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, scientific understanding of the Cambrian biology surpasses that of some later periods.[10]
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 and earlier
TonianHuainan biota 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 most 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
mollusks and
arthropods (albeit not terrestrial) that emerged to graze 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
supercontinentPannotia. The seas were relatively warm, and polar ice was absent for much of the period.
The Cambrian Period followed the
Ediacaran Period and preceded 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,
Newfoundland, and possibly in the western US. 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 (Merioneth or Croixan, 497 ± 0.2 to 485.4 ± 1.9 mya).
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:
Approximate correlation of global and regional stages in Cambrian stratigraphy[28]
*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.[29][30][31]
Dating the Cambrian
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, has led to calls for a more precisely defined base to the Cambrian Period.[32]
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.[32] 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.[32][33]
This formal designation allowed radiometric dates to be obtained from samples across the globe that corresponded to the base of the Cambrian. An early date of 570 million years ago quickly gained favour,[32] 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 to 542 Ma, are more suitable.[32]
Paleogeography
Plate reconstructions suggest a global
supercontinent,
Pannotia, was in the process of breaking up early in the Cambrian,[34][35] with
Laurentia (North America),
Baltica, and
Siberia having separated from the main supercontinent of
Gondwana to form isolated land masses.[36] Most continental land was clustered in the Southern Hemisphere at this time, but was drifting north.[36] Large, high-velocity rotational movement of Gondwana appears to have occurred in the Early Cambrian.[37]
With a lack of sea ice – the great glaciers of the Marinoan
Snowball Earth were long melted[38] – 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.[39]
Glaciers likely existed during the earliest Cambrian at high and possibly even at middle palaeolatitudes,[41] 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.[42] 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.[43]
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).[44] 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.[43]
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 the Wuliuan.[44] 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.[45]
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.[46]
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.[47][48] 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.[49]
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,[50] 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.[51]
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.[52] Around the same time there was a seemingly rapid appearance of representatives of all the mineralized
phyla, including the
Bryozoa,[53] which were once thought to have only appeared in the Lower
Ordovician.[54] 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.[55]
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.[57][58]
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.[59] Fossils of the track-maker of Climactichnites have not been found; however, fossil trackways and resting traces suggest a large,
slug-like
mollusc.[60]
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;[61] and mineralizing animals were rarer than in future periods, in part due to the unfavourable
ocean chemistry.[61]
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"On the Silurian and Cambrian systems, exhibiting the order in which the older sedimentary strata succeed each other in England and Wales," Notices and Abstracts of Communications to the British Association for the Advancement of Science at the Dublin meeting, August 1835, pp. 59–61, in: Report of the Fifth Meeting of the British Association for the Advancement of Science; held in Dublin in 1835 (1836). From p. 60: "Professor Sedgwick then described in descending order the groups of slate rocks, as they are seen in Wales and Cumberland. To the highest he gave the name of Upper Cambrian group. ... To the next inferior group he gave the name of Middle Cambrian. ... The Lower Cambrian group occupies the S.W. coast of Cærnarvonshire,"
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