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51-746: In central Europe the Saxothuringian Zone is situated between the Rhenohercynian Zone to the northwest and the Moldanubian Zone to the southeast. The Hercynian metamorphism of the former zone is generally lower grade; that of the latter zone higher grade than that of the Saxothuringian Zone. West of the Vosges the zone is displaced to the north by a major strike-slip fault , the Bray Fault . The Saxothuringian Zone

102-524: A fold belt of west and central Europe, formed during the Hercynian orogeny (about 350 to 280 million years ago ). The zone consists of folded and thrust Devonian and early Carboniferous sedimentary rocks that were deposited in a back-arc basin along the southern margin of the then existing paleocontinent Laurussia . The Rhenohercynian Zone, named for the Rhine River and

153-519: A deep foreland basin . This filled with the products of erosion in the Hercynian mountains and the contemporaneously uplifted London-Brabant Massif to the north. During the Westphalian, the basin was completely filled and rose above sea level. The Rhenohercynian basin was filled with Devonian and Carboniferous sediments. Sedimentation was often disrupted by tectonic phases , but nevertheless

204-411: 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

255-519: 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

306-709: A high grade (up to eclogite facies ). These allochthonous nappes can probably be correlated with the Teplá terrane in the Moldanubian Zone further south. The Saxothuringian Zone is often also supposed to include the Mid-German Crystalline High , which then forms the northern part of the zone and lies directly next to the Rhenohercynian Northern Phyllite Zone . The Mid-German Crystalline High crops out in

357-490: 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 is evidence that the Silurian icecaps were less extensive than those of

408-423: 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

459-525: 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

510-644: Is in some places transected by Permo - Triassic grabens and intramontane basins filled with Rotliegend sediments and older deposits. The Eger Graben in the northwest of the Czech Republic and the Saar-Nahe Basin in western Germany are examples of such structures. The Saxothuringian Zone consists of early Paleozoic marine sediments that were deposited in the Saxothuringian Basin . They were slightly metamorphosed during

561-561: 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

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612-658: The Belgian coal measures . 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

663-813: The Cadomian orogeny (in the Ediacaran, 650-550 million years ago) and intruded by felsic plutons during the Cambrian and Ordovician (540-420 million years ago). In some places klippes of allochthonous crystalline rocks are found on top of these two units. These klippes are the Münchberg complex , Wildenfels complex and Frankenberg complex . They consisted originally of a sequence of deep-marine ( flysch ) sediments of Ordovician to Devonian age (480-360 million years old) and early Paleozoic mid-oceanic ridge basalts . The latter have been metamorphosed at

714-630: The East-Silesian basin of southern Poland . The sedimentary rocks that were laid down in the basin are often weakly metamorphic ( greenschist facies ). Most geologists consider the South-Portuguese zone to be a continuation of the zone to the west. Whether the Rhenohercynian Basin was a continuous feature or rather a string of temporaneously interconnected smaller basins is not well understood, because in many places

765-663: The Hercynian Forest of Antiquity, forms a narrow zone through western and central Europe, from Cornwall and Ireland in the west to the Harz mountains of central Germany in the east, including the Rhenish Massif ( Ardennes , Taunus , Eifel and Hunsrück ). The total length of this ancient basin (the Rhenohercynian Basin ) could have been more than 2500 km. In the east the basin merges with

816-580: 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

867-772: The folded sequences of Cornwall, the Ardennes , the Eifel and the Harz. From the Frasnian age (380 million years ago) the mafic volcanism ended, and the basin came locally under compressional stress , which led to folding and thrusting in the sedimentary rocks. Somewhere near the end of the Devonian, a subduction zone developed under the Mid-German/Normannian highs and Rhenohercynian crust began to subduct. This

918-799: The Devonian and Carboniferous rock strata are covered with younger deposits. Parts of the basin have their own names, like the Cornwall basin in Cornwall, the Munster basin in Ireland or the Rhenisch basin in Belgium and Germany. The Rhenohercynian Zone is a part of the northern foreland of the Hercynian orogeny. It has a lower grade of metamorphism than the Saxothuringian Zone to

969-708: The Hercynian orogeny. The sedimentary sequence is assumed to be continuous from the Ediacaran to the Visean (330 million years ago). These metasediments form a wide zone north of the city of Dresden in Saxony . Tectonostratigraphically, gneisses (high-grade metamorphic rocks) and granites are found under these metasediments. They crop out as the competent massifs of the Ore Mountains and Saxonian Granulite Massif . They were deformed and recrystallized during

1020-788: The Ligerian cordillera. In the Siegenian/Pragian and Emsian the Rhenohercynic basin was a back-arc basin behind this cordillera. Tectonic subsidence in a system of horsts and grabens together with basaltic volcanism resulted in the creation of new oceanic lithosphere . In the Middle Devonian a second basin, the Saxothuringian / Armorican basin , developed south of the Rhenohercynian basin. To

