[lepton]

Fewer plants. Plants during this period were growing like crazy, pumping huge amounts of oxygen into the atmosphere (hence the really big bugs). At the end of this period, the oxygen levels dropped (maybe from global cooling and drying killing off a lot of the world's vegetation).

but unless you are assuming large amounts of the oxygen was buried, then that doesn't make sense. It's not like we have 10% CO2 floating around now.

[Junior]

Oxygen is reactive and combines with all sorts of other agents. All that extra oxygen is locked up.

[null and void]

As Renfield said in post #97:

This brings up an interesting issue. Arthropods don't have lungs (at least, not in the mammalian sense). They breath through their skin. How could a creature this size have taken in enough oxygen to survive? It could only happen, I suspect, if the air pressure was vastly denser than it is now. We must have lost a lot of air somewhere along the path of history.

I have on occasion seen fossils of flying insects that are orders of magnitude larger than their descendants of today. This also suggests that the air was once much more dense.

Compare the atmospheric density on Earth to Venus, or any other planet in the system except Mercury and Mars...