Clues in craters
Records of cosmic impacts are spotty at best on Earth, a planet that tends to fold evidence inward and destroy it over time. And Morrison said the scarce data that scientists have been able to assemble on mass extinctions tends to make die-offs look more gradual than they were.
Beyond that, nobody knows exactly how big a space rock would be required to exterminate half a planet's species, nor exactly
Several previous studies, going back to 1967, had established the T-J mass extinction. Previous attempts to link it to an impact were based mostly on crater evidence.
But researchers don't agree on whether these craters had anything to do with the mass extinction.
Andrew Glikson of the Australian National University said the period was one of heavy bombardment by space rocks. Four or perhaps five craters, ranging in size from about 5 to 62 miles (9 to 100 kilometers) in diameter, were carved by cosmic impacts roughly 200 million years ago, he said.
This was also a time in history when the Atlantic Ocean was created, as a giant land mass separated into continents. Ample evidence suggests this slow but colossal upheaval was accompanied by tremendous volcanic activity that might have simply welled up from below. This volcanism, possibly unrelated to impacts, might have fueled enough climate change to cause the extinction.
"However, it is possible the volcanic activity rifting and ocean splitting may have been triggered by the impacts," Glikson said.
The chaotic collection of evidence points to another idea that is growing in popularity among researchers. Mass extinctions might not be the result of a single event, but rather are caused by converging catastrophes. An impact might either trigger other killing mechanisms, or it might serve as the death knell for plants and animals already struggling in a choked atmosphere or rapidly changing climate.
Next Page: Details of the study -- finding tropical fossils off Canada's coast
advertisement
