What are causes of mass extinction?
Mass extinctions are caused by a large change in the ecosystem. The first mass extinction that we know about was caused by O2 producing microbes. They consumes CO2 and excreted O2. O2 was poisonous to most other life forms so nearly everything died. The only organisms that survived were those who already had the ability to survive an environment with high concentrations of O2. Other mass extinctions have been caused by internal and external stimuli. A massive asteroid slamming into the planet ~65 MYA caused one extinction and a whole bunch of guys with sharp pointy sticks caused another. Wikipedia has a good list of the causes for the known mass extinctions. They don’t have anything on the unknown mass extinctions.
