The Carboniferous is a geologic period and system that extends from the end of the Devonian Period, about 359.2 ± 2.5 Mya (million years ago), to the beginning of the Permian Period, about 299.0 ± 0.8 Mya
Fossil remains of air-breathing insects, myriapods and arachnids are known from the late Carboniferous. Their large size can be attributed to the moistness of the environment (mostly swampy fern forests) and the fact that the oxygen concentration in the Earth’s atmosphere in the Carboniferous was much higher than today. (The oxygen concentration in the Earth’s atmosphere during the Carboniferous was 35% whereas the oxygen concentration in earth’s current atmosphere is 21%.) This required less effort for respiration and allowed arthropods to grow larger with the up to 2.6 metres long millipede-like Arthropleura being the largest known land invertebrate of all time.