Silurian-Devonian terrestrial colonization

For more than four billion years, the continents were biologically empty. Exposed rock surfaces, subjected to ultraviolet radiation at intensities far higher than today and lacking the buffering chemistry of any soil, offered no foothold for complex multicellular life. By the close of the Devonian period, roughly 359 million years ago, those same continents were clothed in forests, rooted in meters of biologically generated soil, and inhabited by arthropods, arachnids, and the earliest limbed vertebrates. The transformation took little more than 100 million years. No single event in the history of life — save perhaps the origin of photosynthesis itself — restructured Earth’s surface chemistry, climate, and ecology so thoroughly as the terrestrial colonization of the Silurian and Devonian periods.^{4, 7}

The colonization was not a single event but a prolonged, multi-lineage transition in which plants, fungi, and invertebrates each solved the fundamental physical challenges of terrestrial life more or less independently, then became increasingly entangled through ecological and evolutionary feedbacks. Understanding the sequence of that transition — which groups moved first, which barriers proved most difficult to overcome, and what the ecological consequences were — has become one of the central questions of Palaeozoic palaeontology.^{4, 12}

The pre-colonization baseline: a barren world

In the Early Ordovician, approximately 480 million years ago, land surfaces were geologically active but biologically minimal. Microbial crusts of cyanobacteria and simple algae may have colonized damp rock surfaces and stream margins, but these communities left little macroscopic structure and generated only thin, geochemically primitive surface films rather than soil in any meaningful sense.⁷ The obstacles facing any organism attempting to establish a sustained terrestrial presence were severe and interrelated.

Desiccation was the most immediate problem. Aquatic organisms can rely on ambient water to maintain cell hydration; on land, water must be retained within tissues against a gradient that constantly favors evaporation. Without a waxy cuticle or equivalent impermeable outer layer, any terrestrial organism would lose water faster than it could absorb it from the substrate. Related to desiccation was the challenge of structural support: water provides buoyancy in aquatic environments, and organisms that evolved in suspension required entirely new biomechanical strategies to grow upright against gravity on land. Reproduction posed a third barrier, since many aquatic life cycles depend on water as a dispersal medium for gametes or spores, a medium unavailable on dry land. And overarching all of these challenges was ultraviolet radiation. In the absence of an ozone layer substantial enough to screen UV-B wavelengths — and evidence from Ordovician palaeoclimate suggests ozone concentrations were considerably lower before large-scale photosynthesis on land could produce the oxygen needed to sustain stratospheric ozone — surface UV exposure would have been lethal to unprotected cells.¹²

The first land plants: cryptospores and early embryophytes

The earliest convincing evidence for land plants is not body fossils but spores. Fossilized cryptospores — distinctive dyads and tetrads of spore-like structures enclosed in a sporopollenin-resistant wall — appear in Ordovician sediments dated to approximately 470 million years ago in present-day Gondwana, including sites in Argentina and Saudi Arabia.^{1, 2} Sporopollenin, the extraordinarily chemically resistant biopolymer forming the outer wall of these structures, is a hallmark of embryophyte spores and is itself an adaptation to UV exposure: it absorbs and scatters UV-B radiation far more effectively than any competing polymer available to aquatic algae. The organisms producing these early cryptospores were almost certainly bryophyte-grade — non-vascular, low-growing, resembling modern liverworts in their general organization, and dependent on moisture films for reproduction — but they represent the crossing of the single most fundamental threshold: the sustained biological colonization of terrestrial substrate.

Body fossils of early land plants appear somewhat later, with the genus Cooksonia representing the first confirmed vascular plant at approximately 425 million years ago in the Late Silurian.³ Cooksonia was anatomically simple: erect, leafless axes no more than a few centimetres tall, branching dichotomously and terminating in globose or kidney-shaped sporangia. It possessed a central strand of conducting tissue — the primitive xylem and phloem that define vascular plants — allowing water and nutrients to move through the plant body against gravity rather than relying on diffusion alone. This vascular system was the key innovation that allowed plants to grow taller and colonize drier substrates, since taller growth improved both light capture and spore dispersal while the vascular system addressed the water transport limitation inherent in small size.⁴

The Early Devonian Rhynie Chert, an extraordinary silicified peat deposit from Scotland dated to approximately 407 million years ago, preserves a snapshot of the earliest terrestrial ecosystem in unparalleled cellular detail.¹⁴ Within it, several genera of small vascular plants — including Rhynia, Aglaophyton, and Horneophyton — grew in waterlogged, hydrothermally influenced ground alongside fungi, algae, and the earliest known terrestrial arthropods. The Rhynie Chert ecosystem was low-statured and relatively simple, but it demonstrates that by 407 Ma a functioning, multi-trophic terrestrial community already existed, with mycorrhizal fungi colonizing plant axes and facilitating nutrient uptake in the poorly developed soils.¹⁴

The Devonian elaboration: roots, leaves, and seeds

Through the Devonian, plant body plans elaborated at a pace that has no parallel in subsequent plant evolution. Roots, in the sense of branching underground organs with a root cap and the capacity to penetrate and chemically weather consolidated substrate, first appear convincingly in the fossil record around 400 million years ago and diversify markedly across the Middle Devonian.¹³ The ecological significance of roots extended well beyond plant physiology: roots mechanically disrupted rock, creating channels for water infiltration; root-associated fungi secreted organic acids that accelerated mineral dissolution; and decaying root material contributed organic carbon to accumulating soil profiles. The appearance of true roots was, in this sense, as much a geological event as a botanical one.

