The age of coal forests
Carboniferous
Paleozoic Era · 358.9 – 298.9 million years ago
The Carboniferous Period ran from 358.9 to 298.9 million years ago, a span of 60 million years at the end of the Palaeozoic Era. It is named after the coal it left behind: swampy forests of giant club mosses and tree ferns grew, died and were buried in such quantities that they became many of the coal seams that later powered the Industrial Revolution.
It was also a world of extremes. Dragonfly-like insects had wingspans of around 70 centimetres, millipede relatives grew longer than a person, and the first reptiles scurried through the undergrowth, while ice sheets spread across the southern continents and the pieces of a future supercontinent drifted steadily together.
Key facts
- Began
- 358.9 MaMillion years ago
- Ended
- 298.9 MaMillion years ago
- Duration
- 60 million years
- Era
- Paleozoic“Ancient life”
- Global temperature
- ~16°CRoughly 13–19°C · today 14.5°C
- Plants on land
- Forests
- Atmospheric CO₂
- ~300–400 ppm by the late Carboniferous (≈ pre-industrial)
- Oxygen
- Possibly ~30–35% at its peak (vs 21% today); debated
- Sea level
- High early on, then rising and falling with the ice ages
- Continents
- Laurussia and Gondwana colliding to build Pangaea
- Signature life
- Coal-swamp lycopsids, giant insects, first reptiles
- Began with
- Recovery from the end-Devonian extinctions
- Ended with
- Drying climate after the coal forests fragmented
Ages follow the International Chronostratigraphic Chart (ICS). Global temperatures are rough reconstructions after Scotese et al. (2021); ancient CO₂, oxygen and sea-level values carry large uncertainties.
Epochs of the Carboniferous
- Mississippian358.9 Ma – 323.2 Ma
- Pennsylvanian323.2 Ma – 298.9 Ma
Continents and oceans
Two great landmasses dominated the Carboniferous globe. In the north lay Laurussia, also called Euramerica, made of what is now North America, Greenland and much of Europe. To the south sprawled Gondwana, which included South America, Africa, Antarctica, India and Australia, and stretched across the South Pole.
Through the period these two continents closed the ocean between them and collided, crumpling up long mountain belts: the Variscan mountains of Europe and the Alleghanian phase of the Appalachians in North America. The collision was a key step in assembling Pangaea. Around it stretched the vast Panthalassic Ocean, while the Palaeo-Tethys Ocean formed a wedge-shaped embayment on the eastern side.
Much of the coal-forest belt sat near the equator, in low-lying basins along the edges of the growing mountains. Those swampy lowlands are now found in places as far apart as Pennsylvania, Britain, Germany, Ukraine and northern China.
Climate and atmosphere
The Carboniferous began warm but slid into the Late Palaeozoic Ice Age, one of the longest glaciations in Earth's history. Ice sheets grew and shrank across southern Gondwana, and as they did, sea level rose and fell repeatedly. These swings left a distinctive layer-cake of rocks, called cyclothems, in which coal, shale and limestone alternate over and over.
Plants pulled enormous amounts of carbon dioxide out of the air and locked it away as buried peat, which helped cool the planet. Oxygen levels are thought to have climbed well above today's 21%, with many estimates around 30–35%, although the exact peak is still debated. High oxygen may help explain why some insects and other arthropods grew so large.
Life in the Carboniferous
Life on land flourished in the Carboniferous. Forests spread across the tropics, arthropods took to the air, and backboned animals made a decisive step: the evolution of the amniotic egg, which freed them from returning to water to breed.
Plants
The coal swamps were dominated by lycopsids such as Lepidodendron and Sigillaria, tree-sized relatives of today's small club mosses that could reach 30 metres or more. Giant horsetails like Calamites, tree ferns and seed ferns filled out the forest, and early conifers grew on drier ground.
