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Devonian

The Devonian Period is a geologic period and system within the Paleozoic Era of the Phanerozoic Eon, spanning from 419.62 ± 1.36 million years ago to 358.86 ± 0.19 million years ago.^[1] Known as the "Age of Fishes," it marked a time of profound evolutionary innovation among vertebrates, particularly the diversification and dominance of various fish groups in marine, brackish, and freshwater environments.^[2]^[3] During the Devonian, life on Earth underwent transformative changes across multiple domains. In the oceans, jawless fish like osteostracans coexisted with early jawed fishes, including placoderms such as Dunkleosteus terrelli (reaching lengths of about 3.4 meters or 11 feet) and bottom-dwelling species like Bothriolepis canadensis (around 1 foot or 30 cm long), alongside the ancestors of modern sharks and ray-finned fishes.^[3]^[4] Sharks, bony fishes, ammonoids, brachiopods (the most abundant marine invertebrates), and declining trilobites contributed to peak marine faunal diversity in the Paleozoic.^[5] On land, vascular plants exploded in diversity, forming the first forests with species like Archaeopteris and lycophytes that developed roots, leaves, woody tissues, and seeds, while the earliest soils and insects (such as collembolans) and arachnids like spiders and scorpions appeared.^[2]^[5] Early tetrapods, the four-limbed ancestors of land vertebrates, evolved from lobe-finned fish around 375 million years ago, exemplified by transitional "fishapods."^[3]^[5] Geologically, the period featured warm, equable climates with high sea levels that facilitated extensive shallow seas and carbonate deposition, though mid-Devonian global cooling occurred due to plant-driven reductions in atmospheric CO₂.^[5] Tectonic activity intensified as the supercontinents Euramerica (Laurentia-Baltica) and Gondwana collided, forming mountain ranges like the Appalachians and Caledonians, while sediments such as shales, siltstones, sandstones, and limestones accumulated in foreland basins.^[5]^[6] The Devonian concluded with a prolonged mass extinction event, eliminating approximately 20–22% of marine families and up to 75% of species, possibly linked to global cooling, anoxia, or atmospheric changes.^[2]^[5] These developments laid critical foundations for subsequent terrestrial and marine ecosystems in the Carboniferous Period.

Overview

Position and Duration

The Devonian Period represents the fourth chronological division of the Paleozoic Era in the geologic timescale, following the Silurian and preceding the Carboniferous. It spans approximately 60.8 million years, from 419.62 ± 1.36 Ma to 358.86 ± 0.19 Ma, according to the latest International Chronostratigraphic Chart.^[1] This timeframe positions the Devonian as a pivotal interval in Paleozoic history, bridging the late Silurian's end at around 419.62 Ma and the Carboniferous onset at 358.86 Ma, with the period's duration reflecting a relatively stable but eventful phase of Earth evolution.^[1] The lower boundary of the Devonian, marking the Silurian-Devonian transition, is defined at the Global Stratotype Section and Point (GSSP) in the Klonk section near Suchomasty, Czech Republic, by the first appearance datum (FAD) of the graptolite Monograptus uniformis.^[7] This biostratigraphic marker is supplemented by the near-contemporaneous FAD of the conodont Icriodus hesperius, which aids global correlation in shallow-marine sequences.^[8] The upper boundary, delineating the Devonian-Carboniferous transition, is established at the GSSP in the La Serre section, Montagne Noire, France, by the FAD of the conodont Siphonodella sulcata within its evolutionary lineage from S. sandbergi.^[9] This boundary coincides with the Hangenberg extinction event, a major biotic turnover that underscores the period's closure. These chronostratigraphic boundaries have been refined through radiometric dating techniques, particularly U-Pb isotope dilution-thermal ionization mass spectrometry (ID-TIMS) on zircon crystals from volcanic ash beds (bentonites) interlayered with fossiliferous strata. For the lower boundary, U-Pb dates from uppermost Silurian and lowermost Devonian ashes yield ages clustering around 419-422 Ma, with the current calibration incorporating error margins of ±1.36 Ma to integrate biostratigraphy and geochronology. Similarly, for the upper boundary, high-precision U-Pb analyses of uppermost Famennian and lowermost Tournaisian bentonites yield ages around 358-360 Ma, supporting the interpolated boundary age of 358.86 ± 0.19 Ma per the 2024 International Chronostratigraphic Chart.^[1]^[10] These methods enhance precision by minimizing inheritance and lead loss in zircons, ensuring robust anchoring of the Devonian timescale.^[11]

Significance

The Devonian Period is renowned as the "Age of Fishes" due to the remarkable diversification of jawed vertebrates, which marked a profound shift from invertebrate-dominated marine ecosystems to vertebrate prominence. During this time, groups such as placoderms, chondrichthyans (early sharks and rays), and osteichthyans (bony fishes) underwent rapid adaptive radiations, filling diverse ecological niches in ancient oceans and freshwater environments. This evolutionary explosion not only established the foundational lineages of modern fish but also set the stage for subsequent vertebrate transitions to land, fundamentally altering the trajectory of animal life on Earth.^[2]^[3] The period also witnessed the onset of widespread terrestrial ecosystems, with the emergence of the first true forests dominated by vascular plants like Archaeopteris and early lycopods, alongside the development of complex soils through root systems that stabilized landscapes and enhanced nutrient cycling. These innovations transformed Earth's surface, promoting the colonization of land by arthropods and early tetrapods while significantly influencing global biogeochemical cycles; notably, plant-driven photosynthesis contributed to an atmospheric oxygen rise to approximately 20%, enabling more active aerobic life forms and reshaping climate dynamics.^[12]^[13]^[14] Geologically, the Devonian played a crucial role in the assembly of the supercontinent Euramerica through the collision of Laurentia, Baltica, and Avalonia, which influenced ocean circulation patterns and contributed to episodic ocean anoxia events that disrupted marine productivity and biodiversity. These anoxic episodes, linked to tectonic reconfiguration and nutrient runoff from emerging land plants, provide key insights into how continental dynamics can drive global environmental crises, with lasting effects on the distribution of modern biodiversity hotspots.^[15]^[16]^[17] Scientifically, the Devonian offers pivotal evidence for evolutionary transitions, exemplified by fossils like Tiktaalik roseae, a 375-million-year-old sarcopterygian fish with limb-like fins and neck mobility that bridges aquatic and terrestrial vertebrates, supporting Darwinian gradualism in the fish-to-tetrapod shift. Economically, Devonian strata hold substantial hydrocarbon resources, such as the Bakken Formation in the Williston Basin, which has yielded billions of barrels of tight oil and natural gas through advanced extraction techniques, underscoring the period's ongoing resource significance for energy production.^[18]^[19]^[20]^[21]

