So, I have an Earthlike planet with a healthy biosphere, and I need to completely reset all complex life on it. Near as I can tell, one way to do this is by totally destroying the atmosphere (after all, not even the great dying could put an end to complex life on earth.) trouble is, I need the planet to still be habitable afterwards.
What could cause this to happen? Put formally, these are the relevant constraints:
The planet must be habitable to humans before and after the event. (Complex life will be funneled in via magic portals afterwards. I’d like the event itself to be mostly non-magical in nature.)
All multicellular life forms must be wiped out. Colonies of single-celled organisms can stay, but are not required to. (I may make an exception if the multicellular life form cannot easily be distinguished from single-celled organisms.)
The fossil record must be left intact (with an appropriate “great fossil gap” representing the event.)
The event in its entirety must not last longer than 30 million years. Ideally, it should last longer than 20 million years. The planet must remain uninhabitable to multi-cellular life for the duration of that time.
In addition to this, I should like to know what the gap will look like in the resulting rock layers, and what types of life could survive the event.