Cambrian

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Cambrian
538.8 ± 0.2 – 485.4 ± 1.9 Ma
kyaambriy'aan50.png
Earth in the middle of the Cambrian Period
Chronology
Etymology
Name formalityFormal
Usage information
Celestial body Earth
Regional usageGlobal (ICS)
Time scale(s) usedICS Time Scale
Definition
Chronological unit Period
Stratigraphic unit System
First proposed by Adam Sedgwick, 1835
Time span formalityFormal
Lower boundary definitionAppearance of the Ichnofossil Treptichnus pedum
Lower boundary GSSP Fortune Head section, Newfoundland, Canada
47°04′34″N55°49′52″W / 47.0762°N 55.8310°W / 47.0762; -55.8310
Lower GSSP ratified1992 [2]
Upper boundary definition FAD of the Conodont Iapetognathus fluctivagus .
Upper boundary GSSPGreenpoint section, Green Point, Newfoundland, Canada
49°40′58″N57°57′55″W / 49.6829°N 57.9653°W / 49.6829; -57.9653
Upper GSSP ratified2000 [3]
Atmospheric and climatic data
Sea level above present dayRising steadily from 4 m to 90 m [4]

The Cambrian Period ( /ˈkæmbri.ən,ˈkeɪm-/ KAM-bree-ən, KAYM-; sometimes symbolized Ꞓ ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. [5] The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million years ago (mya) to the beginning of the Ordovician Period 485.4 mya. [6] Its subdivisions, and its base, are somewhat in flux. The period was established as "Cambrian series" by Adam Sedgwick, [5] who named it after Cambria, the Latin name for 'Cymru' (Wales), where Britain's Cambrian rocks are best exposed. [7] [8] [9] Sedgwick identified the layer as part of his task, along with Roderick Murchison, to subdivide the large "Transition Series", although the two geologists disagreed for a while on the appropriate categorization. [5] The 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 biology surpasses that of some later periods. [10]

Contents

The 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 (Ediacaran fauna being notable exceptions). 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. [11] The rapid diversification of lifeforms in the Cambrian, known as the Cambrian explosion, produced the first representatives of all modern animal phyla. Phylogenetic analysis has supported the view that before the Cambrian radiation, in the Cryogenian [12] [13] [14] or Tonian, [15] animals (metazoans) evolved monophyletically from a single common ancestor: flagellated colonial protists similar to modern choanoflagellates. [16] 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 [17] and a few molluscs and arthropods (albeit not terrestrial) that emerged to browse on the microbial biofilm. [18] By the end of the Cambrian, myriapods, [19] [20] arachnids, [21] and hexapods [22] started adapting to the land, along with the first plants. [23] [24] 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.

Stratigraphy

The Cambrian Period followed the Ediacaran Period and was followed by the Ordovician Period.

The base of the Cambrian lies atop a complex assemblage of trace fossils known as the Treptichnus pedum assemblage. [25] The use of Treptichnus pedum, a reference ichnofossil to mark the lower boundary of the Cambrian, is problematic because very similar trace fossils belonging to the Treptichnids group are found well below 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. [26] [27]

Subdivisions

The Cambrian is divided into four epochs (series) and ten ages (stages). Currently only three series and six stages are named and have a GSSP (an internationally agreed-upon stratigraphic reference point).

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, 538.8 ± 0.2 to 509 ± 1.9 mya), Middle Cambrian (St Davids or Albertan, 509 ± 0.2 to 497 ± 1.9 mya) and Late Cambrian ( 497 ± 0.2 to 485.4 ± 1.9 mya; also known as Merioneth or Croixan). Trilobite zones allow biostratigraphic correlation in the Cambrian. Rocks of these epochs are referred to as belonging to the Lower, Middle, or Upper Cambrian.

Each of the local series 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:

Correlation of global and regional Cambrian stratigraphy[ citation needed ]
International SeriesChineseNorth AmericanRussian-KazakhianAustralianRegional[ where? ]
C
a
m
b
r
i
a
n		 Furongian Ibexian (part)AyusokkanianDatsonianDolgellian (Trempealeauan, Fengshanian)
Payntonian
SunwaptanSakianIverianFfestiniogian (Franconian, Changshanian)
SteptoanAksayanIdameanMaentwrogian (Dresbachian)
MarjumanBatyrbayanMindyallan
Miaolingian MaozhangianMayanBoomerangian
ZuzhuangianDelamaranAmganUndillian
ZhungxianFlorian
Templetonian
 DyeranOrdian
Cambrian Series 2 LongwangmioanToyonianLenian
ChanglangpuanMontezumanBotomian
QungzusianAtdabanian
Terreneuvian
Meishuchuan
Jinningian		PlacentianTommotian
Nemakit-Daldynian*		Cordubian
Precambrian SinianHadrynianNemakit-Daldynian*
Sakharan		Adelaidean

*Most Russian paleontologists define the lower boundary of the Cambrian at the base of the Tommotian Stage, characterized by diversification and global distribution of organisms with mineral skeletons and the appearance of the first Archaeocyath bioherms. [28] [29] [30]

Dating the Cambrian

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

The International Commission on Stratigraphy lists the Cambrian Period as beginning at 538.8  million years ago and ending at 485.4  million years ago.

