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Did ancient animals cause a mass extinction?

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WASHINGTON — For markers of a mass extinction, the fossils dug up from the Namibian soil are not very impressive. The few animals caught in the rock are ribbon-shaped and worm-like. Many are nothing but impressions in 540-million-year-old sea bed muck, tunnels left behind by burrowing creatures whose soft bodies were not preserved. Others are small craters that erupt out of the ancient sand like exploded zits.

The scientists studying these fossil traces say the indentations could not have been left by bubbling gas -- where gas would leave a pointed impression like a volcano, these holes are concave -- but are where predatory sea anemones once anchored themselves to the sea floor.

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If this fossil trove had a theme, it would be death.

Or, more specifically, eating and out-competing. Despite their mundane appearance, the fossils represent a clash between two worlds. It is one of the few places on the planet where animal remains representing the end of the Ediacaran period and those at the beginning of the Cambrian age coexist.

The scientists believe the fossils are convincing evidence this overlap resulted in one of the earliest mass extinctions — if not the first among animals. (Perhaps only the Great Oxygenation Event had previously thrown the planet's organisms into such biological turmoil, when bacteria struggled to live in a more oxygen-rich world.) Unlike the extinction that deep-sixed the dinosaurs, with an asteroid as wide as New Jersey or a volcanic salvo or possibly a combination thereof, these were living things simply taking their hungry course.

To the Ediacaran critters, what happened 540 million years ago was disaster. To the Cambrian animals, as a team of scientists argue recently in the journal Palaeogeography, Palaeoclimatology, Palaeoecology, it was just lunchtime.

The older Ediacaran animals were some of the first multicellular organisms to live on Earth. And they were utterly weird. Appearing around 600 million years ago, the Ediacaran creatures stuck around for 60 million years. Little more than pipes or fleshy bags glued to the sea floor, the majority of these animals could not move. (It is an unusual way of life, but one that persists today in animals like adult coral.) Paleontologists say the Ediacaran animals looked like fronds, tubes, Frisbees or even lumpy mattresses.

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The diversity of animals found in the Cambrian period, which began around 540 million years ago, coinciding with the Ediacaran end, is famously described in terms of an explosion. The result was a more familiar zoo. Among the arrivals were animals with backbones, insect ancestors, jellyfish, mollusks, and anemones.

Simon Darroch, an environmental scientist and expert on mass extinctions at Vanderbilt University in Nashville, describes the new creatures as ecosystem engineers. They were capable of manipulating the environment in ways the mattress-like Ediacarans could not.

Importantly, many Cambrian animals could move. And they could hunt.

In some places, the fossils show Cambrian critters wrapped around the bottom of the Ediacran frond organisms, said Darroch in a press release.

Even the immobile Cambrian animals likely posed a threat to their neighbors.

''Some of the burrow fossils we've found are usually interpreted as being formed by sea anemones,'' Darroch said, ''which are passive predators that may have preyed upon Ediacaran larvae.''

Determining the cause of an ancient extinction is tricky business. Coexistence is not necessarily evidence one group completely devoured the other, the researchers were clear to point out. In the study, the team of American and Canadian paleontologists also hypothesize that mobile Cambrian animals stirred up nutrients far from the microbial mats on the ocean floor. What once always fell to the seabed could be suspended in a new oceanic ecosystem — one that the Ediacaran animals, fixed in place, could not reach.

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''In general, 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 [organisms] clinging on for grim death,'' said Darroch, ''just as modern-looking animals are diversifying and starting to realize their potential.''

It reinforces a previous idea held by Darroch and other paleontologists that the Cambrian animals pushed their precursors over the edge by sheer dint of existing.

And in the ancient Namibian rock, Darroch sees an omen for modern times.

Of the mass extinctions throughout history, there are five — the so-called Big Five — that stand out as periods when extinction rates greatly increase. Scientists believe we may be at the beginning of a sixth. If so, humans are to blame.

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