The Carboniferous turned swamp forests into coal and let insects take flight
Much of the coal that powered the industrial age grew as swamp forest between about 359 and 299 million years ago. Geologists named the period for that coal, and it was also when four-limbed animals settled on five fingers, insects took to the air, and the ancestors of mammals and reptiles first appeared.
The name simply means coal-bearing, from Latin words for coal and carry. It was the first of the modern geological system names, coined in 1822 by William Conybeare and William Phillips from their study of British rock layers, though the adjective had been used a couple of decades earlier. The period lasted around 60 million years, following the Devonian and preceding the Permian. Americans, noticing a clear change halfway through, split it into two periods named after states: the limestone-rich Mississippian and the coal-rich Pennsylvanian, a division the US Geological Survey adopted in 1953.
Life on land became firmly established. Four-limbed vertebrates, descended from lobe-finned fish, settled on the five-digit limb, and early amphibians diversified so widely that the period is sometimes called the Age of Amphibians. Late in the period came the first amniotes, animals whose eggs could survive away from water, splitting into the line leading to mammals and the line leading to reptiles and birds. Arthropods boomed too, from giant millipede relatives such as Arthropleura to the first flying insects.
Oxygen levels were long assumed to have been high throughout. More recent work suggests they started low and climbed to between 25 and 30 per cent, well above today. In the second half of the period, continents crashed together to build the supercontinent Pangaea, raising mountains, while ice sheets spread and sea levels fell. At the very end, a changing climate caused the Carboniferous rainforest collapse, a modest extinction on land and sea.
Dividing the period precisely is still unfinished business. International geologists agreed on a common framework in 1975, blending British, Russian and American schemes, but several stage boundaries still lack an agreed reference site, partly because falling seas and isolated basins make fossils hard to match across the globe.
Source: Carboniferous
