User blog comment:Lord Loss/Monster Hunter Theory... Blog/@comment-1758220-20110712233022/@comment-1011548-20110713202906

Yeah, I've seen Walking With Monsters (good, but not as good as WW Dinosaurs, in my opinion). It seems to be generally accepted^, however, that the much greater concentration of oxygen in the air (35% to today's 21%) is the major driving force behind the enormous size of the invertebrates; more oxygen means respiration is easier for them, so they can afford to grow bigger and still take in enough oxygen to 'power' all their cells. And 'Carboniferous' just means 'coal-bearing', apparently. Although the enormous insects that lived during it still makes it one of my favourite pre-historic time periods.

^ I googled it and the websites that came up agreed with me. Including Wikipedia.