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150-476: The Late Ordovician glaciation is widely considered to be the leading cause of the Late Ordovician mass extinction , and it is the only glacial episode that appears to have coincided with a major mass extinction of nearly 61% of marine life. Estimates of peak ice sheet volume range from 50 to 250 million cubic kilometres, and its duration from 35 million to less than 1 million years. At its height during

300-580: A global anoxic event , which has been termed the Hirnantian ocean anoxic event (HOAE). Corresponding to widespread anoxia are δ S CAS , δ Mo, δ U, and εNd(t) excursions found in many different regions. At least in European sections, late Hirnantian anoxic waters were originally ferruginous (dominated by ferrous iron) before gradually becoming more euxinic. In the Yangtze Sea, located on

450-434: A TPW of around ~50˚ occurring at a maximum speed of ~55 cm per year, which better explains the rapid motion of the continents than conventional plate tectonics. Ocean heat transport is a major driver in the warming of the poles, taking warm water from the equator and distributing it to higher latitudes. A weakening of this heat transport may have allowed the poles to cool enough to form ice under high CO 2 conditions. Due to

600-489: A biological process which depletes nitrates. The Nitrogen-fixing ability of cyanobacteria would give them an edge over inflexible competitors like eukaryotic algae . At Anticosti Island , a uranium isotope excursion consistent with anoxia actually occurs prior to indicators of receding glaciation. This may suggest that the Hirnantian-Rhuddanian anoxic event (and its corresponding extinction) began during

750-412: A conflicting interpretation of this δC change as being caused by increased weathering of carbonate platforms exposed by sea level fall. This enhanced organic carbon burial resulted in a decrease in the atmospheric CO 2 levels and an inverse greenhouse effect, allowing glaciation to occur more readily. A gamma-ray burst (GRB) has been suggested by some researchers as the cause of the abrupt glaciation at

900-487: A distinctive band around the Earth instead of being randomly scattered, which may have come from debris falling to Earth from the ring. This ring may have lasted for nearly 40 million years. Although volcanic activity often leads to warming through the release of greenhouse gasses, it may also lead to cooling via the production of aerosols , light-blocking particles. There is good evidence for elevated volcanic activity through

1050-471: A finding bolstered by ∆ Hg measurements much higher than would be expected for volcanogenic mercury input. A 2023 paper points to the Deniliquin multiple-ring feature in southeastern Australia, which has been dated to around the start of LOMEI-1, for initiating the intense Hirnantian glaciation and the first pulse of the extinction event. According to the paper, it still requires further research to test

1200-411: A food web based on as-yet-undiscovered detritivores and grazers on micro-organisms. Millipedes from Cowie Formation such as Cowiedesmus and Pneumodesmus were considered as the oldest millipede from the middle Silurian at 428–430 million years ago, although the age of this formation is later reinterpreted to be from the early Devonian instead by some researchers. Regardless, Pneumodesmus

1350-594: A global greenhouse state. At the Katian-Hirnantian boundary, a sudden cooling event caused a rapid expansion of glaciers, resulting in one of the most severe glaciations of the Phanerozoic, an extreme cooling event generally believed to be coincident with the first pulse of the Late Ordovician mass extinction. An δO shift occurs at the start of the Hirnantian; the magnitude of this shift (+2-4‰)

1500-456: A global scale, euxinia was probably one or two orders of magnitude more prevalent than in the modern day. Global anoxia may have lasted more than 3 million years, persisting through the entire Rhuddanian stage of the Silurian period . This would make the Hirnantian-Rhuddanian anoxia one of the longest-lasting anoxic events in geologic time. The cause of the Hirnantian-Rhuddanian anoxic event

1650-519: A high degree of development in relation to the age of its fossil remains. Fossils of this plant have been recorded in Australia, Canada, and China. Eohostimella heathana is an early, probably terrestrial, "plant" known from compression fossils of Early Silurian (Llandovery) age. The chemistry of its fossils is similar to that of fossilised vascular plants, rather than algae. Fossils that are considered as terrestrial animals are also known from

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1800-567: A latitudinal extent of the subtropics and tropics similar to that of today, as evidenced by a steep faunal gradient that is uncharacteristic of greenhouse periods, suggesting that Earth was in a mild icehouse state by the start of the Sandbian, around 460 Ma. Many possible short glaciation occurred during the Katian: three very short glaciations during the early Katian, the Rakvere glaciation during

1950-504: A major factor. Explosive volcanic eruptions, which regularly send debris and volatiles into the stratosphere, would be even more effective at producing sulfate aerosols. Ash beds are common in the Late Ordovician, and Hirnantian pyrite records sulphur isotope anomalies consistent with stratospheric eruptions. The enormous megaeruption that formed the Deicke bentonite layer in particular has been linked to global cooling due to it coinciding with

2100-402: A major positive oxygen isotope excursion and the high sulphur concentration observed in its bentonite layer. One of the possible causes for the temperature drop during this period is a drop in sea level. Sea level must drop prior to the initiation of extensive ice sheets in order for it to be a possible trigger. A drop in sea level allows more land to become available for ice sheet growth. There

2250-404: A meaningful effect on nutrient cycles. Retreating glaciers could expose more land to weathering, which would be a more sustained source of phosphates flowing into the ocean. There is also evidence implicating volcanism as a contributor to Late Hirnantian anoxia. There were few clear patterns of extinction associated with the second extinction pulse. Every region and marine environment experienced

2400-423: A minor mass extinction and associated with rapid sea-level change. Each one leaves a similar signature in the geological record, both geochemically and biologically; pelagic (free-swimming) organisms were particularly hard hit, as were brachiopods , corals , and trilobites , and extinctions rarely occur in a rapid series of fast bursts. The climate fluctuations are best explained by a sequence of glaciations, but

2550-492: A more gradual replacement of the Hirnantia fauna after glaciation. Although this suggests that the second extinction pulse may have been a minor event at best, other paleontologists maintain that an abrupt ecological turnover accompanied the end of glaciation. There may be a correlation between the relatively slow recovery after the second extinction pulse, and the prolonged nature of the anoxic event which accompanied it. On

2700-459: A nearby gamma-ray burst ever happened. Though more commonly associated with greenhouse gases and global warming, volcanoes may have cooled the planet and precipitated glaciation by discharging sulphur into the atmosphere. This is supported by a positive uptick in pyritic Δ S values, a geochemical signal of volcanic sulphur discharge, coeval with LOMEI-1. More recently, in May 2020, a study suggested

2850-557: A negative carbon isotope excursion preserved in Baltican sediments. Toxic metals on the ocean floor may have dissolved into the water when the oceans' oxygen was depleted. An increase in available nutrients in the oceans may have been a factor, and decreased ocean circulation caused by global cooling may also have been a factor. Hg/TOC values from the Peri-Baltic region indicate noticeable spikes in mercury concentrations during

3000-418: A nutrient which would otherwise resupply S from the land. Pyrite forms most easily in anoxic and euxinic environments, while better oxygenation encourages the formation of gypsum instead. As a result, anoxia and euxinia would need to be common in the deep sea to produce enough pyrite to shift the δ S ratio. Thallium isotope ratios can also be used as indicators of anoxia. A major positive ε Tl excursion in

