The Silurian period of the Paleozoic era, 440-410 million years ago, follows the Ordovician period of the Paleozoic, "the age of ancient life." The Silurian was named by the R. I. Murchison in 1835 in honor of the Silure
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tribe of Celts who had inhabited the Welsh borderlands where he first studied these rocks.
A long, warm, stable period followed the ice age and mass extinctions of the Ordovician, when over half of all previous life forms became extinct. Silurian fauna built upon the evolutionary patterns that had preceded it. No major new groups of invertebrates appeared, although a great radiation in number and form of existing invertebrates occurred. It has been said that the evolution of life cannot be separated from the evolution of the planet. In the Silurian, geologic trends greatly influenced animal development. As the two great supercontinents, Laurasia in the Northern Hemisphere and Gondwanaland in the Southern Hemisphere, once again drifted toward one another on their tectonic plates, mountains were heaved up to form distinct ecosystems where species could evolve uninfluenced by one another. And as the glaciers began to melt, warm, shallow seas flooded much of Laurasia, providing ideal conditions for a variety of benthic (bottom-dwelling) species. These included a rich variety of sea lilies, lampshells, trilobites, graptolites, and mollusks. The crinoidal sea lilies and graptolites are particularly interesting. They are both echinoderms: small, soft-bodied wormlike creatures that lack a normal head but have a well-developed nervous system located in a rudimentary notochord ("backchord"), the precursor of a backbone. The presence of a notochord makes these once-abundant sea-floor scavengers the ancestors of the chordates, animals with backbones.
A recurrent theme occurs in the origin of new marine species. They first tend to appear in very shallow waters along the shore, then disperse into deeper habitats. The shoreline is a harsh area of constant tides, storms with silt flows, and temperature fluctuations. These conditions favor species that are resilient and adaptive. Gradually the offspring expand into deeper water. The new forms of all the existing invertebrates followed this pattern: the brachiopods , sponges, bryozoans, arthropods , and echinoderms, as well as the vertebrate fishes. In the deeper waters, mobile predators appeared in unprecedented sizes. The free-swimming nautiloids, which grew up to 3 meters (10 feet) long and the eurypterids, sea-scorpion arthropods at 2-meters (6-feet) long, fed on the vast numbers of early jawed fishes that now appeared. The great reefs destroyed by the ice age were rebuilt, coral by individual coral.
The most noteworthy event of the Silurian (from the human point of view) took place on land. The first minuscule plants began to creep across the previously barren land masses, followed by tiny scorpions and millipedes. The whiskery, or pleated, tracks of arthropods appear in the Silurian rocks of western Australia, and for a brief while these arthropods dominated Earth. The formerly rare agnathans (jawless fishes), became plentiful and began to explore up the brackish estuaries and into the freshwater rivers and upstream pools where they flourished.
see also Geological Time Scale.
Asimov, Isaac. Life and Time. Garden City, NY: Doubleday & Company, 1978.
Fortey, Richard. Fossils: The Key to the Past. Cambridge, MA: Harvard University Press, 1991.
———. Life: A Natural History of the First Four Billion Years of Life on Earth. New York: Viking Press, 1998.
Gould, Stephen Jay, ed. The Book of Life. New York: W. W. Norton & Company, 1993.
Lambert, David. The Field Guide to Prehistoric Life. New York: Facts on File, 1985.
McLoughlan, John C. Synapsida: A New Look Into the Origin of Mammals. New York: Viking Press, 1980.
Steele, Rodney and Anthony Harvey, eds. The Encyclopedia of Prehistoric Life. New York: McGraw Hill, 1979.
Wade, Nicholas, ed. The Science Times Book of Fossils and Evolution. New York: The Lyons Press, 1998.
In geologic time , the Silurian Period, the third period of the Paleozoic Era , covers the time from roughly 440 million years ago (mya) until 410 mya. The name, Silurian, derives from the Silures, an ancient British tribe. The Silurian Period spans two epochs. The Early Silurian Epoch is the most ancient, followed by the Late Silurian Epoch.
The Early Silurian Epoch is divided chronologically (from the most ancient to the most recent) into the Llandoverian and Wenlockian stages. The Late Silurian Epoch is divided chronologically (from the most ancient to the most recent) into the Ludlovian, and Pridolian stages.
In terms of paleogeography (the study of the evolution of the continents from supercontinents and the establishment of geologic features), the Silurian Period featured cleavage of some supercontinent landmass and fusion of plates into Laurussia. Collision with remnants of other continents later formed the supercontinent Laurasia and eventually the super-continent Pangaea.
The fossil record establishes that the preceding Ordovician Period ended with a mass extinction. This mass extinction, approximately 440 mya, marked the end of the Ordovician Period and the start of the Silurian Period. In accord with a mass extinction, many fossils dated to the Ordovician Period are not found in Silurian Period formations. Differentiated by fossil remains and continental movements, the Devonian Period followed the Silurian Period.
The Silurian Period marked a geologically active period for volcanic activity. The accompanying ash deposits and lava flows are clearly evident in Silurian Period strata.
The fossil record indicates that it was during the Silurian Period that marine species made the evolutionary transition to terrestrial (land-based species). The first true insect fossils date to this period, as do fossils of jawed fish. Atmospheric changes, driven by increasingly diverse plant life, allowed the further development of the protective ozone layer, which filters out harmful ultraviolet radiation.
See also Archean; Cambrian Period; Cenozoic Era; Cretaceous Period; Dating methods; Eocene Epoch; Evolution, evidence of; Fossils and fossilization; Historical geology; Holocene Epoch; Jurassic Period; Mesozoic Era; Miocene Epoch; Mississippian Period; Oligocene Epoch; Paleocene Epoch; Pennsylvanian Period; Phanerozoic Eon; Pleistocene Epoch; Pliocene Epoch; Precambrian; Proterozoic Era; Quaternary Period; Tertiary Period; Triassic Period
In the mid 19th century, Silurian was used to denote the third period of the Palaeozoic era, between the Ordovician and Devonian periods, which lasted from about 439 to 409 million years ago. The first true fish and land plants appeared, and the end of the period is marked by the climax of the Caledonian mountain-forming.