Devonian to Carboniferous: Early Forests and Amphibians

Plants were already living on land before the Devonian. Early vegetation could influence rock weathering and local conditions even without large trunks or deep roots. Devonian forests expanded that influence; they did not begin on an entirely lifeless surface.[2]

By the Middle Devonian, forests contained different kinds of trees and rooting systems. Fossil roots at Cairo, New York, reveal an underground landscape as informative as the trunks above it. Some belonged to the Archaeopteris group: plants with wood, leaves, and extensive branching roots that nevertheless reproduced through spores rather than seeds.[3]

“Tree” describes a way of growing, not one unchanging lineage. The earliest forests included combinations of growth forms unfamiliar in modern woodland. Their fossil record captures repeated experiments in building tall plants and supporting them from below.[3]

The change from fins to limbs was not a single moment when a fish climbed onto a riverbank. Different features appeared in different combinations. Stronger appendages and air breathing were already present in some fishes before a fully terrestrial way of life evolved.[4]

These fossils reveal parts of an evolutionary branching pattern. They should not be arranged as a proven chain of direct ancestors. Likewise, the early limbed vertebrates often called “amphibians” in introductory accounts were not simply ancient frogs or salamanders; the relationships among early tetrapods and living groups require more careful distinctions.[6]

Carboniferous wetlands supported striking plant communities. Giant lycopsids such as Lepidophloios were relatives of clubmosses, rather than true mosses. Calamites belonged to the horsetail lineage. Tree ferns grew alongside seed ferns—seed plants with fern-like foliage. A coal forest was a distinctive ecosystem, not a modern rainforest transplanted into the past.[7]

Coal preserves carbon taken up by ancient plants. When it burns, that stored carbon returns largely as carbon dioxide. A lump of coal therefore connects biological growth, sedimentary burial, and a much later release of energy.[7]

Several reconstructions place atmospheric oxygen above today’s roughly 21% near the Carboniferous–Permian interval. Some models give peaks around 25–30%; other estimates differ. These values come from geological evidence and modelling, not samples of preserved Carboniferous air. A fixed “35% oxygen throughout the Carboniferous” overstates what is known.[8]

The familiar image of a steamy coal swamp represents particular tropical lowlands. Elsewhere were seasonally dry landscapes and different plant communities. Ice covered parts of southern Gondwana, while tropical forests existed far away. The late Paleozoic ice age was already underway before the Late Carboniferous.[11]

As continents assembled into Pangaea, geography helped alter circulation and rainfall. Glacial cycles changed sea levels, affecting coastal environments. Wet forests expanded, contracted, or changed composition at different times in different regions; there was no single worldwide moment when every coal forest vanished.[11]

Could changes on land disrupt the sea?

During the Kellwasser crises around 372 million years ago, marine ecosystems suffered major losses. Research explores a connection with expanding vegetation: weathering and runoff could carry more phosphorus to the sea, stimulating biological production. Decomposition of the extra organic matter could then deplete oxygen in marine waters. Modelling supports this as a possible contributor alongside influences such as volcanism; it does not establish forests as the sole cause of every Devonian extinction.[12]

Notice the connection: a change in roots and soils can affect rivers, which can affect marine chemistry. The land and ocean are linked parts of the same planetary system.

Locomotion was only one part of terrestrial life. Reproduction presented another challenge: an embryo needs a suitable environment even when the surrounding habitat is dry. Amniotes evolved membranes that maintain a protected fluid environment around the developing embryo, reducing dependence on reproduction in open water.[13]

Amniotes were present during the Carboniferous, but their earliest history is still being revised. A 2025 study interpreted Australian tracks around 355 million years old as amniote footprints, based partly on claw impressions and foot shape. This would extend their record much earlier than many older textbook accounts. The identification is an interpretation of tracks, not the discovery of a complete animal or its eggs.[6]

A fossil is a surviving piece of evidence, not a complete recording of an ecosystem. Understanding these worlds involves asking what each kind of trace can reveal—and which questions remain open.

Comparing these records makes the history richer. Roots reveal one part of the environment; animal remains reveal another. A convincing reconstruction must respect both the evidence that survives and the gaps between discoveries.

The Devonian and Carboniferous show why an atmosphere cannot be understood from biology alone. Oxygen depended on production, consumption, and the burial and recycling of material. Life altered its surroundings within a wider geological system.[8]

That lesson extends to the search for life beyond Earth. Oxygen on another planet would need to be interpreted alongside its star, atmospheric chemistry, and geological setting. Some nonbiological processes can produce oxygen, while a living world can also have little oxygen in its atmosphere. An oxygen signal alone could not tell us that distant forests or familiar animals existed there.[14]