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The Silurian ( ) is a geologic period and system spanning 23.5 million years from the end of the Ordovician Period, at Ma (million years ago) to the beginning of the Devonian Period, Ma. 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 , 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, fungi started expanding and diversifying along with fungi, and three groups of arthropods (myriapods, arachnids and Hexapoda) became fully terrestrialized.
Another significant evolutionary milestone during the Silurian was the diversification of Gnathostomata, which include , (which gave rise to cartilaginous fish) and osteichthyan (bony fish, further divided into lobe-finned fish and ), although this corresponded to sharp decline of jawless fish such as and .
The English geologist Charles Lapworth resolved the conflict by defining a new Ordovician system including the contested beds. 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-called Upper Cambrian or Lower 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." An alternative name for the Silurian was "Gotlandian" after the strata of the Baltic island of Gotland.The Gotlandian system was proposed in 1893 by the French geologist Albert Auguste Cochon de Lapparent (1839–1908): From p. 748: "D'accord avec ces divisions, on distingue communément dans le silurien trois étages: l'étage inférieur ou cambrien (1); l'étage moyen ou ordovicien (2); l'étage supérieur ou gothlandien (3)." (In agreement with these divisions, one generally distinguishes, within the Silurian, three stages: the lower stage or Cambrian 1; the middle stage or Ordovician 2; the upper stage or Gotlandian 3.)
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 was questioned in 1854 by Edward Forbes, See p. xxxiv. 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.
| +Subdivisions of the Silurian period |
| Trefawr Track near the farm is the site of the GSSP |
| During the Wenlock, the oldest-known Vascular plant 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. The first terrestrial animals also appear in the Wenlock, represented by air-breathing millipedes from Scotland. |
| Homer, Shropshire, England |
| Přídolí is the old name of a Cadastre field area. |
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 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 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.
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. (2025). 9780125986557 ISBN 9780125986557 Early in the Silurian, retreated back into 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 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. However, the land fauna did not have a major impact on the Earth until it diversified in the Devonian.
The first fossil records of , that is, land plants with tissues that carry water and food, appeared in 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 Ma) and has branching stems and needle-like leaves of . 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, Canada, and China. Eohostimella heathana is an early, probably terrestrial, "plant" known from compression fossils of Early Silurian (Llandovery) age., p. 4 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 the Silurian. The definitive oldest record of millipede ever known is Kampecaris and Archidesmus sp. from the late Silurian (425 Ma) of Kerrera. There are also other millipedes, , and Trigonotarbida known from Ludlow Epoch (420 Ma). Predatory would indicate that simple were in place that included non-predatory prey animals. Extrapolating back from Early Devonian biota, Andrew Jeram et al. in 1990 suggested a food web based on as-yet-undiscovered and grazers on micro-organisms.
The first bony fish, the Osteichthyes, appeared, represented by the Acanthodii covered with bony scales. Fish reached considerable diversity and developed movable , adapted from the supports of the front two or three gill arches. A diverse fauna of (sea scorpions)—some of them a few meters in length—prowled the shallow Silurian seas and lakes of North America; many of their 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 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. exhibited significant degrees of endemism to a particular shelf. They also developed symbiotic relationships with cnidarians and stromatolites. Many Bivalvia fossils have also been found in Silurian deposits, and the first deep-boring bivalves are known from this period. saw a peak in diversity during the middle of the Silurian. hederellid enjoyed significant success in the Silurian, with some developing symbioses with the colonial rugose coral Entelophyllum. The Silurian was a heyday for Tentaculita, which experienced an evolutionary radiation focused mainly in Baltoscandia, along with an expansion of their geographic range in 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 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 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 a diverse range of epibionts, including certain hederelloids as aforementioned. Photosymbiotic scleractinians made their first appearance during the Middle Silurian. Reef abundance was patchy; sometimes, fossils are frequent, but at other points, are virtually absent from the rock record.
(1992). 9780226041551, University of Chicago Press. ISBN 9780226041551 Millipedes from Cowie Formation such as Cowiedesmus and Pneumodesmus were considered as the oldest millipede from the middle Silurian at 428–430 Ma, although the age of this formation is later reinterpreted to be from the early Devonian instead by some researchers. Regardless, Pneumodesmus is still an important fossil as the oldest definitive evidence of spiracles to breathe in the air.
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