Old fart almost destroyed the world. Archaean bacteria outgassed.

Although a dozen great meteors set back life on earth, detective work blames the biggest hit of all on a giant methane fart from Archaean bacteria.

Evidence left at the crime scene is abundant and global: Fossil remains show that sometime around 252 million years ago, about 90 percent of all species on Earth were suddenly wiped out -- by far the largest of this planet's five known mass extinctions a form of microbes -- specifically, methane-producing archaea called Methanosarcina -- that suddenly bloomed explosively in the oceans, spewing prodigious amounts of methane into the atmosphere and dramatically changing the climate and the chemistry of the oceans.
Volcanoes are not entirely off the hook, according to this new scenario; they have simply been demoted to accessories.

MIT reports on three clues from the scene:

Methanosarcina

Carbon dioxide increased over ten fold and Methanosarcina processed it, after picking up a gene from some other bacteria.  The limiting nutrient for the processing was nickel which had just been spewed out by the world's greatest ever volcanic upheaval in the Siberian Traps area.  With extra C02, a new gene and a shitload of nickel, Methanosarcina ramped up the production of methane, one of your basic fart gases.

The resulting outburst of methane produced effects similar to those predicted by current models of global climate change: a sudden, extreme rise in temperatures, combined with acidification of the oceans. In the case of the end-Permian extinction, virtually all shell-forming marine organisms were wiped out -- consistent with the observation that such shells cannot form in acidic waters.

Extinction article

Fartology: There's a half dozen ingredients,  Two of them burn.  Hydrogen sulphide is the stinky one.  Methane is made in our gut by bacterial beasties that live down there.