Silurian | |||||||||||||
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![]() A map of Earth as it appeared 430 million years ago during the Silurian Period, Wenlock Epoch | |||||||||||||
Chronology | |||||||||||||
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Etymology | |||||||||||||
Name formality | Formal | ||||||||||||
Synonym(s) | Gotlandian | ||||||||||||
Usage information | |||||||||||||
Celestial body | Earth | ||||||||||||
Regional usage | Global (ICS) | ||||||||||||
Time scale(s) used | ICS Time Scale | ||||||||||||
Definition | |||||||||||||
Chronological unit | Period | ||||||||||||
Stratigraphic unit | System | ||||||||||||
First proposed by | Roderick Murchison, 1835 | ||||||||||||
Time span formality | Formal | ||||||||||||
Lower boundary definition | FAD of the Graptolite Akidograptus ascensus | ||||||||||||
Lower boundary GSSP | Dob's Linn, Moffat, United Kingdom 55°26′24″N3°16′12″W / 55.4400°N 3.2700°W | ||||||||||||
Lower GSSP ratified | 1984 [4] [5] | ||||||||||||
Upper boundary definition | FAD of the Graptolite Monograptus uniformis | ||||||||||||
Upper boundary GSSP | Klonk, Czech Republic 49°51′18″N13°47′31″E / 49.8550°N 13.7920°E | ||||||||||||
Upper GSSP ratified | 1972 [6] | ||||||||||||
Atmospheric and climatic data | |||||||||||||
Sea level above present day | Around 180 m, with short-term negative excursions [7] |
The Silurian ( /sɪˈljʊəri.ən,saɪ-/ sih-LURE-ee-ən, sy-) [8] [9] [10] 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. [11] The Silurian is the third and shortest period of the Paleozoic Era, and the third of twelve periods of 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 is known as the Silurian-Devonian Terrestrial Revolution: vascular plants emerged from more primitive land plants, [12] [13] dikaryan fungi started expanding and diversifying along with glomeromycotan fungi, [14] and three groups of arthropods (myriapods, arachnids and hexapods) became fully terrestrialized. [15]
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), [16] although this corresponded to sharp decline of jawless fish such as conodonts and ostracoderms.
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The Silurian system 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. [17] Whilst the British rocks now identified as belonging to 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'. [18] In 1835 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. [19] 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 the friendship.
The English geologist Charles Lapworth resolved the conflict by defining a new Ordovician system including the contested beds. [20] An alternative name for the Silurian was "Gotlandian" after the strata of the Baltic island of Gotland. [21]
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 the Silurian rocks of Bohemia into eight stages. [22] His interpretation was questioned in 1854 by Edward Forbes, [23] 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.
Epoch | Age | Start (mya) | Etymology of Epochs and Stages | Notes |
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Llandovery | Rhuddanian | 443.8 | Cefn-Rhuddan Farm, Llandovery in Carmarthenshire, Wales | |
Aeronian | 440.8 | Cwm Coed-Aeron Farm, Wales | Trefawr Track near the farm is the site of the GSSP | |
Telychian | 438.5 | Pen-lan-Telych Farm, Llandovery, Wales | ||
Wenlock | Sheinwoodian | 433.4 | Sheinwood village, Much Wenlock and Wenlock Edge, Shropshire, England | During the Wenlock, the oldest-known tracheophytes of the genus Cooksonia , appear. The complexity of slightly later Gondwana plants like Baragwanathia , which resembled a modern clubmoss, indicates a much longer history for vascular plants, extending into the early Silurian or even Ordovician.[ citation needed ] The first terrestrial animals also appear in the Wenlock, represented by air-breathing millipedes from Scotland. [24] |
Homerian | 430.5 | Homer, Shropshire, England | ||
Ludlow | Gorstian | 427.4 | Gorsty village near Ludlow, Shropshire, England | |
Ludfordian | 425.6 | Ludford, Shropshire, England | ||
Přídolí | — | 423.0 | Named after a locality at the Homolka a Přídolí nature reserve near the Prague suburb of Slivenec, Czech Republic. | Přídolí is the old name of a cadastral field area. [25] |
With the supercontinent Gondwana covering the equator and much of the southern hemisphere, a large ocean occupied most of the northern half of the globe. [26] 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. [26]
During the Silurian, Gondwana continued a slow southward drift to high southern latitudes, but there 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 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 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 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 the global climate underwent many drastic fluctuations throughout the Silurian, [27] [28] evidenced by numerous major carbon and oxygen isotope excursions during this geologic period. [29] [30] [31] 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 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. [26]