History of Study

Naming and Discovery

The Devonian period was formally named in 1839 by the British geologists Roderick Impey Murchison and Adam Sedgwick, who designated it after the county of Devon in southwest England, where they extensively studied a thick sequence of sedimentary rocks containing distinctive fossils.^[2] These rocks, primarily marine limestones and shales, were initially examined in the 1830s around Torquay and the South Hams region, revealing a stratigraphic interval intermediate between the underlying Silurian and overlying Carboniferous systems.^[22] Murchison and Sedgwick's joint publication in the Geological Society of London Transactions established the Devonian as a distinct geological system based on these exposures. In the early 19th century, Murchison and Sedgwick's efforts focused on distinguishing Devonian strata from adjacent periods, particularly through comparisons with the non-marine Old Red Sandstone formations in Scotland and Wales, which had previously been misclassified as part of the basal Carboniferous.^[22] Murchison's fieldwork in 1836–1838 mapped these red beds and correlated them with marine equivalents in Devon, while Sedgwick contributed paleontological analyses to separate them from Silurian rocks below.^[23] Further confirmation came from Murchison's 1839 observations in Germany's Rhineland, where he identified similar marine sequences with shared fossil assemblages, solidifying the system's European extent.^[2] Initial fossil evidence supporting the Devonian's recognition emerged in the 1820s and 1830s from European strata, including marine invertebrates such as trilobites (e.g., Phacops species) and brachiopods (e.g., Atrypa and Spirifer), which were collected from Devonian limestones in Devon and Belgium.^[22] Fish remains, particularly from the Old Red Sandstone, were pivotal; Swiss naturalist Louis Agassiz described over 100 species of fossil fishes, including placoderms and osteichthyans, in his 1833–1844 Recherches sur les Poissons Fossiles, linking continental and marine deposits.^[24] These discoveries, including Agassiz's 1834 identification of a fish scale in a marine limestone interlayered with Old Red Sandstone equivalents, provided critical biostratigraphic markers.^[2] Correlation challenges, stemming from regional variations in facies and fossil preservation, were largely resolved in the 1840s through international collaborations. Murchison, accompanied by French paleontologist Édouard de Verneuil and Russian count Alexander von Keyserling, conducted expeditions across Europe and Russia in 1840–1841, mapping Devonian rocks from the Rhine Valley to the Ural Mountains and confirming their global coherence via shared invertebrate and fish faunas.^[25] Their findings, detailed in the 1845 The Geology of Russia in Europe and the Ural Mountains, established the Devonian as a standardized system recognized beyond Britain.^[26]

Key Developments in Research

In the mid-20th century, the emergence of plate tectonics fundamentally reshaped interpretations of Devonian paleogeography, highlighting the dynamics of supercontinent assembly. Edward Bullard and colleagues' 1965 quantitative analysis of continental fits around the Atlantic provided empirical support for continental drift, enabling reconstructions of Euramerica—a key Devonian landmass formed by the amalgamation of Laurentia and Baltica—as a stable craton bordered by active subduction zones.^[27] By the 1970s and 1980s, paleomagnetic data further refined these models, demonstrating how subduction between Gondwana and Euramerica initiated collisions that influenced Devonian sediment distribution and mountain-building, such as the Acadian orogeny.^[28]^[29] Advances in biostratigraphy from the 1950s onward leveraged microfossils like conodonts and graptolites to achieve high-resolution global correlations of Devonian sequences, surpassing earlier lithostratigraphic approaches. Conodont apparatuses, with their rapid evolutionary turnover, became standard for zoning marine deposits, while graptolites offered zonal markers for deeper-water facies.^[30] A pivotal milestone was the 1972 ratification of the Global Stratotype Section and Point (GSSP) at Klonk Hill, Czech Republic, defining the Silurian-Devonian boundary by the first appearance datum of the graptolite Uncinatograptus uniformis uniformis in bed 20, supplemented by conodont biozonation for interregional ties.^[31] Subsequent refinements, including integrated conodont-graptolite schemes, have enhanced precision to the sub-stage level across hemispheres.^[32] Geochemical and isotopic investigations, intensifying from the 1980s, revealed episodic ocean anoxia and carbon cycle disruptions through analyses of δ¹³C excursions in carbonates and organics. Early studies identified positive shifts linked to enhanced organic burial during black shale intervals, such as the mid-Frasnian punctata event, signaling nutrient-driven productivity spikes and euxinic conditions.^[33] By the 1990s and 2000s, high-resolution profiles confirmed multiple excursions—often +2 to +5‰—correlating with eustatic changes and anoxic events like the Kellwasser, providing causal links to biodiversity declines without relying solely on biostratigraphy.^[34]^[35] Post-2000 fossil discoveries and molecular approaches have illuminated Devonian transitions in terrestrial colonization. The 2004 unearthing of Tiktaalik roseae on Ellesmere Island, Canada—a 375-million-year-old sarcopterygian with robust fins, neck mobility, and wrist-like elements—bridged fish and tetrapod anatomies, reshaping narratives of limb evolution in shallow-water habitats.^[36] Concurrently, phylogenomic analyses of extant bryophytes and vascular plants, calibrated against Devonian fossils like those from the Rhynie Chert, uncovered ancient whole-genome duplications around 400 million years ago, driving morphological innovations in early tracheophytes such as vascular tissues and upright growth.^[37]

