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Cambrian

The Cambrian is the first geological period of the Paleozoic Era, and the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran period 538.8 Ma (million years ago) to the beginning of the Ordovician Period Ma.

Most of the continents lay in the southern hemisphere surrounded by the vast Panthalassa Ocean. The assembly of Gondwana during the Ediacaran and early Cambrian led to the development of new convergent plate boundaries and continental-margin arc magmatism along its margins that helped drive up global temperatures. Laurentia lay across the equator, separated from Gondwana by the opening Iapetus Ocean.

The Cambrian was a time of greenhouse climate conditions, with high levels of atmospheric carbon dioxide and low levels of oxygen in the atmosphere and seas. Upwellings of anoxic deep ocean waters into shallow marine environments lead to extinction events, whilst periods of raised oxygenation led to increased biodiversity.

The Cambrian marked a profound change in life on Earth; prior to the Period, the majority of living organisms were small, unicellular and poorly preserved. Complex, multicellular organisms gradually became more common during the Ediacaran, but it was not until the Cambrian that organisms with mineralised shells and skeletons are found in the rock record, and the rapid diversification of lifeforms, known as the Cambrian explosion, produced the first representatives of most modern animal phyla. The Period is also unique in its unusually high proportion of lagerstätte deposits, sites of exceptional preservation where "soft" parts of organisms are preserved as well as their more resistant shells.

By the end of the Cambrian, myriapods, arachnids, and hexapods started adapting to the land, along with the first plants.

Etymology and history
The term Cambrian is derived from the Latin version of Cymru, the Welsh name for Wales, where rocks of this age were first studied. It was named by Adam Sedgwick in 1835, who divided it into three groups; the Lower, Middle, and Upper. He defined the boundary between the Cambrian and the overlying Silurian, together with Roderick Murchison, in their joint paper "On the Silurian and Cambrian Systems, Exhibiting the Order in which the Older Sedimentary Strata Succeed each other in England and Wales". This early agreement did not last.

Due to the scarcity of fossils, Sedgwick used rock types to identify Cambrian strata. He was also slow in publishing further work. The clear fossil record of the Silurian, however, allowed Murchison to correlate rocks of a similar age across Europe and Russia, and on which he published extensively. As increasing numbers of fossils were identified in older rocks, he extended the base of the Silurian downwards into the Sedgwick's "Upper Cambrian", claiming all fossilised strata for "his" Silurian series. Matters were complicated further when, in 1852, fieldwork carried out by Sedgwick and others revealed an unconformity within the Silurian, with a clear difference in fauna between the two. This allowed Sedgwick to now claim a large section of the Silurian for "his" Cambrian and gave the Cambrian an identifiable fossil record. The dispute between the two geologists and their supporters, over the boundary between the Cambrian and Silurian, would extend beyond the life times of both Sedgwick and Murchison. It was not resolved until 1879, when Charles Lapworth proposed the disputed strata belong to its own system, which he named the Ordovician.

The term Cambrian for the oldest period of the Paleozoic was officially agreed in 1960, at the 21st International Geological Congress. It only includes Sedgwick's "Lower Cambrian series", but its base has been extended into much older rocks.

Stratigraphy
Systems, series and stages can be defined globally or regionally. For global stratigraphic correlation, the ICS ratify rock units based on a Global Boundary Stratotype Section and Point (GSSP) from a single formation (a stratotype) identifying the lower boundary of the unit. Currently the boundaries of the Cambrian System, three series and six stages are defined by global stratotype sections and points.

Ediacaran-Cambrian boundary
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.

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. 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.

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 Ma quickly gained favour, though the methods used to obtain this number are now considered to be unsuitable and inaccurate. A more precise analysis using modern radiometric dating yields a date of 538.8 ± 0.2 Ma. 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.

* 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.

Terreneuvian
The Terreneuvian is the lowermost series/epoch of the Cambrian, lasting from 538.8 ± 0.2 Ma to c. 521 Ma. It is divided into two stages: the Fortunian stage, 538.8 ± 0.2 Ma to c. 529 Ma; and the unnamed Stage 2, c. 529 Ma to c. 521 Ma. The name Terreneuvian was ratified by the International Union of Geological Sciences (IUGS) in 2007, replacing the previous "Cambrian Series 1". The GSSP defining its base is at Fortune Head on the Burin Peninsula, eastern Newfoundland, Canada (see Ediacaran - Cambrian boundary above). The Terreneuvian is the only series in the Cambrian to contain no trilobite fossils. Its lower part is characterised by complex, sediment-penetrating Phanerozoic-type trace fossils, and its upper part by small shelly fossils.

Cambrian Series 2
The second series/epoch of the Cambrian is currently unnamed and known as Cambrian Series 2. It lasted from c. 521 Ma to c. 509 Ma. Its two stages are also unnamed and known as Cambrian Stage 3, c. 521 Ma to c. 514 Ma, and Cambrian Stage 4, c. 514 Ma to c. 509 Ma. The base of Series 2 does not yet have a GSSP, but it is expected to be defined in strata marking the first appearance of trilobites in Gondwana. There was a rapid diversification of metazoans during this epoch, but their restricted geographic distribution, particularly of the trilobites and archaeocyaths, have made global correlations difficult, hence ongoing efforts to establish a GSSP.

