Devonian Period

The Devonian Period lasted from 419.62 ± 1.36 to 358.86 ± 0.19 million years ago. It was the fourth period of the Palaeozoic Era, between the Silurian and Carboniferous. During roughly 60.76 million years, jawed fishes diversified, reefs became enormous, the first forests transformed rivers and soils, and vertebrates with digits appeared in aquatic habitats.

“Age of Fishes” is a useful label only if it is not mistaken for a complete description. Devonian seas also contained reefs, ammonoids, brachiopods, trilobites and many smaller organisms. On land, plants, fungi and arthropods built increasingly complex communities before the earliest known tetrapods. The period ended through several biological crises rather than one instantaneous catastrophe.

The Devonian connected two great changes: vertebrate diversification in water and the restructuring of continents by roots, wood and early forests. Neither was a straight march towards modern life.

Why the period is called Devonian

The name comes from Devon in south-west England. Roderick Murchison and Adam Sedgwick introduced the Devonian System in 1839 after studying rocks that lay between their Silurian and Carboniferous sequences. The recognition was controversial because fossils and rock types differed between marine Devon and the Old Red Sandstone of Britain.

Geologists eventually demonstrated that these contrasting deposits were broadly contemporaneous. The episode helped establish that a period can contain different environments and rock types. A red river sandstone and a marine limestone need not have formed in the same place to belong to the same interval.

Where the Devonian begins

The lower boundary is defined at Klonk near Suchomasty in the Czech Republic. The formal level is tied to the first appearance of the graptolite Monograptus uniformis. It is dated to 419.62 ± 1.36 million years ago on the current international chart.

Graptolites are particularly useful in marine rocks, but continental deposits need other tools. Conodonts, spores, fish assemblages, isotope changes, volcanic ash dates and stratigraphic position connect marine and terrestrial successions. Correlation is a network of evidence, not a single fossil name applied everywhere.

Three epochs and seven stages

Regional schemes do not always match the international stages exactly. “Upper Devonian” refers to rocks and “Late Devonian” to time. Numerical ages can improve while the agreed boundary sections remain fixed reference points.

Continents, mountains and changing basins

Laurentia, Baltica and Avalonia had joined to form Laurussia, also called Euramerica, near the equator. Gondwana occupied much of the southern hemisphere, while Siberia and smaller terranes remained separate. The Rheic Ocean narrowed as plate movements brought Gondwana closer to Laurussia.

Mountain building affected sediment and rivers. The Acadian phase of the Appalachian orogeny raised terrain along eastern Laurussia. Erosion supplied the thick continental deposits known as the Old Red Sandstone. These rocks preserve river channels, floodplains, lakes, plants and fishes rather than one continuous desert.

Warm shallow seas flooded continental margins and interior basins. Reefs flourished in suitable clear water, while deeper or restricted basins could become oxygen poor. Geography changed through the 61-million-year period, so no single map or global coastline represents all Devonian stages.

Climate from greenhouse warmth to crisis

Much of the Devonian was warm and lacked large permanent polar ice sheets. That broad description hides strong variation. Isotope records, fossil plants and sediments indicate climatic shifts, while sea level rose and fell repeatedly. Tropical conditions supported carbonate platforms, but some high-latitude and later intervals were cooler.

Expansion of vascular plants affected the carbon cycle. Deeper roots and more developed soils increased chemical weathering, which consumes atmospheric carbon dioxide over geological time. Rivers carried more nutrients into seas, potentially stimulating productivity and oxygen consumption during decay. These links are plausible and supported by several records, but forests were not one global switch that single-handedly caused the Late Devonian extinctions.

Near the end of the period, cooling and evidence for glaciation accompanied the Hangenberg crisis. Earlier Kellwasser events involved warming, sea-level movement and widespread marine anoxia in many basins. Different pulses need not share an identical balance of causes.

Why it is called the Age of Fishes

Jawed vertebrates occupied a widening range of marine and freshwater roles. Placoderms, acanthodians, early chondrichthyans, ray-finned fishes and lobe-finned fishes all diversified. Jawless vertebrates remained important too. The phrase “Age of Fishes” refers to ecological prominence and variety, not the origin of fishes or jaws, both of which have older roots.

Placoderms carried bony armour around the head and front of the trunk, but the group included bottom dwellers, open-water swimmers, small forms and large predators. Dunkleosteus is known mainly from the armoured head and shoulder region. Its total length and rear body outline are modelled rather than preserved in one complete skeleton.

Early chondrichthyans cannot simply be called modern sharks. Their scales, fin spines, teeth and skeletons document branches near the cartilaginous-fish lineage, but the familiar streamlined shark body evolved through a mosaic of forms. Ray-finned and lobe-finned fishes likewise contained many experimental anatomies.

