Sixth mass extinction in Earth’s history discovered


Animals have gone 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. Subsequently two more, which affected oceanic vertebrates and land animals. The last mass extinction, which occurred about 66 million years ago at the end of the Cretaceous, wiped out about 75% of plants and animals, including dinosaurs. However, it seems that another one existed, preceding all the others.

The Ediacaran Period is the third and last geological period of the Neoproterozoic Era , before the Cambrian Period of the Paleozoic Era. We must mentally transport ourselves to 550 million years ago. Scholars have always described this geological era as characterized only by the presence of simpler living organisms (unicellular), even if these statements have been the subject of various controversies since, in 2008, an animal footprint with legs was discovered in Nevada dating to 570 million years ago.

Disputes aside, another is the very interesting discovery about the Ediacaran: it seems to be the period in which the Earth witnessed the disappearance of about 80% of the life forms present, leaving no traces in the fossil record.

Learn from the past

A new study suggests these missing fossils point to a mass extinction event. The revelations indicate that early communities of large and complex animals were killed off by a sharp global decrease in oxygen , a finding that could have implications for modern ocean ecosystems threatened by human activities.

“This represents the oldest known extinction event in the animal fossil record,” said study lead author Scott Evans, a research scientist at Virginia Tech. “It’s consistent with all the big mass extinctions, it’s related to climate change.”


“We looked at the pattern of selectivity — what went extinct, what survived, and what flourished after the extinction,” said study co-author Shuhai Xiao, a professor of geobiology at Virginia Tech. “It has emerged that organisms that cannot tolerate low levels of oxygen have been selectively eliminated.”

But why oxygen levels plummeted in the 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.

The findings could also teach us something about human threats to aquatic life. Various agricultural and wastewater treatment practices have introduced nutrients such as phosphorus and nitrogen into marine and riverine ecosystems, thereby increasing the amount of algae that decompose in the water and consume oxygen . The spread of “dead zones,” where oxygen levels in water are too low to support life, could pose similar challenges to modern animals.

The researchers have published their findings in the journal Proceedings of the National Academy of Sciences


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