A dramatic climate change event which devastated Earth's rainforests just under 300 million years ago also opened the way for the rise of the reptiles, a new study shows.
During the late Carboniferous, Europe and North America formed a single large land mass on the equator. Tropical rainforests effectively stretched from Utah to the Urals. These were the first rainforests to evolve on our planet and their remains are preserved today as coal seams.
The paper, published in the current edition of Geology, explains how one can deduce from the fossil record that the drier hotter conditions reduced habitat for the amphibians, which had until then been the planet's dominant tetrapods - land-based four-legged creatures.
At the same time, lizards thrived and diversified, preparing the ground for the reptile revolution 100 million years later.
Dr Howard Falcon-Lang, part of the research team at Royal Holloway, University of London, in the UK, says that as rainforests collapsed, they contracted into little 'islands' separated by areas of dry scrub prone to frequent fires.
"We think that diversification was driven by this 'island effect'. [Pockets of] forests containing animals got separated from one another and animals in each island evolved in different ways," he says.
Falcon-Lang says that following rainforest collapse, the Earth's atmosphere entered a feedback loop of climate change.
"The rainforests have always been an important 'sink' for carbon dioxide, much of that greenhouse gas getting locked up as coal.
"When the rainforests collapsed, more carbon dioxide entered the atmosphere leading to steady global warming," he says.
Falcon-Lang believes there was probably a period of rapid glaciation before the global warming event, as sea-levels dropped to their lowest in 25 million years.
"There is no doubt that rainforest collapse was triggered by climate change," he says.
"It may have been a combination of processes - a climate double whammy. First climate abruptly cooled, making the tropics much drier, and then steadily warmed to create a world that was too hot for rainforests."
That led to further forest decline.
Falcon-Lang says that the rainforest dieback in turn drove evolution.
"Some animals like the amphibians went to the wall with many species becoming extinct. Other animals like the reptiles did well and diversified."
Amphibians were limited because they needed to return to the water to lay their eggs; but the reptiles, which had already developed scaly skins and tough leathery eggs, were able to move out of the shrinking rainforests and occupy new ecological niches.
Exploring new food sources
Interestingly, up until 300 million years ago tetrapods had been almost exclusively piscivores - fish-eaters - but after the rainforest collapse the surviving amphibians moved away from fish and towards insects. The researchers say this reflects an increasing diversity, abundance and size of insects at the time. Eagle-sized dragonflies typified the late Carboniferous.
Meanwhile the reptiles explored two new food sources: other tetrapods, and plants.
The researchers report that this ecological diversity was the result of tetrapods adapting, to maximise the acquisition of limited resources in a fragmented habitat.
Falcon-Lang says dramatic changes in environment have always been the major driver behind evolution.
"We can conclude that life is resilient to climate change, even the collapse of rainforests", he says, "but not without devastating loss of diversity and a change in the direction of evolution.
"If rainforests were to collapse today, life on Earth would never be the same again. But given sufficient time, new forms would evolve in the new environment that followed What might replace it is uncertain."