A global drop in oxygen levels about 550 million years ago led to Earth's first known mass extinction, new evidence suggests.
The height of the Ediacaran period, about 550 million years ago, was a boom time for life in Earth's oceans. Petalonamids shaped like feathers sucked 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 traces in the fossil record.
Now, a new study suggests that these missing fossils point to the earliest known mass extinction event on Earth. These first communities of large, complex animals were killed by a steep 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 fossil record of animals," said lead study author Scott Evans, a postdoctoral researcher at Virginia Tech. "It is consistent with all major mass extinctions, in being linked to climate change."
Animals have passed through the evolutionary crucible of mass extinctions at least five times. There were the Ordovician-Silurian and the 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 Dying" and Triassic-Jurassic extinctions (250 million and 210 million years ago, respectively), which affected ocean vertebrates and land animals. The most recent mass extinction, about 66 million years ago at the end of the Cretaceous period, wiped out approximately 75% of plants and animals, including nonavian dinosaurs.
Whether one more 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 if that was due to a sudden mass extinction event.
One possible explanation could be that early trilobites armored and often helmet-headed marine arthropods began competing with Ediacaran fauna, causing the latter to die out. Another possible explanation is that Ediacaran fauna lived on, but the conditions necessary for preserving Ediacaran fossils existed only until 550 million years ago. "People recognized that there was a change in biota at this time," Evans said. "But there were significant questions about what the causes might be."
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