Miaolingian
The Miaolingian is the third series/epoch of the Cambrian, lasting from c. 509 Ma to c. 497 Ma, and roughly identical to the middle Cambrian in older literature. It is divided into three stages: the Wuliuan c. 509 Ma to 504.5 Ma; the Drumian c. 504.5 Ma to c. 500.5 Ma; and the Guzhangian c. 500.5 Ma to c. 497 Ma. The name replaces Cambrian Series 3 and was ratified by the IUGS in 2018. It is named after the Miaoling Mountains in southeastern Guizhou Province, South China, where the GSSP marking its base is found. This is defined by the first appearance of the oryctocephalid trilobite Oryctocephalus indicus. Secondary markers for the base of the Miaolingian include the appearance of many acritarchs forms, a global marine transgression, and the disappearance of the polymerid trilobites, Bathynotus or Ovatoryctocara. Unlike the Terreneuvian and Series 2, all the stages of the Miaolingian are defined by GSSPs.

The olenellids, eodiscids, and most redlichiids trilobites went extinct at the boundary between Series 2 and the Miaolingian. This is considered the oldest mass extinction of trilobites.

Furongian
The Furongian, c. 497 Ma to 485.4 ± 1.9 Ma, is the fourth and uppermost series/epoch of the Cambrian. The name was ratified by the IUGS in 2003 and replaces Cambrian Series 4 and the traditional "Upper Cambrian". The GSSP for the base of the Furongian is in the Wuling Mountains, in northwestern Hunan Province, China. It coincides with the first appearance of the agnostoid trilobite Glyptagnostus reticulatus, and is near the beginning of a large positive δ13C isotopic excursion.

The Furongian is divided into three stages: the Paibian, c. 497 Ma to c. 494 Ma, and the Jiangshanian c. 494 Ma to c. 489.5 Ma, which have defined GSSPs; and the unnamed Cambrian Stage 10, c. 489.5 Ma to 485.4 ± 1.9 Ma.

Cambrian–Ordovician boundary
The GSSP for the Cambrian–Ordovician boundary is at Green Point, western Newfoundland, Canada, and is dated at 485.4 Ma. It is defined by the appearance of the conodont Iapetognathus fluctivagus. Where these conodonts are not found the appearance of planktonic graptolites or the trilobite Jujuyaspis borealis can be used. The boundary also corresponds with the peak of the largest positive variation in the δ13C curve during the boundary time interval and with a global marine transgression.

Paleogeography
Reconstructing the position of the continents during the Cambrian is based on palaeomagnetic, palaeobiogeographic, tectonic, geological and palaeoclimatic data. However, these have different levels of uncertainty and can produce contradictory locations for the major continents. This, together with the ongoing debate around the existence of the Neoproterozoic supercontinent of Pannotia, means that while most models agree the continents lay in the southern hemisphere, with the vast Panthalassa Ocean covering most of northern hemisphere, the exact distribution and timing of the movements of the Cambrian continents varies between models.

Most models show Gondwana stretching from the south polar region to north of the equator. Early in the Cambrian, the south pole corresponded with the western South American sector and as Gondwana rotated anti-clockwise, by the middle of the Cambrian, the south pole lay in the northwest African region.

Laurentia lay across the equator, separated from Gondwana by the Iapetus Ocean. Proponents of Pannotia have Laurentia and Baltica close to the Amazonia region of Gondwana with a narrow Iapetus Ocean that only began to open once Gondwana was fully assembled c. 520 Ma. Those not in favour of the existence of Pannotia show the Iapetus opening during the Late Neoproterozoic, with up to c. 6,500 km (c. 4038 miles) between Laurentia and West Gondwana at the beginning of the Cambrian.

Of the smaller continents, Baltica lay between Laurentia and Gondwana, the Ran Ocean (an arm of the Iapetus) opening between it and Gondwana. Siberia lay close to the western margin of Gondwana and to the north of Baltica. Annamia and South China formed a single continent situated off north central Gondwana. The location of North China is unclear. It may have lain along the northeast Indian sector of Gondwana or already have been a separate continent.

Laurentia
During the Cambrian, Laurentia lay across or close to the equator. It drifted south and rotated c. 20° anticlockwise during the middle Cambrian, before drifting north again in the late Cambrian.

After the Late Neoproterozoic (or mid-Cambrian) rifting of Laurentia from Gondwana and the subsequent opening of the Iapetus Ocean, Laurentia was largely surrounded by passive margins with much of the continent covered by shallow seas.

As Laurentia separated from Gondwana, a sliver of continental terrane rifted from Laurentia with the narrow Taconic seaway opening between them. The remains of this terrane are now found in southern Scotland, Ireland, and Newfoundland. Intra-oceanic subduction either to the southeast of this terrane in the Iapetus, or to its northwest in the Taconic seaway, resulted in the formation of an island arc. This accreted to the terrane in the late Cambrian, triggering southeast-dipping subduction beneath the terrane itself and consequent closure of the marginal seaway. The terrane collided with Laurentia in the Early Ordovician.

Towards the end of the early Cambrian, rifting along Laurentia's southeastern margin led to the separation of Cuyania (now part of Argentina) from the Ouachita embayment with a new ocean established that continued to widen through the Cambrian and Early Ordovician.

Gondwana
Gondwana was a massive continent, three times the size of any of the other Cambrian continents. Its continental land area extended from the south pole to north of the equator. Around it were extensive shallow seas and numerous smaller land areas.

The cratons that formed Gondwana came together during the Neoproterozoic to early Cambrian. A narrow ocean separated Amazonia from Gondwana until c. 530 Ma and the Arequipa-Antofalla block united with the South American sector of Gondwana in the early Cambrian. The Kuunga Orogeny between northern (Congo Craton, Madagascar and India) and southern Gondwana (Kalahari Craton and East Antarctica), which began c. 570 Ma, continued with parts of northern Gondwana over-riding southern Gondwana and was accompanied by metamorphism and the intrusion of granites.

Subduction zones, active since the Neoproterozoic, extended around much of Gondwana's margins, from northwest Africa southwards round South America, South Africa, East Antarctica, and the eastern edge of West Australia. Shorter subduction zones existed north of Arabia and India.

