A study raises the possibility that 251 million years ago a giant asteroid hit Earth and wiped out 90% of the lifeforms on Earth. This event would be more catastrophic than the asteroid 66 million years ago said to have killed the dinosaurs.
The researchers base their study on fragments of a meteorite that they discovered in Antarctica. The object hit the Earth during the Permian-Triassic period. The fragments contained a chemical composition which could be found in dust.
The theory is this asteroid hit a Southern landmass, resulting in a gigantic fireball. This impact would send enough tons of dust to shut out the Sun for months. But, iridium, which was in the dinosaur-killing asteroid, was not in this asteroid. Source