The Silurian system was first identified by the Scottish geologistRoderick Murchison, who was examining fossil-bearing sedimentary rockstrata in southWales in the early 1830s. He named the sequences for aCeltic tribe of Wales, theSilures, inspired by his friendAdam Sedgwick, who had named the period of his study theCambrian, from aLatin 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 includedCaer Caradoc andWenlock 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 titleOn 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 moderngeological 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 French geologistJoachim Barrande, building on Murchison's work, used the termSilurian in a more comprehensive sense than was justified by subsequent knowledge. He divided the Silurian rocks ofBohemia into eight stages.[22] His interpretation was questioned in 1854 byEdward 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.
During the Wenlock, the oldest-knowntracheophytes of the genusCooksonia, appear. The complexity of slightly laterGondwana plants likeBaragwanathia, which resembled a modern clubmoss, indicates a much longer history for vascular plants, extending into the early Silurian or evenOrdovician.[citation needed] The first terrestrial animals also appear in the Wenlock, represented by air-breathingmillipedes fromScotland.[24]
With the supercontinentGondwana 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 andglaciers contributed to a rise in sea level, recognizable from the fact that Silurian sediments overlie eroded Ordovician sediments, forming anunconformity. The continents ofAvalonia,Baltica, andLaurentiadrifted together near theequator, starting the formation of a secondsupercontinent known asEuramerica.
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 theCaledonian orogeny, a spate of mountain building that stretched fromNew York State through conjoined Europe and Greenland to Norway. At the end of the Silurian, sea levels dropped again, leaving telltale basins ofevaporites extending from Michigan to West Virginia, and the new mountain ranges were rapidly eroded. TheTeays River, flowing into the shallow mid-continental sea, eroded Ordovician Period strata, forming deposits of Silurian strata in northern Ohio and Indiana.
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 theirHirnantian 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, theEarth entered a warmgreenhouse 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 theSouth Pole until they almost disappeared in the middle of Silurian.[28] Layers of broken shells (calledcoquina) provide strong evidence of a climate dominated by violent storms generated then as now by warm sea surfaces.[33]
The climate andcarbon 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] TheIreviken event,Mulde event, andLau 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 werebrachiopods,corals, andtrilobites, 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 oftillites 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 theLate 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 ofmoss-like miniature forests along lakes and streams and networks of large, mycorrhizalnematophytes, 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]
Diorama of a Silurian seafloor
The first fossil records ofvascular 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 areCooksonia. Most of the sediments containingCooksonia 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 Ma)[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 ofmillipede ever known isKampecaris obanensis andArchidesmus sp. from the late Silurian (425 Ma) ofKerrera.[45] There are also other millipedes,centipedes, andtrigonotarbid arachnoids known fromLudlow (420 Ma).[45][46][47] Predatoryinvertebrates would indicate that simplefood webs were in place that included non-predatory prey animals. Extrapolating back fromEarly Devonian biota, Andrew Jeramet al. in 1990[48] suggested a food web based on as-yet-undiscovereddetritivores and grazers on micro-organisms.[49] Millipedes fromCowie Formation such asCowiedesmus andPneumodesmus were considered as the oldest millipede from the middle Silurian at 428–430 Ma,[24][50][51] although the age of this formation is later reinterpreted to be from the earlyDevonian instead by some researchers.[52][53] Regardless,Pneumodesmus is still an important fossil as the oldest definitive evidence ofspiracles to breathe in the air.[45]
The first bony fish, theOsteichthyes, appeared, represented by theAcanthodians covered with bony scales. Fish reached considerable diversity and developed movablejaws, adapted from the supports of the front two or threegill arches. A diverse fauna ofeurypterids (sea scorpions)—some of them a few meters in length—prowled the shallow Silurian seas and lakes of North America; many of theirfossils have been found inNew 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] Manybivalve 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 coralEntelophyllum.[65] The Silurian was a heyday fortentaculitoids,[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 forcrinoids 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]
Cooksonia, the earliest vascular plant, middle Silurian
^Jeppsson, L.; Calner, M. (2007). "The Silurian Mulde Event and a scenario for secundo—secundo events".Earth and Environmental Science Transactions of the Royal Society of Edinburgh.93 (02):135–154.doi:10.1017/S0263593300000377.
^Munnecke, A.; Samtleben, C.; Bickert, T. (2003). "The Ireviken Event in the lower Silurian of Gotland, Sweden-relation to similar Palaeozoic and Proterozoic events".Palaeogeography, Palaeoclimatology, Palaeoecology.195 (1):99–124.doi:10.1016/S0031-0182(03)00304-3.
