35%. An Arthropod's Atmospheric Garden Spot.

Greetings. The Carboniferous Period lasted from about 359.2 to 299 million years ago. During the Carboniferous, atmospheric oxygen levels attained a peak of around 35%, significantly higher than the current 21% oxygen level, with this high concentration occurring towards the end of the Carboniferous, approximately 300 million years ago. 

The high oxygen levels during the Carboniferous were largely the direct result of the vast planetwide growth of swamp forests which absorbed copious amounts of atmospheric carbon dioxide, releasing a surplus of oxygen into the atmosphere. The high oxygen levels contributed to the large size of insects and other arthropods during this period, as higher oxygen levels facilitate larger body dimensions. 

In addition to having the ideal conditions for the formation of coal, several major biological, geological, and climatic events occurred during this time. One of the greatest evolutionary innovations in the history of complex life on the planet occurred during the Carboniferous: the amniote egg, which allowed for the further exploitation of the land by certain tetrapods. It offered the ancestors of birds, mammals, and reptiles the capability to lay their eggs on land without fear of desiccation. The collision of Laurasia (present-day Europe, Asia, and North America) into Gondwana (present-day Africa, South America, Antarctica, Australia, and India) produced the Appalachian Mountains of eastern North America and the Hercynian Mountains in the United Kingdom. An additional collision of what is now Siberia and eastern Europe created the Ural Mountains of Russia. There was also a gradual trend towards milder temperatures during the Carboniferous, as evidenced by the decrease in lycopods and large insects, and an increase in tree fern diversify. 

Fossil remains of air-breathing insects, myriapods, and arachnids are known from the Carboniferous. Their overall diversity when they do appear, demonstrates that these arthropods were both well-developed, numerous, and specialized. Some arthropods grew to monumental sizes with the 2.6-meter-long millipede-like Arthropleura being the largest known land invertebrate of all time. During the Carboniferous, the oldest known winged insects appear, followed by the huge predatory Protodonata, which includes Meganeura, a giant dragonfly-like insect with a wingspan of approximately 75 centimeters, the largest flying insect ever to exist on the planet. 

Amphibians were diverse and highly successful by the middle of the Carboniferous, with some forms growing as long as 6 meters in length. Some species were aquatic and lived in rivers, while others may have been semi-aquatic, or fully terrestrial.

The collapse of the Carboniferous rainforests slowed the further evolution of amphibians who could thrive in the cooler, drier environmental conditions. However, the earliest sauropsid reptiles (Hylonomus) and the earliest known synapsid (Archaeothyris) continued to evolve and diversify. Synapsids eventually attained large body sizes, and became advanced during the later Permian, but their dominance waned during the Mesozoic. Sauropsids also marched on towards evolutionary diversity, but remained small until the Mesozoic, during which they dominated the skies, the oceans, and the Earth's landmasses.

A substantial amount of the fossil fuels that the human species has been utilizing were initially laid down during the Carboniferous. Their use has greatly contributed to the atmospheric and environmental changes that have occurred in the past few decades. 

Thank you for your time and consideration.