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| Devonian (geology) | |
|---|---|
| Name | Devonian |
| Caption | Global paleogeography during the Devonian |
| Time start | 419.2 |
| Time end | 358.9 |
| Time unit | Ma |
| Color | #ffcc66 |
| Former names | Old Red Sandstone |
| Lithology | marine carbonates, siliciclastics, reef limestones |
| Fauna | Eurypterid, Placodermi, Sarcopterygii, Actinopterygii |
| Flora | Rhyniophyta, Lycopodiophyta, Archaeopteris |
| Preceded by | Silurian |
| Followed by | Carboniferous |
Devonian (geology) The Devonian is a geologic period spanning about 419.2–358.9 million years ago, notable for major diversification of vertebrate lineages, the rise of land plants, and extensive reef building. It witnessed critical faunal turnovers, plate reorganizations that influenced paleogeography, and sedimentary sequences exploited for hydrocarbons and metalliferous ores. Prominent regional stratotypes include successions in Britain, Germany, Canada, United States, and Australia.
The Devonian sits between the Silurian and the Carboniferous in the geologic timescale and is subdivided into Early, Middle, and Late epochs defined by Global Boundary Stratotype Sections and Points set by the International Commission on Stratigraphy. Classic Devonian localities include the Old Red Sandstone of Scotland, the reef complexes of Western Australia, the fossil beds of Miguasha, and the marine strata of the Holy Cross Mountains. The interval records interactions among evolving biotas such as Placodermi, Acanthodii, and early Tetrapoda with changing climates tied to assembly of Euramerica and Gondwana.
Devonian chronostratigraphy is anchored by conodont biostratigraphy and radiometric calibration from bentonite layers in sections like GSSP candidates in Záboří and La Serre. Stage names—Lochkovian, Pragian, Emsian, Eifelian, Givetian, Frasnian, Famennian—derive from type sections in Czech Republic, Germany, France, and Belgium. Lithostratigraphic units include the Old Red Sandstone Supergroup, the Hamilton Group, the Marcellus Formation, the Catskill Delta, and the Mancos Shale equivalents. Biostratigraphic markers encompass conodont genera like Palmatolepis and ammonoid lineages tracked in Morocco, Spain, and Bohemia.
Devonian environments ranged from epicontinental seas over Laurentia to reef-rimmed platforms around Siberia and submerged margins of Gondwana. Climatic conditions shifted from greenhouse warmth with high eustatic sea levels to cooler intervals associated with orogenic uplift in the Variscan belt and glacioeustasy linked to Antarctic-affiliated Gondwanan high latitudes. Depositional facies include carbonate buildups (e.g., Frasnian reefs), black shale anoxia exemplified by the Kellwasser Event horizons, deltaic progradations like the Catskill Delta, and fluvial floodplain deposits such as the Red Bed sequences of Euramerica.
The Devonian is famed as the "Age of Fishes" with diversification of Sarcopterygii including lungfish and Coelacanthiformes, expansion of Actinopterygii, and armored Placodermi dominating marine and estuarine ecosystems. Key fossil localities—Gogo Formation, Miguasha National Park, Rhynie Chert—preserve exceptional specimens of early Tetrapodomorpha, vascular plants like Rhynia and Archaeopteris, and invertebrates including Trilobita and Brachiopoda. Reef communities hosted Stromatoporoidea and Tabulata corals alongside eurypterids such as Pterygotus. Evolutionary milestones include the origin of tetrapods attested by fossils from Acanthostega-bearing beds, radiations of Ostracoderms, and terrestrial colonization by arthropods recorded in Rhynie Chert and Dunedin-area sediments.
Plate tectonic reconfigurations during the Devonian involved collision and suturing events building the Variscan, Acadian, and Antler orogens as Laurentia, Baltica, and Avalonia converged. The assembly of Euramerica altered ocean gateways and shelf architecture, influencing sediment supply to basins like the Rhine Graben and the Dniepr-Donets Basin. Mantle dynamics affected magmatism recorded in Viluy Traps-type provinces and in arc assemblages along the Tethys margin. Paleogeographic reconstructions place extensive shallow seas over Laurentia and passive margins along Gondwana, facilitating widespread carbonate platforms and reef growth.
Devonian strata host economically important resources: petroleum systems within the Marcellus Formation, gas-bearing shales in Bakken Formation-equivalents, and oil in reefal reservoirs such as the Wick-type carbonate buildups. Metallogenic processes concentrated lead, zinc, copper, and gold in strata-hosted deposits and orogenic veins across Europe and North America. Stratabound base metal deposits occur in the Devonian sequences of Ireland and Poland, while reef limestones serve as aquifers and building stone, quarried in regions like Belgium and Germany. Black shale units are source rocks for hydrocarbons and targets for shale-gas exploration in basins including Appalachia.
The Late Devonian encompasses a series of extinction pulses culminating in the Frasnian–Famennian crisis, linked to widespread anoxia, sea-level changes, and biotic stress on reef ecosystems. Correlative events include the Kellwasser Event and later the Hangenberg Event, with global records in Morocco, Kellwasser Limestone-bearing outcrops, and boreal successions. Drivers proposed by researchers include volcanic episodes, changes in terrestrial vegetation altering weathering and nutrient fluxes, and bolide impacts debated in connection with ejecta horizons. The post-extinction recovery set the stage for Carboniferous faunal assemblages and affected vertebrate, invertebrate, and plant evolutionary trajectories.