TIL, upon the advent of wood [400mya], it took fungi 50mil yrs to evolve a way to decompose it. Until then, wood just piled up, never to decay. It is this single fact that led to the Carboniferous period [BBC doc.]
More from the post via reddit:
Today, a geology student added to the Reddit thread some confirming and explanatory notes:
1) This period of elevated oxygen levels (30-35%, versus the 21% of today) lasted from the Carboniferous through the end of the Cretaceous, 65M years ago. It is extremely likely that the large-type dinosaurs simply cannot exist in our current atmosphere. They probably needed these increased oxygen levels to reach the energy production density that these massive creatures are estimated to require.
2) It is because of A) These elevated oxygen levels, and B) the lack of a fungi capable of breaking down lignin, the structural molecule of plant material, that forest growth back in the day was completely rampant. A very large amount of the solid biomass preserved in the entire fossil record came from this one 65-ish million year timespan.
3) Nearly all of the coal beds we exploit today came from the Carboniferous and the other periods with elevated oxygen levels. Guess what? Most of the coal has been judged to have been originally deposited as charcoal. Here's the kicker. Most of the solid biomass from the time period is believed to have lived in wet marshes/semi-permanently raining rainforests. Coal beds from the same (originally rainforest) bed formations have been found on continents separated by entire oceans. Some of these beds have been hundreds of meters thick. These factors imply that global-scale firestorms were a very common occurrence during the Carboniferous, and that these fires occurred in very wet conditions that would be simply impossible during the modern day. This and the lack of a fungal decay mechanism (also, charcoal basically cannot be broken down by fungi even today) is why so very much coal comes from the Carboniferous.
4) This here is the cool piece of information. The Cretaceous ends, geologically, at something called the K-T boundary, which is the few-millimeter thick layer of space dust that marks the Yucatan Peninsula impact. As most of you know, a rare platinum group element, iridium, is found in this thin layer (Iridium is only found in decent amounts in asteroids...). But, recently, investigators have found that a very large amount of soot is also in this layer, to the tune of several weight %. One investigator did some simple projections and calculated that the deposition of this amount of soot worldwide would imply that 25% of the entire biomass of the planet Earth burned after the meteor strike. The asteroid has been found to have caused a global firestorm, a holocaust in the truest sense of the world - one that would not have been possible were it not for the elevated oxygen levels.