A gamma ray burst
Depleted the ozone layer
UV radiation was harmful to life

A decrease in surface dwelling creatures
Indicates it was likely UV radiation

Or was it a galactic bow shock?
Two of our planets mass extinctions
Occurred at about the time
The sun was highest above the plane of our galaxy

During the Ordivician extinction,
The UV rays could have caused
DNA damage and mutations

Wow, thanks YouTube
I learn something new each day
And that makes me gay