During this period, the Earth entered a warm greenhouse phase, supported by high CO2 levels of 4500 ppm, and warm shallow seas covered much of the equatorial land masses. [32] Early in the Silurian, glaciers retreated back into the South Pole until they almost disappeared in the middle of Silurian. [28] Layers of broken shells (called coquina) provide strong evidence of a climate dominated by violent storms generated then as now by warm sea surfaces. [33]
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. [26] The Ireviken event, Mulde event, and Lau event each represent isotopic excursions following a minor mass extinction [34] 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. [26] [31] The climate fluctuations are best explained by a sequence of glaciations, but the lack of tillites in the middle to late Silurian make this explanation problematic. [35]
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. [36]
The Silurian was 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. [12] [13] [37] However, the land fauna did not have a major impact on the Earth until it diversified in the Devonian. [26]
The first fossil records of vascular plants, that is, land plants with tissues that carry water and food, appeared in the second half of the Silurian Period. [38] 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)[ needs update? ] and has branching stems and needle-like leaves of 10–20 centimetres (3.9–7.9 in). The plant shows a high degree of development in relation to the age of its fossil remains. Fossils of this plant have been recorded in Australia, [39] [40] Canada, [41] and China. [42] Eohostimella heathana is an early, probably terrestrial, "plant" known from compression fossils [43] of Early Silurian (Llandovery) age. [44] The chemistry of its fossils is similar to that of fossilised vascular plants, rather than algae. [43]
Fossils that are considered as terrestrial animals are also known from 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. [45] There are also other millipedes, centipedes, and trigonotarbid arachnoids known from Ludlow (420 million years ago). [45] [46] [47] 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 [48] suggested a food web based on as-yet-undiscovered detritivores and grazers on micro-organisms. [49] 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, [24] [50] [51] although the age of this formation is later reinterpreted to be from the early Devonian instead by some researchers. [52] [53] Regardless, Pneumodesmus is still an important fossil as the oldest definitive evidence of spiracles to breath in the air. [45]
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 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. [54] Brachiopods that survived the LOME developed novel adaptations for environmental stress, [55] and they tended to be endemic to a single palaeoplate in the mass extinction's aftermath, but expanded their range afterwards. [56] The most abundant brachiopods were atrypids and pentamerides; [57] atrypids were the first to recover and rediversify in the Rhuddanian after LOME, [58] while pentameride recovery was delayed until the Aeronian. [57] Bryozoans exhibited significant degrees of endemism to a particular shelf. [59] They also developed symbiotic relationships with cnidarians [60] and stromatolites. [61] Many bivalve fossils have also been found in Silurian deposits, [62] and the first deep-boring bivalves are known from this period. [63] Chitons saw a peak in diversity during the middle of the Silurian. [64] Hederelloids enjoyed significant success in the Silurian, with some developing symbioses with the colonial rugose coral Entelophyllum. [65] The Silurian was a heyday for tentaculitoids, [66] which experienced an evolutionary radiation focused mainly in Baltoscandia, [67] along with an expansion of their geographic range in the Llandovery and Wenlock. [68] Trilobites started to recover in the Rhuddanian, [69] 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. [70] The Early Silurian was a chaotic time of turnover for crinoids as they rediversified after LOME. [71] Members of Flexibilia, which were minimally impacted by LOME, took on an increasing ecological prominence in Silurian seas. [72] Monobathrid camerates, like flexibles, diversified in the Llandovery, whereas cyathocrinids and dendrocrinids diversified later in the Silurian. [73] 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. [74] Endobiotic symbionts were common in the corals and stromatoporoids. [75] [76] Rugose corals especially were colonised and encrusted by a diverse range of epibionts, [77] including certain hederelloids as aforementioned. [65] Photosymbiotic scleractinians made their first appearance during the Middle Silurian. [78] Reef abundance was patchy; sometimes, fossils are frequent, but at other points, are virtually absent from the rock record. [26]
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: CS1 maint: DOI inactive as of November 2024 (link)The Devonian is a geologic period and system of the Paleozoic era during the Phanerozoic eon, spanning 60.3 million years from the end of the preceding Silurian period at 419.2 million years ago (Ma), to the beginning of the succeeding Carboniferous period at 358.9 Ma. It is the fourth period of both the Paleozoic and the Phanerozoic. It is named after Devon, South West England, where rocks from this period were first studied.
