The First Known Extinction Event May Have Been Caused by Early Animals

The term "mass extinction" conjures up apocalyptic visions of raging wildfires, erupting volcanoes, and asteroids locked on collision courses with the Earth.

This high-drama characterization fits some major extinction events, like the one that wiped out the non-avian dinosaurs 66 million years ago. But not all ecological die-offs are caused by large-scale natural disasters, according to a new study published in the journal Palaeogeography, Palaeoclimatology, Palaeoecology.

Led by Simon Darroch, a professor of earth and environmental studies at Vanderbilt University, the authors describe newly discovered fossils from Namibia that date back some 540 million years. These fossilized organisms lived in a time of ecological upheaval and transition; defined both by the rapid, spectacular diversification of complex animal life known as the Cambrian explosion, as well as the first major die-off in known evolutionary history, the end-Ediacaran extinction event.

For years, scientists have suspected that the sudden arrival of animals on the evolutionary scene may have edged out the previous complex lifeforms, called the Ediacaran biota.

These Ediacaran-era organisms were fantastically weird creatures, soft-bodied and typically immobile. Some were shaped like rippled fronds, some had tubular frames, and some were curiously disk-shaped. Because they appear to have had no hard skeletal features, Ediacaran biota are extremely rare in the fossil record. Even the few specimens that have been recovered are difficult to interpret and classify.

One thing's for certain, though: The Ediacaran biota appear to drop off the face of the planet when the earliest animals show up. These first "metazoans," the taxonomical category animals fall under, were able to evolve their own means of locomotion, giving them a real competitive edge in the ancient oceans. It's not hard to imagine these mobile critters dominating the biota that came before them, but scientists need fossils that preserve both Ediacaran biota and early animals to prove this hypothesis, and examine its underlying ecological dynamics.

That's exactly why the new Namibian fossils are so exciting. "With this paper, we're narrowing in on causation," Darroch said in a statement. "We've discovered some new fossil sites that preserve both Ediacara biota and animal fossils […] sharing the same communities, which lets us speculate about how these two very different groups of organisms interacted."

Specifically, the team studied fossilized burrows that may have been made by early sea anemones that fed on Ediacaran larvae. Darroch notes that early animals may have choked out Ediacaran organisms by coiling around their bases.

"These new species were 'ecological engineers' who changed the environment in ways that made it more and more difficult for the Ediacarans to survive," he added.

For Darroch and his team, these new fossils not only clarify the dynamics of the end-Ediacaran extinction. They also demonstrate that sometimes, the most efficient way of killing off life is to evolve new forms of it. That is a relevant message to keep in mind as we enter the age of the Anthropocene, a time shaped by a sixth mass extinction event caused by human activity.

"There is a powerful analogy between the Earth's first mass extinction and what is happening today," Darroch said.

"The end-Ediacaran extinction shows that the evolution of new behaviors can fundamentally change the entire planet, and today we humans are the most powerful 'ecosystems engineers' ever known."