About 305 million years ago, five or six million years before the end of the Pennsylvanian Period, the rainforests and coal swamps came to a relatively abrupt end. This was an extinction event that seems to have caused a step-wise decimation of the plant life that was so abundant earlier in the Period.
The cause of this rapid decline in the tropical rainforests is not clear. One good candidate is the increasing glaciation in the southern hemisphere, in Gondwana. A short but intense pulse of glacier formation would have lowered sea level, perhaps enough to affect the swamps as well. And it could have led to cooler overall climates that would be challenging for the warm-weather adapted forests. But it may have been the variability as much as anything – rapid warming following a short glacial episode. Forests acclimated to a pretty standard, unchanging environment might not have been able to cope with the variations. The overall climate appears to have become considerably drier at the same time it cooled, and that too would have impacted rainforests negatively.
Some evidence seems to suggest that the collapse of rainforests may have taken only a few thousand years, and that variable climates followed. There were survivors, but they occupied ecological islands where diversity was restricted for the older rainforest flora. The amount of carbon dioxide in the atmosphere had been declining through the Pennsylvanian, and reached some of the lowest levels in the past 600 million years toward the end of the Pennsylvanian and into the Permian Period. Was that a cause of the decline of the rainforests - less CO2 for them to use? Or was the decline in CO2 a consequence of the abundance of plants, taking it out of the atmosphere? That’s not clear, but the CO2 decline was a long, gradual affair, while the crash of the rainforest ecosystem was quite rapid.
There is a well-known periodicity to extinctions in earth history, at about 26 to 27 million years. It’s suspected to relate to some cosmic-scale phenomenon such as the solar system’s position within the Milky Way Galaxy, but the ultimate cause of cyclic extinction events is not known with certainty. But the Carboniferous collapse of the rainforest ecosystem does fall within one of the maxima of the 26-million-year cycle.
What was a disaster for the ferns and scale trees of the Pennsylvanian swamps may have been a blessing for the tetrapods, the vertebrates that were establishing themselves on land during Mississippian and Pennsylvanian time. A 2010 study by scientists at the University of Bristol and University of London in the U.K. found that following an initial decimation that coincided with the rainforest collapse, land animals diversified rapidly into the new ecological niches created by the plant extinctions. Amphibians were hard hit by the changes, and while they survived, they were outpaced by the animals that were adapted to drier conditions – namely, the ancestors of the reptiles and mammals. Reptiles and mammals did not rely on water to reproduce, and they had skins that allowed for better management of both temperature and moisture. The Pennsylvanian rainforest collapse may have created the circumstances that led to reptilian and mammalian diversification, leading eventually to the dinosaurs, and to us humans too.
—Richard I. Gibson
Rainforest collapse triggers tetrapod diversification
The Carboniferous Crisis