Cambrian

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The Cambrian
Cambrian Period ( /ˈkæmbriən/ or /ˈkeɪmbriən/) was the first geological period of the Paleozoic
Paleozoic Era, of the Phanerozoic
Phanerozoic Eon.[6] The Cambrian
Cambrian lasted 55.6 million years from the end of the preceding Ediacaran
Ediacaran Period 541 million years ago (mya) to the beginning of the Ordovician
Ordovician Period 485.4 mya.[7] Its subdivisions, and its base, are somewhat in flux. The period was established (as “Cambrian series”) by Adam Sedgwick,[6] who named it after Cambria, the Latinised form of Cymru, the Welsh name for Wales, where Britain's Cambrian
Cambrian rocks are best exposed.[8][9][10] The Cambrian
Cambrian is unique in its unusually high proportion of lagerstätte sedimentary deposits, sites of exceptional preservation where "soft" parts of organisms are preserved as well as their more resistant shells. As a result, our understanding of the Cambrian
Cambrian biology surpasses that of some later periods.[11] The Cambrian
Cambrian marked a profound change in life on Earth; prior to the Cambrian, the majority of living organisms on the whole were small, unicellular and simple; the Precambrian
Precambrian Charnia
Charnia being exceptional. Complex, multicellular organisms gradually became more common in the millions of years immediately preceding the Cambrian, but it was not until this period that mineralized—hence readily fossilized—organisms became common.[12] The rapid diversification of lifeforms in the Cambrian, known as the Cambrian
Cambrian explosion, produced the first representatives of all modern animal phyla. Phylogenetic analysis has supported the view that during the Cambrian
Cambrian radiation, metazoa (animals) evolved monophyletically from a single common ancestor: flagellated colonial protists similar to modern choanoflagellates. Although diverse life forms prospered in the oceans, the land is thought to have been comparatively barren—with nothing more complex than a microbial soil crust[13] and a few molluscs that emerged to browse on the microbial biofilm known to have been present.[14] Most of the continents were probably dry and rocky due to a lack of vegetation. 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.

Contents

1 Stratigraphy

1.1 Subdivisions 1.2 Dating the Cambrian

2 Paleogeography 3 Climate 4 Flora 5 Oceanic life 6 Symbol 7 Gallery 8 See also 9 References 10 Further reading 11 External links

Stratigraphy[edit] Further information: Stratigraphy of the Cambrian Despite the long recognition of its distinction from younger Ordovician
Ordovician Period rocks and older Precambrian
Precambrian Supereon rocks, it was not until 1994 that this time period was internationally ratified. The base of the Cambrian
Cambrian lies atop a complex assemblage of trace fossils known as the Treptichnus pedum
Treptichnus pedum assemblage.[15] The use of Treptichnus pedum, a reference ichnofossil to mark the lower boundary of the Cambrian, is difficult as the occurrence of very similar trace fossils belonging to the Treptichnids group are found 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
Ediacaran fossils in Namibia, and probably in Spain.[16][17] Subdivisions [edit] The Cambrian
Cambrian Period followed the Ediacaran
Ediacaran Period and was followed by the Ordovician
Ordovician Period. The Cambrian
Cambrian is divided into four epochs (series) and ten ages (stages). Currently only two series and five 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
Cambrian is divided into three epochs with locally differing names – the Early Cambrian
Cambrian (Caerfai or Waucoban, 541 ± 1.0 to 509 ± 1.7 mya), Middle Cambrian
Cambrian (St Davids or Albertan, 509 ± 1.0 to 497 ± 1.7 mya) and Furongian (497 ± 1.0 to 485.4 ± 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
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, Fengshanian)

Payntonian

Sunwaptan Sakian Iverian Ffestiniogian (Franconian, Changshanian)

Steptoan Aksayan Idamean Maentwrogian (Dresbachian)

Marjuman Batyrbayan Mindyallan

Cambrian
Cambrian Series 3 Maozhangian Mayan Boomerangian

Zuzhuangian Delamaran Amgan Undillian

Zhungxian

Florian

Templetonian

Dyeran

Ordian

Cambrian
Cambrian Series 2 Longwangmioan Toyonian Lenian

Changlangpuan Montezuman Botomian

Qungzusian

Atdabanian

Terreneuvian

Meishuchuan Jinningian Placentian Tommotian Nemakit-Daldynian*

Cordubian

Precambrian Sinian Hadrynian Nemakit-Daldynian* Sakharan Adeladean

*In Russian scientific thought the lower boundary of the Cambrian
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.[18][19][20] Dating the Cambrian[edit]

