The exact causes behind the Permian-Triassic mass extinction have been much debated.
Two separate pulses of CO2 into the atmosphere - a "one-two punch" - may have helped fuel the die-off, new research suggests.
Changes to ocean acidity would have been one of the consequences, according to the study in Science journal.

Computer models suggested that this CO2 may have been released by massive bouts of volcanism from the Siberian Traps, now represented as a large region of volcanic rock in northern Eurasia.
The Permian-Triassic mass extinction, which took place 252 million years ago, wiped out more than 90% of marine species and more than two-thirds of the animals living on land.
The event is thought to have played out over a 60,000-year period and acidification of the oceans lasted for about 10,000 years.