Subdivisions

Early Devonian

The Early Devonian Epoch, the initial subdivision of the Devonian Period, encompasses three chronostratigraphic stages: the Lochkovian, Pragian, and Emsian. The Lochkovian Stage spans from 419.62 ± 1.36 Ma to 413.02 ± 1.91 Ma, the Pragian from 413.02 ± 1.91 Ma to 410.62 ± 1.95 Ma, and the Emsian from 410.62 ± 1.95 Ma to 393.47 ± 0.99 Ma.^[38] These stages are defined primarily through conodont biostratigraphy, with the base of the Lochkovian (and thus the Devonian Period) marked by the first appearance datum of the conodont Icriodus woschmidti at the Global Stratotype Section and Point (GSSP) located at Klonk in the Czech Republic.^[39] The Pragian base is defined by the first occurrence of Eognathodus sulcatus sulcatus, while the Emsian-Pragian boundary relies on zones such as the Icriodus steinhaueri assemblage.^[40] The climate during the Early Devonian transitioned from the relatively warm Silurian conditions, maintaining overall greenhouse-like warmth with limited evidence of polar ice caps, though atmospheric CO₂ levels began a gradual decline toward the epoch's end.^[41] Global sea levels were generally high, with eustatic fluctuations including an initial rise following late Silurian regressions, leading to widespread shallow marine inundations across equatorial regions.^[42] In Euramerica, this transgressive regime facilitated the initial deposition of the Old Red Sandstone, a thick sequence of red-bed continental sediments comprising sandstones, conglomerates, and mudstones formed in alluvial and lacustrine settings, reflecting arid to semi-arid conditions with episodic fluvial input.^[43] Tectonically, the Early Devonian marked a phase of relative stability following the Acadian phase of the Caledonian Orogeny, which had culminated in the late Silurian collision of Laurentia and Baltica to form Euramerica.^[44] This post-orogenic relaxation led to extensional collapse in foreland basins, promoting the development of shallow marine shelves, deltaic systems, and intracratonic basins without intense compressional deformation.^[45] Environments were dominated by low-gradient fluvial-deltaic plains transitioning seaward into epicontinental seas, with sediment provenance largely from the eroding Caledonide highlands.^[46] Biologically, the Early Devonian saw the continued diversification of jawless fishes (agnathans), including ostracoderms such as pteraspids and thelodonts, which adapted to nearshore and freshwater habitats amid stable marine conditions.^[47] Jawed vertebrates, particularly primitive placoderms like the antiarchs and arthrodires, underwent initial radiations, originating in the late Silurian but achieving greater ecological roles in shallow-water ecosystems by the Lochkovian and Pragian.^[48] No major extinction events disrupted these communities, allowing steady evolutionary expansion. On land, palynological records indicate an increase in trilete spores from early vascular plants, signaling enhanced terrestrial colonization and sporophyte dominance in cooksonioid and protolepidodendroid forms.^[49]

Middle Devonian

The Middle Devonian Epoch, spanning approximately 393.5 to 382.3 million years ago, is subdivided into the Eifelian Stage (393.47 ± 0.99 to 387.95 ± 1.04 Ma) and the Givetian Stage (387.95 ± 1.04 to 382.31 ± 1.36 Ma).^[38] These divisions are defined primarily through conodont biostratigraphy, with the base of the Eifelian marked by the first appearance datum (FAD) of the conodont Polygnathus costatus partitus at the Global Stratotype Section and Point (GSSP) in Wetteldorf, Germany.^[50] The Eifelian-Givetian boundary is delineated by the FAD of Polygnathus hemiansatus at the GSSP in Jebel Mech Irdane, Morocco.^[51] Conodont taxa such as Polygnathus serotinus also characterize assemblages near the Emsian-Eifelian transition, providing key markers for global correlation of Middle Devonian strata.^[52] This epoch marked the acme of Devonian tropical climates, characterized by warm global temperatures and elevated sea levels that inundated continental margins, fostering expansive epicontinental seas.^[42] Highstand conditions promoted the proliferation of carbonate platforms across low-latitude regions, where stromatoporoid-coral buildups contributed to thick sequences of limestones, as seen in the Appalachian and Michigan basins.^[53] Restricted circulation in these shallow basins led to the deposition of evaporites, including halite and anhydrite in formations like the Prairie Evaporite in the Williston Basin, reflecting arid to semi-arid conditions in interior seaways.^[54] A prominent feature was the Taghanic transgression in the middle Givetian (~385 Ma), a eustatic sea-level rise that flooded vast continental areas, particularly along the eastern margin of Laurentia, resulting in the deposition of organic-rich shales and enhanced marine incursion.^[55] At the Eifelian-Givetian boundary (~388 Ma), the Kačák Event represented a minor extinction pulse, linked to sea-level fluctuations and transient anoxia, which impacted ~20% of brachiopod genera and other benthic invertebrates while facilitating biotic turnover.^[56] This period also witnessed the evolutionary radiation of advanced armored fishes, notably placoderms such as arthrodires, which diversified in shallow marine habitats and achieved ecological dominance among early gnathostomes.^[57]