The lower boundary of the 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 substantially earlier, led to calls for a more precisely defined base to the Cambrian Period. [31]

Despite the long recognition of its distinction from younger Ordovician rocks and older Precambrian rocks, it was not until 1994 that the Cambrian system/period was internationally ratified. After decades of careful consideration, a continuous sedimentary sequence at Fortune Head, Newfoundland was settled upon as a formal base of the Cambrian Period, which was to be correlated worldwide by the earliest appearance of Treptichnus pedum . [31] Discovery of this fossil a few metres below the 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. [31] [32]

This formal designation allowed radiometric dates to be obtained from samples across the globe that corresponded to the base of the Cambrian. Early[ when? ] dates of 570  million years ago quickly gained favour, [31] 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 538.8  ± 0.2 million years ago. [6] 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 fossils (Namacalathus, Cloudina). Nevertheless, there are arguments that the dated horizon in Oman does not correspond to the Ediacaran-Cambrian boundary, but represents a facies change from marine to evaporite-dominated strata – which would mean that dates from other sections, ranging from 544 or 542 Ma, are more suitable. [31]

Paleogeography

Plate reconstructions suggest a global supercontinent, Pannotia, was in the process of breaking up early in the Cambrian, [33] [34] with Laurentia (North America), Baltica, and Siberia having separated from the main supercontinent of Gondwana to form isolated land masses. [35] Most continental land was clustered in the Southern Hemisphere at this time, but was drifting north. [35] Large, high-velocity rotational movement of Gondwana appears to have occurred in the Early Cambrian. [36]

With a lack of sea ice – the great glaciers of the Marinoan Snowball Earth were long melted [37]  – 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. [38]

In Baltoscandia a Lower Cambrian transgression transformed large swathes of the Sub-Cambrian peneplain into an epicontinental sea. [39]

Climate

Glaciers likely existed during the earliest Cambrian at high and possibly even at middle palaeolatitudes, [40] possibly due to the ancient continent of Gondwana covering the South Pole and cutting off polar ocean currents. Middle Terreneuvian deposits, corresponding to the boundary between the Fortunian and Stage 2, show evidence of glaciation. [41] However, other authors believe these very early, pretrilobitic glacial deposits may not even be of Cambrian age at all but instead date back to the Neoproterozoic, an era characterised by numerous severe icehouse periods. [42] The beginning of Stage 3 was relatively cool, with the period between 521 and 517 Ma being known as the Cambrian Arthropod Radiation Cool Event (CARCE). [43] The Earth was generally very warm during Stage 4; its climate was comparable to the hot greenhouse of the Late Cretaceous and Early Palaeogene, as evidenced by a maximum in continental weathering rates over the last 900 million years and the presence of tropical, lateritic palaeosols at high palaeolatitudes during this time. [42] The Archaecyathid Extinction Warm Event (AEWE), lasting from 511 to 510.5 Ma, was particularly warm. Another warm event, the Redlichiid-Olenid Extinction Warm Event, occurred at the beginning of Stage 5. [43] It became even warmer towards the end of the period, and sea levels rose dramatically. This warming trend continued into the Early Ordovician, the start of which was characterised by an extremely hot global climate. [44]

Flora

The Cambrian flora was little different from the Ediacaran. The principal taxa were the marine macroalgae Fuxianospira , Sinocylindra , and Marpolia . No calcareous macroalgae are known from the period. [45]

No land plant (embryophyte) fossils are known from the Cambrian. However, biofilms and microbial mats were well developed on Cambrian tidal flats and beaches 500 mya, [17] and microbes forming microbial Earth ecosystems, comparable with modern soil crust of desert regions, contributing to soil formation. [46] [47] Although molecular clock estimates suggest terrestrial plants may have first emerged during the Middle or Late Cambrian, the consequent large-scale removal of the greenhouse gas CO2 from the atmosphere through sequestration did not begin until the Ordovician. [48]

Oceanic life

The Cambrian explosion was a period of rapid multicellular growth. Most animal life during the Cambrian was aquatic. Trilobites were once assumed to be the dominant life form at that time, [49] 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. [50]

The period marked a steep change in the diversity and composition of Earth's biosphere. The Ediacaran biota suffered a mass extinction at the start of the Cambrian Period, which corresponded with 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. 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. [51] Around the same time there was a seemingly rapid appearance of representatives of all the mineralized phyla, including the Bryozoa, [52] which were once thought to have only appeared in the Lower Ordovician. [53] 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. [54]