3150-548: A periodicity characteristic of Milankovitch cycles and have been interpreted as reflecting cyclic waxing and waning of polar ice caps. A speculated glaciation in the middle Darriwilian corresponds to the MDICE positive carbon isotope excursion. Sea level changes likely reflective of glacioeustasy are known from this geologic stage, around 467 Ma. However, there are no known Middle Ordovician glacial deposits that would provide direct geological evidence of glaciation. Isotopic evidence from

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3300-558: A planktonic lifestyle were more exposed to more UV radiation than groups that lived on the seabed. It is estimated that 20% to 60% of the total phytoplankton biomass on Earth would have been killed in such an event because the oceans were mostly oligotrophic and clear during the Late Ordovician. This is consistent with observations that planktonic organisms suffered severely during the first extinction pulse. In addition, species dwelling in shallow water were more likely to become extinct than species dwelling in deep water, also consistent with

3450-480: A positive feedback loop of inorganic carbon sequestration. A hypothetical large igneous province emplaced during the Katian whose existence is unproven has been speculated to have been the sink that absorbed carbon dioxide and precipitated Hirnantian cooling. Alternatively, volcanic activity may have caused the cooling by supplying sulphur aerosols to the atmosphere and generating severe volcanic winters that triggered

3600-459: A runaway ice-albedo positive feedback loop. In addition, volcanic fertilisation of the oceans with phosphorus may have increased populations of photosynthetic algae and enhanced biological sequestration of carbon dioxide from the atmosphere. Increased burial of organic carbon is another method of drawing down carbon dioxide from the air that may have played a role in the Late Ordovician. Other studies point to an asteroid strike and impact winter as

3750-463: A third of bryozoan genera went extinct, but most families survived the extinction interval and the group as a whole recovered in the Silurian. The hardest-hit subgroups were the cryptostomes and trepostomes , which never recovered the full extent of their Ordovician diversity. Bryozoan extinctions started in coastal regions of Laurentia, before high extinction rates shifted to Baltica by the end of

3900-500: A worldwide perspective these correspond to local events. Some Chinese sections record an early Hirnantian increase in the abundance of Mo-98, a heavy isotope of molybdenum . This shift can correspond to a balance between minor local anoxia and well-oxygenated waters on a global scale. Other trace elements point towards increased deep-sea oxygenation at the start of the glaciation. Oceanic current modelling suggest that glaciation would have encouraged oxygenation in most areas, apart from

4050-413: Is connected to many other mass extinctions throughout geological time. It may have also had a role in the first pulse of the Late Ordovician mass extinction, though support for this hypothesis is inconclusive and contradicts other evidence for high oxygen levels in seawater during the glaciation. Some geologists have argued that anoxia played a role in the first extinction pulse, though this hypothesis

4200-481: Is considered to have a lower surface elevation, and though it would be better for initiation during high CO 2 , it would have a harder time maintaining glacial coverage. From what we know about tectonic movement, the time span required to allow the southward movement of Gondwana toward the South Pole would have been too long to trigger this glaciation. Tectonic movement tends to take several million years, but

4350-488: Is controversial. In the early Hirnantian, shallow-water sediments throughout the world experience a large positive excursion in the δ S ratio of buried pyrite . This ratio indicates that shallow-water pyrite which formed at the beginning of the glaciation had a decreased proportion of S, a common lightweight isotope of sulfur . S in the seawater could hypothetically be used up by extensive deep-sea pyrite deposition. The Ordovician ocean also had very low levels of sulfate ,

4500-433: Is evidence that the Silurian icecaps were less extensive than those of the late-Ordovician glaciation. The southern continents remained united during this period. The melting of icecaps and glaciers contributed to a rise in sea level, recognizable from the fact that Silurian sediments overlie eroded Ordovician sediments, forming an unconformity . The continents of Avalonia , Baltica , and Laurentia drifted together near

4650-820: Is known as the Silurian-Devonian Terrestrial Revolution : vascular plants emerged from more primitive land plants, dikaryan fungi started expanding and diversifying along with glomeromycotan fungi, and three groups of arthropods ( myriapods , arachnids and hexapods ) became fully terrestrialized. Another significant evolutionary milestone during the Silurian was the diversification of jawed fish , which include placoderms , acanthodians (which gave rise to cartilaginous fish ) and osteichthyan ( bony fish , further divided into lobe-finned and ray-finned fishes )， although this corresponded to sharp decline of jawless fish such as conodonts and ostracoderms . The Silurian system

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4800-520: Is known from the Katian. Shifts in isotopic ratios of carbon and neodymium that correspond to graptolite biostratigraphy lend further evidence in favour of the existence of glacioeustatic cycles during the Katian, as do conodont apatite δO fluctuations from Kentucky and Quebec that likely reflect glacioeustatic sea level changes. However, the existence of glacials during the Katian remains controversial. Katian brachiopod and seawater δO values from Cincinnati Arch indicate ocean temperatures characteristic of

4950-492: Is more likely to produce glacial ice at high CO 2 concentrations, and the Ashgillian is more likely to produce glacial ice at low CO 2 concentrations. The height of the land mass above sea level also plays an important role, especially after ice sheets have been established. A higher elevation allows ice sheets to remain with more stability, but a lower elevation allows ice sheets to develop more readily. The Caradocian

5100-570: Is often considered to be the second-largest known extinction event just behind the end-Permian mass extinction , in terms of the percentage of genera that became extinct. Extinction was global during this interval, eliminating 49–60% of marine genera and nearly 85% of marine species. Under most tabulations, only the Permian-Triassic mass extinction exceeds the Late Ordovician mass extinction in biodiversity loss . The extinction event abruptly affected all major taxonomic groups and caused

5250-735: Is recorded in Late Ordovician strata (predominantly Ashgillian ) in West Africa (Tamadjert Formation of the Sahara), in Morocco ( Tindouf Basin ) and in west-central Saudi Arabia, all areas at polar latitudes at the time. From the Late Ordovician to the Early Silurian the centre of glaciation moved from northern Africa to southwestern South America." Continental glaciers developed in Africa and eastern Brazil, while alpine glaciers formed in

5400-522: Is recorded, however; clathrodictyids increased in abundance relative to labechiids. Sponges thrived and dominated marine ecosystems in South China immediately after the extinction event, colonising depauperate, anoxic environments in the earliest Rhuddanian. Their pervasiveness in marine environments after the biotic crisis has been attributed to drastically decreased competition and an abundance of vacant niches left behind by organisms that perished in

5550-561: Is still an important fossil as the oldest definitive evidence of spiracles to breath in the air. The first bony fish, the Osteichthyes , appeared, represented by the Acanthodians covered with bony scales. Fish reached considerable diversity and developed movable jaws , adapted from the supports of the front two or three gill arches. A diverse fauna of eurypterids (sea scorpions)—some of them several meters in length—prowled

5700-477: Is stored in marine sediments. This has often been linked to the Taconic Orogeny , a mountain-building event on the east coast of Laurentia (present-day North America). Another hypothesis is that a hypothetical large igneous province in the Katian led to basaltic flooding caused by high continental volcanic activity during that period. In the short term, this would have released a large amount of CO 2 into

