Prehistoric Rainforest Collapse Dramatically Changed the Course of Evolution

The collapse of rainforests during the Carboniferous period 300 million years ago triggered great changes in the evolutionary paths of plants and animals.

Artist's recreation of Dimetrodon in its habitat | Simon Stalenhag
Artist's recreation of Dimetrodon in its habitat | Simon Stalenhag

It is estimated that more than 150 acres of rainforest are lost each minute of every day, totaling some 78 million acres of loss annually. Should this current rate of tropical deforestation continue, NASA's Earth Observatory projects that rainforests will vanish within a century, "causing unknown effects on global climate and eliminating the majority of plant and animal species on the planet."

Although the present level of human-caused deforestation is unprecedented, this is not the first time that rainforests have dramatically shrunk in size. Tropical rainforests collapsed around 307 million years ago, toward the end of the Carboniferous period (359–299 million years ago).

New research on this prehistoric rainforest collapse, published in the journal Proceedings of the Royal Society B, finds that it forever changed the course of evolution for terrestrial species, with effects still felt today.

"There have been several studies over the last 40 years on how diversity has changed through time, but it is only in the last few years that researchers have begun to acknowledge the limitations of fossil data," lead author Emma Dunne of the University of Birmingham told Seeker.

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She added, "Newly developed methods have been allowing researchers to look for genuine patterns of diversity amongst this patchy fossil data, and we thought it was time to look more closely at diversity patterns of the first vertebrates to live on land."

A key source of information for Dunne and her colleagues is the ever-growing Paleobiology Database, which is the largest international collaborative project among paleontologists all over the world. It has already supported more than 300 papers and is being used now more than ever by scientists.

Throughout her work on the new study, Dunne actually updated, and added to, the database's information on early tetrapods: vertebrates with two pairs of limbs, including those that lost one or both pairs over evolutionary time, such as whales and snakes.

Around 310 million years ago, before the early rainforest collapse, North America and Europe were part of a single landmass located at the equator. There, dense tropical rainforests flourished. This was at a time before the first dinosaurs and mammals evolved.

The ample vegetation and warm, humid climate supported amphibian-like early tetrapods, which quickly diversified into many different species. Giant dragonflies, millipedes, the first amniotes — egg-producing animals that lay them on land or within the mother — and even early cockroaches were also around then.

But the tropical paradise began to change.

"Carbon dioxide levels dropped in the late Carboniferous, and this led to the cooling and drying of the climate," Dunne explained.

Artist's recreation of a forest during the Carboniferous period | Mark Ryan

The event proved to be catastrophic to plants, and rainforests began to disappear. What happened to animals has been debated for many decades.

A prevailing theory has been that tetrapod diversity reduced markedly before endemism occurred. Endemism refers to species evolving in defined geographic locations, such as on an island, in a particular country, or in some other defined zone. In short, the species are basically confined to a certain place where they evolve to live.

The fossil data that Dunne and her team compiled and obtained from the massive Paleontology Database, however, counters that long-held theory.

"Instead of tetrapod communities of species becoming more isolated (endemism), we found that they became even more well connected, a pattern called cosmopolitanism," Dunne said. "These better-connected communities suggest that species were more able to move around between different areas."

At first the findings surprised Dunne and her team.

"But when coupled with the diversity decrease, these results make a lot of sense," she said.

The Carboniferous rainforest collapse was one of two mass extinctions recorded in the fossil record for plants. The other one occurred in the mid-late Permian (299–252 million years ago).

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Once tetrapods recovered from the changes, the Permian led to some of the most interesting animals ever to walk the earth. Amphibians and reptiles known as anapsids, diapsids, and synapsids grew to tremendous sizes.

Particularly flashy were pelycosaurs like the predator Dimetrodon, which was adorned with a line of long, bony spines along its back that supported a sail-like crest. Dinosaurs later evolved similar features.

While the dramatic climate change at the end of the Carboniferous was devastating to plants, animals seemed to recover pretty well over time. One might then wonder what lessons might be learned that could apply to today's environmental conditions.

It appears that animals highly evolved for survival in a specific habitat do not fare well during times of profound change, since they lack the flexibility to quickly adapt to new climates and environments.

"I think it's generally considered the case that species with broader niches do better in environmental crises, and there is potential to use the fossil record to test hypotheses like these, and therefore inform current environmental work," Dunne said.

"However," she quickly added, "I think we would want to be careful about using data from ecosystems like those of the Carboniferous that are so fundamentally different to today. It's probably more useful to look at the much more recent fossil record."

Dunne and many others believe that conservation of today's remaining rainforests is critical. "Rainforests today are incredible hotspots of both animal and plant diversity,” she said, “and it is very important to protect them, and indeed other threatened habitats."