A large meteorite collided 380 million years ago into what is now the Moroccan desert and may have caused one of the five known mass extinctions of life on Earth, researchers report.

Until now the Chixculub crater in the Gulf of Mexico, where a 10 km asteroid struck 65 million years ago, was the only known impact convincingly connected to a mass extinction, an American-Moroccan team write in the latest issue of the journal Science.

The new discovery strengthens support for the idea that multiple catastrophes - some of them extraterrestrial in origin - have shaped the evolution of life on Earth.

"We know that meteors have struck the Earth hundreds of times," said team leader, Professor Brooks Ellwood of Louisiana State University in Baton Rouge, USA. "If I had to guess, I would say that once every five million years, a meteor big enough to cause a mass extinction hits the Earth."

The extinction event in this case, in the Middle Devonian era, was already known to geologists, but its cause has long been uncertain. Most life at the time was confined to the sea, with only small plants, wingless insects and spiders inhabiting the land: the fossil record shows that about 40% of all animal life subsequently disappeared.

Ellwood's team has now found strong physical and chemical evidence that a meteor-like object dealt the fatal blow. But he cautions that just because the extinction and the meteor collision happened at the same time does not prove the impact caused the extinction, although he believes it is strongly suggestive of it.

The researchers studied carbon isotope ratios, mineral concentrations, rock deformations and other evidence and found them to be almost identical to those found at Chixculub and other craters. Fossils found in rock layers just above the Middle Devonian impact layer suggest that many new species appeared after the extinction event.

Ellwood said one of the problems was determining whether an extinction was a local event, such as a volcanic eruption, or occurred on a global scale, such as an asteroid impact. Key to this was identifying the same strata of rock at different locations around the globe. The team did manage to find such corroborating evidence for an impact in several places, including Australia.

Other scientists have suggested that more than one impact may have occurred within a short geological time, such as the Woodleigh crater in Western Australia which also been tentatively linked with the same extinction event. That crater is 120 km across, and is thought to have been produced by an asteroid about 5 km wide.

The tell-tale layers of material left by an impact could also be extremely thin, Ellwood said. The relevant layer in this case was found near the top of a barren plateau in the Anti Atlas desert, near Rissani in Morocco, and was about the thickness of a felt-tipped marker pen and only distinguishable from the soil around it by its reddish colour.

Ellwood's research group clinched its findings, however, through its special interest in magnetic susceptibility - the signatures left by globally uniform changes in the Earth's magnetic field in rocks that can be used to help date geological events and archaeological findings.

Large craters identified in many countries show that Earth has a long history of impacts from space, and many scientists suspect that one of these was also linked to the most severe mass extinction at the end of the Permian era about 245 million years ago, when up to 95% of all marine species and three-quarters of all vertebrate animal families on land died out.

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