Murchison, Roderick Impey (1835)."On the Silurian system of rocks".Philosophical Magazine. 3rd series.7 (37):46–52.doi:10.1080/14786443508648654. From p. 48: " … I venture to suggest, that as the great mass of rocks in question, trending from south-west to north-east, traverses the kingdom of our ancestors the Silures, the term "Silurian system" should be adopted … "
^Murchison, Roderick (1835)."On the Silurian System of Rocks".The London and Edinburgh Philosophical Magazine and Journal of Science. Third Series, Vol. 7:46–52 – via Biodiversity Heritage Library.
^Lapworth, Charles (1879)."On the tripartite classification of the Lower Palaeozoic rocks".Geological Magazine. 2nd series.6 (1):1–15.Bibcode:1879GeoM....6....1L.doi:10.1017/s0016756800156560.S2CID129165105. From pp. 13–14: "North Wales itself – at all events the whole of the great Bala district where Sedgwick first worked out the physical succession among the rocks of the intermediate or so-calledUpper Cambrian orLower Silurian system; and in all probability much of the Shelve and the Caradoc area, whence Murchison first published its distinctive fossils – lay within the territory of the Ordovices; … Here, then, have we the hint for the appropriate title for the central system of the Lower Palaeozoics. It should be called the Ordovician System, after this old British tribe."
^The Gotlandian system was proposed in 1893 by the French geologistAlbert Auguste Cochon de Lapparent (1839–1908):Lapparent, A. de (1893).Traité de Géologie (in French). Vol. 2 (3rd ed.). Paris, France: F. Savy. p. 748. From p. 748:"D'accord avec ces divisions, on distingue communément dans le silurien trois étages: l'étage inférieur oucambrien (1); l'étage moyen ouordovicien (2); l'étage supérieur ougothlandien (3)." (In agreement with these divisions, one generally distinguishes, within the Silurian, three stages: the lower stage orCambrian [1]; the middle stage orOrdovician [2]; the upper stage orGotlandian [3].)
^abGambacorta, G.; Menichetti, E.; Trincianti, E.; Torricelli, S. (March 2019). "The Silurian climatic transition recorded in the epicontinental Baltica Sea".Palaeogeography, Palaeoclimatology, Palaeoecology.517:16–29.Bibcode:2019PPP...517...16G.doi:10.1016/j.palaeo.2018.12.016.S2CID135118794.
^abTrotter, Julie A.; Williams, Ian S.; Barnes, Christopher R.; Männik, Peep; Simpson, Andrew (February 2016). "New conodont δ18O records of Silurian climate change: Implications for environmental and biological events".Palaeogeography, Palaeoclimatology, Palaeoecology.443:34–48.Bibcode:2016PPP...443...34T.doi:10.1016/j.palaeo.2015.11.011.
^Nealon, T.; Williams, D. Michael (30 April 2007). "Storm-influenced shelf deposits from the silurian of Western Ireland: A reinterpretation of deep basin sediments".Geological Journal.23 (4):311–320.doi:10.1002/gj.3350230403.
^Samtleben, C.; Munnecke, A.; Bickert, T. (2000). "Development of facies and C/O-isotopes in transects through the Ludlow of Gotland: Evidence for global and local influences on a shallow-marine environment".Facies.43 (1):1–38.Bibcode:2000Faci...43....1S.doi:10.1007/BF02536983.S2CID130640332.
^Hueber, F.M. (1983). "A new species ofBaragwanathia from the Sextant Formation (Emsian) Northern Ontario, Canada".Botanical Journal of the Linnean Society.86 (1–2):57–79.doi:10.1111/j.1095-8339.1983.tb00717.x.
^Bora, Lily (2010).Principles of Paleobotany. Mittal Publications. pp. 36–37.
^Edwards, D. & Wellman, C. (2001), "Embryophytes on Land: The Ordovician to Lochkovian (Lower Devonian) Record", in Gensel, P. & Edwards, D. (eds.),Plants Invade the Land: Evolutionary and Environmental Perspectives, New York: Columbia University Press, pp. 3–28,ISBN978-0-231-11161-4{{citation}}: CS1 maint: publisher location (link), p. 4
^DiMichele, William A; Hook, Robert W (1992)."The Silurian". In Behrensmeyer, Anna K. (ed.).Terrestrial Ecosystems Through Time: Evolutionary Paleoecology of Terrestrial Plants and Animals. University of Chicago Press. pp. 207–10.ISBN978-0-226-04155-1.
Emiliani, Cesare. (1992).Planet Earth : Cosmology, Geology, & the Evolution of Life & the Environment. Cambridge University Press. (Paperback EditionISBN0-521-40949-7)
Mikulic, DG, DEG Briggs, and J Kluessendorf. 1985. A new exceptionally preserved biota from the Lower Silurian of Wisconsin, USA. Philosophical Transactions of the Royal Society of London, 311B:75-86.
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![Shenacanthus vermiformis[69] is jawed stem-chondrichthyan genus from the early Silurian (Telychian) of China](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aShenacanthus_vermiformis.jpg)
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