The Famatinian continental arc stretched from central Peru in the north to central Argentina in the south. Subduction beneath this proto-Andean margin began by the late Cambrian.

Along the northern margin of Gondwana, between northern Africa and the Armorican Terranes of southern Europe, the continental arc of the Cadomian Orogeny continued from the Neoproterozoic in response to the oblique subduction of the Iapetus Ocean. This subduction extended west along the Gondwanan margin and by c. 530 Ma may have evolved into a major transform fault system.

At c. 511 Ma the continental flood basalts of the Kalkarindji large igneous province (LIP) began to erupt. These covered an area of > 2.1 × 106 km2 across northern, central and Western Australia regions of Gondwana making it one of the largest, as well as the earliest, LIPs of the Phanerozoic. The timing of the eruptions suggests they played a role in the early to middle Cambrian mass extinction.

Ganderia, East and West Avalonia, Carolinia and Meguma Terranes
The terranes of Ganderia, East and West Avalonia, Carolinia and Meguma lay in polar regions during the early Cambrian, and high-to-mid southern latitudes by the mid to late Cambrian. They are commonly shown as an island arc-transform fault system along the northwestern margin of Gondwana north of northwest Africa and Amazonia, which rifted from Gondwana during the Ordovician. However, some models show these terranes as part of a single independent microcontinent, Greater Avalonia, lying to the west of Baltica and aligned with its eastern (Timanide) margin, with the Iapetus to the north and the Ran Ocean to the south.

Baltica
During the Cambrian, Baltica rotated more than 60° anti-clockwise and began to drift northwards. This rotation was accommodated by major strike-slip movements in the Ran Ocean between it and Gondwana.

Baltica lay at mid-to-high southerly latitudes, separated from Laurentia by the Iapetus and from Gondwana by the Ran Ocean. It was composed of two continents, Fennoscandia and Sarmatia, separated by shallow seas. The sediments deposited in these unconformably overlay Precambrian basement rocks. The lack of coarse-grained sediments indicates low lying topography across the centre of the craton.

Along Baltica's northeastern margin subduction and arc magmatism associated with the Ediacaran Timanian Orogeny was coming to an end. In this region the early to middle Cambrian was a time of non-deposition and followed by late Cambrian rifting and sedimentation.

Its southeastern margin was also a convergent boundary, with the accretion of island arcs and microcontinents to the craton, although the details are unclear.

Siberia
Siberia began the Cambrian close to western Gondwana and north of Baltica. It drifted northwestwards to close to the equator as the Ægir Ocean opened between it and Baltica. Much of the continent was covered by shallow seas with extensive archaeocyathan reefs. The then northern third of the continent (present day south; Siberia has rotated 180° since the Cambrian) adjacent to its convergent margin was mountainous.

From the Late Neoproterozoic to the Ordovician, a series of island arcs accreted to Siberia's then northeastern margin, accompanied by extensive arc and back-arc volcanism. These now form the Altai-Sayan terranes. Some models show a convergent plate margin extending from Greater Avalonia, through the Timanide margin of Baltica, forming the Kipchak island arc offshore of southeastern Siberia and curving round to become part of the Altai-Sayan convergent margin.

Along the then western margin, Late Neoproterozoic to early Cambrian rifting was followed by the development of a passive margin.

To the then north, Siberia was separated from the Central Mongolian terrane by the narrow and slowly opening Mongol-Okhotsk Ocean. The Central Mongolian terrane's northern margin with the Panthalassa was convergent, whilst its southern margin facing the Mongol-Okhotsk Ocean was passive.

Central Asia
During the Cambrian, the terranes that would form Kazakhstania later in the Paleozoic were a series of island arc and accretionary complexes that lay along an intra-oceanic convergent plate margin to the south of North China.

To the south of these the Tarim microcontinent lay between Gondwana and Siberia. Its northern margin was passive for much of the Paleozoic, with thick sequences of platform carbonates and fluvial to marine sediments resting unconformably on Precambrian basement. Along its southeast margin was the Altyn Cambro–Ordovician accretionary complex, whilst to the southwest a subduction zone was closing the narrow seaway between the North West Kunlun region of Tarim and the South West Kunlun terrane.

North China
North China lay at equatorial to tropical latitudes during the early Cambrian, although its exact position is unknown. Much of the craton was covered by shallow seas, with land in the northwest and southeast.

Northern North China was a passive margin until the onset of subduction and the development of the Bainaimiao arc in the late Cambrian. To its south was a convergent margin with a southwest dipping subduction zone, beyond which lay the North Qinling terrane (now part of the Qinling Orogenic Belt).

South China and Annamia
South China and Annamia formed a single continent. Strike-slip movement between it and Gondwana accommodated its steady drift northwards from offshore the Indian sector of Gondwana to near the western Australian sector. This northward drift is evidenced by the progressive increase in limestones and increasing faunal diversity.

The northern margin South China, including the South Qinling block, was a passive margin.

Along the southeastern margin, lower Cambrian volcanics indicate the accretion of an island arc along the Song Ma suture zone. Also, early in the Cambrian, the eastern margin of South China changed from passive to active, with the development of oceanic volcanic island arcs that now form part of the Japanese terrane.

Climate
The distribution of climate-indicating sediments, including the wide latitudinal distribution of tropical carbonate platforms, archaeocyathan reefs and bauxites, and arid zone evaporites and calcrete deposits, show the Cambrian was a time of greenhouse climate conditions. During the late Cambrian the distribution of trilobite provinces also indicate only a moderate pole-to-equator temperature gradient. There is evidence of glaciation at high latitudes on Avalonia. However, it is unclear whether these sediments are early Cambrian or actually late Neoproterozoic in age.

