Silurian period
Silurian period
| Noun | 1. |  Silurian period - from 425 million to 405 million years ago; first air-breathing animals |
| 单词 | silurian period | |||
| 释义 | Silurian periodSilurian period
Silurian periodSee also: Geologic Timescale (table)Geologic Timescale Silurian period(sĭlo͝or`ēən, sī–) [from the Silures, ancient tribe of S Wales, where the period was first studied; named by the British geologist R. I. MurchisonMurchison, Sir Roderick Impey, 1792–1871, British geologist. He served in the Napoleonic Wars but after the peace turned his attention to science. In the 1830s he undertook the investigation of previously undifferentiated rock strata in Wales and England; as a result of ..... Click the link for more information. ], third period of the Paleozoic eraPaleozoic era , a major division (era) of geologic time (see Geologic Timescale, table) occurring between 570 to 240 million years ago. It is subdivided into six periods, the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian (see each listed individually). ..... Click the link for more information. of geologic time (see Geologic TimescaleGeologic Timescale Era Period Epoch Approximate duration (millions of years) Approximate number of years ago (millions of years) Cenozoic Quaternary Holocene 10,000 years ago to the present Pleistocene 2 . ..... Click the link for more information. , table) lasting from 405 to 435 million years ago. The continents in the Silurian period remained much as they had been in the preceding Ordovician periodOrdovician period [from the Ordovices, ancient tribe of N Wales], second period of the Paleozoic era of geologic time (see Geologic Timescale, table) from 505 to 438 million years ago. ..... Click the link for more information. , with approximately the same areas being subject to flooding by shallow seas. The earth was relatively tectonically inactive during the Silurian. The Appalachian Mountains, which uplifted during the Ordovician, were being eroded. Large coral reefs and algae were abundant, indicating that warm, shallow seas predominated. Major evaporite basins, including the circular Michigan Basin, showed evidence of subsidence. The transition between the Ordovician and Silurian rocks is not clearly marked in the United States. The Medina sandstone extending from New York to Alabama has been assigned to both periods but is generally considered to be Silurian. Three main series, based on the succession of strata in New York state, are usually distinguished—the lower Silurian (Medinan, or Alexandrian, series), the middle Silurian (Niagaran series), and the upper Silurian (Cayugan series). The early Silurian deposits in the East are commonly sandstone, shale, and conglomerate, comprising erosion products from high-standing mountains; in the West, marine limestone predominates. There were also desert conditions, under which the Salinan "red beds" of the Appalachian area and the salt deposits of New York, Michigan, Ontario, and Ohio were formed. Some areas were later reflooded, depositing Cobleskill and Rondout limestone of New York. The Silurian of the Far West is as yet not well established. In North America, the Silurian ended quietly; however, in the British Isles, Scandinavia, and France, as a result of the Caledonian disturbance, great mountains continued to be thrust up. Economic resources of the Silurian strata, besides salt, are iron ore (near Birmingham, Ala.) and quartz sandstone, used in glass manufacture. Dominating the life of the Silurian were marine invertebrates, including crinoids and cystoids, mollusks, and eurypterids, invertebrates related to crabs and insects. Members of the trilobite family were still numerous; primitive fishes increased in number. Also notable in the Silurian fauna were scorpions, possibly the first animals to live on land and take their oxygen from the air. Silurian periodSilurian periodthe period of the PALAEOZOIC ERA lasting from 445 million BP to 415 million BP, during which trilobites declined. The first land plants appeared, though they probably evolved in the Ordovician. Armoured jawless fish and eurypterids were common. see GEOLOGICAL TIME.Silurian period
Synonyms for Silurian period
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