Leaves — expanded photosynthetic organs — are absent from the earliest vascular plants and appear to have evolved independently in several lineages during the Devonian. The lycopsid lineage produced microphylls, simple single-veined structures, while the fern and seed-plant lineages produced megaphylls, more complex organs with branching vein systems derived from the fusion of lateral branch systems.⁴ Modelling studies and empirical evidence from the fossil record suggest that the evolution of megaphyllous leaves was delayed by the high CO₂ atmosphere of the early Devonian, which reduced the selective advantage of expanded photosynthetic area; as CO₂ declined through the Devonian, selection favoured larger leaf area, and megaphyll diversity increased accordingly.⁸

Seeds represent the final major Devonian innovation in plant reproduction. A seed encases the female gametophyte and developing embryo within the parent sporophyte’s tissues, providing nutrition, physical protection, and in many cases dormancy capabilities that allow dispersal across time as well as space. The earliest seed plants — the “seed ferns” or pteridosperms — appear in the fossil record by the Late Devonian, approximately 365 million years ago.⁴ Seeds freed plant reproduction entirely from dependence on free water for fertilisation, enabling colonisation of drier upland environments that remained inaccessible to spore-bearing plants.

The first terrestrial animals: arthropods onto land

Animals faced the same suite of challenges in colonising land as plants, and in most cases appear to have followed plants onto the terrestrial surface by tens of millions of years, exploiting the biomass and humid microhabitats that plant communities created. The earliest confirmed air-breathing terrestrial animal known from the fossil record is Pneumodesmus newmani, a myriapod (millipede) from the Silurian of Stonehaven, Scotland, dated to approximately 428 million years ago.⁵ Pneumodesmus preserves direct anatomical evidence of air-breathing in the form of spiracles — openings on the lateral surface of body segments connecting to internal respiratory tubules (tracheae) — making it the oldest animal demonstrably adapted for gas exchange in air rather than water.

The colonisation of land by arthropods was not a single event but reflects multiple independent crossings by different lineages across the Silurian and Devonian. Arachnids — the clade that today encompasses spiders, scorpions, mites, and their relatives — appear in Silurian and Early Devonian deposits, with early scorpions such as Dolichophonus known from the Silurian and showing a mosaic of aquatic and terrestrial features that mirrors the transitional forms seen in vertebrate lineages later in the Devonian.¹² By the time of the Rhynie Chert community (~407 Ma), mites and a primitive arachnid known as Palaeocharinus were already established terrestrial predators, indicating that multiple arthropod lineages had achieved the transition to full terrestriality within the first quarter of the Devonian.

The ecological significance of early terrestrial arthropods extended beyond their roles as consumers. Millipedes and other detritivores were early processors of plant litter, contributing to the decomposition and nutrient cycling that allowed organic carbon to accumulate in soils rather than simply mineralising at the surface. The presence of arthropod detritivores was a prerequisite for the development of nutrient-rich soils capable of supporting larger, deeper-rooted plants later in the Devonian.^{7, 12}

The Devonian forest revolution: Archaeopteris and the first canopy

The most dramatic transformation of Devonian landscapes was the appearance of tree-sized plants and, with them, the first structurally complex forest ecosystems. The pivotal genus is Archaeopteris, a progymnospermatous plant that achieved trunk diameters of up to one metre and heights of 30 metres or more, making it the first known tree-sized plant in the fossil record.⁶ Archaeopteris forests were widespread across tropical and subtropical Gondwana and Laurussia by approximately 385 million years ago during the Late Devonian (Frasnian Stage), and the organism combined features of ferns — it reproduced by spores — with a secondary xylem (wood) anatomy that prefigured the gymnosperms and seed plants that would dominate subsequent Palaeozoic and Mesozoic landscapes. The wood of Archaeopteris was anatomically modern, and its capacity to produce multiple growth rings indicates it was a perennial organism capable of overwintering, a significant departure from the annuals and geophytes of the earlier Devonian.

The establishment of Archaeopteris-dominated forests marked the first appearance of several ecological structures that now characterise forests globally: a closed canopy creating pronounced light gradients and shading of the forest floor; deep root networks reaching metres into developing soils; a substantial standing biomass that sequestered carbon for years to decades; and a litter layer supporting complex detritivore communities.^{6, 11} Earlier Devonian plant communities — dominated by small, shallow-rooted vascular plants — had begun these processes on a small scale. Archaeopteris forests intensified them by orders of magnitude.

