The Ordovician (/ɔːr.dəˈvɪʃ.i.ən, -d-, -ˈvɪʃ.ən/ or-də-VISH-ee-ən, -doh-, -VISH-ən)[9] is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period 485.4 million years ago (Mya) to the start of the Silurian Period 443.8 Mya.[10]

485.4 ± 1.9 – 443.8 ± 1.5 Ma
Name formalityFormal
Name ratified1960
Usage information
Celestial bodyEarth
Regional usageGlobal (ICS)
Time scale(s) usedICS Time Scale
Chronological unitPeriod
Stratigraphic unitSystem
First proposed byCharles Lapworth, 1879
Time span formalityFormal
Lower boundary definitionFAD of the Conodont Iapetognathus fluctivagus
Lower boundary GSSPGreenpoint section, Green Point, Newfoundland, Canada
49.6829°N 57.9653°W / 49.6829; -57.9653
GSSP ratified2000[5]
Upper boundary definitionFAD of the Graptolite Akidograptus ascensus
Upper boundary GSSPDob's Linn, Moffat, U.K.
55.4400°N 3.2700°W / 55.4400; -3.2700
GSSP ratified1984[6][7]
Atmospheric and climatic data
Mean atmospheric O
c. 13.5 vol %
(68 % of modern)
Mean atmospheric CO
c. 4200 ppm
(15 times pre-industrial)
Mean surface temperaturec. 16 °C
(2 °C above modern)
Sea level above present day180 m; rising to 220 m in Caradoc and falling sharply to 140 m in end-Ordovician glaciations[8]

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.[11] Lapworth recognized that the fossil fauna 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 did 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. 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. Life had yet to diversify on land. About 100 times as many meteorites struck the Earth per year during the Ordovician compared with today.[12]