c. 320 million years ago
The giant forests of the Carboniferous
During the Carboniferous, giant tree-ferns and horsetails blanketing continental interiors accumulated over millions of years to form the coal seams we use today. These forests produced high oxygen levels while also setting the stage for the evolution of giant insects and the first amniotes.
The Carboniferous period (approximately 359β299 million years ago) encompasses one of the most extraordinary biosphere episodes in Earth history. The equatorial regions of the assembling Pangaea supercontinent (335-mya-pangaea) were blanketed in tropical swamp forests composed of plant groups that no longer exist anywhere on Earth.
Tree-ferns such as Lepidodendron and Sigillaria could reach 30β40 metres in height, while giant horsetails of the genus Calamites formed dense stands along swamp margins. Cordaitaleans and seed ferns also achieved their evolutionary peak in this period, representing an accelerating botanical conquest of land that had begun with the first terrestrial plants (470-mya-land-plants).
These forests had a consequence that would directly shape human civilisation millions of years later: coal. Carboniferous plants contained a tough structural polymer called lignin which the terrestrial bacteria of the time had not yet fully evolved the enzymes to decompose. Plants that died and fell into the oxygen-poor swamp waters accumulated undecayed, forming thick peat and eventually coal seams over geological time. The majority of the coal deposits that powered the Industrial Revolution originated in this period.
A second remarkable legacy of Carboniferous forests involves atmospheric oxygen. Vigorous photosynthesis combined with reduced microbial decomposition drove atmospheric Oβ well above its present-day level of 21 percent β estimates reach as high as 35 percent. This elevated oxygen concentration allowed some animals to grow to extraordinary size: dragonfly-like Meganeuropsis with wingspans of 70 centimetres and giant scorpions lived in this period. High oxygen also intensified forest fires; abundant charcoal in Carboniferous rock sequences provides the geological evidence.
A further critical evolutionary step occurred during this period: the first amniotes β vertebrates capable of laying eggs on land β appeared in the late Carboniferous. This development defined the common ancestor of reptiles, birds, and mammals.
Sources
- Collapse of Carboniferous rainforest ecosystems and the subsequent recovery β Geology
- Atmospheric oxygen over Phanerozoic time β Proceedings of the Royal Society B
- Carboniferous Period β Encyclopedia Britannica β Encyclopaedia Britannica