5850-682: Is supported by glacial deposits and large land formations found in Ghat, Libya which is part of the Murzuq Basin . As the Ice sheets began to increase the weathering of silicate rocks and basaltic important to carbon sequestration (the silicates through the Carbonate–silicate cycle , the basalt through forming calcium carbonate ) decreased, which caused CO 2 levels to rise again, this in turned helped push deglaciation. This deglaciation cause

6000-433: Is uncertain. Like most global anoxic events, an increased supply of nutrients (such as nitrates and phosphates ) would encourage algal or microbial blooms that deplete oxygen levels in the seawater. The most likely culprits are cyanobacteria , which can use nitrogen fixation to produce usable nitrogen compounds in the absence of nitrates. Nitrogen isotopes during the anoxic event record high rates of denitrification ,

6150-516: Is wide debate on the timing of sea level change, but there is some evidence that a sea level drop started before the Ashgillian , which would have made it a contributing factor to glaciation. The possible setup of the paleogeography during the period from 460 Ma to 440 Ma falls in a range between the Caradocian and the Ashgillian. The choice of setup is important, because the Caradocian setup

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6300-597: The Cryogenian ice ages (720–630 Ma, the Sturtian and Marinoan glaciations ), often referred to as Snowball Earth , and followed by the Karoo Ice Age (350–260 Ma). One of the factors that hindered glaciation during the early Paleozoic was atmospheric CO 2 concentrations, which at the time were somewhere between 8 and 20 times pre-industrial levels. However, solar irradiance was significantly lower during

6450-674: The Hirnantian , the ice age is believed to have been significantly more extreme than the Last Glacial Maximum occurring during the terminal Pleistocene . Glaciation of the Northern Hemisphere was minimal because a large amount of the land was in the Southern Hemisphere . The earliest evidence for possible glaciation comes from Floian conodont apatite oxygen isotope fluctuations, which display

6600-400: The Late Ordovician mass extinction . This period is the only known glaciation to occur alongside of a mass extinction event. The extinction event consisted of two discrete pulses. The first pulse of extinctions is thought to have taken place because of the rapid cooling, and increased oxygenation of the water column. This first pulse was the larger of the two and caused the extinction of most of

6750-481: The Lilliput effect in response to the extinction, this phenomenon was not particularly widespread compared to other mass extinctions. Trilobites were hit hard by both phases of the extinction, with about 70% of genera and 50% of families going extinct between the Katian and Silurian. The extinction disproportionately affected deep water species and groups with fully planktonic larvae or adults. The order Agnostida

6900-662: The Paleo-Tethys ocean . Devastation of the Dicranograptidae-Diplograptidae-Orthograptidae (DDO) graptolite fauna, which was well adapted to anoxic conditions, further suggests that LOMEI-1 was associated with increased oxygenation of the water column and not the other way around. Deep-sea anoxia is not the only explanation for the δ S excursion of pyrite. Carbonate-associated sulfate maintains high S levels, indicating that seawater in general did not experience S depletion during

7050-490: The Phanerozoic Eon. 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. One important event in this period was the initial establishment of terrestrial life in what

7200-658: The Pridoli that marked the end of the Andean-Saharan glaciation saw further floral expansion. Late Ordovician mass extinction The Late Ordovician mass extinction ( LOME ), sometimes known as the end-Ordovician mass extinction or the Ordovician-Silurian extinction , is the first of the "big five" major mass extinction events in Earth's history , occurring roughly 445 million years ago (Ma). It

7350-580: The South Pole until they almost disappeared in the middle of Silurian. Layers of broken shells (called coquina ) provide strong evidence of a climate dominated by violent storms generated then as now by warm sea surfaces. The climate and carbon cycle appear to be rather unsettled during the Silurian, which had a higher frequency of isotopic excursions (indicative of climate fluctuations) than any other period. The Ireviken event , Mulde event , and Lau event each represent isotopic excursions following

7500-566: The equator , starting the formation of a second supercontinent known as Euramerica . When the proto-Europe collided with North America, the collision folded coastal sediments that had been accumulating since the Cambrian off the east coast of North America and the west coast of Europe. This event is the Caledonian orogeny , a spate of mountain building that stretched from New York State through conjoined Europe and Greenland to Norway. At

7650-537: The glaciation were responsible for much of the Late Ordovician extinction. First, the cooling global climate was probably especially detrimental because the biota were adapted to an intense greenhouse, especially because most shallow sea habitats in the Ordovician were located in the tropics. The southward shift of the polar front severely contracted the available latitudinal range of warm-adapted organisms. Second, sea level decline, caused by sequestering of water in

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7800-512: The Alborz LIP of northern Iran, as well as a warming phase known as the Boda event. However, other research still suggests the Boda event was a cooling event instead. Increased volcanic activity during the early late Katian and around the Katian-Hirnantian boundary is also implied by heightened mercury concentrations relative to total organic carbon. Marine bentonite layers associated with

7950-469: The Andean-Saharan glaciation. Following a peak in diversity in the late Darriwilian, chitinozoans declined in diversity as the Late Ordovician progressed. An exception to this declining trend of chitinozoan diversity was exhibited in Laurentia due to its low latitude position and warmer climate. The Late Ordovician Glaciation coincided with the second largest of the five major extinction events , known as

8100-466: The Andes. In western South America (Peru, Bolivia and northern Argentina) were found glacio-marine diamictites interbedded with turbidites, shales, mud flows and debris flows, dated as early Silurian (Llandonvery), with a southward extension into northern Argentina and western Paraguay, and with a probably northern extension into Peru, Ecuador and Colombia. A major ice age, the Andean-Saharan was preceded by

8250-470: The Arabian Peninsula. In all areas of North Africa where Early Silurian shale occurs, Late Ordovician glaciogenic deposits occur beneath, likely due to the anoxia promoted in these basins. At the end of the Hirnantian, an abrupt retreat of glaciers concurrent with the second pulse of the Late Ordovician mass extinction occurred, after which Earth receded back into a much warmer climate during

8400-540: The Earth over a short time interval, eliminating and altering the relative diversity and abundance of certain groups. The Cambrian-type evolutionary fauna nearly died out, and was unable to rediversify after the extinction. Brachiopod diversity and composition was strongly affected, with the Cambrian-type inarticulate brachiopods ( linguliforms and craniiforms ) never recovering their pre-extinction diversity. Articulate ( rhynchonelliform ) brachiopods, part of

8550-429: The Hirnantian, based on anomalously high concentrations of mercury (Hg) in many areas. Sulphur dioxide (SO 2 ) and other sulphurous volcanic gasses are converted into sulphate aerosols in the stratosphere , and short, periodic large igneous province eruptions may be able to account for cooling in this way. Although there is no direct evidence for a large igneous province during the Hirnantian, volcanism could still be

8700-551: The Hirnantian, so their unexpected extinction points towards the abrupt loss of their specific habitat. During the glaciation, a high-latitude brachiopod assemblage, the Hirnantia fauna, established itself along outer shelf environments in lower latitudes, probably in response to cooling. However, the Hirnantia fauna would meet its demise in the second extinction pulse, replaced by Silurian-style assemblages adapted for warmer waters. The brachiopod survival intervals following

