by The peak of the Ediacaran Period, about 550 million years ago, was a heyday for life in Earth’s oceans. Petalonamides in the form of feathers suck nutrients from the water, slug-like Kimberella 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 these missing fossils point to the earliest known mass extinction event on Earth. These first communities of large, complex animals were killed off by a sharp global decrease 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 new tab), a postdoctoral researcher at Virginia Tech. “It’s consistent with all major mass extinctions, because it’s related to climate change.”
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Animals have been through the evolutionary melting pot of mass extinctions at least five times. There was the Ordovician-Silurian and Devonian extinctions (440 million and 365 million years ago, respectively), which killed off many marine organisms. Then there were the Permian-Triassic – also known as the “Great die— and Triassic-Jurassic extinctions (250 million and 210 million years ago, respectively), which affected ocean and terrestrial vertebrates. The most recent mass extinction, about 66 million years ago at the end of the Cretaceous erawiped out about 75% of plants and animals, including non-Asian dinosaurs.
Whether another mass extinction should be added to that 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 that was due to a sudden mass extinction.
One possible explanation could be that early trilobites – armored and often helmet-headed marine arthropods – began to compete with the Ediacaran fauna, driving the latter to extinction. Another possible explanation is that Ediacaran fauna lived on, but the conditions necessary for the preservation of Ediacaran fossils existed only until 550 million years ago. “People recognized that there was a change in biota at this point,” 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, sorting each item by factors such as geographic location, body size and feeding mode. The team cataloged 70 animal genera that lived 550 million years ago and found that only 14 of those genera survived about 10 million years later. They noted no significant changes in the conditions necessary for fossil preservation, nor did they find the kind of differences in diets that would suggest that the Ediacaran animals became extinct due to competition with early Cambrian animals, such as trilobites.
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But there was one common thread among the organisms that survived: body shapes with large surface area relative to volume, which can help animals cope with low oxygen conditions. That observation, combined with geochemical evidence of a decrease 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 Procedures of the National Academy of Sciences (opens in new tab).
“We examined the selectivity pattern — what went extinct, what survived, and what thrived after the extinction,” said study co-author Shuhai Xiao (opens in new tab), a geobiology professor at Virginia Tech. “It turns out that organisms that cannot tolerate low oxygen levels have been selectively removed.”
Why oxygen levels plummeted in the waning years of the Ediacaran remains a mystery. Volcanic eruptions, tectonic plate movements and asteroid effects 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 originated.
“Ediacaran animals are pretty strange — most of them look nothing like the animals we know,” Evans said. “After this extinction event, we’re starting to see more and more animals that look like today. It may be that this early event 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, increasing the amount of algae that decompose in the water and consume oxygen. The proliferation of “dead zones,” where water oxygen levels are too low to sustain life, may present similar challenges for modern animals.
“This study helps us understand the long-term ecological and geological consequences of oxygen deprivation events,” said Xiao.
