Ordovician

EtymologyName formalityFormalName ratified1960Usage informationCelestial bodyEarthRegional usageGlobal (ICS)Time scale(s) usedICS Time ScaleDefinitionChronological unitPeriodStratigraphic unitSystemFirst proposed byCharles Lapworth, 1879Time span formalityFormalLower boundary definitionFAD of the ConodontIapetognathus fluctivagusLower boundary GSSPGreenpoint section, Green Point, Newfoundland, Canada
49°40′58″N57°57′55″W / 49.6829°N 57.9653°W / 49.6829; -57.9653Lower GSSP ratifiedJanuary 2000Upper boundary definitionFAD of the GraptoliteUpper boundary GSSPDob's Linn, Moffat, U.K.
55°26′24″N3°16′12″W / 55.4400°N 3.2700°W / 55.4400; -3.2700Upper GSSP ratified1984Atmospheric and climatic dataSea level above present day180 m; rising to 220 m in Caradoc and falling sharply to 141 m in end-Ordovician glaciations

The Ordovician ( ) is a geologic period and system, the second of six periods of the PaleozoicEra, and the second of twelve periods of the PhanerozoicEon. The Ordovician spans 43.75 million years from the end of the Cambrian Period 486.85 Ma (million years ago) to the start of the Silurian Period 443.1 Ma.

The Ordovician, named after the Welsh tribe of the Ordovices, was defined by Charles Lapworth in 1879 to resolve a dispute between followers of Adam Sedgwick and Roderick Murchison, who were placing the same rock beds in North Wales in the Cambrian and Silurian systems, respectively. Lapworth recognized that the fossilfauna in the disputed strata were different from those of either the Cambrian or the Silurian systems, and placed them in a system of their own. The Ordovician received international approval in 1960 (forty years after Lapworth's death), when it was adopted as an official period of the Paleozoic Era by the International Geological Congress.

Life continued to flourish during the Ordovician as it had in the earlier Cambrian Period, although the end of the period was marked by the Ordovician–Silurian extinction events. Invertebrates, namely molluscs and arthropods, dominated the oceans, with members of the latter group probably starting their establishment on land during this time, becoming fully established by the Devonian. The first land plants are known from this period. The Great Ordovician Biodiversification Event considerably increased the diversity of life. Fish, the world's first true vertebrates, continued to evolve, and those with jaws may have first appeared late in the period. About 100 times as many meteorites struck the Earth per year during the Ordovician compared with today in a period known as the Ordovician meteor event. It has been theorized that this increase in impacts originated from a break up of an asteroid during this time.