Posted on 06/17/2003 7:48:04 AM PDT by Mike Darancette
Rock from space might have hit life hard 380 million years ago. 13 June 2003
JOHN WHITFIELD
About 380 million years ago, a rock from space smashed into the Earth, say geologists. They believe that the impact wiped out a large fraction of life.
The idea could strengthen the controversial connection between mass extinctions and impacts. Up to now, the only candidate for a link is the meteor 65 million years ago that some believe helped exterminate the dinosaurs.
Signs of an earlier catastrophe coincide with a disappearance of many animals, says Brooks Ellwood of Louisiana State University, Baton Rouge: "It doesn't mean that the impact killed off the critters, but it's suggestive that it had something to do with it." It's not known where a rock struck, although it's possible that traces of a crater might be found, he adds.
Other researchers agree that there was an impact around that time, but feel the evidence for a mass extinction is much weaker.
Rocks in Morocco laid down about 380 million years ago bear a layer of sediment that looks like the debris from a cataclysmic explosion, Ellwood's team found. The sediment has unusual magnetic properties, and contains grains of quartz that seem to have experienced extreme stresses.
Around this time, about 40% of marine animal groups vanish from the fossil record, say Ellwood's team. Ellwood posits an asteroid slightly smaller than the 10-kilometre rock suspected of killing the dinosaurs. The evidence for an impact is compelling, says geologist Paul Wignall of Leeds University, UK. And linking it to a mass extinction would be a major finding. "The potential lethality of impacts would be greatly increased," he says.
But it's not clear how much disappeared around the time of the impact - the death toll may be far lower than Ellwood's team suggest, says Wignall. He thinks palaeontologists should search the rocks for a better picture of what happened at that time.
Even a figure of 40% is a typical extinction rate for that period of the Earth's history, agrees palaeontologist Norman MacLeod, who studies mass extinctions at the Natural History Museum, London. "It's not a mass extinction, it's part of a much longer-term pattern," he says.
MacLeod doubts that mass extinctions are the result of extraterrestrial intervention. "Impacts are quite a common phenomenon," he says. "But they don't correlate significantly with peaks in extinction."
References Name, A.B. Impact ejecta layer from the mid-Devonian: possible connection to global mass extinctions. Science, 300, 1734 - 1737, (2003).
Copyright 2003, Nature
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