Calculations of global average temperatures (GAT) vary depending on which techniques are used. Whilst some measurements show GAT over c. 40 °C models that combine multiple sources give GAT of c. 20 - 22 C in the Terreneuvian increasing to c. 23 - 25 C for the rest of the Cambrian. The warm climate was linked to elevated atmospheric carbon dioxide levels. Assembly of Gondwana led to the reorganisation of the tectonic plates with the development of new convergent plate margins and continental-margin arc magmatism that helped drive climatic warming. The eruptions of the Kalkarindji LIP basalts during Stage 4 and into the early Miaolingian, also released large quantities of carbon dioxide, methane and sulphur dioxide into the atmosphere leading to rapid climatic changes and elevated sea surface temperatures.

There is uncertainty around the maximum sea surface temperatures. These are calculated using δ18O values from marine rocks, and there is an ongoing debate about the levels δ18O in Cambrian seawater relative to the rest of the Phanerozoic. Estimates for tropical sea surface temperatures vary from c. 28 - 32 C, to c. 29 - 38 C. Modern average tropical sea surface temperatures are 26 °C.

Atmospheric oxygen levels rose steadily rising from the Neoproterozoic due to the increase in photosynthesising organisms. Cambrian levels varied between c. 3% and 14% (present day levels are c. 21%). Low levels of atmospheric oxygen and the warm climate resulted in lower dissolved oxygen concentrations in marine waters and widespread anoxia in deep ocean waters.

There is a complex relationship between oxygen levels, the biogeochemistry of ocean waters, and the evolution of life. Newly evolved burrowing organisms exposed anoxic sediments to the overlying oxygenated seawater. This bioturbation decreased the burial rates of organic carbon and sulphur, which over time reduced atmospheric and oceanic oxygen levels, leading to widespread anoxic conditions. Periods of higher rates of continental weathering led to increased delivery of nutrients to the oceans, boosting productivity of phytoplankton and stimulating metazoan evolution. However, rapid increases in nutrient supply led to eutrophication, where rapid growth in phytoplankton numbers result in the depletion of oxygen in the surrounding waters.

Pulses of increased oxygen levels are linked to increased biodiversity; raised oxygen levels supported the increasing metabolic demands of organisms, and increased ecological niches by expanding habitable areas of seafloor. Conversely, incursions of oxygen-deficient water, due to changes in sea level, ocean circulation, upwellings from deeper waters and/or biological productivity, produced anoxic conditions that limited habitable areas, reduced ecological niches and resulted in extinction events both regional and global.

Overall, these dynamic, fluctuating environments, with global and regional anoxic incursions resulting in extinction events, and periods of increased oceanic oxygenation stimulating biodiversity, drove evolutionary innovation.

Geochemistry
During the Cambrian, variations in isotope ratios were more frequent and more pronounced than later in the Phanerozoic, with at least 10 carbon isotope (δ13C) excursions (significant variations in global isotope ratios) recognised. These excursions record changes in the biogeochemistry of the oceans and atmosphere, which are due to processes such as the global rates of continental arc magmatism, rates of weathering and nutrients levels entering the marine environment, sea level changes, and biological factors including the impact of burrowing fauna on oxygen levels.

Base of Cambrian
The basal Cambrian δ13C excursion (BACE), together with low δ238U and raised δ34S indicates a period of widespread shallow marine anoxia, which occurs at the same time as the extinction off the Ediacaran acritarchs. It was followed by the rapid appearance and diversification of bilaterian animals.

Cambrian Stages 2 and 3
During the early Cambrian, 87Sr/86Sr rose in response to enhanced continental weathering. This increased the input of nutrients into the oceans and led to higher burial rates of organic matter. Over long timescales, the extra oxygen released by organic carbon burial is balanced by a decrease in the rates of pyrite (FeS2) burial (a process which also releases oxygen), leading to stable levels of oxygen in the atmosphere. However, during the early Cambrian, a series of linked δ13C and δ34S excursions indicate high burial rates of both organic carbon and pyrite in biologically productive yet anoxic ocean floor waters. The oxygen-rich waters produced by these processes spread from the deep ocean into shallow marine environments, extending the habitable regions of the seafloor. These pulses of oxygen are associated with the radiation of the small shelly fossils and the Cambrian arthropod radiation isotope excursion (CARE). The increase in oxygenated waters in the deep ocean ultimately reduced the levels of organic carbon and pyrite burial, leading to a decrease in oxygen production and the re-establishment of anoxic conditions. This cycle was repeated several times during the early Cambrian.

Cambrian Stage 4 to early Miaolingian
The beginning of the eruptions of the Kalkarindji LIP basalts during Stage 4 and the early Miaolingian released large quantities of carbon dioxide, methane and sulphur dioxide into the atmosphere. The changes these wrought are reflected by three large and rapid δ13C excursions. Increased temperatures led to a global sea level rise that flooded continental shelves and interiors with anoxic waters from the deeper ocean and drowned carbonate platforms of archaeocyathan reefs, resulting in the widespread accumulation of black organic-rich shales. Known as the Sinsk anoxic extinction event, this triggered the first major extinction of the Phanerozoic, the 513 - 508 Ma Botoman-Toyonian Extinction (BTE), which included the loss of the archaeocyathids and hyoliths and saw a major drop in biodiversity. The rise in sea levels is also evidenced by a global decrease in 87Sr/86Sr. The flooding of continental areas decreased the rates of continental weathering, reducing the input of 87Sr to the oceans and lowering the 87Sr/86Sr of seawater.

The base of the Miaolingian is marked by the Redlichiid–Olenellid extinction carbon isotope event (ROECE), which coincides with the main phase of Kalkarindji volcanism.