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

While the early Cambrian showed such diversification that it has been named the 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 was surprisingly barren and showed 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 Biodiversification Event. [56] [57]

Some Cambrian organisms ventured onto land, producing the trace fossils Protichnites and Climactichnites . Fossil evidence suggests that euthycarcinoids, an extinct group of arthropods, produced at least some of the Protichnites. [58] Fossils of the track-maker of Climactichnites have not been found; however, fossil trackways and resting traces suggest a large, slug-like mollusc. [59]

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; [60] and mineralizing animals were rarer than in future periods, in part due to the unfavourable ocean chemistry. [60]

Many modes of preservation are unique to the Cambrian, and some preserve soft body parts, resulting in an abundance of Lagerstätten . These include Sirius Passet, [61] [62] the Sinsk Algal Lens, [63] the Maotianshan Shales, [64] the Emu Bay Shale, [65] and the Burgess Shale, [66] [67] [68] .

Symbol

The United States Federal Geographic Data Committee uses a "barred capital C" ⟨Ꞓ⟩ character to represent the Cambrian Period. [69] The Unicode character is U+A792ꞒLATIN CAPITAL LETTER C WITH BAR. [70] [71]

See also

Related Research Articles

<span class="mw-page-title-main">Ediacaran</span> Third and last period of the Neoproterozoic Era

The Ediacaran Period is a geological period that spans 96 million years from the end of the Cryogenian Period 635 million years ago (Mya), to the beginning of the Cambrian Period 538.8 Mya. It marks the end of the Proterozoic Eon, and the beginning of the Phanerozoic Eon. It is named after the Ediacara Hills of South Australia.

The Ordovician is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period 485.4 million years ago (Mya) to the start of the Silurian Period 443.8 Mya.

<span class="mw-page-title-main">Silurian</span> Third period of the Paleozoic Era 444-419 million years ago

The Silurian is a geologic period and system spanning 24.6 million years from the end of the Ordovician Period, at 443.8 million years ago (Mya), to the beginning of the Devonian Period, 419.2 Mya. The Silurian is the shortest period of the Paleozoic Era. As with other geologic periods, the rock beds that define the period's start and end are well identified, but the exact dates are uncertain by a few million years. The base of the Silurian is set at a series of major Ordovician–Silurian extinction events when up to 60% of marine genera were wiped out.

<span class="mw-page-title-main">Trilobite</span> Class of extinct, Paleozoic arthropods

Trilobites are extinct marine arthropods that form the class Trilobita. Trilobites form one of the earliest known groups of arthropods. 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 before slipping into a long decline, when, during the Devonian, all trilobite orders except the Proetida died out. The last extant trilobites finally disappeared in the mass extinction at the end of the Permian about 252 million years ago. Trilobites were among the most successful of all early animals, existing in oceans for almost 270 million years, with over 22,000 species having been described.

<i>Marrella</i> Extinct genus of arthropods

Marrella is an extinct genus of marrellomorph arthropod known from the middle Cambrian Burgess Shale of British Columbia. It is the most common animal represented in the Burgess Shale, with tens of thousands of specimens collected. Much rarer remains are also known from deposits in China.

<span class="mw-page-title-main">Trace fossil</span> Geological record of biological activity

A trace fossil, also known as an ichnofossil, is a fossil record of biological activity but not the preserved remains of the plant or animal itself. Trace fossils contrast with body fossils, which are the fossilized remains of parts of organisms' bodies, usually altered by later chemical activity or mineralization. The study of such trace fossils is ichnology and is the work of ichnologists.

<span class="mw-page-title-main">Cambrian–Ordovician extinction event</span> Mass extinction event about 488 million years ago

The Cambrian–Ordovician extinction event occurred approximately 488 million years ago (mya). This early Phanerozoic Eon extinction event eliminated many brachiopods and conodonts, and severely reduced the number of trilobite species. The Period in the Cambrian extinction in which most of the extinction occurred was the Caerfai Period.

<i>Anomalocaris</i> Extinct genus of anomalocaridid (also extinct)

Anomalocaris is an extinct genus of radiodont, an order of early-diverging stem-group arthropods. The first fossils of Anomalocaris were discovered in the Ogygopsis Shale of the Stephen Formation in British Columbia, Canada by Joseph Frederick Whiteaves, with more examples found by Charles Doolittle Walcott in the Burgess Shale unit of the Stephen Formation. Other closely related fossils have been found in the older Emu Bay Shale of Australia, as well as possibly elsewhere. Originally several fossilized parts discovered separately were thought to be three separate creatures, a misapprehension corrected by Harry B. Whittington and Derek Briggs in a 1985 journal article. With a body length close to 40 centimetres, A. canadensis is thought to be one of the earliest examples of an apex predator, though others have been found in older Cambrian lagerstätten deposits.