1071-683: 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

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1122-490: 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

1173-784: The Odenwald, the Spessart and the northern Vosges. It consists of Proterozoic orthogneisses and early Paleozoic volcanic ( amphibolites with MORB- protoliths and tuffs ) and sedimentary ( pelites , calcareous schists and marbles ) rocks that were metamorphosed at high grade during the Hercynian orogeny (up to amphibolite facies ). These rocks were intruded by two generations of plutons: Silurian to Early Devonian (440-400 million years old) granitoids and middle Carboniferous (Hercynian, 340-325 million years old) granites. Rhenohercynian Zone The Rhenohercynian Zone or Rheno-Hercynian zone in structural geology describes

1224-596: 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 the friendship. The English geologist Charles Lapworth resolved

1275-475: 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 the two men presented a joint paper, under the title On the Silurian and Cambrian Systems, Exhibiting

1326-486: 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 was first identified by the Scottish geologist Roderick Murchison , who

1377-518: 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

1428-650: 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

1479-497: 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 the Silurian rocks of Bohemia into eight stages. His interpretation

1530-609: The continental margin. It separated the London-Brabant Massif to the north from the Normannian and Mid-German Highs to the south. In the Middle Devonian (from 390 million years ago) a subduction zone existed south of Laurussia, where oceanic lithosphere of the Rheic Ocean subducted beneath the Mid-German/Normannian highs. Volcanism above the subduction zone created a cordillera -type mountain chain,

1581-588: The continents of Laurussia (north) and Gondwana (south) from the Silurian onwards. The southern margin of Laurussia was formed during the Caledonian orogeny of the Silurian period, about 420 million years ago . In the Gedinnian/Lochkovian (Early Devonian) the southern part of the Caledonian mountain belt became a region of north–south extension . An elongated basin was formed parallel to

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1632-473: The destruction of the last oceanic crust of the basin. Its sedimentary fill was, however, not (totally) subducted but instead thrust northward. During the later part of the Carboniferous period ( Westphalian and Stephanian ) the Rhenohercynian zone formed the foreland of a relatively fast-developing Hercynian mountainbelt to the south. Isostatic subsidence of the foreland resulted in the development of

1683-459: 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

1734-461: 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

1785-509: 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

1836-639: 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

1887-451: 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

1938-464: 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

1989-402: 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

2040-561: 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

2091-495: 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

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2142-516: 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

2193-479: The south, meaning its rocks have generally been at smaller depths and under lower temperatures. The Subvariscan Zone north of the Rhenohercynian Zone was untouched by Hercynian metamorphism. During the Hercynian orogeny, the Rhenohercynian zone was folded and thrust internally. It was thrust over the foreland to the north (the London-Brabant Massif and other Avalonian terranes ). From the south it

2244-463: 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

2295-411: The total thickness of the sediments can in some places be more than several kilometers. When a foreland basin was formed in the Rhenohercynian zone, this was filled with upper Carboniferous ( Pennsylvanian ) flysch and molasse sediments. The Namurian is characterized by flysch, in the Westphalian this gradually grades into molasse and other continental deposits, among which the thick coal layers of

2346-608: The west some crustal convergence took place, and the Normannian High was partly thrust over the sedimentary basin fill of the Rhenohercynian basin. The Rhenohercynian basin disappeared when the continent Gondwana collided with Laurussia in the course of the Carboniferous period (the Hercynian orogeny ). The sedimentary rocks in the basin were thrust in a series of piggyback basins over the northern foreland (the London-Brabant Massif). These rocks now form

2397-601: 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 the Silurian System and the lands now thought to have been inhabited in antiquity by

2448-408: Was overthrust by the Mid-German Crystalline High , part of the Saxothuringian Zone. The metamorphic grade or degree of metamorphism increases towards the south or southeast. The southern edge of the Rhenish Massif lies in the Northern Phyllite Zone , which has a higher grade than other parts of the zone. The Rhenohercynian basin was situated north of the Rheic Ocean , the ocean that spread between

2499-409: 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 the equator and much of the southern hemisphere,

2550-422: 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, 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

2601-485: Was the short Bretonnic phase of the Hercynian orogeny. It was followed, from the Tournaisian (early Carboniferous, 355 million years ago) till the end of the Visean by a new period of extension. During the Sudetic (main) phase of the Hercynian orogeny (330-320 million years ago, Late-Visean and Namurian / Serpukhovian ) compressional tectonics had the upper hand again. In the Namurian age full-scale continental collision between Laurussia and Gondwana resulted in

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