Roughly 300 million years ago during the Late Carboniferous the world as we know it was quite different; there were no mammals, no dinosaurs, reptiles as a group were still new. There was only one continent, Pangaea, much of which was covered in huge swamp laden forests. Primitive amphibians dominated the waterways while the land was ruled by super sized bugs.
The abundance of plant life during this time is thought to have lead to higher than normal oxygen content in the atmosphere, with levels peaking at 35%, compared to the relatively low levels of today of only 21%. Given that many arthropods do not have lungs and instead absorb oxygen through a series of tube-like structures called trachea. These tubes are spread throughout their bodies, the larger those bodies are the less efficient this system is at absorbing enough oxygen. With oxygen levels being higher during the Carboniferous it is thought that this was the major contributing factor to the increased size of invertebrates during that time.
The only flying animals at this time were insects, the largest by far being Meganeura monyi, a relative of modern day dragonflies. Like it's modern day cousins it was a predator, feeding on other insects and small reptiles and amphibians. It's likely that they would dive bomb unsuspecting prey and trap them with spiny structures on their legs.
Fossil specimens of Meganeura have been found with wingspans reaching upward of 28 inches, meaning that the largest members of this group would be about the same width as your average door frame.
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https://www.patreon.com/posts/67141694
