Carboniferous forests

The story

A world of swamps

About 320 million years ago, on the lands we now call Europe and North America, the landscape was very different. It was hot and extremely humid. Great swamps stretched near the equator, and in those still waters grew dense, green forests. They were not made of oaks or pines, which did not exist yet, but of tree ferns, giant horsetails and above all of club mosses, plants that are small and creeping today but that then grew into trees.

The period is called the Carboniferous and lasts from about 359 to 299 million years ago. The name was given in 1822 by two English geologists, William Conybeare and William Phillips, because rocks of that age contained a lot of coal: “Carboniferous” means exactly “coal-bearing”.

Strange and immense trees

The biggest club mosses, such as Lepidodendron and Sigillaria, were up to about 30 metres tall. They had trunks that did not branch for much of their height and were covered with diamond-shaped scales, like the skin of a reptile. At the top, forked branches opened out with long, ribbon-like leaves. They had little wood, and grew very fast, and then died. Beside them were the giant horsetails, such as Calamites, 20 metres tall, and tree ferns, with broad, ragged leaves.

The leaves and trunks fell into the dark, oxygen-poor water of the swamps. There the remains did not rot completely: they piled up in layers of plant material, called peat. With time, the ground sank and the sea or rivers covered the peat with sand and mud. Under the weight and the heat, over millions of years, the peat turned into coal. To make a layer of coal one metre thick took several metres of peat, and to form that, many thousands of years.

Scientists have long argued about why so much coal formed in just that period. One hypothesis says that the fungi and bacteria able to break down wood were not yet efficient enough, so a lot of wood stayed intact; today many scholars think that the humid climate and the movement of the continents, which created basins where the remains piled up, mattered more. The debate is open.

Giant animals in oxygen-rich air

The Carboniferous was the golden moment of the arthropods. In the forests lived gigantic dragonfly-like insects such as Meganeura, with wings up to about 70 centimetres across, and enormous millipedes such as Arthropleura, more than two metres long, the biggest land invertebrate that has ever existed. These giants are thought to have been helped by the air, which according to calculations held more oxygen than today, between 25 and 35 per cent according to the models, against 21: insects breathe through tiny tubes and richer air lets bodies grow bigger.

Vertebrates walked among the plants too. Amphibians were numerous and some were as big as crocodiles. And, around 315 million years ago, the first reptile-like animals appeared: the best-known example is Hylonomus, a lizard-like animal about 20 centimetres long, whose remains were found inside a fossil tree trunk at Joggins, in Nova Scotia, Canada. Reptiles laid eggs with a shell and a membrane that protected the embryo, and this allowed them to live far from water.

The end of the swamp forests

Towards the end of the Carboniferous the climate became drier and colder, partly because of great glaciers forming in the south of the supercontinent Gondwana. About 305 million years ago the wet forests of club mosses shrank a great deal and broke up; the reptiles, better suited to dry climates, took advantage. The coal stayed underground, waiting.