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| Ordovician–Silurian extinction events | |
|---|---|
| Name | Ordovician–Silurian extinction events |
| Date | ~445–443 million years ago |
| Cause | Glaciation, sea-level fall, anoxia, volcanism (debated) |
| Severity | Second-largest Phanerozoic extinction by diversity loss |
Ordovician–Silurian extinction events The Ordovician–Silurian extinction events were a major pair of Paleozoic biodiversity crises that eliminated a large fraction of marine taxa near the end of the Ordovician and the beginning of the Silurian. These events coincide with profound changes in paleoclimate, sea level, and ocean chemistry recorded across worldwide geologic formations, and they shaped subsequent evolution during the Silurian period and Devonian radiation. Scholarly debate involves links to glaciation on Gondwana, episodes of ocean anoxia, and contemporaneous tectonic and volcanic episodes.
The extinction couplet comprises two principal pulses that reduced global marine biodiversity, affecting numerous brachiopod, bryozoan, trilobite, and graptolite lineages documented in the Fossil record, Burgess Shale-era comparisons, and regional biostratigraphic schemes such as the Caradoc and Ashgill stages. Paleontologists working in contexts of the Paleozoic macrofossil databases, museum collections like the Natural History Museum, London, and field programs in regions such as the Baltic region, Laurentia, and Gondwana have contributed crucial faunal lists and extinction metrics. International collaborations using frameworks from organizations like the International Commission on Stratigraphy inform correlation across separate sedimentary basins.
High-resolution stratigraphy ties the two main pulses to late Katian and early Hirnantian intervals of the Ordovician, with a subsequent minor crisis at the Ordovician–Silurian boundary recorded in Llandovery successions. Radiometric calibration that incorporates dates from zirconbearing ashbeds and isotopic ties to the GSSP network refine timing to a window centered near 445–443 Ma. Biostratigraphic markers such as graptolite zonation and conodont turnover delineate the pulses across the Iapetus Ocean margins, the Avalonia terrane, and the Perunica microcontinent.
Leading hypotheses emphasize glacio-eustatic sea-level fall associated with growth of continental ice on Gondwana, driven by changes in atmospheric carbon dioxide and weathering fluxes influenced by mountain-building events like the Taconic orogeny and Caledonian orogeny. Alternative or complementary mechanisms include widespread marine anoxia tied to ocean circulation changes, perturbations in the marine carbon cycle recorded in carbonate sequences, and episodic volcanism such as large igneous province emplacement comparable in role to the Siberian Traps in later crises. Solar luminosity, orbital forcing (Milankovitch cycles), and biotic feedbacks involving early terrestrial plants are also evaluated in multidisciplinary studies by researchers from institutions including University of Cambridge and Smithsonian Institution.
The extinction disproportionately affected sessile benthic communities—brachiopods, bryozoans, echinoderms, and reef-forming organisms—while nektonic and planktonic groups such as graptolites and planktonic trilobites show significant turnover. Recovery during the early Silurian involved faunal restructuring, opportunistic radiations, and the emergence of new reef frameworks documented in the Wenlock and Llandovery faunas; these patterns are central to macroevolutionary studies by scholars using methods from the Paleobiology Database and quantitative models developed at universities such as Harvard University. The events set the stage for later ecological innovations culminating in Silurian-Devonian vertebrate and terrestrial plant expansions documented in classic localities like Lagerstätten and regional sequences in North America and Europe.
Stratigraphic records from carbonate platforms and black shale successions in areas including the Czech Republic, Scotland, Morocco, China, and the Appalachians preserve facies changes, unconformities, and biotic turnovers that correlate with the extinction pulses. Key sections such as the Dob's Linn locality historically informed debates and GSSP discussions, while conodont and graptolite biozones help align records from Baltica and Siberia. Sequence stratigraphy demonstrates repeated regression–transgression cycles, while sedimentary indicators like glacial diamictites on Gondwana margins record ice-advance phases.
Isotopic excursions in carbon (δ13C), oxygen (δ18O), and strontium (87Sr/86Sr) are globally documented in Ordovician–Silurian successions and used to infer carbon-cycle perturbations, cooling trends, and enhanced weathering. Trace-metal enrichments (e.g., molybdenum, uranium) and pyrite framboid data in black shales serve as redox proxies for ocean anoxia and euxinia, while mercury anomalies have been investigated for links to volcanic emissions. Paleotemperature reconstructions employ clumped isotope thermometry and conodont apatite oxygen isotopes, integrated with climate modeling studies conducted on platforms used by groups affiliated with NASA and major research universities.
The Ordovician–Silurian extinction events reshaped Paleozoic ecosystems, eliminating dominant Ordovician reef builders and enabling Silurian ecospace reorganization that influenced later diversification trajectories culminating in the Devonian fish and plant radiations. The events are a reference point in mass extinction theory alongside the End-Permian extinction and Cretaceous–Paleogene extinction, and they continue to inform debates on climate sensitivity, extinction selectivity, and recovery dynamics addressed in publications from outlets like Nature (journal) and Science (journal). Paleontological and stratigraphic work in global field sites and museum collections keeps refining the timeline and mechanisms that made these pulses a pivotal episode in Earth history.
Category:Mass extinctions Category:Paleozoic events