During the Miaolingian, orogenic events along the Australian-Antarctic margin of Gondwana led to an increase in weathering and an influx of nutrients into the ocean, raising the level of productivity and organic carbon burial. These can be seen in the steady increase in 87Sr/86Sr and δ13C.

Early Furongian
Continued erosion of the deeper levels of the Gondwanan mountain belts led to a peak in 87Sr/86Sr and linked positive δ13C and δ34S excursions, known as the Steptoean positive carbon isotope excursion (SPICE). This indicates similar geochemical conditions to Stages 2 and 3 of the early Cambrian existed, with the expansion of seafloor anoxia enhancing the burial rates of organic matter and pyrite. This increase in the extent of anoxic seafloor conditions led to the extinction of the marjumiid and damesellid trilobites, whilst the increase in oxygen levels that followed helped drive the radiation of plankton.

87Sr/86Sr fell sharply near the top of the Jiangshanian Stage, and through Stage 10 as the Gondwanan mountains were eroded down and rates of weathering decreased.

Magnesium/calcium isotope ratios in seawater
The mineralogy of inorganic marine carbonates has varied through the Phanerozoic, controlled by the Mg2+/Ca2+ values of seawater. High Mg2+/Ca2+ result in calcium carbonate precipitation dominated by aragonite and high-magnesium calcite, known as aragonite seas, and low ratios result in calcite seas where low-magnesium calcite is the primary calcium carbonate precipitate. The shells and skeletons of biomineralising organisms reflect the dominant form of calcite.

During the late Ediacaran to early Cambrian increasing oxygen levels led to a decrease in ocean acidity and an increase in the concentration of calcium in sea water. However, there was not a simple transition from aragonite to calcite seas, rather a protracted and variable change through the Cambrian. Aragonite and high-magnesium precipitation continued from the Ediacaran into Cambrian Stage 2. Low-magnesium calcite skeletal hard parts appear in Cambrian Age 2, but inorganic precipitation of aragonite also occurred at this time. Mixed aragonite–calcite seas continued through the middle and late Cambrian, with fully calcite seas not established until the early Ordovician.

These variations and slow decrease in Mg2+/Ca2+ of seawater were due to low oxygen levels, high continental weathering rates and the geochemistry of the Cambrian seas. In conditions of low oxygen and high iron levels, iron substitutes for magnesium in authigenic clay minerals deposited on the ocean floor, slowing the removal rates of magnesium from seawater. The enrichment of ocean waters in silica, prior to the radiation of siliceous organisms, and the limited bioturbation of the anoxic ocean floor increased the rates of deposition, relative to the rest of the Phanerozoic, of these clays. This, together with the high input of magnesium into the oceans via enhanced continental weathering, delayed the reduction in Mg2+/Ca2+ and facilitated continued aragonite precipitation.

The conditions that favoured the deposition of authigenic clays were also ideal for the formation of lagerstätten, with the minerals in the clays replacing the soft body parts of Cambrian organisms.

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.

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, and microbes forming microbial Earth ecosystems, comparable with modern soil crust of desert regions, contributing to soil formation. 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.

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, 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.

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. Around the same time there was a seemingly rapid appearance of representatives of all the mineralized phyla, including the Bryozoa, which were once thought to have only appeared in the Lower Ordovician. 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.

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 Ma, 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 Ma, 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. 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. Fossils of the track-maker of Climactichnites have not been found; however, fossil trackways and resting traces suggest a large, slug-like mollusc.

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.

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, the Sinsk Algal Lens, the Maotianshan Shales, the Emu Bay Shale, and the Burgess Shale.

Symbol
The United States Federal Geographic Data Committee uses a "barred capital C" $⟨Ꞓ⟩$ character to represent the Cambrian Period. The Unicode character is.

title: Cambrian


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Theory of categories

In ontology, the theory of categories concerns itself with the categories of being: the highest genera or kinds of entities according to Amie Thomasson. To investigate the categories of being, or simply categories, is to determine the most fundamental and the broadest classes of entities. A distinction between such categories, in making the categories or applying them, is called an ontological distinction. Various systems of categories have been proposed, they often include categories for substances, properties, relations, states of affairs or events. A representative question within the theory of categories might articulate itself, for example, in a query like, "Are universals prior to particulars?"

Early development
The process of abstraction required to discover the number and names of the categories of being has been undertaken by many philosophers since Aristotle and involves the careful inspection of each concept to ensure that there is no higher category or categories under which that concept could be subsumed. The scholars of the twelfth and thirteenth centuries developed Aristotle's ideas. For example, Gilbert of Poitiers divides Aristotle's ten categories into two sets, primary and secondary, according to whether they inhere in the subject or not: Furthermore, following Porphyry’s likening of the classificatory hierarchy to a tree, they concluded that the major classes could be subdivided to form subclasses, for example, Substance could be divided into Genus and Species, and Quality could be subdivided into Property and Accident, depending on whether the property was necessary or contingent. An alternative line of development was taken by Plotinus in the second century who by a process of abstraction reduced Aristotle's list of ten categories to five: Substance, Relation, Quantity, Motion and Quality. Plotinus further suggested that the latter three categories of his list, namely Quantity, Motion and Quality correspond to three different kinds of relation and that these three categories could therefore be subsumed under the category of Relation. This was to lead to the supposition that there were only two categories at the top of the hierarchical tree, namely Substance and Relation. Many supposed that relations only exist in the mind. Substance and Relation, then, are closely commutative with Matter and Mind--this is expressed most clearly in the dualism of René Descartes.
 * Primary categories: Substance, Relation, Quantity and Quality
 * Secondary categories: Place, Time, Situation, Condition, Action, Passion

Aristotle
One of Aristotle’s early interests lay in the classification of the natural world, how for example the genus "animal" could be first divided into "two-footed animal" and then into "wingless, two-footed animal". He realised that the distinctions were being made according to the qualities the animal possesses, the quantity of its parts and the kind of motion that it exhibits. To fully complete the proposition "this animal is ..." Aristotle stated in his work on the Categories that there were ten kinds of predicate where ...