Late Devonian

The Late Devonian epoch, spanning the Frasnian and Famennian stages, represents the final subdivision of the Devonian Period, characterized by significant stratigraphic developments and environmental transitions. The Frasnian Stage extends from approximately 382.31 ± 1.36 Ma to 372.15 ± 0.46 Ma, with its base defined at the first appearance datum (FAD) of the conodont Ancyrodella rotundiloba in the lower asymmetricus Zone at the Global Stratotype Section and Point (GSSP) in the Col du Puech de la Suque section, Montagne Noire, France.^[58]^[59] The succeeding Famennian Stage ranges from 372.15 ± 0.46 Ma to 358.86 ± 0.19 Ma, its base marked by the FAD of the conodont Palmatolepis triangularis in the lower triangularis Zone at the GSSP above the upper Coumiac Quarry, also in the Montagne Noire, France, coinciding with the extinction of genera such as Ancyrodella and Ozarkodina.^[58]^[60] These boundaries, delineated through conodont biostratigraphy, highlight the epoch's reliance on microfossil zonations for precise correlation across global sections, including zones like the Frasnian Palmatolepis punctata for intra-stage events.^[61] Environmental conditions during the Late Devonian shifted toward cooler climates and regressive sea levels, driven primarily by a marked decline in atmospheric CO₂ levels that dominated temperature evolution and promoted at least episodic glaciation.^[42] This cooling trend facilitated a major eustatic sea-level fall exceeding 100 m by the late Famennian, culminating in the deposition of the Hangenberg black shales, which record widespread marine anoxia and restricted basin conditions at the Devonian-Carboniferous boundary.^[62] Concurrently, oceanic anoxia expanded globally, as evidenced by organic-rich black shales and geochemical proxies indicating photic-zone euxinia and nutrient-driven stagnation, particularly during events like the Kellwasser and Hangenberg.^[63] The intensification of the Acadian phase of the Appalachian orogeny, reaching its climax in the Late Devonian, contributed to these dynamics through enhanced sediment influx and tectonic uplift along eastern Laurentia, influencing basin evolution and coastal sediment supply. Biologically, the Late Devonian prelude to biotic turnover featured a peak in fish diversity, with jawed vertebrates achieving unprecedented marine proliferation before the Kellwasser event around 372 Ma severely impacted placoderms and other groups.^[64] This diversification underscored the epoch's role as a critical juncture for vertebrate evolution, including the initial appearances of early tetrapods in the Famennian, such as stem-group forms with limb-like fins transitioning from aquatic to semi-terrestrial habitats.^[65]

Paleoenvironment

Climate

The Devonian Period was characterized by overall greenhouse conditions, with atmospheric CO₂ concentrations estimated between 1,000 and 2,000 ppm, declining progressively from the Early to Late Devonian due to increasing vascular plant coverage and silicate weathering.^[42] This decline is evidenced by stomatal indices from fossil lycophytes and other early land plants, which show an inverse relationship with CO₂ levels, transitioning from higher densities in the Early Devonian to lower ones by the Late Devonian. These elevated CO₂ levels contributed to a warm global climate, with no evidence of widespread polar ice caps until the subsequent Carboniferous. In the Early and Middle Devonian, tropical sea surface temperatures exceeded 28°C, with global mean surface air temperatures around 21–22°C, reflecting the intense greenhouse effect.^[42] Oxygen isotope (δ¹⁸O) analyses of brachiopod shells and conodont apatite from low-latitude deposits indicate seawater temperatures of 25–35°C in tropical regions, supporting the prevalence of warm, equable conditions across much of the globe.^[66] By the Late Devonian, cooling ensued as CO₂ dropped toward 1,000 ppm, with global means falling to approximately 19°C and hints of polar cooling near Gondwana, including possible minor glaciations inferred from conodont δ¹⁸O records.^[67] Monsoon-like circulation patterns influenced southern high latitudes around Gondwana, driving seasonal precipitation and storm intensity as evidenced by sedimentary records of enhanced fluvial and storm deposits.^[68] Sea levels fluctuated significantly throughout the period, reaching a peak highstand of approximately +180–200 m relative to present during the Middle Devonian, driven by thermal expansion from high global temperatures and tectonic subsidence along passive margins. These rises facilitated widespread shallow marine inundation, while subsequent falls in the Late Devonian, linked to cooling and reduced thermal effects, exposed more continental shelves. Proxy data from sequence stratigraphy confirm these eustatic changes, with no full-scale glaciation until the Carboniferous.^[66]

Paleogeography

During the Devonian Period, the major continental configurations were dominated by the supercontinents Euramerica (also termed Laurussia) and Gondwana, with several smaller terranes influencing global geography. Euramerica formed in the Early Devonian through the collision and suturing of Laurentia and Baltica following the closure of the Iapetus Ocean during the late Silurian to Early Devonian Caledonian orogeny. This assembly created a large landmass that extended across tropical to subtropical latitudes, oriented roughly east-west along the equator, and featured extensive sedimentary basins such as the Appalachian Basin in eastern North America, where clastic and carbonate deposits accumulated due to post-orogenic subsidence.^[69] Gondwana, the southern supercontinent encompassing present-day South America, Africa, India, Antarctica, and Australia, occupied high southerly latitudes throughout the Devonian, positioned near the South Pole in paleogeographic reconstructions. This high-latitude setting is evidenced by the appearance of glacial deposits in Late Devonian strata across Gondwanan margins, particularly in South America and Africa, indicating the onset of cooler conditions that intensified toward the period's end.^[70] In contrast, smaller Asian blocks like Siberia and Kazakhstania (or Paleo-Kazakhstan) remained independent terranes, located at mid to high northern latitudes and drifting northward progressively through the period; Siberia originated from low southern latitudes in the Early Paleozoic and accelerated its northward motion, while Kazakhstania approached Siberia's southern margin by the Late Devonian. Avalonia, a peri-Gondwanan terrane, had already sutured to the southern margin of Laurussia by the Early Devonian as part of the Acadian phase of the Caledonides.^[71]^[69]^[72] The ocean basins reflected these continental arrangements, with the remnant Iapetus Ocean effectively closed between Laurentia and Baltica-Avalonia by the Early Devonian, limiting its extent to narrow seaways. The dominant ocean was Panthalassa, a vast circum-global basin encircling the equatorial supercontinents and connecting to smaller proto-Tethys seaways in the east. Paleomagnetic studies reveal dynamic continental motion, with latitude shifts on the order of 30° for blocks like Avalonia and parts of Gondwana during the Devonian, driven by plate drift and contributing to evolving geographic patterns.^[73]^[74]

