‘Biological engineers’ caused first mass extinction

Changes to the environment caused by newly-evolved organisms were responsible for the first-ever mass extinction on Earth, according to new research led by a team of Vanderbilt University scientists and published in the journal Palaeogeography, Palaeoclimatology, Palaeoecology.

As the authors of that paper explained in a statement, single-celled microbes dominated the Earth for more than three billion years before the first multicellular organisms emerged approximately 600 million years ago. Among the most successful of those early creatures were nearly immobile marine organisms shaped life discs, tubes, and fronds and called Ediacarans.

Ediacarans spread around the world, but 60 million years after they emerged, the first animals – metazoans – emerged. Metazoans could move independently at some point during their lifetime, and fed on either the products of other organisms or the organisms themselves, they added.

This eventually led to an event that paleontologists refer to as the Cambrian explosion: a period spanning 25 million years during which vertebrates, mollusks, arthropods, annelids and most of the other modern animal families emerged. It was these events that ultimately resulted in what is known as the end-Ediacaran extinction, as these new organisms made such radical changes to the environment that they caused their less complex predecessors to die out completely.

“These new species were ‘ecological engineers’ who changed the environment in ways that made it more and more difficult for the Ediacarans to survive,” explained Simon Darroch, the lead investigator of the new study as well as an assistant professor of earth and environmental sciences at Vanderbilt.

Drawing parallels with modern-day environmental changes

Darroch and his colleagues previously reported on fossils which appeared to show stressed-looking Ediacara communities associated with animal burrows. Now, their new study confirms that they have discovered a well-preserved mixed community of Ediacarans and animals – the best evidence to date that the two groups shared a close ecological association.

“Until this, the evidence for an overlapping ecological association between metazoans and soft-bodied Ediacaran organisms was limited,” the Vanderbilt professor said. “Here, we describe new fossil localities from southern Namibia that preserve soft-bodied Ediacara biota, enigmatic tubular organisms thought to represent metazoans and vertically oriented metazoan trace fossils.”

“With this paper we’re narrowing in on causation,” he added. “We’ve discovered some new fossil sites that preserve both Ediacara biota and animal fossils (both animal burrows – ‘trace fossils’ – and the remains of animals themselves) sharing the same communities, which lets us speculate about how these two very different groups of organisms interacted.”

While they have not been able to verify the identity of the creature responsible for those trace fossils, the researchers noted that the burrow-like structures are similar to those associated with Conichnus, a cone-shaped organism that has been discovered in the Cambrian period. Some of these burrows, Darroch said, are often believed to have been formed by sea anemones, passive predators which could have fed on the larvae of Ediacarans.

“These new fossil sites reveal a snapshot of a very unusual ‘transitional’ ecosystem existing right before the Cambrian explosion, with the last of the Ediacara biota clinging on for grim death, just as modern-looking animals are diversifying and starting to realize their potential,” he said, noting that there is “a powerful analogy between the Earth’s first mass extinction and what is happening today. 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.”

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Image credit: Simon Darroch / Vanderbilt

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