<i>Treptichnus</i> Preserved burrow of an animal

Treptichnus is the preserved burrow of an animal. As such, it is regarded as the earliest widespread complex trace fossil. Its earliest appearance, around 542 mya, which was contemporaneous with the last of the Ediacaran biota, is used to help define the dividing line, considered geologically at 541 mya, between the Ediacaran and Cambrian periods. It is last seen in the fossil record during the Cenomanian.

<i>Parvancorina</i>

Parvancorina is a genus of shield-shaped bilaterally symmetrical fossil animal that lived in the late Ediacaran seafloor. It has some superficial similarities with the Cambrian trilobite-like arthropods.

In the geological timescale, the Llandovery Epoch occurred at the beginning of the Silurian Period. The Llandoverian Epoch follows the massive Ordovician-Silurian extinction events, which led to a large decrease in biodiversity and an opening up of ecosystems.

<span class="mw-page-title-main">Ediacaran biota</span> All organisms of the Ediacaran Period (c. 635–538.8 million years ago)

The Ediacaranbiota is a taxonomic period classification that consists of all life forms that were present on Earth during the Ediacaran Period. These were composed of enigmatic tubular and frond-shaped, mostly sessile, organisms. Trace fossils of these organisms have been found worldwide, and represent the earliest known complex multicellular organisms.

<i>Diplichnites</i>

Diplichnites are arthropod trackways with two parallel rows of blunt to elongate, closely spaced tracks oriented approximately perpendicularly to the mid-line of the trackway. The term is more often used for the ichnofossils of this description; however, similar trackways from recent arthropods are sometimes given this name as well.

<span class="mw-page-title-main">Euthycarcinoidea</span> Extinct order of arthropods

Euthycarcinoidea are an enigmatic group of extinct possibly amphibious arthropods that ranged from Cambrian to Triassic times. Fossils are known from Europe, North America, Argentina, Australia and Antarctica.

The Burgess Shale of British Columbia is famous for its exceptional preservation of mid-Cambrian organisms. Around 69 other sites have been discovered of a similar age, with soft tissues preserved in a similar, though not identical, fashion. Additional sites with a similar form of preservation are known from the Ediacaran and Ordovician periods.

The end-Ediacaran extinction is a mass extinction believed to have occurred near the end of the Ediacaran period, the final period of the Proterozoic eon. Evidence suggesting that such a mass extinction occurred includes a massive reduction in diversity of acritarchs, the sudden disappearance of the Ediacara biota and calcifying organisms, and the time gap before Cambrian organisms "replaced" them. Some lines of evidence suggests that there may have been two distinct pulses of the extinction event, one occurring 550 million years ago and the other 539 million years ago.

The small shelly fauna, small shelly fossils (SSF), or early skeletal fossils (ESF) are mineralized fossils, many only a few millimetres long, with a nearly continuous record from the latest stages of the Ediacaran to the end of the Early Cambrian Period. They are very diverse, and there is no formal definition of "small shelly fauna" or "small shelly fossils". Almost all are from earlier rocks than more familiar fossils such as trilobites. Since most SSFs were preserved by being covered quickly with phosphate and this method of preservation is mainly limited to the late Ediacaran and early Cambrian periods, the animals that made them may actually have arisen earlier and persisted after this time span.

The Cambrian explosion, Cambrian radiation,Cambrian diversification, or the Biological Big Bang refers to an interval of time approximately 538.8 million years ago in the Cambrian Period when practically all major animal phyla started appearing in the fossil record. It lasted for about 13 – 25 million years and resulted in the divergence of most modern metazoan phyla. The event was accompanied by major diversification in other groups of organisms as well.

The fossils of the Burgess Shale, like the Burgess Shale itself, formed around 505 million years ago in the Mid Cambrian period. They were discovered in Canada in 1886, and Charles Doolittle Walcott collected over 65,000 specimens in a series of field trips up to the alpine site from 1909 to 1924. After a period of neglect from the 1930s to the early 1960s, new excavations and re-examinations of Walcott's collection continue to reveal new species, and statistical analysis suggests that additional discoveries will continue for the foreseeable future. Stephen Jay Gould's book Wonderful Life describes the history of discovery up to the early 1980s, although his analysis of the implications for evolution has been contested.

<span class="mw-page-title-main">Stratigraphy of the Cambrian</span>

The Stratigraphy of the Cambrian period currently has several schemes used for ordering geologic formations from the period. The International Commission on Stratigraphy−ICS scheme has set a stratotype section for the base of the Cambrian, dated quite accurately to 538.8 ± 0.2 million years ago. Russian and Chinese scientists have developed a different scheme.

References

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