High Quality Content by WIKIPEDIA articles! Coal forests were the vast swathes of wetlands that extended over much of the tropical land areas during late Carboniferous (Pennsylvanian) and Permian times. These forests got their name because they accumulated enormous deposits of peat which later changed into coal. As much of the carbon in this enormous amount of peat came from photosynthesis splitting existing carbon dioxide, it is thought that the accompanying split-off oxygen went into the atmosphere, greatly increasing its ppO2: one estimate says to about 35%, making it easier for animals to breathe air as a source of oxygen, as seen in the size of Meganeura compared to modern dragonflies.