Archeocyathids from the Poleta formation
Poleta formation in the Death Valley
Death Valley area

The International Commission on Stratigraphy list the Cambrian
Cambrian period as beginning at 541 million years ago and ending at 485.4 million years ago. The lower boundary of the Cambrian
Cambrian was originally held to represent the first appearance of complex life, represented by trilobites. The recognition of small shelly fossils before the first trilobites, and Ediacara biota
Ediacara biota substantially earlier, led to calls for a more precisely defined base to the Cambrian
Cambrian period.[21] After decades of careful consideration, a continuous sedimentary sequence at Fortune Head, Newfoundland
Newfoundland was settled upon as a formal base of the Cambrian
Cambrian period, which was to be correlated worldwide by the earliest appearance of Treptichnus pedum.[21] Discovery of this fossil a few metres below the GSSP
GSSP led to the refinement of this statement, and it is the T. pedum ichnofossil assemblage that is now formally used to correlate the base of the Cambrian.[21][22] This formal designation allowed radiometric dates to be obtained from samples across the globe that corresponded to the base of the Cambrian. Early dates of 570 million years ago quickly gained favour,[21] though the methods used to obtain this number are now considered to be unsuitable and inaccurate. A more precise date using modern radiometric dating yield a date of 541 ± 0.3 million years ago.[23] The ash horizon in Oman from which this date was recovered corresponds to a marked fall in the abundance of carbon-13 that correlates to equivalent excursions elsewhere in the world, and to the disappearance of distinctive Ediacaran
Ediacaran fossils (Namacalathus, Cloudina). Nevertheless, there are arguments that the dated horizon in Oman does not correspond to the Ediacaran- Cambrian
Cambrian boundary, but represents a facies change from marine to evaporite-dominated strata — which would mean that dates from other, more suitable sections, ranging from 544 or 542 Ma, are more suitable.[21] Paleogeography[edit] Plate reconstructions suggest a global supercontinent, Pannotia, was in the process of breaking up early in the period,[24][25] with Laurentia
Laurentia (North America), Baltica, and Siberia having separated from the main supercontinent of Gondwana
Gondwana to form isolated land masses.[26] Most continental land was clustered in the Southern Hemisphere at this time, but was drifting north.[26] Large, high-velocity rotational movement of Gondwana
Gondwana appears to have occurred in the Early Cambrian.[27] With a lack of sea ice – the great glaciers of the Marinoan Snowball Earth
Snowball Earth were long melted[28] – the sea level was high, which led to large areas of the continents being flooded in warm, shallow seas ideal for sea life. The sea levels fluctuated somewhat, suggesting there were 'ice ages', associated with pulses of expansion and contraction of a south polar ice cap.[29] In Baltoscandia
Baltoscandia a Lower Cambrian
Cambrian transgression transformed large swathes of the Sub-Cambrian peneplain
Sub-Cambrian peneplain into a epicontinental sea.[30] Climate[edit] The Earth was generally cold during the early Cambrian, probably due to the ancient continent of Gondwana
Gondwana covering the South Pole
South Pole and cutting off polar ocean currents. However, average temperatures were 7 degrees Celsius higher than today. 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
Ordovician period. Flora[edit] Although there were a variety of macroscopic marine plants[which?][citation needed] no land plant (embryophyte) fossils are known from the Cambrian. However, biofilms and microbial mats were well developed on Cambrian
Cambrian tidal flats and beaches 500 mya.,[31] and microbes forming microbial Earth ecosystems, comparable with modern soil crust of desert regions, contributing to soil formation.[32][33] Oceanic life[edit]

Life
Life timeline

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-4500 — – -4000 — – -3500 — – -3000 — – -2500 — – -2000 — – -1500 — – -1000 — – -500 — – 0 —

water

Single-celled life

photosynthesis

Eukaryotes

Multicellular life

Land life

Dinosaurs

Mammals

Flowers

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Earliest Earth (−4540)

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Earliest water

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Earliest life

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LHB meteorites

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Earliest oxygen

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Atmospheric oxygen

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Oxygen
Oxygen crisis

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Earliest sexual reproduction

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Ediacara biota

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Cambrian
Cambrian explosion

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Earliest humans

P h a n e r o z o i c

P r o t e r o z o i c

A r c h e a n

H a d e a n

Pongola

Huronian

Cryogenian

Andean

Karoo

Quaternary

Axis scale: million years Orange labels: ice ages. Also see: Human
Human timeline and Nature timeline