Co-evolution of plants and soil: the geochemical consequences

The spread of rooted vascular plants across continental surfaces during the Devonian drove a geochemical transformation of unprecedented scale. Roots and their fungal associates accelerated the chemical weathering of silicate rocks, a process in which atmospheric CO₂ dissolves in water to form carbonic acid, which in turn reacts with silicate minerals to release calcium and magnesium ions that are subsequently transported to the ocean and deposited as carbonate sediments. This weathering reaction is the primary mechanism by which CO₂ is removed from the atmosphere on geological timescales, and the expansion of rooted plants increased weathering rates substantially by delivering organic acids directly to rock surfaces and by creating extensive networks of water-conducting channels through the substrate.⁸

Palaeosol (fossil soil) records from the Devonian document the progressive deepening and chemical enrichment of continental soils as plant root depths increased from centimetres in the Early Devonian to metres by the Late Devonian.⁷ Early Devonian palaeosols are thin, weakly developed, and chemically similar to the substrate rock; Late Devonian palaeosols associated with Archaeopteris forests show deep root traces, abundant organic carbon, and pronounced mineral depletion horizons consistent with intense biological weathering. The transition from raw rock to biologically mature soil that took geological epochs to accomplish on pre-plant continents was being driven by biology in only tens of millions of years.

The biogeochemical model developed by Robert Berner and colleagues, known as GEOCARB, quantifies the effect of this plant-driven weathering increase on atmospheric CO₂ through the Devonian. The model indicates that atmospheric CO₂ declined from approximately 10 to 15 times present pre-industrial levels (~2,800–4,200 ppm) in the Early Devonian to near modern levels or below by the end of the Devonian and into the Carboniferous, representing one of the largest and most rapid CO₂ drawdowns in the geological record.⁸ The direct consequence was global cooling. Glacial deposits associated with the Gondwanan ice sheet appear in the Late Devonian and become extensive in the earliest Carboniferous, marking a transition from the warm greenhouse conditions that had characterised most of the Palaeozoic to a cooler icehouse state. The plants colonising the continents had, in effect, altered the planetary thermostat.

Vertebrates on land and the Late Devonian crisis

The final major lineage to achieve terrestriality during the Devonian was the vertebrates, through the fish-to-tetrapod transition documented by sarcopterygian fishes and early limbed vertebrates spanning approximately 385 to 360 million years ago. Transitional forms such as Tiktaalik roseae (~375 Ma) and the earliest true tetrapods Acanthostega and Ichthyostega (~365 Ma) record the progressive acquisition of weight-bearing limbs, a mobile neck, and lung-dominated respiration.^{9, 10, 15} Critically, these earliest tetrapods were not fully terrestrial animals: both Acanthostega and Ichthyostega retained internal gills and were likely amphibious at best, spending most of their lives in shallow, heavily vegetated water. The terrestrial arthropod communities and the soil ecosystems that plant colonisation had built may have provided the biomass that made sustained terrestrial foraging increasingly profitable for vertebrates, but fully terrestrial vertebrate lifestyles were largely a Carboniferous development.

The ecological and climatic consequences of terrestrial colonisation intersected catastrophically at the close of the Devonian in the Late Devonian mass extinction — specifically the Kellwasser event (~372 Ma) and Hangenberg event (~359 Ma). These extinction pulses eliminated roughly 70 to 80 percent of marine invertebrate species, most placoderm fishes, and much of the acanthodian diversity that had characterised the earlier Devonian seas.¹¹ The mechanisms remain debated, but the dominant hypotheses centre on plant-driven changes to the global carbon cycle: the rapid burial of forest biomass and the CO₂ drawdown associated with forest expansion cooled the climate and reduced ocean oxygen levels through changes in thermohaline circulation, creating widespread oceanic anoxia. The forests that had transformed the land had, indirectly, devastated the sea.

Significance of the transition

The colonisation of land during the Silurian and Devonian was not merely a chapter in the evolutionary history of particular lineages; it was a planetary-scale event that fundamentally altered the chemistry of the atmosphere, the structure of the lithosphere, and the distribution of biological productivity on Earth. Before terrestrial colonisation, the continents were geologically active but biologically passive surfaces contributing relatively little to global biogeochemical cycles beyond inorganic weathering. After it, the land surface became an active participant in the carbon cycle, the nitrogen cycle, and the water cycle, stabilising soil against erosion, modulating river chemistry, and providing the first complex three-dimensional habitats on the continental surface.

Each of the key transitions — cryptospore-producing bryophytes in the Ordovician, Cooksonia-grade vascular plants in the Late Silurian, deep-rooted Archaeopteris forests in the Late Devonian, and the earliest tetrapod explorers of the terrestrial margin — built upon what preceded it. Plants created the soil and litter that arthropods required; arthropod decomposers and mycorrhizal fungi created the nutrient cycling that allowed plant communities to grow taller and spread further; and the expanding forest biomass altered the hydrology and microclimate of continental interiors in ways that both facilitated and constrained the subsequent expansion of animal diversity. The terrestrial world that all land-dwelling organisms today inhabit was not inherited from a pre-existing landscape; it was constructed, incrementally and irreversibly, by the organisms that colonised it.