"... each signifies either substance or quantity or quality or relation or where or when or being-in-a-position or having or acting or being acted upon".

He realised that predicates could be simple or complex. The simple kinds consist of a subject and a predicate linked together by the "categorical" or inherent type of relation. For Aristotle the more complex kinds were limited to propositions where the predicate is compounded of two of the above categories for example "this is a horse running". More complex kinds of proposition were only discovered after Aristotle by the Stoic, Chrysippus, who developed the "hypothetical" and "disjunctive" types of syllogism and these were terms which were to be developed through the Middle Ages and were to reappear in Kant's system of categories.

Category came into use with Aristotle's essay Categories, in which he discussed univocal and equivocal terms, predication, and ten categories:
 * Substance, essence (ousia) – examples of primary substance: this man, this horse; secondary substance (species, genera): man, horse
 * Quantity (poson, how much), discrete or continuous – examples: two cubits long, number, space, (length of) time.
 * Quality (poion, of what kind or description) – examples: white, black, grammatical, hot, sweet, curved, straight.
 * Relation (pros ti, toward something) – examples: double, half, large, master, knowledge.
 * Place (pou, where) – examples: in a marketplace, in the Lyceum
 * Time (pote, when) – examples: yesterday, last year
 * Position, posture, attitude (keisthai, to lie) – examples: sitting, lying, standing
 * State, condition (echein, to have or be) – examples: shod, armed
 * Action (poiein, to make or do) – examples: to lance, to heat, to cool (something)
 * Affection, passion (paschein, to suffer or undergo) – examples: to be lanced, to be heated, to be cooled

Plotinus
Plotinus in writing his Enneads around AD 250 recorded that "Philosophy at a very early age investigated the number and character of the existents ... some found ten, others less ... to some the genera were the first principles, to others only a generic classification of existents." He realised that some categories were reducible to others saying "Why are not Beauty, Goodness and the virtues, Knowledge and Intelligence included among the primary genera?" He concluded that such transcendental categories and even the categories of Aristotle were in some way posterior to the three Eleatic categories first recorded in Plato's dialogue Parmenides and which comprised the following three coupled terms:
 * Unity/Plurality
 * Motion/Stability
 * Identity/Difference

Plotinus called these "the hearth of reality" deriving from them not only the three categories of Quantity, Motion and Quality but also what came to be known as "the three moments of the Neoplatonic world process":
 * First, there existed the "One", and his view that "the origin of things is a contemplation"
 * The Second "is certainly an activity ... a secondary phase ... life streaming from life ... energy running through the universe"
 * The Third is some kind of Intelligence concerning which he wrote "Activity is prior to Intellection ... and self knowledge"

Plotinus likened the three to the centre, the radii and the circumference of a circle, and clearly thought that the principles underlying the categories were the first principles of creation. "From a single root all being multiplies." Similar ideas were to be introduced into Early Christian thought by, for example, Gregory of Nazianzus who summed it up saying "Therefore, Unity, having from all eternity arrived by motion at duality, came to rest in Trinity."

Modern development
Kant and Hegel accused the Aristotelian table of categories of being 'rhapsodic', derived arbitrarily and in bulk from experience, without any systematic necessity.

The early modern dualism, which has been described above, of Mind and Matter or Subject and Relation, as reflected in the writings of Descartes underwent a substantial revision in the late 18th century. The first objections to this stance were formulated in the eighteenth century by Immanuel Kant who realised that we can say nothing about Substance except through the relation of the subject to other things.

For example: In the sentence "This is a house" the substantive subject "house" only gains meaning in relation to human use patterns or to other similar houses. The category of Substance disappears from Kant's tables, and under the heading of Relation, Kant lists inter alia the three relationship types of Disjunction, Causality and Inherence. The three older concepts of Quantity, Motion and Quality, as Peirce discovered, could be subsumed under these three broader headings in that Quantity relates to the subject through the relation of Disjunction; Motion relates to the subject through the relation of Causality; and Quality relates to the subject through the relation of Inherence. Sets of three continued to play an important part in the nineteenth century development of the categories, most notably in G.W.F. Hegel's extensive tabulation of categories, and in C.S. Peirce's categories set out in his work on the logic of relations. One of Peirce's contributions was to call the three primary categories Firstness, Secondness and Thirdness which both emphasises their general nature, and avoids the confusion of having the same name for both the category itself and for a concept within that category.

In a separate development, and building on the notion of primary and secondary categories introduced by the Scholastics, Kant introduced the idea that secondary or "derivative" categories could be derived from the primary categories through the combination of one primary category with another. This would result in the formation of three secondary categories: the first, "Community" was an example that Kant gave of such a derivative category; the second, "Modality", introduced by Kant, was a term which Hegel, in developing Kant's dialectical method, showed could also be seen as a derivative category; and the third, "Spirit" or "Will" were terms that Hegel and Schopenhauer were developing separately for use in their own systems. Karl Jaspers in the twentieth century, in his development of existential categories, brought the three together, allowing for differences in terminology, as Substantiality, Communication and Will. This pattern of three primary and three secondary categories was used most notably in the nineteenth century by Peter Mark Roget to form the six headings of his Thesaurus of English Words and Phrases. The headings used were the three objective categories of Abstract Relation, Space (including Motion) and Matter and the three subjective categories of Intellect, Feeling and Volition, and he found that under these six headings all the words of the English language, and hence any possible predicate, could be assembled.

