The peak of the Ediacaran period, about 550 million years ago, was a boom time for life in Earth’s oceans. Petals shaped like feathers sucked nutrients from the water like snails Kimberella grazed on microbial mats, and the jellyfish’s ancestors were just beginning to make waves.
But then 80% of life on Earth disappeared, leaving no trace in the fossil record.
Now a new study suggests these missing fossils point to the earliest known mass extinction event on Earth. These first communities of large, complex animals were killed by a precipitous global decline in oxygen levels – a finding that could have implications for modern marine ecosystems threatened by human activities.
“This represents the oldest recognized major extinction event in the animal fossil record,” said the study’s lead author Scott Evans (opens in new tab), a postdoctoral fellow at Virginia Tech. “It’s consistent with all major mass extinctions as it’s linked to climate change.”
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Animals have passed through the evolutionary crucible of mass extinction at least five times. There were Ordovician-Silurian and Devonian extinctions (440 million and 365 million years ago, respectively) that killed many marine organisms. Then there was the Permian-Triassic – also known as “Big Dying” – and Triassic-Jurassic extinctions (250 million and 210 million years ago, respectively), which affected marine vertebrates and terrestrial animals. The most recent mass extinction occurred about 66 million years ago at the end of the Cretaceouswiped out about 75% of plants and animals, including non-avian dinosaurs.
Whether another mass extinction should be added to this list has been an open question among paleontologists for some time. Scientists have long known about the sudden decline in fossil diversity 550 million years ago, but it was unclear if it was due to a sudden mass extinction.
A possible explanation could be that early trilobites – armored and often helmet-headed marine arthropods – began to compete with Ediacaran fauna, causing the latter to become extinct. Another possible explanation is that Ediacaran fauna lived on, but the conditions for preserving Ediacaran fossils only existed until 550 million years ago. “People realized that there was a change in the biota at that time,” Evans said. “But there were significant questions about what the causes might be.”
To answer these questions, Evans and his colleagues compiled a database of Ediacaran fossils that other researchers had previously described in the scientific literature, sorting each entry by factors such as geographic location, body size, and diet. The team cataloged 70 animal genera that lived 550 million years ago and found that only 14 of those genera still existed about 10 million years later. They did not notice any significant changes in the conditions required for the preservation of fossils, nor did they find any differences in diets that would suggest the Ediacara animals became extinct due to competition with early Cambrian animals such as trilobites.
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But there was one commonality among the surviving organisms: body plans with a large surface area to volume ratio that may help the animals cope with low-oxygen conditions. This observation, combined with geochemical evidence of a decline in oxygen 550 million years ago, suggests that the Ediacaran may have ended in a mass extinction caused by low oxygen availability in the ocean. The researchers published their findings online Nov. 7 in the journal Proceedings of the National Academy of Sciences (opens in new tab).
“We looked at the selectivity pattern – what went extinct, what survived and what thrived after the extinction,” said the study’s co-author Shuhai Xiao (opens in new tab), Professor of Geobiology at Virginia Tech. “It turned out that organisms that couldn’t cope with low oxygen levels were selectively removed.”
Why the oxygen content dropped in the last years of the Ediacara remains a mystery. volcanic eruptions, tectonic plate movements and asteroid Impacts are all possibilities, Evans said, as well as less dramatic explanations like changes in nutrient levels in the ocean.
Regardless of how it happened, this mass extinction likely influenced the subsequent evolution of life on Earth and could have implications for scientists studying how animal life began.
“Ediacaran animals are pretty weird — most don’t look anything like the animals we know,” Evans said. “After this extinction event, we see more and more animals that look like they do today. It may be that this early event paved the way for more modern animals.”
Findings may also contain lessons about human threats to aquatic life. Various agricultural and wastewater practices have introduced nutrients such as phosphorus and nitrogen into marine and river ecosystems, increasing the amount of algae that decompose in the water and use up oxygen. The proliferation of “dead zones,” where oxygen levels in the water are too low to support life, could pose similar challenges to modern animals.
“This study helps us understand the long-term ecological and geological impacts of oxygen starvation events,” Xiao said.
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