The Age of Giant Bugs

The first amphibians evolved from fish, insects formed from arthropods, and the first primitive trees formed.

The Carboniferous period saw the highest oxygen levels ever known, the largest insects and arachnids and climate change on truly unprecedented levels all vying for dominance in a lush rainforest world.

Highlights of the Carboniferous

• Diversification of insects and arachnids
• Highest ever oxygen levels
• Evolution of the first reptiles
• Climate change leads to rainforest collapse
• 350 million years ago: A cataclysm raises numerous new land masses, such as Ponape and R’lyeh. The disaster also destroys several marine Elder Thing cities. Cthulhu and his kin arrive from the star Xoth, and settle on the newly-risen landmasses. The Elder Things war with Cthulhu, but they eventually make peace. Cthulhu is permitted to keep his current surface territories, and the Elder Things keep the rest of the planet.
• The Deep Ones enter the service of Cthulhu, and help build the city of R’lyeh. 
• According to legend, the foundation of the underground civilization of K’n-yan dates back to this time.

Imprisonment of the Great Old Ones

• 300 million years ago: A cosmic cataclysm (possibly a certain configuration of the stars, possibly a war with the Elder Gods) occurs, resulting in R’lyeh sinking beneath the waves. Cthulhu is imprisoned within the city. In all likelihood, the other Great Old Ones are imprisoned around the same time. 
• Reptiles arise on Earth. Yig is allegedly involved in their creation. 

Vast Swamp Forests Span a Tropical World

Sharks were now the apex predators of the vast Panthalassic Ocean. Among these were the stethacanthids. 

As life recovered in the Early Carboniferous oceans, vast swathes of swamp forest colonised freshwater regions. Creating lush rainforests across equatorial regions and far beyond. Swampy rainforests covered millions of square miles of land. Their rapid and unhindered evolution continued to pump vast amounts of oxygen into the air.

The Early Carboniferous saw the continued development of true amphibians. Which continued to grow throughout the Period, eventually leading to giants like the eryops.

Later in the Carboniferous, the very first reptiles evolved. 

Supersized Bugs

Not since the Great Oxygenation Event of 2.3 billion years ago did oxygen have such a profound effect on the evolution of life on Earth as it did during the Carboniferous. With the unprecedented greening of the land came rapidly rising oxygen levels. This record high that allowed arthropods to evolve to monstrous proportions.

First among the terrifying Carboniferous creatures were giant scorpions. Among their contemporaries was the equally freakishly  giant centipedes. 

Although the first flying insects likely evolved during the Devonian Period, it was not until the Late Carboniferous that they really took off, giant dragonflys were one of the largest flying insects that ever lived.

As oxygen levels spiralled out of control, it wasn’t just giant bugs that were the horrors of the time. Forest fires are also rife, since such high levels of oxygen would allow even damp biomass to catch alight easily.

Conclusion

The Carboniferous began as a hot and humid world of ever-rising oxygen levels only to end with glaciations transforming the climate and redefining the course of evolution. Although the end of the period was not marked by a major extinction event as many other periods are, a great deal of species disappeared as icy deserts crept up from the South Pole into far higher latitudes. 

The largest trees in history grow including Yggdrasil, elder treants tend the land creating great forests.