Ordovician

Ordovician
Name	Ordovician
Period	Paleozoic
Time start	485.4
Time end	443.8
Unit	Period
Preceded by	Cambrian
Followed by	Silurian

Ordovician The Ordovician was a geologic period of the Paleozoic Era lasting from about 485.4 to 443.8 million years ago. It followed the Cambrian and preceded the Silurian, and is characterized by major diversification of marine faunas, widespread carbonate platform deposition, and a profound end‑period glaciation that precipitated one of the largest Phanerozoic extinction events. Key figures and institutions in modern study include researchers at the Smithsonian Institution, the United States Geological Survey, and universities such as Cambridge University, University of Oxford, and University of Chicago.

Overview and Definition

The Ordovician was formalized during 19th‑century stratigraphic work by geologists linked to the Geological Society of London and stratigraphers at institutions like the British Geological Survey and the Geological Survey of Canada. Global chronostratigraphic boundaries were ratified by the International Commission on Stratigraphy with stages correlated using type sections in regions including Wales, Scotland, Morocco, and parts of China. Paleontological biostratigraphy often relies on taxa described in classic monographs from museums such as the Natural History Museum, London and the American Museum of Natural History.

Geology and Stratigraphy

Ordovician stratigraphy is recorded in marine successions such as the Bentonites of North America, the carbonate platforms of the Baltic Basin, and shales of the Tremadocian and Arenig equivalencies. Lithostratigraphic units include formations described by researchers at the United States Geological Survey and mapped in regions like the Appalachian Basin, the Cleveland Basin, and the Fennoscandian Shield. Volcaniclastic horizons correlated with ash beds (bentonites) have been tied to volcanic centers identified in reconstructions involving the Iapetus Ocean margins near Ireland and Greenland.

Paleoclimate and Oceanography

Climate reconstructions for the Ordovician integrate isotope work carried out at laboratories in institutions such as Stanford University, Princeton University, and the Max Planck Institute for Chemistry. Oxygen and carbon isotope excursions recorded in carbonate and phosphate minerals from sites like Anticosti Island, Svalbard, and Sichuan document a transition from greenhouse conditions to cooling culminating in Hirnantian glaciation tied to ice on Gondwana. Oceanographic models developed by teams at the National Oceanic and Atmospheric Administration and the Woods Hole Oceanographic Institution show changes in thermohaline circulation and stratification that influenced nutrient distributions and anoxic events recorded in black shales of the Cincinnati Basin.

Biodiversity and Major Life Forms

The Ordovician witnessed the Great Ordovician Biodiversification Event (GOBE), chronicled by paleontologists at the Smithsonian Institution and in journals edited by the Royal Society. Marine communities expanded with diverse trilobites, brachiopods, bryozoans, cephalopods, and early crinoids known from classic localities including Burgess Shale analogues, Montañón Blanco, and the Floian successions. Graptolites used for biostratigraphy were described from outcrops in Scotland and Argentina. Early land plants and fungal remains reported from Antarctica and South China suggest terrestrial colonization preceding the Silurian. Studies from teams at Yale University and Harvard University advanced understanding of morphological disparity among cephalopods, while researchers at the Natural History Museum, London documented bryozoan radiations.

Extinction Events and Biotic Turnover

The end‑Ordovician mass extinction, widely analyzed by researchers at the University of California, Berkeley and the British Antarctic Survey, involved two closely spaced pulses associated with cooling and subsequent warming. Isotopic studies by groups at the University of Wyoming and the University of Leeds link glacioeustatic sea‑level fall and habitat loss on continental shelves to turnovers in benthic and pelagic faunas. The event reshaped trilobite assemblages and brachiopod dominance, with ecological recovery patterns traced in post‑extinction faunas preserved in the Arenig and Llanvirn sequences.

Paleogeography and Plate Tectonics

Ordovician paleogeography maps produced by teams at the Paleomap Project and the Utrecht University place major continental masses such as Laurentia, Baltica, Siberia, and Gondwana in configurations that influenced ocean basins like the Iapetus and the Paleo‑Tethys Ocean. Plate reconstructions using paleomagnetic data from expeditions associated with Lamont‑Doherty Earth Observatory and the Geological Survey of Japan document continental drift, terrane accretion, and the closing of oceanic gateways that affected climate and biogeography. Collisional events along margins now preserved in the Caledonides and Taconic orogen shaped sediment supply to basins such as the Cleveland Basin.

Economic and Scientific Significance

Ordovician rocks are economically important for resources studied by the United States Geological Survey and energy companies, hosting reservoirs, mineral deposits, and source rocks: notable examples include phosphate beds of Morocco, lead‑zinc ores of the Mississippi Valley Type, and oil and gas plays in the Williston Basin. Scientific significance encompasses contributions to understanding macroevolutionary patterns, driven by work published in outlets like Nature and Science and by collaborations involving the International Union of Geological Sciences and the Paleontological Society. Ongoing field programs in regions such as Wales, Sichuan, Anticosti Island, and Svalbard continue to refine stratigraphic frameworks and evolutionary narratives.

Category:Geologic periods