8850-509: The Hirnantian. Bryozoan biodiversity loss appears to have been a prolonged process which partially preceded the Hirnantian extinction pulses. Extinction rates among Ordovician bryozoan genera were actually higher in the early and late Katian, and origination rates sharply dropped in the late Katian and Hirnantian. About 70% of crinoid genera died out. Early studies of crinoid biodiversity loss by Jack Sepkoski overestimated crinoid biodiversity losses during LOME. Most extinctions occurred in

9000-537: The L-chondrite parent body may have had a near-miss encounter with Earth, causing a part of it to break off from Earth's gravitational pull. This debris may have formed a planetary ring , and down-falling debris from the ring may have shaded Earth from the sun's rays and triggering significant cooling. Evidence for this comes from the fact that craters dating from the Ordovician meteor event appear to cluster in

9150-582: The Late Ordovician saw a loss of 85% of marine animal species and 26% of animal families. The deglaciation at the end of the Homerian glacial interval was coeval with the first major radiation of trilete spore-producing plants, harbingering the dawn of the Silurian-Devonian Terrestrial Revolution . The later middle Ludfordian glaciation caused a sea level drop that created vast areas of new terrestrial habitats that were promptly colonised by land plants, further facilitating their diversification. The warming during

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9300-431: The Late Ordovician; 450 million years ago, solar irradiance of Earth was about 1312.00 W m compared to 1360.89 W m in the present day. Furthermore, CO 2 concentrations are thought to have dropped significantly in the Hirnantian, which could have induced widespread glaciation during an overall cooling trend. Methods for the removal of CO 2 during this time were not well known, and are still hotly debated, with

9450-624: The Llandovery and Wenlock. Trilobites started to recover in the Rhuddanian, and they continued to enjoy success in the Silurian as they had in the Ordovician despite their reduction in clade diversity as a result of LOME. The Early Silurian was a chaotic time of turnover for crinoids as they rediversified after LOME. Members of Flexibilia, which were minimally impacted by LOME, took on an increasing ecological prominence in Silurian seas. Monobathrid camerates, like flexibles, diversified in

9600-490: The Llandovery, whereas cyathocrinids and dendrocrinids diversified later in the Silurian. Scyphocrinoid loboliths suddenly appeared in the terminal Silurian, shortly before the Silurian-Devonian boundary, and disappeared as abruptly as they appeared very shortly after their first appearance. Endobiotic symbionts were common in the corals and stromatoporoids. Rugose corals especially were colonised and encrusted by

9750-470: The Ordovician-Silurian boundary. Mercury anomalies at the end of the Ordovician relative to total organic carbon, or Hg/TOC, that some researchers have attributed to large-scale volcanism have been reinterpreted by some to be flawed because the main mercury host in the Ordovician was sulphide, and thus Hg/TS should be used instead; Hg/TS values show no evidence of volcanogenic mercury loading,

9900-627: The Paleozoic evolutionary fauna, were more variable in their response to the extinction. Some early rhynchonelliform groups, such as the Orthida and Strophomenida , declined significantly. Others, including the Pentamerida , Athyridida , Spiriferida , and Atrypida, were less affected and took the opportunity to diversify after the extinction. Additionally, brachiopods with higher abundance were more likely to survive. The extinction pulse at

10050-407: The Rhuddanian. Late Hirnantian warming was marked by a similarly meteoric shift in δO towards more negative values. δC values likewise fall sharply at the beginning of the Silurian . Following the relatively warm Rhuddanian, glacial events occurred during the early and latest Aeronian. A further glaciation occurred from the late Telychian to middle Sheinwoodian. From the early to late Homerian, Earth

10200-620: The Sandbian reveals three possible glaciations: an early Sandbian glaciation, a middle Sandbian glaciation, a late Sandbian glaciation. Although biostratigraphy dating the glacial deposits in Gondwana has been problematic, there is evidence suggesting the presence of glaciation by the Sandbian stage (approximately 451–461 Ma). Graptolite distribution during the time interval delineated by the Nemacanthus gracilis graptolite biozone indicates

10350-522: The Silurian System and the lands now thought to have been inhabited in antiquity by the Silures show little correlation ( cf . Geologic map of Wales , Map of pre-Roman tribes of Wales ), Murchison conjectured that their territory included Caer Caradoc and Wenlock Edge exposures - and that if it did not there were plenty of Silurian rocks elsewhere 'to sanction the name proposed'. In 1835

10500-431: The Silurian rocks of Bohemia into eight stages. His interpretation was questioned in 1854 by Edward Forbes , and the later stages of Barrande; F, G and H have since been shown to be Devonian. Despite these modifications in the original groupings of the strata, it is recognized that Barrande established Bohemia as a classic ground for the study of the earliest Silurian fossils. With the supercontinent Gondwana covering

10650-518: The Silurian. The definitive oldest record of millipede ever known is Kampecaris obanensis and Archidesmus sp. from the late Silurian (425 million years ago) of Kerrera . There are also other millipedes, centipedes , and trigonotarbid arachnoids known from Ludlow (420 million years ago). Predatory invertebrates would indicate that simple food webs were in place that included non-predatory prey animals. Extrapolating back from Early Devonian biota, Andrew Jeram et al. in 1990 suggested

10800-650: The Tethys, the Proto-Tethys and Paleo-Tethys , the Rheic Ocean , the Iapetus Ocean (a narrow seaway between Avalonia and Laurentia), and the newly formed Ural Ocean . The Silurian period was once believed to have enjoyed relatively stable and warm temperatures, in contrast with the extreme glaciations of the Ordovician before it and the extreme heat of the ensuing Devonian; however, it is now known that

10950-457: The atmosphere, which may explain a warming pulse in the Katian. However, in the long term flood basalts would have left behind plains of basaltic rock, replacing exposures of granitic rock. Basaltic rocks weather substantially faster than granitic rocks, which would quickly remove CO 2 from the atmosphere at a much faster rate than before the volcanic activity. CO 2 levels could also have decreased due to accelerated silicate weathering caused by

11100-421: The atmosphere, which would have occurred as a result of the nitrogen dioxide reacting with hydroxyl and raining back down to Earth as nitric acid , precipitating large quantities of nitrates that would have enhanced wetland productivity and sequestration of carbon dioxide. Although the gamma-ray burst hypothesis is consistent with some patterns at the onset of extinction, there is no unambiguous evidence that such

11250-566: The atmosphere. Additionally, the GRB would have initiated a major depletion of ozone , another potent greenhouse gas, through its reaction with nitric oxide produced as a result of the GRB's dissociation of diatomic nitrogen and subsequent reaction of nitrogen atoms with oxygen. The breakup of the L-chondrite parent body caused a rain of extraterrestrial material onto the Earth called the Ordovician meteor event . This event increased stratospheric dust by 3 or 4 orders of magnitude and may have triggered

