mya,...

 supercontinent to form isolated landmasses. Most continental land mass was clustered in the southern hemisphere. Large, high-velocity rotational movement of Gondwana appears to have occurred in the Early Cambrian.

With a lack of sea ice – the great glaciers of the Marinoan Snowball Earth
Snowball Earth
The Snowball Earth hypothesis posits that the Earth's surface became entirely or nearly entirely frozen at least once, some time earlier than 650 Ma . Proponents of the hypothesis argue that it best explains sedimentary deposits generally regarded as of glacial origin at tropical...

 were long melted – 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
Ice cap
An ice cap is an ice mass that covers less than 50 000 km² of land area . Masses of ice covering more than 50 000 km² are termed an ice sheet....

.

Climate


While the Cambrian period was, on the whole, rather warm, it was not entirely without glaciation.

Fauna


The Cambrian 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
Cambrian substrate revolution
The "Cambrian substrate revolution" or "Agronomic revolution", evidenced in trace fossils, is the diversification of animal burrowing during the early Cambrian period....

 which transformed the seabed
Seabed
The seabed is the bottom of the ocean.- Ocean structure :Most of the oceans have a common structure, created by common physical phenomena, mainly from tectonic movement, and sediment from various sources...

 ecosystem
Ecosystem
An ecosystem is a biological environment consisting of all the organisms living in a particular area, as well as all the nonliving , physical components of the environment with which the organisms interact, such as air, soil, water and sunlight....

s. Before Cambrian, the sea floor was covered by microbial mat
Microbial mat
A microbial mat is a multi-layered sheet of micro-organisms, mainly bacteria and archaea. Microbial mats grow at interfaces between different types of material, mostly on submerged or moist surfaces but a few survive in deserts. They colonize environments ranging in temperature from –40°C to +120°C...

s. By the end of the period, burrowing animals had destroyed the mats through bioturbation
Bioturbation
In oceanography, limnology, pedology, geology , and archaeology, bioturbation is the displacement and mixing of sediment particles and solutes by fauna or flora . The mediators of bioturbation are typically annelid worms , bivalves In oceanography, limnology, pedology, geology (especially...

, 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.
Around the same time there was a seemingly rapid appearance of representatives of all the mineralized phyla
Phylum
In biology, a phylum The term was coined by Georges Cuvier from Greek φῦλον phylon, "race, stock," related to φυλή phyle, "tribe, clan." is a taxonomic rank below kingdom and above class. "Phylum" is equivalent to the botanical term division....

. 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.

There are also suggestions that some Cambrian organisms ventured onto land, producing the trace fossils Protichnites
Protichnites
Protichnites is a genus of trace fossil consisting of the imprints made by the walking activity of arthropods. It is likely that more than one type of arthropod was responsible for these tracks. Euthycarcinoids, aglaspidids and eurypterids are possible contributors. Protichnites consists of two...

and Climactichnites
Climactichnites
Climactichnites is an enigmatic, late Cambrian fossil formed on or within sandy tidal flats around .It has been interpreted in many different ways in the past, but is now known to be a trace fossil of a slug-like organism, thought to have moved by crawling on near-shore or on-shore surfaces or...

.

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; and mineralizing animals were rarer than in future periods, in part due to the unfavourable ocean chemistry. (Most Cambrian carbonates were formed by microbial or non-biological processes.)

Many modes of preservation are unique to the Cambrian period, resulting in an abundance of lagerstätte
Lagerstätte
A Lagerstätte is a sedimentary deposit that exhibits extraordinary fossil richness or completeness.Palaeontologists distinguish two kinds....

; see the list at the end of the article.

Flora


Generally it is accepted that there were no land plants at this time, although it is likely that a microbial "scum" comprising fungi, algae
Algae
Algae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. They are photosynthetic like plants, and "simple" because their tissues are not organized into the many...

, and possibly lichens covered the land.

See also

  • End Botomian extinction event—circa 517 m.y.a.
  • Dresbachian
    Dresbachian
    The Dresbachian is the lower stage of the Late or Upper Cambrian Period in North America, equivalent to the Chinese Guzhangian which spans about 4 million years, from...

     extinction event—circa 502 m.y.a.
  • Cambro-Ordovician extinction event
    Cambrian-Ordovician extinction events
    ‎The Cambrian–Ordovician extinction event occurred approximately 488 million years ago . This early Phanerozoic Eon extinction event eliminated many brachiopods and conodonts, and severely reduced the number of trilobite species....

     – circa 488 m.y.a.
  • List of fossil sites (with link directory)
  • Type locality (geology)
    Type locality (geology)
    Type locality , also called type area or type locale, is the where a particular rock type, stratigraphic unit, fossil or mineral species is first identified....

    , the locality where a particular rock type, stratigraphic unit, fossil or mineral species is first identified

Further reading

  • 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 Gould
    Stephen Jay Gould was an American paleontologist, evolutionary biologist, and historian of science. He was also one of the most influential and widely read writers of popular science of his generation....

    , Stephen Jay; Wonderful Life: the Burgess Shale
    Burgess Shale
    The Burgess Shale Formation, located in the Canadian Rockies of British Columbia, is one of the world's most celebrated fossil fields, and the best of its kind. It is famous for the exceptional preservation of the soft parts of its fossils...

     and the Nature of Life (New York: Norton, 1989)

External links