The Ordovician is a geologic period and system, the second of six periods of the Paleozoic Era, and the second of twelve periods of the Phanerozoic Eon. The Ordovician spans 41.6 million years from the end of the Cambrian Period 485.4 Ma to the start of the Silurian Period 443.8 Ma.
The PaleozoicEra is the first of three geological eras of the Phanerozoic Eon. Beginning 538.8 million years ago (Ma), it succeeds the Neoproterozoic and ends 251.9 Ma at the start of the Mesozoic Era. The Paleozoic is subdivided into six geologic periods, Cambrian, Ordovician, Silurian, Devonian, Carboniferous and Permian. Some geological timescales divide the Paleozoic informally into early and late sub-eras: the Early Paleozoic consisting of the Cambrian, Ordovician and Silurian; the Late Paleozoic consisting of the Devonian, Carboniferous and Permian.
Approximately 251.9 million years ago, the Permian–Triassicextinction event forms the boundary between the Permian and Triassic geologic periods, and with them the Paleozoic and Mesozoic eras. It is Earth's most severe known extinction event, with the extinction of 57% of biological families, 83% of genera, 81% of marine species and 70% of terrestrial vertebrate species. It is also the greatest known mass extinction of insects. It is the greatest of the "Big Five" mass extinctions of the Phanerozoic. There is evidence for one to three distinct pulses, or phases, of 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 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 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 the first 5 million years of the Silurian period.
The Late Devonian extinction consisted of several extinction events in the Late Devonian Epoch, which collectively represent one of the five largest mass extinction events in the history of life on Earth. The term primarily refers to a major extinction, the Kellwasser event, also known as the Frasnian-Famennian extinction, which occurred around 372 million years ago, at the boundary between the Frasnian age and the Famennian age, the last age in the Devonian Period. Overall, 19% of all families and 50% of all genera became extinct. A second mass extinction called the Hangenberg event, also known as the end-Devonian extinction, occurred 359 million years ago, bringing an end to the Famennian and Devonian, as the world transitioned into the Carboniferous Period.
Tentaculites is an extinct genus of conical fossils of uncertain affinity, class Tentaculita, although it is not the only member of the class. It is known from Lower Ordovician to Upper Devonian deposits both as calcitic shells with a brachiopod-like microstructure and carbonaceous 'linings'. The "tentaculites" are also referred to as the styliolinids.
The Hirnantian glaciation, also known as the Andean-Saharan glaciation, Early Paleozoic Ice Age (EPIA), the Early Paleozoic Icehouse, the Late Ordovician glaciation, or the end-Ordovician glaciation, occurred during the Paleozoic from approximately 460 Ma to around 420 Ma, during the Late Ordovician and the Silurian period. The major glaciation during this period was formerly thought only to consist of the Hirnantian glaciation itself but has now been recognized as a longer, more gradual event, which began as early as the Darriwilian, and possibly even the Floian. Evidence of this glaciation can be seen in places such as Arabia, North Africa, South Africa, Brazil, Peru, Bolivia, Chile, Argentina, and Wyoming. More evidence derived from isotopic data is that during the Late Ordovician, tropical ocean temperatures were about 5 °C cooler than present day; this would have been a major factor that aided in the glaciation process.
Tabulata, commonly known as tabulate corals, are an order of extinct forms of coral. They are almost always colonial, forming colonies of individual hexagonal cells known as corallites defined by a skeleton of calcite, similar in appearance to a honeycomb. Adjacent cells are joined by small pores. Their distinguishing feature is their well-developed horizontal internal partitions (tabulae) within each cell, but reduced or absent vertical internal partitions. They are usually smaller than rugose corals, but vary considerably in shape, from flat to conical to spherical.