Kant
In the Critique of Pure Reason (1781), Immanuel Kant argued that the categories are part of our own mental structure and consist of a set of a priori concepts through which we interpret the world around us. These concepts correspond to twelve logical functions of the understanding which we use to make judgements and there are therefore two tables given in the Critique, one of the Judgements and a corresponding one for the Categories. To give an example, the logical function behind our reasoning from ground to consequence (based on the Hypothetical relation) underlies our understanding of the world in terms of cause and effect (the Causal relation). In each table the number twelve arises from, firstly, an initial division into two: the Mathematical and the Dynamical; a second division of each of these headings into a further two: Quantity and Quality, and Relation and Modality respectively; and, thirdly, each of these then divides into a further three subheadings as follows.

Table of Judgements

Mathematical Dynamical
 * Quantity
 * Universal
 * Particular
 * Singular
 * Quality
 * Affirmative
 * Negative
 * Infinite
 * Relation
 * Categorical
 * Hypothetical
 * Disjunctive
 * Modality
 * Problematic
 * Assertoric
 * Apodictic

Table of Categories

Mathematical Dynamical
 * Quantity
 * Unity
 * Plurality
 * Totality
 * Quality
 * Reality
 * Negation
 * Limitation
 * Relation
 * Inherence and Subsistence (substance and accident)
 * Causality and Dependence (cause and effect)
 * Community (reciprocity)
 * Modality
 * Possibility
 * Existence
 * Necessity

Criticism of Kant's system followed, firstly, by Arthur Schopenhauer, who amongst other things was unhappy with the term "Community", and declared that the tables "do open violence to truth, treating it as nature was treated by old-fashioned gardeners", and secondly, by W.T.Stace who in his book The Philosophy of Hegel suggested that in order to make Kant's structure completely symmetrical a third category would need to be added to the Mathematical and the Dynamical. This, he said, Hegel was to do with his category of concept.

Hegel
G.W.F. Hegel in his Science of Logic (1812) attempted to provide a more comprehensive system of categories than Kant and developed a structure that was almost entirely triadic. So important were the categories to Hegel that he claimed the first principle of the world, which he called the "absolute", is "a system of categories the categories must be the reason of which the world is a consequent".

Using his own logical method of sublation, later called the Hegelian dialectic, reasoning from the abstract through the negative to the concrete, he arrived at a hierarchy of some 270 categories, as explained by W. T. Stace. The three very highest categories were "logic", "nature" and "spirit". The three highest categories of "logic", however, he called "being", "essence", and "notion" which he explained as follows:

 * Being was differentiated from Nothing by containing with it the concept of the "other", an initial internal division that can be compared with Kant's category of disjunction. Stace called the category of Being the sphere of common sense containing concepts such as consciousness, sensation, quantity, quality and measure.
 * Essence. The "other" separates itself from the "one" by a kind of motion, reflected in Hegel's first synthesis of "becoming". For Stace this category represented the sphere of science containing within it firstly, the thing, its form and properties; secondly, cause, effect and reciprocity, and thirdly, the principles of classification, identity and difference.
 * Notion. Having passed over into the "Other" there is an almost neoplatonic return into a higher unity that in embracing the "one" and the "other" enables them to be considered together through their inherent qualities. This according to Stace is the sphere of philosophy proper where we find not only the three types of logical proposition: disjunctive, hypothetical, and categorical but also the three transcendental concepts of beauty, goodness and truth.

Schopenhauer's category that corresponded with "notion" was that of "idea", which in his Four-Fold Root of Sufficient Reason he complemented with the category of the "will". The title of his major work was The World as Will and Idea. The two other complementary categories, reflecting one of Hegel's initial divisions, were those of Being and Becoming. At around the same time, Goethe was developing his colour theories in the Farbenlehre of 1810, and introduced similar principles of combination and complementation, symbolising, for Goethe, "the primordial relations which belong both to nature and vision". Hegel in his Science of Logic accordingly asks us to see his system not as a tree but as a circle.

Twentieth-century development
In the twentieth century the primacy of the division between the subjective and the objective, or between mind and matter, was disputed by, among others, Bertrand Russell and Gilbert Ryle. Philosophy began to move away from the metaphysics of categorisation towards the linguistic problem of trying to differentiate between, and define, the words being used. Ludwig Wittgenstein’s conclusion was that there were no clear definitions which we can give to words and categories but only a "halo" or "corona" of related meanings radiating around each term. Gilbert Ryle thought the problem could be seen in terms of dealing with "a galaxy of ideas" rather than a single idea, and suggested that category mistakes are made when a concept (e.g. "university"), understood as falling under one category (e.g. abstract idea), is used as though it falls under another (e.g. physical object). With regard to the visual analogies being used, Peirce and Lewis, just like Plotinus earlier, likened the terms of propositions to points, and the relations between the terms to lines. Peirce, taking this further, talked of univalent, bivalent and trivalent relations linking predicates to their subject and it is just the number and types of relation linking subject and predicate that determine the category into which a predicate might fall. Primary categories contain concepts where there is one dominant kind of relation to the subject. Secondary categories contain concepts where there are two dominant kinds of relation. Examples of the latter were given by Heidegger in his two propositions "the house is on the creek" where the two dominant relations are spatial location (Disjunction) and cultural association (Inherence), and "the house is eighteenth century" where the two relations are temporal location (Causality) and cultural quality (Inherence). A third example may be inferred from Kant in the proposition "the house is impressive or sublime" where the two relations are spatial or mathematical disposition (Disjunction) and dynamic or motive power (Causality). Both Peirce and Wittgenstein introduced the analogy of colour theory in order to illustrate the shades of meanings of words. Primary categories, like primary colours, are analytical representing the furthest we can go in terms of analysis and abstraction and include Quantity, Motion and Quality. Secondary categories, like secondary colours, are synthetic and include concepts such as Substance, Community and Spirit.

