Cambrian

The term Cambrian is derived from the Latin version of Cymru, the Welsh name for Wales, where rocks of this age were first studied. Cambria was the name given to the ancient Roman province of the country now known as Wales. The geological term was named by Adam Sedgwick based on work done in the summer of 1831 in North Wales. (1836). The proposal to label the period Cambrian was based on a segment of rock strata that represented a period of geological time. 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. == Geology ==

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. 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, Canada, was settled upon as a formal base of the Cambrian Period, which was to be correlated worldwide by the earliest appearance of Treptichnus pedum. 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, Miaolingian The Miaolingian is the third series/epoch of the Cambrian, lasting from c. 506.5 Ma to c. 497 Ma, and roughly identical to the middle Cambrian in older literature. It is divided into three stages: the Wuliuan c. 506.5 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. 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. Impact structures Major meteorite impact structures include: the early Cambrian (c. 535 Ma) Neugrund crater in the Gulf of Finland, Estonia, a complex meteorite crater about 20 km in diameter, with two inner ridges of about 7 km and 6 km diameter, and an outer ridge of 8 km that formed as the result of an impact of an asteroid 1 km in diameter; the 5 km diameter Gardnos crater (500±10 Ma) in Buskerud, Norway, where post-impact sediments indicate the impact occurred in a shallow marine environment with rock avalanches and debris flows occurring as the crater rim was breached not long after impact; the 24 km diameter Presqu'ile crater (500 Ma or younger) Quebec, Canada; the 19 km diameter Glikson crater (c. 508 Ma) in Western Australia; the 5 km diameter Mizarai crater (500±10 Ma) in Lithuania; and the 3.2 km diameter Newporte structure (c. 500 Ma or slightly younger) in North Dakota, U.S.A. == 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. 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. 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. 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. and the Arequipa-Antofalla block united with the South American sector of Gondwana in the early Cambrian. 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. This subduction extended west along the Gondwanan margin and by c. 530 Ma may have evolved into a major transform fault system. |alt=Paleogeographic map showing Gondwana close to the south pole, Siberia, North and South China near the equator and Baltica to the south of Siberia. Baltica During the Cambrian, Baltica rotated more than 60° anti-clockwise and began to drift northwards. Its southeastern margin was also a convergent boundary, with the accretion of island arcs and microcontinents to the craton, although the details are unclear. 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), together with Qilian-Qaidam, Altyn belts, and South West Kunlun terranes. South China and Annamia '' is an extinct fish-like craniate that lived in what is now China approximately 518 million ago 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. 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. Estimates for tropical sea surface temperatures vary from c. , 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. 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 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. Land plants may have emerged during the Cambrian, but the evidence for this is fragmentary and contested and the oldest unamibiguous evidence for land plants is from the following Ordovician. Molecular clock estimates have also led some authors to suggest that arthropods colonised land during the Cambrian, but again the earliest physical evidence of this is during the following 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. '' 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 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. Marine life lived under low and fluctuating levels of oxygen in the ocean. Upwellings of anoxic deep ocean waters into shallow marine environments could push organisms over the edge into mass extinctions, leading ultimately to increased biodiversity. and mineralizing animals were rarer than in future periods, in part due to the unfavourable ocean chemistry. 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" character to represent the Cambrian Period. ==Gallery==

File:CambrianStromatolites.jpg|Stromatolites of the Pika Formation (Middle Cambrian) near Helen Lake, Banff National Park, Canada File:Elrathia kingii growth series.jpg|Trilobites, like these Elrathia kingii were very common arthropods during this time File:20191203 Anomalocaris canadensis.png|Anomalocaris was an early marine predator, a member of the stem-arthropod group Radiodonta File:20191108 Opabinia regalis.png|Opabinia was a bizarre stem-arthropod that possessed five stalked eyes, and a fused proboscis tipped with a claw-like appendage. File:Protichnites, Blackberry Hill, Wisconsin, Cambrian 2.jpg|Protichnites were the trackways of arthropods that walked Cambrian beaches File:20210830 Hallucigenia sparsa diagrammatic reconstruction.png|Hallucigenia sparsa was a member of the group lobopodia, that is considered to be related to modern velvet worms. File:20200329 Cambroraster falcatus.png|Cambroraster falcatus was a hurdiid radiodont that bore a large horseshoe-shaped carapace. File:Schematic anatomical reconstruction of Pikaia.png|Pikaia was a stem-chordate from the Middle Cambrian File:Amiskwia sagittiformis restoration.png|Amiskwia sagittiformis was a large bodied gnathiferan from Canada and China File:Haplophrentis.png|Haplophrentis was a hyolith, a group of conical shelled lophotrochozoans that were potentially related to either lophophorates or mollusks. File:Halkieria reconstruction.png|Halkieria was a bizarre invertebrate that was an early member of the mollusk group == See also ==