The forests and wetlands behind Carboniferous coal

Carboniferous coal swamps were humid lowland forests, not a single marsh covering the whole planet. Tree-sized lycopsids, horsetail relatives and ferns grew in waterlogged basins where plant material could accumulate. Later burial transformed part of that organic matter into coal.

Where the coal forests grew

During the Carboniferous Period, about 359 to 299 million years ago, broad tropical lowlands included river plains, coastal basins and wetlands. Their flat terrain and high water table allowed water to linger in soils. Floods and shifting channels repeatedly covered or exposed plant communities.

Conditions differed between regions and through time. Humid coal forests flourished in some equatorial basins, while other parts of the assembling supercontinent became seasonal or dry. The Carboniferous was a varied interval, not a continuous swamp under one uniform climate.

Plants that built the wetlands

Many of the largest plants belonged to groups unlike familiar modern trees. Lepidodendron and Sigillaria were tree-sized lycopsids, relatives of today's small clubmosses. Their patterned trunks rose above wet ground and formed extensive stands.

Calamites, large relatives of horsetails, grew with jointed stems. Ferns and seed ferns occupied other levels of the vegetation. These forests had no grasses or flowering plants. Their structure and reproduction differed substantially from those of a modern tropical swamp.

From plant litter to coal

Dead leaves, roots and trunks accumulated in waterlogged soils. Limited oxygen slowed many decay processes, allowing peat to build up. If a basin subsided or sediment covered it, the peat was buried. Pressure and heat then altered it over millions of years, producing coal of different ranks.

Coal formation was not simply a result of ancient fungi being unable to digest wood. Fungi capable of breaking down plant material existed, and research rejects that popular single-cause explanation for the major Carboniferous coal deposits. Basin formation, climate, plant productivity, burial and preservation all mattered.

Animals in a wetland landscape

Wet forests supported many arthropods as well as fishes and amphibian-grade vertebrates. Some Carboniferous arthropods reached exceptional sizes. High oxygen availability may have helped large insects exchange gases, but it did not determine body size by itself. Temperature, ecology, food, competition and evolutionary history also played roles.

Some early amniotes were part of the Carboniferous vertebrate record. Their reproductive biology was becoming less dependent on open water, although not every lineage occupied dry ground. Forest margins and wetlands brought together animals with different degrees of dependence on aquatic habitats.

Why the coal swamps changed

Late in the Carboniferous and into the Permian, climate and sea level shifted. Some tropical regions became more seasonal or dry, and wetland forests contracted or changed composition. These changes were not synchronous everywhere, and coal-forming environments persisted in some places after disappearing in others.

The surviving coal seams preserve a major chapter in the history of land plants and carbon cycling. They also provide evidence for how local habitats, sedimentation and long-term geological processes combined to make the Carboniferous world different from both the earlier Devonian forests and the later Permian landscapes.