Apart from these, the categorial scheme of Alfred North Whitehead and his Process Philosophy, alongside Nicolai Hartmann and his Critical Realism, remain one of the most detailed and advanced systems in categorial research in metaphysics.

Peirce
Charles Sanders Peirce, who had read Kant and Hegel closely, and who also had some knowledge of Aristotle, proposed a system of merely three phenomenological categories: Firstness, Secondness, and Thirdness, which he repeatedly invoked in his subsequent writings. Like Hegel, C.S. Peirce attempted to develop a system of categories from a single indisputable principle, in Peirce's case the notion that in the first instance he could only be aware of his own ideas. "It seems that the true categories of consciousness are first, feeling ... second, a sense of resistance ... and third, synthetic consciousness, or thought". Elsewhere he called the three primary categories: Quality, Reaction and Meaning, and even Firstness, Secondness and Thirdness, saying, "perhaps it is not right to call these categories conceptions, they are so intangible that they are rather tones or tints upon conceptions":
 * Firstness (Quality): "The first is predominant in feeling ... we must think of a quality without parts, e.g. the colour of magenta ... When I say it is a quality I do not mean that it "inheres" in a subject ... The whole content of consciousness is made up of qualities of feeling, as truly as the whole of space is made up of points, or the whole of time by instants".
 * Secondness (Reaction): "This is present even in such a rudimentary fragment of experience as a simple feeling ... an action and reaction between our soul and the stimulus ... The idea of second is predominant in the ideas of causation and of statical force ... the real is active; we acknowledge it by calling it the actual".
 * Thirdness (Meaning): "Thirdness is essentially of a general nature ... ideas in which thirdness predominate [include] the idea of a sign or representation ... Every genuine triadic relation involves meaning ... the idea of meaning is irreducible to those of quality and reaction ... synthetical consciousness is the consciousness of a third or medium".

Although Peirce's three categories correspond to the three concepts of relation given in Kant's tables, the sequence is now reversed and follows that given by Hegel, and indeed before Hegel of the three moments of the world-process given by Plotinus. Later, Peirce gave a mathematical reason for there being three categories in that although monadic, dyadic and triadic nodes are irreducible, every node of a higher valency is reducible to a "compound of triadic relations". Ferdinand de Saussure, who was developing "semiology" in France just as Peirce was developing "semiotics" in the US, likened each term of a proposition to "the centre of a constellation, the point where other coordinate terms, the sum of which is indefinite, converge".

Others
Edmund Husserl (1962, 2000) wrote extensively about categorial systems as part of his phenomenology.

For Gilbert Ryle (1949), a category (in particular a "category mistake") is an important semantic concept, but one having only loose affinities to an ontological category.

Contemporary systems of categories have been proposed by John G. Bennett (The Dramatic Universe, 4 vols., 1956–65), Wilfrid Sellars (1974), Reinhardt Grossmann (1983, 1992), Johansson (1989), Hoffman and Rosenkrantz (1994), Roderick Chisholm (1996), Barry Smith (ontologist) (2003), and Jonathan Lowe (2006).

Selected bibliography

 * Aristotle, 1953. Metaphysics. Ross, W. D., trans. Oxford University Press.
 * , 2004. Categories, Edghill, E. M., trans. Uni. of Adelaide library.
 * John G. Bennett, 1956–1965. The Dramatic Universe. London, Hodder & Stoughton.
 * Gustav Bergmann, 1992. New Foundations of Ontology. Madison: Uni. of Wisconsin Press.
 * Browning, Douglas, 1990. Ontology and the Practical Arena. Pennsylvania State Uni.
 * Butchvarov, Panayot, 1979. Being qua Being: A Theory of Identity, Existence, and Predication. Indiana Uni. Press.
 * Roderick Chisholm, 1996. A Realistic Theory of Categories. Cambridge Uni. Press.
 * Feibleman, James Kern, 1951. Ontology. The Johns Hopkins Press (reprinted 1968, Greenwood Press, Publishers, New York).
 * Grossmann, Reinhardt, 1983. The Categorial Structure of the World. Indiana Uni. Press.
 * Grossmann, Reinhardt, 1992. The Existence of the World: An Introduction to Ontology. Routledge.
 * Haaparanta, Leila and Koskinen, Heikki J., 2012. Categories of Being: Essays on Metaphysics and Logic. New York: Oxford University Press.
 * Hoffman, J., and Rosenkrantz, G. S.,1994. Substance among other Categories. Cambridge Uni. Press.
 * Edmund Husserl, 1962. Ideas: General Introduction to Pure Phenomenology. Boyce Gibson, W. R., trans. Collier.
 * --, 2000. Logical Investigations, 2nd ed. Findlay, J. N., trans. Routledge.
 * Johansson, Ingvar, 1989. Ontological Investigations. Routledge, 2nd ed. Ontos Verlag 2004.
 * Kahn, Charles H., 2009. Essays on Being, Oxford University Press.
 * Immanuel Kant, 1998. Critique of Pure Reason. Guyer, Paul, and Wood, A. W., trans. Cambridge Uni. Press.
 * Charles Sanders Peirce, 1992, 1998. The Essential Peirce, vols. 1,2. Houser, Nathan et al., eds. Indiana Uni. Press.
 * Gilbert Ryle, 1949. The Concept of Mind. Uni. of Chicago Press.
 * Wilfrid Sellars, 1974, "Toward a Theory of the Categories" in Essays in Philosophy and Its History. Reidel.
 * Barry Smith, 2003. "Ontology" in Blackwell Guide to the Philosophy of Computing and Information. Blackwell.

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title: Theory of categories