Tectonic Activity

During the Devonian Period, the closure of the Rheic Ocean marked a pivotal phase in global plate tectonics, initiating the convergence of major landmasses toward the assembly of Pangea. Subduction of the Rheic oceanic lithosphere began in the Early Devonian, with northward-directed subduction beneath the southern margin of Euramerica (the amalgamated Laurentia-Baltica continent) and southward-directed subduction beneath northwestern Gondwana. This process generated precursors to the Variscan orogeny in Europe and the Alleghanian orogeny in North America, involving the accretion of peri-Gondwanan terranes and the development of magmatic arcs. Evidence for this subduction is preserved in ophiolite complexes within the Appalachian belt, such as those dated to approximately 395–370 Ma in southern Britain and Iberia, which represent fragments of the Rheic Ocean floor emplaced during closure.^[75]^[76] The Acadian orogeny, occurring primarily in the Early to Middle Devonian (approximately 419–380 Ma), represented a major collisional event along the eastern margin of Laurentia. This orogeny resulted from the oblique collision of the Avalonia microcontinent—a fragment derived from the Gondwanan margin—with the Laurentian craton, following the earlier closure of the Iapetus Ocean. The convergence led to intense northwest-directed thrusting, regional metamorphism, and the emplacement of synorogenic plutons, forming a fold-and-thrust belt that extended from Newfoundland to New England. Foreland basins developed in response to crustal loading, accumulating thick sequences of clastic sediments derived from the rising orogenic highlands, which reached elevations sufficient to influence regional drainage patterns.^[77]^[78] Hints of extensional tectonics emerged in Gondwana during the Late Devonian, signaling early rifting along its northern and eastern margins as a prelude to the broader Pangea assembly. Alkaline volcanism and intra-plate extension, particularly in regions now part of North Africa and the Arabian plate, indicate localized lithospheric thinning and magmatism associated with the initial separation of terranes like South China from Gondwana. Concurrently, the Siberian craton experienced accretionary processes, with the development of rift systems such as the Viluy rift system accompanied by extensive trap volcanism around 380–370 Ma, reflecting the approach of Siberia toward Euramerica and the onset of convergence that would culminate in Pangea's formation by the Late Carboniferous. These dynamics contributed to the reconfiguration of northern Pangaea precursors, with Siberia's meridional motion facilitating its integration into the supercontinent framework.^[79]^[80]^[81] Volcanism and magmatism during the Devonian were prominent indicators of deeper mantle processes, including potential plume activity beneath Laurentia. Kimberlite pipes emplaced around 400 Ma in the Wyoming craton and eastern North American regions, such as the Slave and Superior provinces, represent ultramafic magmas derived from the asthenospheric mantle, often linked to low-degree partial melting triggered by mantle plumes or edge-driven convection. These pipes, dated precisely via U-Pb methods to the Late Devonian (e.g., 386–400 Ma in the Iron Mountain field), carried deep-seated xenoliths and diamonds, providing evidence of sublithospheric sources and transient thermal perturbations that influenced cratonic stability. Such events underscore the role of intraplate magmatism in the tectonic evolution of Laurentia during this period.^[82]^[83]^[84]

Life

Marine Biota

The marine biota of the Devonian Period (419–359 million years ago) was characterized by extraordinary diversification, particularly among jawed vertebrates, which earned the era the nickname "Age of Fishes." This radiation occurred against a backdrop of expanding shallow seas and nutrient-rich waters that supported a wide array of open-ocean and soft-bottom communities. Jawless fish, such as osteostracans and anaspids, diversified alongside early jawed forms, filling niche roles in marine and freshwater environments.^[3] Invertebrates continued to dominate in terms of sheer abundance, while planktonic forms underwent significant shifts, setting the stage for evolutionary innovations that influenced later ecosystems.^[85] Vertebrates, especially fishes, underwent a major adaptive radiation during the Devonian, with placoderms emerging as apex predators in many marine settings. These armored jawed fishes, such as Dunkleosteus terrelli, could attain lengths of up to 3.4–4.1 meters, featuring robust bony plates and powerful shearing jaws adapted for crushing prey.^[4] Chondrichthyans, the cartilaginous fishes, also diversified early in the period, exemplified by cladoselachians like Cladoselache, which displayed streamlined bodies and multiple fins suited for agile swimming in open waters.^[86] Meanwhile, osteichthyans (bony fishes) proliferated, including primitive sarcopterygians such as Eusthenopteron foordi, a lobe-finned form from the Late Devonian whose robust fins and skeletal structure hinted at transitional features toward terrestrial locomotion.^[87] Invertebrate communities were equally vibrant, with trilobites remaining a staple of benthic habitats; species like Phacops rana were common in Middle Devonian deposits, known for their compound eyes that provided enhanced vision in dimly lit seafloors.^[88] Brachiopods thrived as filter feeders, with Atrypa being a representative genus that anchored to substrates in shallow marine environments throughout much of the period.^[89] Crinoids, or sea lilies, formed dense assemblages on soft bottoms, their feathery arms capturing plankton in currents, while coral-like rugosans—solitary or colonial anthozoans—achieved peak diversity, often solitary forms dotting the seafloor.^[90]^[91] Ammonoids, coiled cephalopods ancestral to later forms, began diversifying notably in the Late Devonian, adding to the predatory pressures on smaller invertebrates.^[92] Planktonic elements saw notable changes, as graptolites—colonial hemichordates that had dominated earlier Paleozoic oceans—declined progressively through the Devonian, becoming scarce by the Late stage due to ecological shifts and competition.^[93] They were largely replaced by conodont animals, eel-like chordates whose microscopic tooth-like elements served as vital biostratigraphic tools and indicate a turnover in microfossil assemblages.^[94] This era also witnessed pivotal evolutionary milestones among gnathostomes (jawed vertebrates), including the refinement of jaw structures from gill arch elements, which facilitated diverse feeding strategies from filter feeding to predation.^[85] In osteichthyans, the emergence of the swim bladder—a gas-filled organ derived from the gut—enabled precise buoyancy regulation, allowing fishes to exploit vertical water columns more efficiently without constant swimming.^[95]

