Carboniferous period: coal forests in an ice age

Lasted 358.86 to 298.9 million years ago about 60 million years; the boundaries carry errors of ± 0.19 and ± 0.15 million years [1]

Name Latin for "coal-bearing" coined by Conybeare and Phillips in 1822 from the British rock succession [1]

Two halves Mississippian, then Pennsylvanian the split falls 323.4 million years ago [4]

Tallest plants scale trees, 30 to 50 m one museum page says over 54 m; trunks often over 1 m across [5]

Ice age about 360 to 255 million years ago ice sheets over Gondwana while swamps grew near the equator [7]

Global average temperature about 22 °C down to about 13 °C 358 to 353 million years ago, against the glacial peak of 299 to 293 million years ago [1]

Oxygen 15 to 20%, rising to highs of 25 to 30% all models show the rise; the exact figures differ [1]

Oldest amniote tracks about 350 to 356 million years old clawed footprints from Victoria, Australia; the university release says about 350, news reports 356 [18]

Forest turnover about 305 million years ago in the earliest Kasimovian, the stage dated 307.0 to 303.7 million years ago [10]

Length of a day about 22 hours about 390 days a year, 300 million years ago, from daily layers in fossil coral [22]

About the pictures. Painted as a coal swamp about 310 million years ago, from the forest floor and from high above; as a thicket of sea lilies on a shallow sea floor about 340 million years ago; and as a small early reptile, loosely based on Hylonomus, on a rotting log. Coal seams and trunks buried upright inform the swamps, and fossil crinoids and brachiopods the sea floor. The trees give only the mood of such a forest: fossils show straight poles with diamond-patterned bark that fork only at the top, not these leafy crowns and furrowed trunks. The sea lilies are simplified, and the colours, scales, mist and light are imagined.

The Carboniferous ran from 358.86 to 298.9 million years ago, while the continents were closing to form Pangaea. Its name is Latin for coal-bearing, and the majority of Earth’s coal dates from its later part and the early Permian. Swamp forests filled the wet lowlands near the equator while ice sheets spread over Gondwana in the south. American geologists split the period in two: the mostly limestone Mississippian, when crinoids thronged the seas, and the coal-bearing Pennsylvanian, from 323.4 million years ago.

Trees that were not trees

The tallest plants were scale trees such as Lepidodendron, cousins of today’s tiny club mosses. Height estimates run from about 30 m to over 50 m. For most of its life a scale tree was a single unbranched pole, its bark patterned with the diamond-shaped leaf bases behind its name. Only late in life did the top fork repeatedly into a crown, tipped with cones that shed spores rather than seeds. The trunk held little wood.

Around them stood Calamites, giant horsetails up to 20 m tall, with tree ferns and seed ferns. Forests may already have had layers: Sanfordiacaulis, about 350 million years old and described in 2024 from New Brunswick, grew a few metres tall under a crown 5 to 6 m wide.

Coal made by ice

Coal begins as peat: plant debris piling up in waterlogged, oxygen-poor swamps, then buried and compressed. Why so much formed then is still debated. One idea is that trees evolved lignin, the tough material in wood, before fungi could break it down, and a 2012 genetic study placed the origin of that ability around the end of the Carboniferous. In 2016 Nelsen and colleagues rejected this: much Carboniferous coal comes from scale-tree bark with little lignin, and it built up just as fast when lignin-rich plants took over. They point instead to geography: tropics that were always wet, and wide sinking basins, as Pangaea assembled. The rising Central Pangean Mountains drew in moist air from the Paleo-Tethys.

The ice age, from about 360 to 255 million years ago, helped with the burying. Ice spread from up to 30 centres in Gondwana, advancing and retreating on a cycle of about 100 000 years. As it grew, sea level fell and swamps spread across the exposed shelves. As it melted, the sea came back and buried them in sediment. Each round left a cyclothem, a package of land and marine rocks.

Oxygen and the giants

All models show oxygen rising, from 15 to 20 per cent to highs of 25 to 30 per cent. This long served to explain the griffinfly Meganeura and the 2.6 m Arthropleura. The explanation is weakening. In 2026 Snelling and colleagues measured the finest air tubes in the flight muscle of 44 insect species. They fill about 1 per cent or less in most species and barely increase with body size, so oxygen supply seems not to cap growth.

Eggs away from water

Amniotes, the group that later gave rise to reptiles, birds and mammals, enclose the embryo so that it does not dry out, which frees them from breeding in water. The classic early example is Hylonomus, 20 to 25 cm long and about 318 million years old, found in the hollow stumps of scale trees at Joggins in Nova Scotia. In 2025 Long and colleagues described clawed footprints from Victoria, Australia, about 350 to 356 million years old, which push the first amniotes back by at least 35 to 40 million years. Footprints are indirect evidence, so the maker is inferred.

