Geobiologists shed new light on Earth’s first known mass extinction event 550 million years ago

A new study by geobiologists at Virginia Tech traces the cause of the first known mass extinction of animals to a decrease in the global availability of oxygen, leading to the loss of a majority of animals present towards the end of the period. Ediacaran about 550 million years ago.

Research led by Scott Evans, a postdoctoral researcher in the Department of Geosciences, part of the Virginia Tech College of Science, shows this first mass extinction of around 80% of animals during this interval. “This included the loss of many different types of animals, but those whose body plans and behaviors indicate that they relied on significant amounts of oxygen appear to have been particularly affected,” Evans said. “This suggests that the extinction event was environmentally controlled, as are all other mass extinctions in the geological record.”

Evans’ work was published Nov. 7 in the Proceedings of the National Academy of Sciences, a peer-reviewed journal of the National Academy of Sciences. The study was co-authored by Shuhai Xiao, also a professor in the Department of Geosciences, and several researchers led by Mary Droser of the Department of Earth and Planetary Sciences at the University of California, Riverside, where Evans obtained her master’s and doctorate.

“Environmental changes, such as global warming and deoxygenation events, can lead to mass animal extinctions and profound ecosystem disruption and reorganization,” said Xiao, an affiliate member of the Global Change Center, which is part of from Virginia Tech Fralin Life. Institute of Sciences. “This has been repeatedly demonstrated in the study of Earth’s history, including this work on the first documented extinction in the fossil record. This study thus tells us about the long-term impact of current environmental changes. on the biosphere.

What exactly caused the drop in global oxygen? It is still to be debated. “The short answer to how it happened is we don’t really know,” Evans said. “It could be any number and combination of volcanic eruptions, tectonic plate movements, asteroid impact, etc., but what we’re seeing is that the animals that are disappearing appear to be responding to a decrease in global oxygen availability.”

Evans and Xiao’s study is more timely than one might think. In an independent study, Virginia Tech scientists recently found that anoxia, the loss of oxygen availability, affects the world’s fresh waters. The cause? Warming waters caused by climate change and excessive runoff of pollutants from land use. Warming waters decrease freshwater’s ability to hold oxygen, while the degradation of nutrients in runoff by freshwater microbes gobbles up oxygen.

“Our study shows that, as with all other mass extinctions in Earth’s past, this new and first mass extinction of animals was caused by a major climate change – another in a long list of cautionary tales demonstrating the dangers of our current climate crisis for animal life,” said Evans, who is a geobiology fellow at the Agouron Institute.

Some perspectives: the Ediacaran period spanned about 96 million years, ending on either side with the end of the Cryogenian period, 635 million years ago, and the beginning of the Cambrian period , 539 million years ago. The extinction event occurs just before a significant break in the geological record, from the Proterozoic eon to the Phanerozoic eon.

There are five known mass extinctions that stand out in animal history, the “Big Five”, according to Xiao, including the Ordovician-Silurian extinction (440 million years ago), the Late Devonian extinction (370 million years ago), Permian-Triassic extinction (250 million years ago), Triassic-Jurassic extinction (200 million years ago) and the Cretaceous-Paleogene extinction (65 million years ago).

“Mass extinctions are well recognized as milestones in the evolutionary trajectory of life on this planet,” Evans and his team wrote in the study. Whatever the cause behind the mass extinction, the result was multiple major changes in environmental conditions. “In particular, we find support for declining global oxygen availability as the mechanism responsible for this extinction. This suggests that abiotic controls have had significant impacts on diversity patterns throughout history. over 570 million years of animals on this planet,” the authors wrote. .

Fossil footprints in the rock tell researchers what the creatures that perished during this extinction event would have looked like. And they looked, in Evans’ words, “strange.”

“These organisms occur so early in the evolutionary history of animals that in many cases they appear to be experimenting with different ways of building large, sometimes mobile, multicellular bodies,” Evans said. “There are many ways to recreate their appearance, but the bottom line is that before this extinction, the fossils we find don’t always match up perfectly with how we classify animals today. Essentially, this extinction may have to be instrumental in paving the way for the evolution of animals as we know them.”

The study, like dozens of other recent publications, emerged from the COVID-19 pandemic. Because Evans, Xiao, and their team couldn’t access the field, they decided to build a global database based primarily on published records to test ideas about the evolution of diversity. “Others had suggested there might be an extinction at that time, but there was a lot of speculation, so we decided to put together everything we could to try and test those ideas.” said Evans. Much of the data used in the study was collected by Droser and several graduate students at the University of California Riverside.