Reef Systems

During the Devonian Period, reef systems emerged as complex, wave-resistant carbonate platforms that dominated shallow marine environments, reaching their peak development in the Middle Devonian (Givetian stage, approximately 388–382 million years ago). These reefs were primarily constructed by stromatoporoids—extinct, hypercalcifying sponge-like organisms that formed massive, encrusting colonies—alongside tabulate corals such as Favosites, which contributed branching and massive growth forms to the framework.^[96] Additional builders included calcifying algae, bryozoans, and microbial mats, particularly in the early developmental stages where they stabilized substrates and facilitated initial accretion.^[97] A notable example is the Miette Platform in Alberta, Canada, which formed as a large carbonate buildup up to 400–500 meters thick, encompassing bioherms and biostromes that created expansive reef complexes.^[98] Reef zonation reflected environmental gradients, with fringing reefs developing along continental shelves in high-energy settings and barrier reefs forming in deeper intracratonic basins, often enclosing lagoons with restricted circulation.^[97] These structures served as biodiversity hotspots, supporting diverse assemblages of frame-builders, encrusters, and borers, with reef-associated communities exhibiting two- to threefold higher genus-level diversity compared to surrounding non-reef habitats.^[99] The ecological role of these reefs was profound, providing habitats for symbiotic relationships, such as photosymbiosis in tabulate corals, which enhanced calcification and structural integrity under warm, nutrient-limited conditions.^[96] In the Late Devonian (Frasnian–Famennian stages), reef systems underwent a dramatic decline, triggered by episodes of marine anoxia and global cooling associated with mass extinction events like the Kellwasser crisis around 372 million years ago.^[96]^[100] This led to the collapse of stromatoporoid and coral frameworks, reducing reef volume and diversity, with surviving assemblages shifting toward lower-integration forms less suited to reef-building.^[100] Economically, Devonian reef limestones, such as those in the Leduc Formation of Alberta, have proven vital as hydrocarbon reservoirs, hosting significant oil and gas deposits that contribute to resources like the Alberta oil sands.^[101]

Terrestrial Biota

The Devonian period marked the initial greening of terrestrial landscapes, beginning with the colonization of land by early vascular plants in the Early Devonian. Rhyniophytes, such as Cooksonia, represented the pioneering tracheophytes, featuring simple, leafless stems and terminal sporangia adapted for spore dispersal in subaerial environments, with fossils dating to approximately 410 million years ago (Ma).^[102] These primitive plants lacked true roots but possessed rhizoids for anchorage and limited water absorption, facilitating the transition from aquatic algal ancestors to emersed life forms. By the Middle Devonian, plant diversity expanded with the emergence of more complex groups, including early lycopods that formed small herbaceous stands, contributing to initial soil stabilization through organic matter accumulation.^[12] In the Late Devonian, terrestrial vegetation underwent a profound transformation with the rise of progymnosperms, culminating in vast forests dominated by Archaeopteris, which reached heights of up to 30 meters and featured woody trunks, extensive root systems, and fern-like fronds for efficient photosynthesis.^[103] These trees, reproducing via spores rather than seeds, formed the first widespread woodlands, enhancing carbon sequestration and habitat complexity. Ferns and more advanced lycopods also proliferated during this epoch, with lycopods evolving arborescent forms that supported diverse understory ecosystems, while ferns developed larger fronds for improved light capture in shaded forest floors.^[12] This floral diversification fundamentally altered terrestrial nutrient dynamics by increasing organic inputs to soils. Arthropod faunas paralleled plant colonization, with myriapods among the earliest invaders. The millipede Pneumodesmus newmani, from late Silurian deposits in Scotland dated to about 428 Ma, is recognized as the oldest known air-breathing terrestrial animal, equipped with spiracles for atmospheric respiration and adapted for detritivory in moist litter layers.^[104] Early arachnids, including trigonotarbids like those preserved in the Rhynie Chert (~410 Ma), exhibited book lungs for gas exchange and chelicerae for predation on smaller invertebrates, thriving in the humid, vegetated lowlands of the Early Devonian.^[105] By the Middle Devonian, wingless insects appeared, exemplified by Rhyniognatha hirsti from the Rhynie Chert (~400 Ma), a pterygote-like form with mandibles suited for masticating plant material, indicating an adaptive radiation tied to emerging vegetation.^[106] The advent of terrestrial vertebrates occurred late in the period, with the first tetrapods emerging in the Famennian stage (~375 Ma). Ichthyostegids, such as Ichthyostega, and acanthostegians, including Acanthostega, possessed robust limbs with polydactylous feet derived from sarcopterygian fish fins, enabling limited terrestrial excursions but primarily suited for aquatic propulsion in freshwater environments.^[107] These early amphibians retained gills alongside lungs and exhibited skeletal adaptations like a reinforced vertebral column for weight-bearing, marking a pivotal shift toward amniote ancestry while remaining dependent on moist habitats for reproduction and respiration.^[107] Soil development advanced significantly during the Devonian due to plant-root penetration and organic decay, fostering the formation of early paleosols that promoted chemical weathering and nutrient cycling. The proliferation of rooted vegetation intensified silicate weathering, releasing essential elements like phosphorus into ecosystems and stabilizing sediments against erosion.^[108] This biogenic activity, driven by photosynthetic fixation of carbon dioxide, contributed to a marked rise in atmospheric oxygen levels, reaching approximately 15-24% by the Late Devonian, which in turn supported the metabolic demands of larger terrestrial organisms.^[108]