11400-455: The average size of many organisms, likely attributable to the Lilliput effect , and the disappearance of many relict taxa from the Ordovician indicate a third extinction interval linked to an expansion of anoxic conditions into shallower shelf environments, particularly in Baltica. This sharp decline in dissolved oxygen concentrations was likely linked to a period of global warming documented by

11550-491: The beginning of the Hirnantian and shifted the Earth from a greenhouse to icehouse climate . Cooling and a falling sea level brought on by the glaciation led to habitat loss for many organisms along the continental shelves , especially endemic taxa with restricted temperature tolerance and latitudinal range. During this extinction pulse, there were also several marked changes in biologically responsive carbon and oxygen isotopes . Marine life partially rediversified during

11700-427: The beginning of the Hirnantian. The effects of a ten second GRB occurring within two kiloparsecs of Earth would have delivered it a fluence of 100 kilojoules per square metre. This would have resulted in large amounts of nitric acid raining down on Earth's surface in the aftermath of the gamma-ray burst, causing blooms of nitrate-limited photosynthesisers that would have sequestered large amounts of carbon dioxide from

11850-574: The catastrophe. Sponges may have assisted the recovery of other sessile suspension feeders: by helping stabilise sediment surfaces, they enabled bryozoans, brachiopods, and corals to recolonise the seafloor. The first pulse of the Late Ordovician Extinction has typically been attributed to the Late Ordovician Glaciation , which is unusual among mass extinctions and has made LOME an outlier. Although there

12000-419: The cold period and a new cold-water ecosystem, the " Hirnantia fauna", was established. The second pulse (interval) of extinction, referred to as LOMEI-2, occurred in the later half of the Hirnantian as the glaciation abruptly receded and warm conditions returned. The second pulse was associated with intense worldwide anoxia (oxygen depletion) and euxinia (toxic sulfide production), which persisted into

12150-894: The comparative abundance of highly reactive iron compounds which are only stable without oxygen. Most geological sections corresponding to the beginning of the Hirnantian glaciation have Fe HR /Fe T below 0.38, indicating oxygenated waters. However, higher Fe HR /Fe T values are known from a few deep-water early Hirnantian sequences found in China and Nevada . Elevated Fe Py /Fe HR values have also been found in association with LOMEI-1, including ones above 0.8 that are tell-tale indicators of euxinia. Glaciation could conceivably trigger anoxic conditions, albeit indirectly. If continental shelves are exposed by falling sea levels, then organic surface runoff flows into deeper oceanic basins. The organic matter would have more time to leach out phosphate and other nutrients before being deposited on

12300-400: The course of LOMEI-1. Also, the weathering of nutrient-rich volcanic rocks emplaced during the middle and late Katian likely enhanced the reduction in dissolved oxygen. Intense volcanism also fits in well with the attribution of euxinia as the main driver of LOMEI-2; sudden volcanism at the Ordovician-Silurian boundary is suggested to have supplied abundant sulphur dioxide, greatly facilitating

12450-412: The course of the biotic recovery. The region around what is today Oslo was a hotbed of atrypide rediversification. Brachiopod recovery consisted mainly of the reestablishment of cosmopolitan brachiopod taxa from the Late Ordovician. Progenitor taxa that arose following the mass extinction displayed numerous novel adaptations for resisting environmental stresses. Although some brachiopods did experience

12600-440: The culprit for the glaciation. True polar wander and the associated rapid palaeogeographic changes have also been proposed as a cause. Other studies have even suggested that shading of the sun's rays by a temporary planetary ring formed from the partial breakup of a large meteor in the atmosphere may have caused the glaciation, which would also link it to the Ordovician meteor event . Two environmental changes associated with

12750-495: The development of euxinia. Other papers have criticised the volcanism hypothesis, claiming that volcanic activity was relatively low in the Ordovician and that superplume and LIP volcanic activity is especially unlikely to have caused the mass extinction at the end of the Ordovician. A 2022 study argued against a volcanic cause of LOME, citing the lack of mercury anomalies and the discordance between deposition of bentonites and redox changes in drillcores from South China straddling

12900-469: The disappearance of one third of all brachiopod and bryozoan families, as well as numerous groups of conodonts , trilobites , echinoderms , corals , bivalves , and graptolites . Despite its taxonomic severity, the Late Ordovician mass extinction did not produce major changes to ecosystem structures compared to other mass extinctions, nor did it lead to any particular morphological innovations. Diversity gradually recovered to pre-extinction levels over

13050-488: The earth's precession, and eccentricity, could have set the off the tipping point for initiation of glaciation. The Orbit at this time is thought to have been in a cold summer orbit for the Southern Hemisphere. This type of orbital configuration is a change in the orbital precession such that during the summer when the hemisphere is tilted toward the sun (in this case the earth) the earth is furthest away from

13200-408: The edge of the continental shelf) can be identified, and the highest Silurian sea level was probably around 140 metres (459 ft) higher than the lowest level reached. During this period, the Earth entered a warm greenhouse phase, supported by high CO 2 levels of 4500 ppm, and warm shallow seas covered much of the equatorial land masses. Early in the Silurian, glaciers retreated back into

13350-466: The end of the Katian stage and the start of the Hirnantian stage. The second pulse of extinction occurred in the later part of the Hirnantian stage, coinciding with the Metabolograptus persculptus zone. Each extinction pulse affected different groups of animals and was followed by a rediversification event. Statistical analysis of marine losses at this time suggests that the decrease in diversity

13500-422: The end of the Katian was selective in its effects, disproportionally affecting deep-water species and tropical endemics inhabiting epicontinental seas . The Foliomena fauna, an assemblage of thin-shelled species adapted for deep dysoxic (low oxygen) waters, went extinct completely in the first extinction pulse. The Foliomena fauna was formerly widespread and resistant to background extinction rates prior to

13650-461: The end of the Silurian, sea levels dropped again, leaving telltale basins of evaporites extending from Michigan to West Virginia, and the new mountain ranges were rapidly eroded. The Teays River , flowing into the shallow mid-continental sea, eroded Ordovician Period strata, forming deposits of Silurian strata in northern Ohio and Indiana. The vast ocean of Panthalassa covered most of the northern hemisphere. Other minor oceans include two phases of

13800-411: The equator and much of the southern hemisphere, a large ocean occupied most of the northern half of the globe. The high sea levels of the Silurian and the relatively flat land (with few significant mountain belts) resulted in a number of island chains, and thus a rich diversity of environmental settings. During the Silurian, Gondwana continued a slow southward drift to high southern latitudes, but there

13950-551: The expansion of terrestrial non-vascular plants. Vascular plants only appeared 15 Ma after the glaciation. Isotopic evidence points to a global Hirnantian positive shift in δC at nearly the same time as the positive shift in marine carbonate δO. This shift is known as the Hirnantian Isotopic Carbon Excursion (HICE). The positive shift in δC implies a change in the carbon cycle leading to more burial of organic carbon, though some researchers hold

14100-479: The first 5 million years of the Silurian period . The Late Ordovician mass extinction is traditionally considered to occur in two distinct pulses. The first pulse (interval), known as LOMEI-1, began at the boundary between the Katian and Hirnantian stages of the Late Ordovician epoch . This extinction pulse is typically attributed to the Late Ordovician glaciation , which abruptly expanded over Gondwana at

