The height of the Ediacaran period, around 550 million years ago, was a boom time for life in Earth’s oceans. Feather-like petalonamides sucked nutrients from the water, like slugs kimberella they grazed on microbial mats, and the ancestors of jellyfish 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 that these missing fossils point to the oldest known mass extinction event on Earth. These first communities of large, complex animals were killed off by a sharp global decline in oxygen, a finding that may 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, being related to climate change.”
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 wiped out many marine organisms. Then, there was the Permian-Triassic, also known as the “great die“— and Triassic-Jurassic (250 million and 210 million years ago, respectively), which affected oceanic vertebrates and land animals. The most recent mass extinction, about 66 million years ago at the end of the cretaceous periodit wiped out about 75% of plants and animals, including non-avian dinosaurs.
Whether to add one more mass extinction to that list has been an open question among paleontologists for some time. Scientists have known for a long time about the sudden decline in fossil diversity 550 million years ago, but it was not clear if that was due to a sudden mass extinction event.
One 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 become extinct. Another possible explanation is that the Ediacaran fauna survived, but the conditions necessary to preserve Ediacaran fossils existed only until 550 million years ago. “People recognized that there was a change in the biota at this time,” Evans said. “But there were important questions about what the causes might be.”
To answer those questions, Evans and his colleagues compiled a database of Ediacaran fossils that other researchers had previously described in the scientific literature, classifying 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 were still around 10 million years later. They did not notice significant changes in the conditions necessary to preserve the fossils, nor did they find the kind of differences in feeding modes that would suggest that Ediacaran animals became extinct due to competition with early Cambrian animals such as trilobites.
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But there was a common thread among the organisms that survived: body designs with a high surface area to volume ratio, which can help animals cope with low-oxygen conditions. That observation, combined with geochemical evidence for a decline in oxygen 550 million years ago, suggests that the Ediacaran may have ended in a mass extinction event 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 a new tab).
“We examined the pattern of selectivity: what went extinct, what survived, and what flourished after extinction,” said the study co-author. Shuhai Xiao (opens in a new tab)a professor of geobiology at Virginia Tech. “It turns out that organisms that can’t cope with low oxygen levels were selectively killed.”
Why oxygen levels plummeted in the late 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.
“Ediacaran animals are quite strange, most are nothing like the animals we know,” Evans said. “After this extinction event, we started to see more and more animals that look similar to those that exist today. It may be that this early event has paved the way for more modern animals.”
The findings may also hold 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, thereby 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 support life, could pose similar challenges to modern animals.
“This study helps us understand the long-term geological and ecological impacts of oxygen deficiency events,” Xiao said.