|Paleozoic era (542 - 251 mya)|
444 - 416 million years ago
The Silurian period is an interval of about 28 million years defined on the geologic timescale as spanning roughly from 444 to 416 million years ago (mya) and lying between the earlier Ordovician period and the later Devonian period. As with other geologic periods, the rock beds defining the period's start and end are well identified, but the exact dates are uncertain by five to ten million years.
The base of the Silurian is set at a major extinction event when 60 percent of marine species were wiped out. (See Ordovician-Silurian extinction event.) As with all development, which takes place in stages, the Silurian flora and fauna came on the foundation of earlier stages, including this extinction, and in turn helped lay the foundation for life to the present. Silurian strata are the source of some present-day deposits of oil, gas, and iron ore.
The Silurian system of fossil-bearing rock strata was originally identified in Wales, and it carries the name of a Welsh tribe, the Silures. During the Silurian period, the supercontinent Gondwanaland drifted slowly toward the southern pole, temperatures rose, sea ice and glaciers melted, proto-Europe and North America collided pushing up mountains extant today from New York state to Greenland and Norway, and sea levels were mostly high and stable with some short-lived declines as the capacity of the ocean basins shifted. Over long intervals, the high sea levels flooded vast, low-lying regions of the continents, producing warm, shallow seas that nurtured the recovery of life after the devastating Ordovician-Silurian extinction. During the Silurian period, life diversified considerably developing such various new life forms as coral reefs, bony fish, fish with movable jaws, sea scorpions, early relatives of spiders and millipedes, and vascular plants.
The Silurian system was first identified by Sir 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, extending the convention his friend Adam Sedgwick had established for the Cambrian.
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. This was the germ of the modern geological time scale. As it was first identified, however, the "Silurian" series when traced farther afield quickly came to overlap Sedgwick's "Cambrian" sequence, provoking furious disagreements that ended the friendship.
Charles Lapworth eventually resolved the conflict by defining a new Ordovician system, including the contested beds.
The Silurian period is usually divided into early and late subdivisions with each of those comprising two epochs: The early Silurian comprising the Llandovery and Wenlock epochs and the late Silurian comprising the Ludlow and Pridoli epochs. In addition, some alternate schemes divide the Silurian period into three parts: Early Silurian (Llandovery epoch), middle Silurian (Wenlock epoch), and late Silurian (Ludlow and Pridoli epochs) breakdown. These faunal stages (divisions based on fossils) are characterized by their index fossils, new species of colonial marine Graptolites that appear in each stage.
Epochs of time are correlated with series of rocks just as periods of time are correlated with larger systems of rocks that usually comprise several series of rocks. Following a rock column from its base upwards, it is commonly divided roughly into lower, middle, and upper parts that would correspond, for example to early, middle, and late Silurian time. The epochs and stages of the Silurian period from youngest to oldest are:
- Late Silurian: Pridoli epoch—no stages defined
- Late Silurian: Ludlow epoch—divided into
- Ludfordian (late Ludlow-late Silurian)
- Gorstian (early Ludlow-late Silurian)
- Early or middle Silurian: Wenlock epoch—divided into
- Late Wenlock: Homerian
- Early Wenlock: Sheinwoodian
- Early Silurian: Llandovery epoch—divided into
- Late Llandovery: Telychian
- Mid Llandovery: Aeronian
- Early Llandovery: Rhuddanian
|Rhuddalian | Aeronian
|Sheinwoodian | Homerian||Gorstian | Ludfordian|
In North America a different suite of regional stages is used:
- Late Silurian: Ludlow-Cayugan
- Middle Silurian: Wenlock
- Early Silurian: Llandovery)
During the Silurian, the hypothesized supercontinent 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 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. Other cratons and continent fragments 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 what is now 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 in a basin 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 strata, leaving traces in the Silurian strata of northern Ohio and Indiana.
The vast ocean of Panthalassa covered most of the northern hemisphere. Other minor oceans include Proto-Tethys, Paleo-Tethys, Rheic Ocean, a seaway of Iapetus Ocean (now in between Avalonia and Laurentia), and newly formed Ural Ocean.
During this period, the Earth entered a long, warm greenhouse phase, and warm shallow seas covered much of the equatorial land masses. The period witnessed a relative stabilization of the Earth's general climate, ending the previous pattern of erratic climatic fluctuations. Layers of broken shells (called coquina) provide strong evidence of a climate dominated by violent storms generated then as now by warm sea surfaces.
Silurian high sea levels and warm shallow continental seas provided a hospitable environment for marine life of all kinds. Silurian beds are oil and gas producers in some areas. Extensive beds of Silurian hematite—an iron ore—in eastern North America were important to the early American colonial economy.
Coral reefs made their first appearance during this time, built by extinct tabulate and rugose corals. The first bony fish, the Osteichthyes appeared, represented by the Acanthodians covered with bony scales. Fishes reached considerable diversity and developed movable jaws, which are considered to have developed from the supports of the front two or three gill arches. A diverse fauna of Eurypterus (Sea Scorpions)—some of them several meters in length—prowled the shallow Silurian seas of North America; many of their fossils have been found in New York State. Brachiopods, bryozoa, molluscs, and trilobites were abundant and diverse.
Myriapods became the first proper terrestrial animals. The terrestrial ecosystems included the first multicellular terrestrial animals that have been identified—relatives of modern spiders and millipedes whose fossils were discovered in the 1990s.
The first fossil records of vascular plants—that is, land plants with tissues that carry food—are considered to have appeared in the Silurian period. The earliest known representatives of this group are the Cooksonia (mostly from the northern hemisphere) and Baragwanathia (from Australia). A primitive Silurian land plant with xylem and phloem but no differentiation in root, stem or leaf, was much-branched Psilophyton. It is considered to have been reproducing by spores and breathing through stomata on every surface, and probably photosynthesizing in every tissue exposed to light. Rhyniophyta and primitive lycopods were other land plants that first appear during this period.
- Emiliani, C. Planet Earth: Cosmology, Geology and the Evolution of Life and Environment. Cambridge, UK: Cambridge University Press, 1992. ISBN 0-521-40949-7.
- International Commission on Stratigraphy (ICS). 2005. Retrieved September 22, 2007.
- Ogg, J. Overview of Global Boundary Stratotype Sections and Points (GSSP's). 2004. Retrieved September 22, 2007.
- Stanley, S. M. Earth System History. New York: W.H. Freeman and Company, 1999. ISBN 0-7167-2882-6.
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