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Three hundred and eighty millions years ago a large asteroid or comet may have collided with the Earth and caused the extinction of up to 40% of all sea creatures, says geologist Brooks Ellwood of Louisiana State University. The research, published in the journal Science, reports the discovery of a layer in the mid-Devonian rocks of Morocco that testifies to a large collision. The layer of debris has unusual magnetic properties, however, it is its quartz grains that give away its formation in an impact. These grains show characteristic lines of damage produced by the enormous pressures generated when asteroids or comets collide with the Earth's surface at speeds of many kilometres per second. Ellwood and his team suggest the catastrophic effects of the impact may have caused a mass extinction.
Only the K/T impact 65 million years ago is known to relate directly to a mass extinction, the discovery of a second, suggests Ellwood, strengtens claims that collisions may have had a significant affect on the evolution of life on Earth. Other scientists, however, are sceptical about direct links between mass extinctions and impacts. Dr Norman McLeod, a palaeontologist at the Natural History Museum in London, suggests the disappearance of so many sea creatures in the Devonian may not be a mass extinction at all but part of a longer-term pattern of decline.
Mass extinctions produced by impacts remain controversial. Although evidence for more large impacts are being found recorded in the rocks of the geological column by their characteristic impact layers, the record of extinctions is less easily determined since it involves measuring the period of time over which species disappear. Just how long species take to become extinct may be crucial in answering how much impacts have influenced life on Earth.
More info: Science
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