Reefs reached enormous scale

Middle Devonian reef complexes could extend for hundreds of kilometres along suitable platforms. Stromatoporoid sponges, tabulate corals, rugose corals, microbes and encrusters built and bound the framework. These were not modern coral reefs, even though they created similarly complex physical habitats.

Reefs supplied firm surfaces, cavities, sheltered water and feeding sites. Brachiopods, molluscs, crinoids, trilobites and fishes used different parts of the structure. Storms, sediment and changes in water chemistry could interrupt growth. A reef limestone records construction, breakage, transport and cementation over time.

Late Devonian crises devastated many tropical reef systems. Stromatoporoid-coral frameworks declined dramatically, and comparable large reefs did not immediately return. Their loss changed habitats well beyond the builder organisms themselves.

The Rhynie Chert preserves an early land ecosystem

The Early Devonian Rhynie Chert of Scotland formed around a geothermal wetland about 407 million years ago. Silica-rich water mineralised organisms rapidly and preserved cells in three dimensions. The deposit contains vascular plants, fungi, algae, cyanobacteria, mites, springtails, harvestman-like arachnids and crustacean relatives.

Plants such as Rhynia, Aglaophyton and Asteroxylon differed in conducting tissues and branching. Fungi formed associations with plants, decomposed organic matter and sometimes acted as parasites. These interactions show that terrestrial ecosystems were already networks rather than a sequence of plants appearing first and animals later.

Rhynie was an unusual hot-spring environment. It cannot stand for every Devonian landscape. Its exceptional preservation reveals anatomy normally destroyed, but geothermal flooding, silica precipitation and local disturbance also selected which organisms were entombed.

Roots and wood created the first forests

Early Devonian vegetation remained low in many settings. Through the Middle and Late Devonian, several plant lineages independently evolved greater height, wood and more extensive rooting. Cladoxylopsids such as Wattieza formed tree-sized trunks with crowns of branches. Archaeopteridaleans later combined woody trunks, deep roots and leafy branches.

The Gilboa site in New York preserves cladoxylopsid bases and a forest floor. Cairo, also in New York, preserves an older extensive root system associated with archaeopterid-type trees. These sites show that “first forest” depends on which feature is meant: tree-sized plants, a closed stand, deep roots or a particular modern-style architecture.

Roots stabilised banks, broke rock and produced new soil structure. Fallen plant matter supplied detritus. Shaded, debris-rich channels differed from earlier rivers with weak banks and little woody obstruction. Vegetation therefore changed physical landscapes as well as atmospheric chemistry.

Leaves, seeds and changing reproduction

Leaves evolved in more than one lineage. Small microphylls characterised lycopsids, while larger megaphyll-like organs developed along other branches. Their origin involved changes in branching, flattening and tissue growth rather than the sudden appearance of a modern leaf.

Late Devonian seed plants enclosed the developing female gametophyte and embryo within protective tissues. Seeds reduced dependence on free water during fertilisation and dispersal, though early forms differed greatly from modern seeds. Spore-producing plants remained diverse and dominant in many habitats.

Plant fossils are often incomplete. A trunk, root, leaf and reproductive organ found separately may receive different form names until connections are demonstrated. Reconstructions must distinguish an articulated plant from parts assembled through comparison.

Arthropods already occupied terrestrial food webs

Millipedes, arachnids, mites, springtails and other arthropods inhabited Devonian land. Some processed decaying material or grazed microbial surfaces; others were predators. Direct evidence includes bodies, mouthparts, coprolites, plant damage and tracks, but exact diets are secure only in favourable specimens.

Insects are present by the Devonian, although the record is sparse and some famous fossils have been reinterpreted. Wings may have evolved by the Late Devonian or early Carboniferous, but fragmentary material makes exact timing difficult. A missing fossil is not proof that a lineage did not exist; equally, molecular estimates alone do not supply the anatomy of an unseen animal.

Reproduction in placoderms was more complex than expected

The Gogo Formation of Western Australia preserves Late Devonian fishes in three dimensions, sometimes including muscles, nerves and reproductive structures. Materpiscis attenboroughi contains an embryo connected by a mineralised umbilical structure and is strong evidence for live birth with internal fertilisation.

Other Gogo placoderms preserve claspers and embryos, indicating that internal fertilisation was not an isolated anomaly. It does not follow that every placoderm gave birth to live young. Reproductive anatomy varied, and absence of an embryo in most fossils is expected because pregnancy was temporary and preservation exceptional.

Lobe-finned fishes and the origin of limbs

Sarcopterygians include coelacanth relatives, lungfish and the tetrapodomorph branch. Their paired fins contained internal bones homologous in broad arrangement with parts of tetrapod limbs. This did not make every lobe-finned fish a walking ancestor. Different branches used robust fins for manoeuvring, bottom contact, vegetation-filled shallows or other functions.