Extinction Events

Mid-Devonian Extinctions

The Mid-Devonian extinctions encompassed a series of lesser-known biotic crises, including the Choteč event at the Emsian-Eifelian boundary around 393 Ma and the Taghanic event in the early Givetian around 387 Ma. These pulses collectively eliminated approximately 15-20% of marine genera, with disproportionate losses among trilobites and orthoceratid cephalopods, which saw significant declines in diversity due to their sensitivity to environmental perturbations.^[109] Faunal evidence for these events is prominent in the records of conodonts and brachiopods, where sharp turnovers reflect abrupt shifts in assemblage composition, such as the replacement of regional endemics with more cosmopolitan forms. These changes are closely linked to early anoxic episodes in the oceans, characterized by the widespread deposition of organic-rich black shales and dysoxic sediments that indicate expanded oxygen minimum zones.^[110]^[111] The extinctions exhibited strong selectivity, disproportionately impacting stenotopic (narrowly adapted) species while favoring the persistence of eurytopic taxa capable of enduring fluctuating oxygen levels and temperatures; trilobites, for instance, lost several families adapted to stable shelf environments. Terrestrial ecosystems, including early vascular plants, showed negligible disruption during these marine-dominated crises.^[112] Recovery was swift, spanning less than a million years in many basins, and coincided with the rapid diversification of actinopterygian (ray-finned) fishes, which exploited newly available niches in post-anoxic waters. Notably, these events lacked the pronounced global carbon isotope anomalies seen in later Devonian crises, suggesting more localized drivers like regional eutrophication rather than widespread perturbation of the carbon cycle.^[110] In contrast to the more devastating Late Devonian extinctions, these mid-Devonian pulses represented lower-severity disruptions that nonetheless reshaped marine communities.

Late Devonian Extinctions

The Late Devonian extinctions represent one of the most severe biotic crises in Earth history, characterized by a series of pulsed events that profoundly disrupted marine ecosystems over approximately 13 million years. These events, part of the broader "Big Five" mass extinctions, eliminated an estimated 70-80% of marine species across the period, with the two most intense pulses being the Kellwasser event at the Frasnian-Famennian boundary and the Hangenberg event near the Devonian-Carboniferous boundary.^[113]^[62] The Kellwasser event, dated to around 372 Ma, consisted of lower and upper phases marked by widespread deposition of organic-rich black shales indicative of oceanic anoxia, leading to the loss of roughly 50% of marine species globally.^[114]^[115] In contrast, the Hangenberg event, occurring at approximately 359 Ma, was a more abrupt crisis that affected about 70% of marine invertebrate genera, ranking it among the largest extinction pulses in the Phanerozoic.^[64]^[62] The ecological impacts were particularly devastating for shallow-water and reef-associated communities. Reef-building organisms, such as stromatoporoids, suffered near-total collapse, with these sponges becoming extinct by the Hangenberg event after dominating Devonian reefs for over 100 million years.^[116] Brachiopods, a dominant group in Devonian seas, lost over 80% of their genera, with warm-water taxa hit hardest due to habitat destruction in tropical settings.^[117] Placoderms, the armored jawed fishes that epitomized Devonian vertebrate diversity, were decimated, with most lineages vanishing during the Kellwasser and the remainder in the Hangenberg, paving the way for the rise of other fish groups.^[114] Terrestrial ecosystems were less severely affected overall, though early forests experienced disruption from associated climate shifts, including nutrient runoff and soil erosion that indirectly influenced global biogeochemical cycles.^[118] Multiple interconnected causes likely drove these extinctions, with marine anoxia playing a central role, as evidenced by the widespread black shales deposited during both pulses, which record expanded oxygen minimum zones and stratified oceans.^[119] Eutrophication from the proliferation of rooted land plants increased phosphorus delivery to oceans, promoting algal blooms and exacerbating hypoxia around 372 Ma.^[118] Volcanism associated with the Viluy Traps large igneous province in Siberia, dated to the Late Devonian, released massive greenhouse gases and mercury, contributing to global warming, acidification, and toxicity that intensified anoxic conditions.^[118]^[120] Bolide impacts have been hypothesized as triggers, based on potential ejecta layers and tsunamites, but remain unconfirmed for the Late Devonian pulses due to lack of definitive craters.^[113] Recovery into the Early Carboniferous was protracted, lasting several million years, with marine biodiversity rebounding slowly amid lingering anoxic episodes. Ammonoids, which had diversified in the Devonian, underwent a temporary resurgence before a final Hangenberg-related setback, eventually stabilizing as key predators.^[121] Bony fishes (Osteichthyes) emerged as dominant vertebrates, filling niches vacated by placoderms and driving further aquatic innovation.^[114] Carbon isotope excursions, with δ¹³C shifts up to +4‰ in carbonates, signal major perturbations to the global carbon cycle, reflecting enhanced organic burial during anoxic intervals and subsequent ecosystem reorganization.^[122]