The Tournaisian is in the ICS geologic timescale the lowest stage or oldest age of the Mississippian, the oldest subsystem of the Carboniferous. The Tournaisian age lasted from 358.9 Ma to 346.7 Ma. It is preceded by the Famennian and is followed by the Viséan. In global stratigraphy, the Tournaisian contains two substages: the Hastarian and Ivorian. These two substages were originally designated as European regional stages.
The late Paleozoic icehouse, also known as the Late Paleozoic Ice Age (LPIA) and formerly known as the Karoo ice age, was an ice age that began in the Late Devonian and ended in the Late Permian, occurring from 360 to 255 million years ago (Mya), and large land-based ice sheets were then present on Earth's surface. It was the second major icehouse period of the Phanerozoic, after the Late Ordovician Andean-Saharan glaciation.
The Lau event was the last of three relatively minor mass extinctions during the Silurian period. It had a major effect on the conodont fauna, but barely scathed the graptolites, though they suffered an extinction very shortly thereafter termed the Kozlowskii event that some authors have suggested was coeval with the Lau event and only appears asynchronous due to taphonomic reasons. It coincided with a global low point in sea level caused by glacioeustasy and is closely followed by an excursion in geochemical isotopes in the ensuing late Ludfordian faunal stage and a change in depositional regime.
The Hangenberg event, also known as the Hangenberg crisis or end-Devonian extinction, is a mass extinction that occurred at the end of the Famennian stage, the last stage in the Devonian Period. It is usually considered the second-largest extinction in the Devonian Period, having occurred approximately 13 million years after the Late Devonian mass extinction at the Frasnian-Famennian boundary. The event is named after the Hangenberg Shale, which is part of a sequence that straddles the Devonian-Carboniferous boundary in the Rhenish Massif of Germany.
The Great Ordovician Biodiversification Event (GOBE), was an evolutionary radiation of animal life throughout the Ordovician period, 40 million years after the Cambrian explosion, whereby the distinctive Cambrian fauna fizzled out to be replaced with a Paleozoic fauna rich in suspension feeder and pelagic animals.
Trimerellida is an extinct order of craniate brachiopods, containing the sole superfamily Trimerelloidea and the families Adensuidae, Trimerellidae, and Ussuniidae. Trimerellidae was a widespread family of warm-water brachiopods ranging from the Middle Ordovician to the late Silurian (Ludlow). Adensuidae and Ussuniidae are monogeneric families restricted to the Ordovician of Kazakhstan. Most individuals were free-living, though some clustered into large congregations similar to modern oyster reefs.
Olev Vinn is an Estonian paleobiologist and paleontologist.
The Lilliput effect is an observed decrease in animal body size in genera that have survived a major extinction. There are several hypotheses as to why these patterns appear in the fossil record, some of which are:
The Silurian-Devonian Terrestrial Revolution, also known as the Devonian Plant Explosion (DePE) and the Devonian explosion, was a period of rapid colonization, diversification and radiation of land plants and fungi on dry lands that occurred 428 to 359 million years ago (Mya) during the Silurian and Devonian periods, with the most critical phase occurring during the Late Silurian and Early Devonian.
The Taghanic event was an extinction event that occurred about 386 million years ago during the Givetian faunal stage of the Middle Devonian geologic period in the Paleozoic era. It was caused by hypoxia from an anoxic event. The event had a period in which dissolved oxygen in the Earth's oceans was depleted. The Taghanic event caused a very high death rate of corals. The loss of the coral reefs caused a high loss of animals that lived in and around the reefs. The extinction rate has been placed between 28.5 and 36%, making the event the 8th largest extinction event recorded. The reduced oxygen levels resulted from a period of global warming caused by Milankovitch cycles. In the Taghanic event sea levels were higher. After the Taghanic Event, sea life recovered in the Frasnian faunal stage starting 382.7 million years ago. Two other events near this period were the Kellwasser event and the Hangenberg event.
Atrypida is an extinct order of rhynchonelliform brachiopods. They first appeared in middle Ordovician and survived the Ordovician-Silurian extinction, becoming the dominant brachiopods of the Silurian alongside the order Pentamerida. They would survive into the Late Devonian before going extinct at the end of the Frasnian.