Eusthenopteron preserves a humerus, radius and ulna pattern inside the fin. Panderichthys had a flattened head and body suited to shallow water. Tiktaalik, about 375 million years old, combined fins with a mobile neck, robust ribs and wrist-like joints capable of supporting the front of the body.

Tiktaalik was not necessarily the direct ancestor of every land vertebrate. It is a close relative of the lineage leading to tetrapods and documents one combination of transitional features. Evolutionary relationships form a branching tree, not a ladder made from the few named genera that happen to fossilise.

Tracks may predate the known skeletons

Track-like structures at Zachełmie in Poland occur in Middle Devonian rocks about 390 million years old. The original study interpreted them as trails made by digit-bearing tetrapods. If correct, such animals existed much earlier than Tiktaalik and the best-known Late Devonian tetrapod skeletons.

The distinction between find and inference matters. Sequential impressions occur in a well-studied layer. Their maker is not preserved. Some researchers accept tetrapod footprints, while others propose feeding or locomotion traces made by fishes. A reconstruction should therefore show the surface without inventing a visible track maker.

Trace fossils record behaviour but seldom identify a body. Skeletons reveal anatomy without necessarily recording the group’s first appearance. The early tetrapod story depends on comparing both archives.

Acanthostega and Ichthyostega still depended on water

Late Devonian tetrapods possessed true digits. Acanthostega had eight digits on each limb, yet weak wrists, a fish-like tail fin and its ribcage indicate a mainly aquatic life. Digits therefore evolved before a fully terrestrial walking gait.

Ichthyostega had stronger ribs and limbs but did not move exactly like a modern salamander. Its body combined aquatic adaptations with the ability to support itself on a substrate. These animals show that “fishes walked onto land” compresses a long, branching transition. Plants, fungi and arthropods already occupied land, and early tetrapods remained linked to shallow water.

Late Devonian crises were a sequence

The Devonian ended through repeated biological crises, especially in marine settings. The Lower and Upper Kellwasser events cluster around the Frasnian to Famennian boundary. The Hangenberg crisis occurred close to the end of the period. Treating them as one moment obscures differences in timing, affected groups and environmental signals.

Organic-rich dark rocks occur in many crisis intervals, and geochemistry indicates expansion of anoxic, sometimes sulphidic water. Anoxia describes an immediate dangerous environment but not necessarily the original trigger. Climate change could alter circulation; nutrients could raise plankton productivity and oxygen demand; forests and soils could increase riverine phosphorus; volcanism could add carbon dioxide and mercury; and sea-level movement could remove or reconnect shallow habitats.

Evidence for volcanic forcing is not equally strong in every Hangenberg section. Short cooling and glaciation are especially plausible near that crisis. A cautious explanation treats the Late Devonian as a long interval of ecological pressure with several pulses, not a single event caused everywhere by one mechanism.

Who suffered and who crossed the boundary

Kellwasser events badly damaged tropical reef communities. Stromatoporoid and coral frameworks contracted, while brachiopods, trilobites, ammonoids, conodonts and fish lineages lost diversity. Large reef systems took a long time to recover.

The Hangenberg crisis affected other parts of marine biota and coincided with the disappearance of the last placoderms. Early ray-finned, cartilaginous and lobe-finned fishes crossed into the Carboniferous, but their communities were reorganised. Continental losses are harder to measure because the land record is patchier.

Placoderms did not disappear because they were “primitive”. They had occupied many successful roles for tens of millions of years. Rapid environmental change and food-web disruption selected among lineages without following a planned progression towards supposedly superior animals.

Important Devonian fossil sites

No locality answers every question. Rhynie reveals microscopic terrestrial anatomy, Gogo preserves fish interiors, and track surfaces record movement without skeletons. Devonian history emerges by combining independent kinds of evidence.

What did not exist in the Devonian

Dinosaurs did not exist. The first dinosaurs appeared in the Late Triassic roughly 125 million years after the Devonian ended. There were no birds, mammals, flowering plants, grasslands or modern coral reefs. Digit-bearing vertebrates were not yet modern amphibians and largely remained aquatic.

Land nevertheless supported complex food webs, woody plants and the first forests. The period did not make Earth modern, but it established important components of forest soils, branching river habitats and the anatomy inherited by land vertebrates.

How to read Devonian reconstructions

A placoderm skull shield, Tiktaalik fin skeleton, root trace or fossil embryo is direct evidence. A complete rear body for Dunkleosteus, skin colour and an exact hunting sequence have different degrees of uncertainty. Good artwork keeps organisms within one time and habitat, avoids modern plants, labels disputed track makers and does not turn a plausible posture into an observed event.