Answer:
Nobody knows the real reason but the
scientists believe that any one of the following could have triggered mass
extinction of species in the past.
(i) Fail of sea levels.
(ii) Change in temperature (freezing
or warming).
(iii) Asteroid/meteorite hitting
the planet.
(iv) Poisonous hydrogen sulphide
emissions from the sea.
(v) Nova/super nova/gamma ray
burst.
(vi) Plate
tectonics.
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