The Devonian extinction saw the oceans choke to death

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The Devonian extinction saw the oceans choke to death

Post by klr » Thu Jun 25, 2015 10:02 pm

Not as well known as the end-Permian or K-T mass extinctions ...
Before the first animals had crawled out onto land, marine animals suffered a catastrophic mass extinction. They may have literally asphyxiated

It was the age of fish. 360 million years ago, there were no large land animals, and the biggest animals were still in the oceans. One of the largest was Dunkleosteus.

This was a sea monster like no other. Dunkleosteus was a powerful fish that grew up to 10m long and was covered with thick armour. It was a predator, and had sharp bones sticking out of its jaw instead of teeth. It literally ate sharks for breakfast.

You might think that this tank-like fish could withstand anything, but time was running out for Dunkleosteus. Throughout the oceans species began dying out, and by the time it was all over between 79% and 87% of all species had gone extinct – including Dunkleosteus.

It was one of the worst mass extinctions in Earth's history. Clearly, something dreadful must have happened, but what? There is no smoking gun, no sign of anything dramatic. Instead it seems the trigger was something utterly innocuous – but which led to a series of knock-on effects that ultimately proved catastrophic.

The "age of fish" is technically called the Devonian period. It lasted from 419 to 359 million years ago. This was a time long before even the dinosaurs walked the Earth.

But while there were few land animals, the oceans were staggeringly rich in life. A fossilised reef in Western Australia, the Gogo Formation, captures life in the Devonian seas.

"It's teeming with placoderm fish and they're exquisitely preserved, in three dimensions," says Michael Coates, an evolutionary biologist at the University of Chicago in Illinois. "They're beautiful and weird and there is nothing like that around today."

All this is now gone, wiped out in the extinction that struck at the end of the Devonian.

The corals that built the Devonian reefs, the famous bottom-feeding creatures known as trilobites and ammonites, and the armoured placoderm fish such as Dunkleosteus: all of them took a clobbering. But it's not clear why.

When the dinosaurs went extinct 65 million years ago, there was an obvious culprit. A massive asteroid impact is thought to have strewn debris across the planet, blocking out the Sun and playing havoc with the climate. We have even found the crater from the impact, at Chicxulub in Mexico.

There is no impact crater dating from the Late Devonian, so we can't blame a rock from outer space. The culprit must have been something on Earth.

Thomas Algeo of the University of Cincinnati has put forward a remarkable idea. Maybe the Devonian sea life was killed off by marauding plants.

continued: ... ceans-died
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