Not quite a collapse

In the earliest Kasimovian stage, about 305 million years ago, the coal forests of Europe and North America changed. A glaciation of under a million years took carbon dioxide as low as 180 ppm, and the climate turned cooler and drier. Within one glacial cycle, peat swamps ruled by scale trees passed to tree ferns and seed ferns, and diversity fell. This is the Carboniferous rainforest collapse, a name DiMichele and colleagues consider exaggerated, because one kind of rainforest replaced another. Lepidodendron survived in Euramerica until the end of the period, and in what is now China until about 252 million years ago. Arthropleura and large griffinflies outlived the change.

Sahney and colleagues argued that the forest broke into islands, each evolving its own fauna. A 2018 reanalysis by Dunne and colleagues, correcting for uneven sampling, found more mixing instead. A permanent shift to seasonally dry woodland came at the boundary with the Permian.

Where it sits in deep time

Carboniferous period: 358.86 to 298.9 million years ago, in the Carboniferous.

Carboniferous period

Earth forms, 4.57 billion years agoToday

Each step to the right is ten times closer to today, so the last few million years get as much room as the first billion.

If Earth's history were one day

Squeeze all of Earth's history, about 4.6 billion years, into 24 hours, starting at midnight. Carboniferous period runs from 22:06:51 to 22:25:46, ending 1 h 34 min before midnight.

On this clock one second is about 52 862 years. All of written history fits into the last tenth of a second.

How tall

The tallest plants of the coal forests next to a person.

Scale tree, upper estimate 50 m
Scale tree, lower estimate 30 m
Giant horsetail Calamites 20 m
Person, for scale 1.7 m

Sources differ on how tall scale trees grew: the Carboniferous article cited gives 30 m, the Lepidodendron article up to 50 m, and one museum page over 54 m.

Reconstructions

A misty swamp forest of tall trunks with furrowed bark and a few spreading, leafy crowns rising from dark water, ferns along the banks and shafts of green light through the haze, an impression of a coal swamp forest about 310 million years ago. AI-generated painting.

Seen from high above, an endless swamp forest stretching to the horizon, cut by winding channels of still water and wrapped in mist, an impression of the coal swamp forests that covered the tropical lowlands about 310 million years ago. AI-generated painting.

A dense thicket of sea lilies, animals with long, thin stalks topped by fan-shaped crowns of feathery arms, rising from a sandy seabed scattered with ribbed shells under shafts of sunlight, an impression of a Carboniferous sea floor about 340 million years ago. AI-generated painting.

A small, slender, lizard-like reptile with brown, scaly skin and a long tail resting on a rotting log among ferns in a misty green forest, an impression of one of the earliest reptiles in a Carboniferous coal swamp about 310 million years ago. AI-generated painting.

Questions people ask

When was the Carboniferous period?

From 358.86 to 298.9 million years ago, about 60 million years, between the Devonian and the Permian. In the United States it is divided into the Mississippian and the Pennsylvanian, with the boundary at 323.4 million years ago.

Why is it called the Carboniferous?

The name is Latin for coal-bearing. Conybeare and Phillips coined it in 1822 from the rocks of Britain, after the coal beds formed at that time. The majority of Earth's coal dates from the late Carboniferous and early Permian.

How much oxygen was in the air in the Carboniferous?

All models show a rise, from a low of 15 to 20 per cent at the start of the period to highs of 25 to 30 per cent. It was not a steady climb, and methods disagree on the exact numbers.

Did high oxygen make Carboniferous animals giant?

That was the classic explanation, but it has weakened. A 2026 study found that the finest air tubes fill about 1 per cent or less of the flight muscle in most insects, so oxygen supply probably does not cap their size. Predators or the mechanics of the exoskeleton are more likely limits.

Were there dinosaurs in the Carboniferous?

No. The oldest known dinosaurs are about 230 million years old, far younger than the end of the period. The land belonged to amphibians, giant arthropods and the first amniotes, the group that later gave rise to reptiles, birds and mammals.

What was the Carboniferous rainforest collapse?

A turnover in the coal forests of Europe and North America about 305 million years ago, as the climate turned cooler and drier. Peat swamps ruled by scale trees passed to tree ferns and seed ferns. Rainforest did not vanish, and scale trees lasted in what is now China until about 252 million years ago, by then a rare part of the flora.