Note that although species can be described as tree-like, they didn't quite look like modern trees do. Also, much of the world was swamp, and much of the dead plant material sank into these bogs and decayed into peat.

The amount of CO2 trapped during this period caused the atmosphere to be around 35% oxygen. This allowed life with inefficient respiratory systems to grow much bigger in size without suffocating, mainly insects. Think woodlice 6 feet long, spiders the size of dogs, millipedes as big as cars, and dragonflies as big as eagles.