14250-509: The first deep-boring bivalves are known from this period. Chitons saw a peak in diversity during the middle of the Silurian. Hederelloids enjoyed significant success in the Silurian, with some developing symbioses with the colonial rugose coral Entelophyllum . The Silurian was a heyday for tentaculitoids , which experienced an evolutionary radiation focused mainly in Baltoscandia, along with an expansion of their geographic range in

14400-411: The first extinction pulse is controversial and not widely accepted. The late Hirnantian experienced a dramatic increase in the abundance of black shales. Coinciding with the retreat of the Hirnantian glaciation, black shale expands out of isolated basins to become the dominant oceanic sediment at all latitudes and depths. The worldwide distribution of black shales in the late Hirnantian is indicative of

14550-643: The first period to see megafossils of extensive terrestrial biota in the form of moss -like miniature forests along lakes and streams and networks of large, mycorrhizal nematophytes , heralding the beginning of the Silurian-Devonian Terrestrial Revolution. However, the land fauna did not have a major impact on the Earth until it diversified in the Devonian. The first fossil records of vascular plants , that is, land plants with tissues that carry water and food, appeared in

14700-415: The first pulse began not during the rapid Hirnantian ice cap expansion but in an interval of deglaciation following it. Another heavily-discussed factor in the Late Ordovician mass extinction is anoxia , the absence of dissolved oxygen in seawater. Anoxia not only deprives most life forms of a vital component of respiration , it also encourages the formation of toxic metal ions and other compounds. One of

14850-466: The first pulse of mass extinction was caused by volcanism which induced global warming and anoxia, rather than cooling and glaciation. Higher resolution of species diversity patterns in the Late Ordovician suggest that extinction rates rose significantly in the early or middle Katian stage, several million years earlier than the Hirnantian glaciation. This early phase of extinction is associated with large igneous province (LIP) activity, possibly that of

15000-477: The first pulse. However, data supporting deep-water anoxia during the glaciation contrasts with more extensive evidence for well-oxygenated waters. Black shales , which are indicative of an anoxic environment, become very rare in the early Hirnantian compared to surrounding time periods. Although early Hirnantian black shales can be found in a few isolated ocean basins (such as the Yangtze platform of China), from

15150-425: The first pulse. However, they rediversified quickly in tropical areas and reacquired their pre-extinction diversity not long into the Silurian. Many other echinoderms became very rare after the Ordovician, such as the cystoids , edrioasteroids , and other early crinoid-like groups. Stromatoporoid generic and familial taxonomic diversity was not significantly impacted by the mass extinction. A change in abundance

15300-494: The friendship. The English geologist Charles Lapworth resolved the conflict by defining a new Ordovician system including the contested beds. An alternative name for the Silurian was "Gotlandian" after the strata of the Baltic island of Gotland . The French geologist Joachim Barrande , building on Murchison's work, used the term Silurian in a more comprehensive sense than was justified by subsequent knowledge. He divided

15450-516: The glaciation, not after it. Cool temperatures can lead to upwelling , cycling nutrients into productive surface waters via air and ocean cycles. Upwelling could instead be encouraged by increasing oceanic stratification through an input of freshwater from melting glaciers. This would be more reasonable if the anoxic event coincided with the end of glaciation, as supported by most other studies. However, oceanic models argue that marine currents would recover too quickly for freshwater disruptions to have

15600-572: The glaciation. Even if pyrite burial did increase at that time, its chemical effects would have been far too slow to explain the rapid excursion or extinction pulse. Instead, cooling may lower the metabolism of warm-water aerobic bacteria, reducing decomposition of organic matter. Fresh organic matter would eventually sink down and supply nutrients to sulfate-reducing microbes living in the seabed. Sulfate-reducing microbes prioritize S during anaerobic respiration , leaving behind heavier isotopes. A bloom of sulfate-reducing microbes can quickly account for

15750-451: The global climate underwent many drastic fluctuations throughout the Silurian, evidenced by numerous major carbon and oxygen isotope excursions during this geologic period. Sea levels rose from their Hirnantian low throughout the first half of the Silurian; they subsequently fell throughout the rest of the period, although smaller scale patterns are superimposed on this general trend; fifteen high-stands (periods when sea levels were above

15900-538: The greenhouse effect and promoting the transition of the climatic system to the glacial mode. Heavy silicate weathering of the uplifting Appalachians and Caledonides occurred during the Late Ordovician, which sequestered CO 2 . In the Hirnantian stage the volcanism diminished, and the continued weathering caused a significant and rapid draw down of CO 2 coincident with the rapid and short ice age. As Earth cooled and sea levels dropped, highly weatherable carbonate platforms became exposed above water, enkindling

16050-469: The hypothetical effects of a galactic gamma-ray burst. A gamma-ray burst could also explain the rapid expansion of glaciers, since the high energy rays would cause ozone , a greenhouse gas , to dissociate and its dissociated oxygen atoms to then react with nitrogen to form nitrogen dioxide , a darkly-coloured aerosol which cools the planet. It would also cohere with the major δ13C isotopic excursion indicating increased sequestration of carbon-12 out of

16200-405: The ice age by reflecting sunlight back into space. A 2023 paper has proposed that the Hirnantian glaciation could have come about due to an impact winter generated by the impact that formed the Deniliquin multiple-ring feature in what is now southeastern Australia, although this hypothesis currently remains untested. A 2024 study suggests that rather than a complete breakup or outright impact,

16350-599: The ice cap, drained the vast epicontinental seaways and eliminated the habitat of many endemic communities. The dispersed positions of the continents, in contrast to their position during the much less extinction-inducing Pleistocene glaciations, made glacioeustatic marine regression especially hazardous to marine life. Falling sea levels may have acted as a positive feedback loop accelerating further cooling; as shallow seas receded, carbonate-shelf production declined and atmospheric carbon dioxide levels correspondingly decreased, fostering even more cooling. Ice caps formed on

16500-411: The ice. The glaciotectonic fold and thrust belt eventually led to ice sheet collapse and retreat of the ice to south of Ghat. Once stabilized south of Ghat, the ice sheet began advancing north again. This cycle slowly shrank more south each time which lead to further retreat and further collapse of glacial conditions. This recursion allowed the melting of the ice sheet, and rising sea level. This hypothesis

16650-618: The idea. Silurian The Silurian ( / s ɪ ˈ lj ʊər i . ən , s aɪ -/ sih- LURE -ee-ən, sy- ) 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 third and shortest period of the Paleozoic Era, and the third of twelve periods of

16800-557: The initial extinctions could have been caused by a gamma-ray burst originating from a hypernova in a nearby arm of the Milky Way galaxy , within 6,000 light-years of Earth. A ten-second burst would have stripped the Earth's atmosphere of half of its ozone almost immediately, exposing surface-dwelling organisms, including those responsible for planetary photosynthesis , to high levels of extreme ultraviolet radiation. Under this hypothesis, several groups of marine organisms with