On land
Arthropleura, a millipede relative up to about 2.5 metres long, was the largest land invertebrate known. Large amphibians lurked in the swamps, and the first reptiles appeared: Hylonomus, from around 315 million years ago, is one of the oldest known. Before the period ended, the amniotes had already split into the lines leading to reptiles and to mammals.
In the air
Insects were the only flying animals, and some were huge. Meganeura, a griffinfly related to dragonflies, had a wingspan of roughly 65–70 centimetres. Cockroach-like insects were also abundant on the forest floor.
In the seas
Warm, shallow seas held crinoid meadows, brachiopods, ammonoids and reef-building organisms. Sharks and their relatives went through a burst of diversity, producing odd forms such as Stethacanthus, with its flat-topped, bristly dorsal fin.
Timeline of key events
- 358.9 Ma
Carboniferous begins
Life recovers from the extinction pulses that closed the Devonian, including the Hangenberg event.
- 330 Ma
Southern ice sheets expand
Glaciers spread across southern Gondwana as the Late Palaeozoic Ice Age takes hold.
- 323.2 Ma
Pennsylvanian subperiod begins
A major drop in sea level marks the boundary between the Mississippian and Pennsylvanian.
- 315 Ma
First reptiles appear
Hylonomus, one of the oldest known reptiles, lives in the lycopsid forests of what is now Nova Scotia.
- 305 Ma
Carboniferous Rainforest Collapse
A drier, more seasonal climate breaks the tropical coal forests into isolated patches.
- 300 Ma
Giant griffinflies take wing
Meganeura, with a wingspan of about 70 centimetres, flies over the swamps of what is now France.
- 298.9 Ma
Permian Period begins
The Carboniferous ends without a mass extinction as Pangaea nears completion.
How the Carboniferous ended
The Carboniferous did not end with a mass extinction. Instead, the climate grew drier and more seasonal, and around 305 million years ago the great tropical rainforests broke up into isolated patches, an event known as the Carboniferous Rainforest Collapse. Many amphibians suffered, while reptiles, better suited to dry ground, spread.
Geologists place the boundary with the Permian at 298.9 million years ago, marked by the first appearance of a particular conodont, a tiny eel-like animal, in rocks at Aidaralash Creek in Kazakhstan.
Where to see Carboniferous fossils
The Joggins Fossil Cliffs in Nova Scotia, a UNESCO World Heritage Site, preserve fossil trees still standing where they grew, along with early reptiles like Hylonomus. Mazon Creek in Illinois is famous for ironstone nodules holding delicate plants, insects, jellyfish and the mysterious Tully monster.
In Europe, coal-mining regions of Britain, Germany and Belgium have yielded countless plant fossils, and Commentry in France produced Meganeura. The Bear Gulch Limestone of Montana is renowned for its exquisitely preserved Carboniferous fishes and sharks.
Surprising facts about the Carboniferous
- Some of the earliest known reptiles were found inside the fossilised stumps of lycopsid trees at Joggins, Nova Scotia, apparently trapped after the hollow stumps filled with sediment.
- Arthropleura, a millipede relative up to about 2.5 metres long, was the largest invertebrate ever to live on land.
- The Tully monster, a soft-bodied animal from Mazon Creek with eyes on a stalk-like bar and a long toothed snout, is so strange that scientists still argue about what kind of animal it was. It is the state fossil of Illinois.
- Much of the black coal mined in Europe and eastern North America formed from Carboniferous swamp plants, so the period quite literally fuelled the Industrial Revolution.
- The name Carboniferous, meaning coal-bearing, was coined in 1822 by the British geologists William Conybeare and William Phillips. In North America it is often split into two periods, the Mississippian and the Pennsylvanian.
- Meganeura was not a true dragonfly but a griffinfly, a member of an extinct group related to dragonflies, and it was a predator of other insects.
Where it sits in deep time
The globe beside this article shows Earth 320 million years ago, reconstructed from the PALEOMAP paleo-elevation models. Step to another period and watch the continents drift into place.