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[PDF] doe/et/12040-151 paleoecology of the devonian-mississippian biack ...
Because of their nektoplanktic nature and rapid evolution during the Late. Devonian, conodonts make excellent Late Devonian index fossils. Although very little ...
[93]
Evolution of Air Breathing: Oxygen Homeostasis and the Transitions ...
In some species the swim bladder gave rise to an air-breathing ... evolved from trigonotarbids, which were contemporary with Devonian aquatic scorpions.
[94]
Functional consequences of Palaeozoic reef collapse - PMC
Jan 26, 2022 · Here, we examine changes in the functional diversity of tabulate coral assemblages over a 35 million year period from the middle Devonian to the ...
[95]
Reefs - Corals of the World
These reefs had little wave resistance and did not form solid platforms although stromatoporoids and tabulate corals formed massive colonies several metres in ...
[96]
Microbial carbonates as indicators of environmental change and ...
2). The Miette and Ancient Wall platforms, located in the western part of the Alberta basin, are 400–500 m thick (Fig. 1, ...
[97]
Reefal regions were biodiversity hotspots throughout the Phanerozoic
### Summary of Devonian Reef Biodiversity from https://www.science.org/doi/10.1126/sciadv.adv9793
[98]
Functional consequences of Palaeozoic reef collapse - Nature
Jan 26, 2022 · The collapse of taxonomic diversity and reef building in the late Devonian had minimal impact on functional richness of coral assemblages.
[99]
Chapter 12 - Devonian Woodbend-Winterburn Strata
The Leduc Formation of the Woodbend Group is well known as a host to prolific reserves of oil and gas within Alberta. Distribution and thicknesses of the Leduc ...
[100]
The southwesternmost record of late Silurian (Pridolian) early land ...
Sep 27, 2024 · Late Silurian and Early Devonian are critical times for primitive land plants radiation. The fossil record of this time is scarce and restricted ...Silurian Flora In Argentina · Results · Taphonomic Features Of The...<|separator|>
[101]
Low atmospheric CO2 levels before the rise of forested ecosystems
Dec 20, 2022 · Recently, atmospheric CO2 levels were found to be only modestly elevated in the late Ordovician (~400–700 ppm) and Mid-Devonian (~700–1400 ppm) ...
[102]
A U-Pb zircon age constraint on the oldest-recorded air-breathing ...
Jun 28, 2017 · Here we report the youngest, most concordant 238U-206Pb zircon age from ash below the fish bed of 413.7±4.4 Ma (±2σ), whereas the youngest age ...
[103]
Microanatomy of Early Devonian book lungs | Biology Letters
Jan 15, 2008 · The book lungs of an exceptionally preserved fossil arachnid (Trigonotarbida) from the Early Devonian (approx. 410 Myr ago) Rhynie cherts of Scotland were ...
[104]
New light shed on the oldest insect - Nature
### Summary on Rhyniognatha
[105]
Paleoenvironments of Late Devonian tetrapods in China - Nature
Nov 21, 2023 · By this metric (see Methods), progymnosperms of the two deep calcic (Fugun) paleosols near the tetrapod level were 6.5 ± 0.7 m and 3.9 ± 0.7 m ...Introduction · Results · Discussion
[106]
Devonian rise in atmospheric oxygen correlated to the radiations of ...
Despite these differences, both predict a Devonian oxygen increase in pO2 (15, 40), reflecting the radiation of vascular plants.Missing: photosynthesis | Show results with:photosynthesis
[107]
[PDF] THE MIDDLE DEVONIAN KAČÁK EVENT - The Ussher Society
In 1985 eight events in the Devonian were named where extinction or change in ammonoid faunas appeared to be related to widespread hypoxic sedimentary events ( ...
[108]
Paleotemperature record of the Middle Devonian Kačák Episode
Aug 16, 2021 · While the shallow marine carbonate platform setting (Val di Collina section) might have been affected by higher evaporation translating into ...Missing: evaporites | Show results with:evaporites
[109]
Devonian upper ocean redox trends across Laurussia - Frontiers
Oct 10, 2022 · The Devonian is characterized by major changes in ocean-atmosphere O2 concentrations, colonialization of continents by plants and animals, ...
[110]
(PDF) The Kačák event (late Eifelian, Middle Devonian) on the ...
Aug 9, 2025 · ... event, a turnover is observed with. several genera becoming extinct, whereas other radiates. (e.g. Agoniatites, Maenioceras, Cabrieroceras ...
[111]
Devonian extinctions | Definition & Facts - Britannica
Devonian extinctions, a series of global extinction events primarily affecting marine communities of the Devonian Period.
[112]
End-Devonian extinction and a bottleneck in the early evolution of ...
The subsequent recovery of previously diverse groups (including placoderms, sarcopterygian fish, and acanthodians) was minimal. Tetrapods, actinopterygians, and ...Missing: agnathans plant spores
[113]
Anchoring the Late Devonian mass extinction in absolute time by ...
Jul 31, 2020 · This study confirms that the succession of events within the Upper Kellwasser event is paced by obliquity, under a low-eccentricity orbit. Hence ...
[114]
Late Devonian mass extinction - Wikipedia
The Late Devonian mass extinction, also known as the Kellwasser event, was a mass extinction event which occurred around 372 million years ago
[115]
Mass Extinctions Through Geologic Time - National Park Service
Feb 28, 2025 · Overall, about 86% of species, 57% of genera, and 27% of taxonomic families died out, making this the second largest extinction in the ...Missing: Taghanic 15-20%<|control11|><|separator|>
[116]
Late Devonian - an overview | ScienceDirect Topics
Warm-water groups, particularly brachiopods and reef forms, suffered the greatest extinction, whereas groups adapted to high latitudes and colder waters seem to ...
[117]
The expansion of land plants during the Late Devonian contributed ...
Nov 29, 2023 · The Late Devonian mass extinction around 372 million years ago, may have been linked to terrestrial release of the nutrient phosphorus driven by newly-rooted ...<|control11|><|separator|>
[118]
Extensive marine anoxia associated with the Late Devonian ...
Mar 1, 2020 · Various causes for these extinctions have been proposed, including volcanism, climate change, sea-level changes, global cooling, submarine ...Missing: eutrophication | Show results with:eutrophication
[119]
Precisely dating the Frasnian-Famennian boundary: implications for ...
The current Geological Time Scale date for the FF boundary is 372.2 ± 1.6 Ma ... Kellwasser Event (~372.6 Ma: Fig. 5). Consequently, if faunal extinctions ...
[120]
Geochemical evidence for major environmental change at the ...
A third extinction affected ammonoids just after they had started to recover in the basalmost Carboniferous (top of German Stockum level, top of globally ...Missing: bony | Show results with:bony
[121]
Phosphorus-cycle disturbances during the Late Devonian anoxic ...
Globally documented positive excursions in sedimentary carbon-isotope (δ13C) values of up to 4‰ also highlight that these events were associated with ...