Main article: Cambrian
Cambrian explosion Most animal life during the Cambrian
Cambrian was aquatic. Trilobites were once assumed to be the dominant life form at that time,[34] but this has proven to be incorrect. Arthropods were by far the most dominant animals in the ocean, but trilobites were only a minor part of the total arthropod diversity. What made them so apparently abundant was their heavy armor reinforced by calcium carbonate (CaCO3), which fossilized far more easily than the fragile chitinous exoskeletons of other arthropods, leaving numerous preserved remains.[35] The period marked a steep change in the diversity and composition of Earth's biosphere. The Ediacaran
Ediacaran biota suffered a mass extinction at the start of the Cambrian
Cambrian Period, which corresponded 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 the Cambrian, the sea floor was covered by microbial mats. By the end of the Cambrian, burrowing animals had destroyed the mats in many areas through bioturbation, and gradually turned the seabeds into what they are today.[clarification needed] As a consequence, many of those organisms that were dependent on the mats became extinct, while the other species adapted to the changed environment that now offered new ecological niches.[36] Around the same time there was a seemingly rapid appearance of representatives of all the mineralized phyla except the Bryozoa, which appeared in the Lower Ordovician.[37] However, many of those 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.[38]

A reconstruction of Margaretia
Margaretia dorus from the Burgess Shale, which were once believed to be green algae, but are now understood to represent hemichordates.[39]

While the early Cambrian
Cambrian showed such diversification that it has been named the Cambrian
Cambrian Explosion, this changed later in the period, when there occurred a sharp drop in biodiversity. About 515 million years ago, the number of species going extinct exceeded the number of new species appearing. Five million years later, the number of genera had dropped from an earlier peak of about 600 to just 450. Also, the speciation rate in many groups was reduced to between a fifth and a third of previous levels. 500 million years ago, oxygen levels fell dramatically in the oceans, leading to hypoxia, while the level of poisonous hydrogen sulfide simultaneously increased, causing another extinction. The later half of Cambrian
Cambrian was surprisingly barren and show evidence of several rapid extinction events; the stromatolites which had been replaced by reef building sponges known as Archaeocyatha, returned once more as the archaeocyathids became extinct. This declining trend did not change until the Great Ordovician
Ordovician Biodiversification Event.[40][41] Some Cambrian
Cambrian organisms ventured onto land, producing the trace fossils Protichnites
Protichnites and Climactichnites. Fossil evidence suggests that euthycarcinoids, an extinct group of arthropods, produced at least some of the Protichnites.[42][43] Fossils of the track-maker of Climactichnites
Climactichnites have not been found; however, fossil trackways and resting traces suggest a large, slug-like mollusc.[44][45] In contrast to later periods, the Cambrian
Cambrian fauna was somewhat restricted; free-floating organisms were rare, with the majority living on or close to the sea floor;[46] and mineralizing animals were rarer than in future periods, in part due to the unfavourable ocean chemistry.[46] Many modes of preservation are unique to the Cambrian, and some preserve soft body parts, resulting in an abundance of Lagerstätten. Symbol[edit] The United States Federal Geographic Data Committee
Federal Geographic Data Committee uses a "barred capital C" ⟨Ꞓ⟩ character to represent the Cambrian
Cambrian Period.[47] The Unicode
Unicode character is U+A792 Ꞓ LATIN CAPITAL LETTER C WITH BAR.[48][49] Gallery[edit]

Stromatolites of the Pika Formation (Middle Cambrian) near Helen Lake, Banff National Park, Canada

Trilobites were very common during this time

Anomalocaris
Anomalocaris was an early marine predator, among the various arthropods of the time.

Pikaia
Pikaia was an early chordate from the Middle Cambrian

Opabinia
Opabinia was a creature with an unusual body plan; it was probably related to arthropods

Protichnites
Protichnites were the trackways of arthropods that walked Cambrian beaches.

Hallucigenia
Hallucigenia is maybe an early ancestor of the Velvet worms. Reconstructions of H. sparsa, H. hongmeia, and H. fortis

Size comparison of different Cambrian
Cambrian species

See also[edit]

Part of a series on

The Cambrian
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
Cambrian substrate revolution

Themes

Cladistics Convergent evolution Stem and crown groups

v t e

Cambro- Ordovician
Ordovician extinction event – circa 488 mya Dresbachian
Dresbachian extinction event—circa 502 mya End Botomian extinction event—circa 517 mya List of fossil sites
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

Orsten Doushantuo type Bitter Springs type Burgess Shale
Burgess Shale type Beecher's Trilobite
Trilobite Bed type Ediacaran
Ediacaran type

Conventional

Small shelly fossils Acritarchs Trace fossils

References[edit]

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