What it is
A griffinfly with wings wider than your shoulders. Nobody has ever found a whole one. The biggest wing anyone has dug up is a fragment 28 centimeters long. Double it, add a body, and you get a wingspan near 70 centimeters. Griffinflies are an extinct group, cousins of today's dragonflies.
Why it matters
The usual explanation is the air, which held more oxygen than it does today, perhaps 28 percent of every breath instead of 21. An insect lets air fall into it, down tubes that open along its sides. Richer air means thinner tubes, and thinner tubes leave room for more animal. That is the best mechanism anyone has, and it is not proven. And the sky was empty. No bird. No bat. Insects had the only wings on Earth for nearly a hundred million years.
A ceiling made of fire
Air that rich burns. Below about 16 percent oxygen, a forest will not carry a fire at all, however dry it gets. Near 28 percent, even wet wood can burn. In some coal seams of this age, half the volume is charcoal, wildfire after wildfire. Every fire pulled the oxygen back down, so the forests that made the air also set its limit.