The height of the Ediacaran Period, about 550 million years ago, was a boom time for life in Earth’s oceans. The feather-like petalamides sucked nutrients from the water, looking like slugs Kimberella were grazing on microbial mats, and the ancestors of jellyfish were just starting to make waves.
But 80% of life on Earth has disappeared, leaving no trace in the fossil record.
Now, a new study suggests these missing fossils point to the oldest known mass extinction event on Earth. These early communities of large, complex animals were killed off by a sharp global drop in oxygen — a finding that could have implications for modern ocean 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 a new tab), a postdoctoral researcher at Virginia Tech. “It is consistent with all major mass extinctions, in that it is linked to climate change.”
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Animals have passed through the evolutionary crucible of mass extinctions at least five times. There were the Ordovician-Silurian and Devonian extinctions (440 million and 365 million years ago, respectively), which killed many marine organisms. Then there was the Permian-Triassic — also known as “Great dying— and the Triassic-Jurassic extinctions (250 million and 210 million years ago, respectively), which affected oceanic vertebrates and land animals. The most recent mass extinction, around 66 million years at the end of the Cretaceous periodwiped 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 whether this was due to a sudden mass extinction.
A possible explanation could be that the first trilobites – armored and often helmet-headed marine arthropods – began to compete with the Ediacaran fauna, causing the latter to disappear. Another possible explanation is that the Ediacaran fauna survived, but the conditions necessary for the preservation of Ediacaran fossils only existed until 550 million years ago. “People recognized 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 mode of feeding. The team cataloged 70 genera of animals that lived 550 million years ago and found that only 14 of those genera still existed around 10 million years later. They didn’t notice any significant changes in the conditions needed to preserve the fossils, or find the kind of differences in feeding patterns that would suggest that Ediacaran animals became extinct due to competition with earlier ones. Cambrian animals, such as trilobites.
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But there was a common thread among the organisms that survived: body planes with a high surface area to volume ratio, which can help 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 with a mass extinction event caused by low availability of oxygen in the ocean. The researchers published their findings online Nov. 7 in the journal Proceedings of the National Academy of Sciences (opens in a new tab).
“We looked at the selectivity model – what disappeared, what survived and what thrived after extinction,” said the study’s co-author. Shuhai Xiao (opens in a new tab), professor of geobiology at Virginia Tech. “It turns out that organisms that can’t cope with low oxygen levels have been selectively eliminated.”
Why oxygen levels dropped in the later Ediacaran years remains a mystery. Volcanic eruptions, tectonic plate movements and asteroid impacts are all possibilities, Evans said, as are less dramatic explanations, such as 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 may have implications for scientists studying how animal life began.
“The Ediacaran animals are quite strange – most look nothing like the animals we know,” Evans said. “After this extinction event, we begin to see more and more animals that resemble those of today. It may be that this early event paved the way for more modern animals.”
The results may also contain lessons about human threats to aquatic life. Various agricultural and sewage treatment practices have introduced nutrients such as phosphorus and nitrogen into marine and river ecosystems, increasing the amount of algae that decomposes in the water and consumes oxygen. The spread of “dead zones,” where oxygen levels in the water are too low to sustain 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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