'Mother of all extinctions' took place before the dinosaurs died

The cessation of dinosaurs was not the greatest mass extinction off all time, say scientists who now suggest that a meteorite caused an even bigger annihilation of animals and plants when it hit the Earth 251 million years ago.

Scientists believe they have found the "smoking gun" that points to a collision with a chunk of space rock, at least seven miles wide, as the cause.

If the evidence is confirmed by other scientists, it would be the second time that a giant meteorite has been shown to be the cause of a sudden and otherwise inexplicable mass death across the planet.

Most scientists agree that a giant meteorite killed off the dinosaurs some 65 million years ago but a team of geologists has now suggested that another meteorite caused even more damage to life on Earth.

That event dates back to the boundary between the Permian and the Triassic geological periods. Asish Basu and colleagues of Rochester University in New York believe they have found microscopic fragments of the meteorite buried within the rock layer dating to that violent moment in history.

Dr Basu said: "An ancient meteorite body, one from the days when the solar system was still forming, struck the Earth 251 million years ago. This was possibly the worst day in the history of life on Earth."

Back then, most of the planet's landmass formed a giant supercontinent called Pangea. Lumbering four-legged reptiles dominated the land and giant fern trees grew in abundance until they were suddenly wiped out in an event known as "the great dying".

During the Permian period, the Earth teemed with strange and exotic life forms, such as giant sharks and reptiles, ancestral dinosaurs, seabed-dwelling creatures called trilobites, and lush prehistoric vegetation.

Estimates indicate that, during the Permian-Triassic (P-T) boundary, about 90 per cent of the marine species and 70 per cent of the land species vanished in the geological equivalent of an instant - the greatest of all the five mass extinctions that are recorded by the fossil record. Luann Becker of the University of California, Santa Barbara, a member of the team, said: "This was the mother of all extinctions. What makes it remarkable is that virtually all marine life and a good portion of land life-forms were eliminated in a very short period." Just as the demise of the dinosaurs allowed the rise of the mammals, the demise of the dominant species 251 million years ago allowed the dinosaurs to dominate the land.

Dr Basu's team has found dozens of microscopic grains of the meteorite within the P-T boundary layer, which he has excavated in Antarctica.

He found an iron alloy in the fragments that does not occur naturally on Earth and matched them with similar meteorite fragments found within the same geological layer excavated in China and Japan.

Dr Basu said: "At the end of the Permian era, Antarctica was close to its present position as the southernmost part of the ancient supercontinent, while south China was at the equator and Japan was to the north of the equator. Such a wide global distribution of these metal [meteorite] grains ... strongly suggests that these grains mark a major impact of a celestial body at that time."

Gauging the exact size of the meteorite is impossible but it is considered to be bigger than the one that killed off the dinosaurs, which was about 6.2 miles wide. "The effect of the impact was more catastrophic than the one that killed off the dinosaurs," Dr Basu said.

The scientists found the meteorite fragments in an area called Graphite Peak but they believe that the actual site of the impact was possibly in what is now western Australia.