16950-400: The lack of tillites in the middle to late Silurian make this explanation problematic. The Silurian period has been viewed by some palaeontologists as an extended recovery interval following the Late Ordovician mass extinction (LOME), which interrupted the cascading increase in biodiversity that had continuously gone on throughout the Cambrian and most of the Ordovician. The Silurian was

17100-399: The late Katian, just before the Katian-Hirnantian boundary, likely reflects a global enlargement of oxygen minimum zones. During the late Katian, thallium isotopic perturbations indicating proliferation of anoxic waters notably preceded the appearance of other geochemical indicators of the expansion of anoxia. A more direct proxy for anoxic conditions is Fe HR /Fe T . This ratio describes

17250-528: The late early Katian, a middle Katian glaciation, the Early Ashgill glaciation of the early late Katian, and a latest Katian glaciation that was followed by a rapid warming event in the Paraorthograptus pacificus graptolite biozone immediately before the Hirnantian glaciation itself. Evidence of major changes in bottom water formation, which usually indicates a sudden change in global climate,

17400-453: The lower late Katian, the Katian-Hirnantian boundary, and the late Hirnantian. The toxic metals may have killed life forms in lower trophic levels of the food chain , causing a decline in population, and subsequently resulting in starvation for the dependent higher feeding life forms in the chain. A minority hypothesis to explain the first burst has been proposed by Philip Ball, Adrian Lewis Melott , and Brian C. Thomas, suggesting that

17550-422: The marine animal species that existed in the shallow and deep oceans. The second phase of extinction was associated with strong sea level rise , and due to the atmospheric conditions, namely oxygen levels being at or below 50% of present-day levels, high levels of anoxic waters would have been common. This anoxia would have killed off many of the survivors of the first extinction pulse. In all the extinction event of

17700-561: The mass extinction's aftermath, but expanded their range afterwards. The most abundant brachiopods were atrypids and pentamerides; atrypids were the first to recover and rediversify in the Rhuddanian after LOME, while pentameride recovery was delayed until the Aeronian. Bryozoans exhibited significant degrees of endemism to a particular shelf. They also developed symbiotic relationships with cnidarians and stromatolites. Many bivalve fossils have also been found in Silurian deposits, and

17850-416: The most common of these poisonous chemicals is hydrogen sulfide , a biological waste product and major component of the sulfur cycle . Oxygen depletion when combined with high levels of sulfide is called euxinia . Though less toxic, ferrous iron (Fe ) is another substance which commonly forms in anoxic waters. Anoxia is the most common culprit for the second pulse of the Late Ordovician mass extinction and

18000-551: The most cosmopolitan in the Phanerozoic, biogeographic patterns that persisted throughout most of the Silurian. LOME had few of the long-term ecological impacts associated with the Permian–Triassic and Cretaceous–Paleogene extinction events. Furthermore, biotic recovery from LOME proceeded at a much faster rate than it did after the Permian-Triassic extinction. Nevertheless, a large number of taxa disappeared from

18150-680: The next pulse of glaciation, eliminating biological diversity at each change. In the North African strata, five pulses of glaciation from seismic sections are recorded. In the Yangtze Platform, a relict warm-water fauna continued to persist because South China blocked the transport of cold waters from Gondwanan waters at higher latitudes. This incurred a shift in the location of bottom water formation, shifting from low latitudes , characteristic of greenhouse conditions, to high latitudes, characteristic of icehouse conditions, which

18300-465: The other hand, the occurrence of euxinic pulses similar in magnitude to LOMEI-2 during the Katian without ensuing biological collapses has caused some researchers to question whether euxinia alone could have been LOMEI-2's driver. Deposition of black graptolite shales continued to be common into the earliest Rhuddanian, indicating that anoxia persisted well into the Llandovery . A sharp reduction in

18450-415: The paleogeographic configuration of the continents, global ocean heat transport is thought to have been stronger in the Late Ordovician. However, research shows that in order for glaciation to occur, poleward heat transport had to be lower, which creates a discrepancy in what is known. Orbital parameters may have acted in conjunction with some of the above parameters to help start glaciation. The variation of

18600-451: The radiation of terrestrial plants, enhanced oceanic organic carbon burial, and a reduction in volcanic outgassing of carbon dioxide having been proposed. It could have been possible for glaciation to initiate with high levels of CO 2 , but it would have depended highly on continental configuration. Long-term silicate weathering is a major mechanism through which CO 2 is removed from the atmosphere, converting it into bicarbonate which

18750-485: The relatively warm climate conditions of a greenhouse earth . The cause of the glaciation is heavily debated. The late Ordovician glaciation was preceded by a fall in atmospheric carbon dioxide (from 7,000 ppm to 4,400 ppm). Atmospheric and oceanic CO 2 levels may have fluctuated with the growth and decay of Gondwanan glaciation. The appearance and development of terrestrial plants and microphytoplankton, which consumed atmospheric carbon dioxide, may have diminished

18900-477: The scale of the glaciation seems to have occurred in less than 1 million years, but the exact time frame of glaciation ranges from less than 1 million years to 35 million years, so it could still be possible for tectonic movement to have triggered this glacial period. Alternatively, true polar wander (TPW) and not conventional plate motion may have been responsible for the initiation of the Hirnantian glaciation. Palaeomagnetic data from between 450 and 440 Ma indicates

19050-498: The seabed. Increased phosphate concentration in the seawater would lead to eutrophication and then anoxia. Deep-water anoxia and euxinia would impact deep-water benthic fauna, as expected for the first pulse of extinction. Chemical cycle disturbances would also steepen the chemocline , restricting the habitable zone of planktonic fauna which also go extinct in the first pulse. This scenario is congruent with both organic carbon isotope excursions and general extinction patterns observed in

19200-416: The second extinction pulse to some extent. Many taxa which survived or diversified after the first pulse were finished off in the second pulse. These include the Hirnantia brachiopod fauna and Mucronaspis trilobite fauna, which previously thrived in the cold glacial period. Other taxa such as graptolites and warm-water reef denizens were less affected. Sediments from China and Baltica seemingly show

19350-495: The second half of the Silurian Period. The earliest-known representatives of this group are Cooksonia . Most of the sediments containing Cooksonia are marine in nature. Preferred habitats were likely along rivers and streams. Baragwanathia appears to be almost as old, dating to the early Ludlow (420 million years) and has branching stems and needle-like leaves of 10–20 centimetres (3.9–7.9 in). The plant shows

19500-425: The second pulse spanned the terminal Hirnantian to the middle Rhuddanian, after which the recovery interval began and lasted until the early Aeronian. Overall, the brachiopod recovery in the late Rhuddanian was rapid. Brachiopod survivors of the mass extinction tended to be endemic to one palaeoplate or even one locality in the survival interval in the earliest Silurian, though their ranges geographically expanded over

19650-516: The shallow Silurian seas and lakes of North America; many of their fossils have been found in New York state . Brachiopods were abundant and diverse, with the taxonomic composition, ecology, and biodiversity of Silurian brachiopods mirroring Ordovician ones. Brachiopods that survived the LOME developed novel adaptations for environmental stress, and they tended to be endemic to a single palaeoplate in

