Geology

Silurian

November 25, 2011, 3:42 pm

The Silurian (443 to 417 million years ago) was a time when the Earth underwent considerable changes that had important repercussions for the environment and life within it. The Silurian witnessed a relative stabilization of Earth's general climate, ending the previous pattern of erratic climatic fluctuations. One result of these changes was the melting of large glacial formations. This contributed to a substantial rise in the levels of the major seas.

Coral reefs made their first appearance during this time, and the Silurian was also a remarkable time in the evolution of fishes. Not only does this time period mark the wide and rapid spread of jawless fish, but also the highly significant appearances of both the first known freshwater fish as well as the first fish with jaws. It is also at this time that our first good evidence of life on land is preserved, including relatives of spiders and centipedes, and also the earliest fossils of vascular plants.

caption The chart shows the major subdivisions of the Silurian Period. This chart is mapped, to allow you to travel back to the Ordovician or forward to the Devonian.The Silurian Period is part of the Paleozoic Era (Source: UCMP)

Stratigraphy of the Silurian

The Silurian lasted from about 443 to 417 million years before present. Its stratigraphy is subdivided into four epochs (from oldest to youngest): the Llandovery, the Wenlock, the Ludlow, and the Pridoli. Each epoch is distinguished from the others by the appearance of new species of graptolites. Graptolites are a group of extinct colonial, aquatic animals that put in their first appearance in the {C}Cambrian period (543 -490 million years ago) and persisted into the Early Carboniferous (354-290 million years ago). The beginning of the Silurian (and the Llandovery) is marked by the appearance of Parakidograptus acuminatus, a species of graptolite.

The Llandovery (443-428 million years ago) preserves its fossils in shale, sandstone, and gray mudstone sediment. Its base (beginning) is marked by the appearance of the graptolites Parakidograptus acuminatus and Akidograptus ascensus. The Llandoverian epoch is subdivided into the Rhuddanian, Aeronian, and Telychian stages.

At the close of the Telychian stage, the appearance of Cyrtograptus centrifugus marks the start of the Wenlockian epoch (428-423 million years ago). The fossils are found in siltstone and mudstone under limestone. Missing from the fossil record of the Wenlock was the conodont Pterospathodus amorphognathoides, present in earlier strata. This is an epoch with excellent preservations of brachiopod, coral, trilobite, clam, bryozoan, and crinoid fossils. The Wenlock is subdivided into the Sheinwoodian and Homerian stages.

The Ludlow (423-419 million years ago) consists siltstone and limestone strata, marked by the appearance of Neodiversograptus nilssoni. There is an abundance of shelly animal fossils. The Gorstian and Ludfordian stages make up the Ludlow epoch.

Platy limestone strata rich in cephalopods and bivalves characterize the Pridolian (419-417 million years ago), the final epoch of the Silurian. It is marked by the appearance of the index fossil Monograptus parultimus, and also by two new species of chitinozoans (marine plankton), Urnochitina urna and Fungochitina kosovensis, which appear at the base or just above the base of the Pridoli.

Tectonics and paleoclimate of the Silurian

During the Silurian, the Earth witnessed many changes in the way in which landmasses were distributed around the globe. Although there were no major volcanic events, a deglaciation and rise in sea levels occurring at that time produced varying periods of continent coverage and exposure. The variation of ocean levels occurred alongside the process of continental fragmentation and grouping that occurred from the {C}Cambrian to the present.

At that time, the continents were distributed very differently than they are today. The Silurian world consisted of a vast north polar ocean and a south polar supercontinent (Gondwana) with a ring of approximately six continents. By the Silurian period, a large portion of the Rodinian landmass had become fragmented, and those fragments migrated toward the equatorial region. Most of these fragments were eventually assembled by a series of plate collisions into the super-continents of Laurussia and Laurasia. The modern Philippine islands were most likely inside the Arctic Circle, while Australia and Scandinavia resided in the tropics; South America and Africa were probably over the South Pole.

There was no major volcanic activity during the Silurian; however, the period is marked by major {C}orogenic (mountain-building) events in eastern North America and in northwestern Europe, resulting in the formation of the mountain chains there. This was called the Caledonian Orogeny. In other areas, large igneous rock formations of the Middle Silurian arose, such as those in Central Europe, as well as light sedimentation throughout the Baltic region. While not characterized by dramatic tectonic activity, the Silurian world experienced gradual continental changes that would be the basis for greater global consequences in the future, such as those that created terrestrial ecosystems.

The Silurian oceans are also of particular interest for activity between the regions known as Laurentia, Baltica and Avalonia. The ocean basins between these areas substantially closed together, continuing a geologic trend that had begun much earlier. The new marine habitats produced by these profound changes in the Silurian seas provided the framework for significant biological events in the evolution of life. Coral reefs, for example, made their first appearances in the fossil record during this time.

The Silurian period was a time when the earth underwent considerable changes that had important repercussions for the environment and the life within it. The Silurian witnessed a relative stabilization of the world's general climate, ending the previous pattern of erratic climatic fluctuations. One significant feature of these changes was the melting of large glacial formations. This contributed to a substantial and significant rise in the levels of the major seas, creating many new marine habitats.

The Silurian period's condition of low continental elevations with a high global stand in sea level can be strongly distinguished from the present-day environment. This is a result of the flood of 65% of the shallow seas in North America during the Llandovery and Wenlock times. The shallow seas ranged from tropical to subtropical in climate. Commonly present in the shallow seas were coral mound reefs with associated carbonate sediments. Due to reduced circulation during the Ludlow and Pridoli times, the process of deposition of evaporites (salts) was set in motion. Some of these deposits are still found in northern Europe, Siberia, South China and Australia.

Further reading

The genesis of this article on the Silurian Period was written by Dan Fischer, Tammy (Yue) Liu, Emily Yip and Korsen Yu. Editors from the Encyclopedia of Earth have expanded and modified the original content.

Glossary

Citation

Paleontology, U. (2011). Silurian. Retrieved from http://www.eoearth.org/view/article/156027

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