19800-605: The southern supercontinent Gondwana as it drifted over the South Pole . Correlating rock strata have been detected in Late Ordovician rock strata of North Africa and then-adjacent northeastern South America, which were south-polar locations at the time. Glaciation locks up water from the world-ocean and interglacials free it, causing sea levels repeatedly to drop and rise ; the vast, shallow Ordovician seas withdrew, which eliminated many ecological niches , then returned, carrying diminished founder populations lacking many whole families of organisms. Then they withdrew again with

19950-623: The subduction of the Junggar Ocean underneath the Yili Block have been dated to the late Katian, close to the Katian-Hirnantian boundary. Volcanic activity could also provide a plausible explanation for anoxia during the first pulse of the mass extinction. A volcanic input of phosphorus, which was insufficient to enkindle persistent anoxia on its own, may have triggered a positive feedback loop of phosphorus recycling from marine sediments, sustaining widespread marine oxygen depletion over

20100-509: The subsequent Rhuddanian stage of the Silurian Period . Some researchers have proposed the existence of a third distinct pulse of the mass extinction during the early Rhuddanian, evidenced by a negative carbon isotope excursion and a pulse of anoxia into shelf environments amidst already low background oxygen levels. Others, however, have argued that Rhuddanian anoxia was simply part of the second pulse, which according to this view

20250-547: The subsequent Silurian period, with the last glacial phase occurring in the Late Silurian. One of the possible causes for the end of the Hirnantian glaciation is that during the glacial maximum, the ice reached out too far and began collapsing on itself. The ice sheet initially stabilized once it reached as far north as Ghat, Libya and developed a large proglacial fan-delta system. A glaciotectonic fold and thrust belt began to form from repeated small-scale fluctuations in

20400-480: The summer. The cause for the end of the Late Ordovician Glaciation is a matter of intense research, but evidence shows that the deglaciation in the terminal Hirnantian may have occurred abruptly, as Silurian strata marks a significant change from the glacial deposits left during the Late Ordovician. Though the Hirnantian glaciation ended rapidly, milder glaciations continued to occur throughout

20550-441: The sun, and orbital eccentricity such that the orbit of the earth is more elongated which would enhance the effect of precession. Coupled models have shown that in order to maintain ice at the pole in the Southern Hemisphere, the earth would have to be in a cold summer configuration. The glaciation was most likely to start during a cold summer period because this configuration enhances the chance of snow and ice surviving throughout

20700-460: The time of the extinction, around 100 marine families became extinct, covering about 49% of genera (a more reliable estimate than species). The brachiopods and bryozoans were strongly impacted, along with many of the trilobite , conodont and graptolite families. The extinction was divided into two major extinction pulses. The first pulse occurred at the base of the global Metabolograptus extraordinarius graptolite biozone , which marks

20850-408: The transformation of silicates exposed to the air (thus given the opportunity to bind to its CO 2 ) and weathering of basaltic rock to start back up which caused glaciation to occur again. Even before the mass extinction at the end of the Ordovician, which resulted in a significant drop in chitinozoan diversity and abundance, the biodiversity of chitinozoans was adversely impacted by the onset of

21000-631: The two men presented a joint paper, under the title On the Silurian and Cambrian Systems, Exhibiting the Order in which the Older Sedimentary Strata Succeed each other in England and Wales, which was the germ of the modern geological time scale . As it was first identified, the "Silurian" series when traced farther afield quickly came to overlap Sedgwick's "Cambrian" sequence, however, provoking furious disagreements that ended

21150-417: The western margins of the South China microcontinent, the second extinction pulse occurred alongside intense euxinia which spread out from the middle of the continental shelf. Mercury loading in South China during LOMEI-2 was likely related to euxinia. However, some evidence suggests that the top of the water column in the Ordovician oceans remained well oxygenated even as the seafloor became deoxygenated. On

21300-418: The δ S excursion in marine sediments without a corresponding decrease in oxygen. A few studies have proposed that the first extinction pulse did not begin with the Hirnantian glaciation, but instead corresponds to an interglacial period or other warming event. Anoxia would be the most likely mechanism of extinction in a warming event, as evidenced by other extinctions involving warming. However, this view of

21450-549: Was a longer cooling trend in Middle and Lower Ordovician, the most severe and abrupt period of glaciation occurred in the Hirnantian stage, which was bracketed by both pulses of the extinction. The rapid continental glaciation was centered on Gondwana , which was located at the South Pole in the Late Ordovician. The Hirnantian glaciation is considered one of the most severe ice ages of the Paleozoic , which previously maintained

21600-491: Was accompanied by increased deep-ocean currents and oxygenation of the bottom water. An opportunistic fauna briefly thrived there, before anoxic conditions returned. The breakdown in the oceanic circulation patterns brought up nutrients from the abyssal waters. Surviving species were those that coped with the changed conditions and filled the ecological niches left by the extinctions. However, not all studies agree that cooling and glaciation caused LOMEI-1. One study suggests that

21750-497: Was completely wiped out, and the formerly diverse Asaphida survived with only a single genus, Raphiophorus . A cool-water trilobite assemblage, the Mucronaspis fauna, coincides with the Hirnantia brachiopod fauna in the timing of its expansion and demise. Trilobite faunas after the extinction were dominated by families that appeared in the Ordovician and survived LOME, such as Encrinuridae and Odontopleuridae . Over

21900-524: Was extraordinary. Its direction implies glacial cooling and possibly increases in ice-volume. The observed shifts in the δO isotopic indicator would require a sea-level fall of 100 meters and a drop of 10 °C in tropical ocean temperatures to have occurred during this glacial episode. Sedimentological data shows that Late Ordovician ice sheets glacierized the Al Kufrah Basin . Ice sheets also probably formed continuous ice cover over North Africa and

22050-635: Was first identified by the Scottish geologist Roderick Murchison , who was examining fossil-bearing sedimentary rock strata in south Wales in the early 1830s. He named the sequences for a Celtic tribe of Wales, the Silures , inspired by his friend Adam Sedgwick , who had named the period of his study the Cambrian , from a Latin name for Wales. Whilst the British rocks now identified as belonging to

22200-682: Was in yet another glacial phase. The last major glaciation of the EPIA occurred during the Ludfordian and was associated with the Lau event. During this period, glaciation is known from Arabia, Sahara, West Africa, the south Amazon, and the Andes, and the centre of glaciation is known to have migrated from the Sahara in the Ordovician (450–440 Ma) to South America in the Silurian (440–420 Ma). According to Eyles and Young, "A major glacial episode at c. 440 Ma ,

22350-547: Was longer and more drawn out than most authors suggest. The Late Ordovician mass extinction followed the Great Ordovician Biodiversification Event (GOBE), one of the largest surges of increasing biodiversity in the geological and biological history of the Earth. At the time of the extinction, most complex multicellular organisms lived in the sea, and the only evidence of life on land are rare spores from small early land plants . At

22500-437: Was mainly caused by a sharp increase in extinctions, rather than a decrease in speciation . Following such a major loss of diversity, Silurian communities were initially less complex and broader niched. Nonetheless, in South China, warm-water benthic communities with complex trophic webs thrived immediately following LOME. Highly endemic faunas, which characterized the Late Ordovician, were replaced by faunas that were amongst

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