Permian-Triassic Mass Extinction event

The Permian-Triassic (P-Tr) Mass Extinction event occurred 252 million years ago.

Because of its severe level, it was nicknamed "The Great Dying" and well known as "the mother of all mass extinction events" . In this event, no class of life was escaped from the extinction: trees, lizards, plants, insects, fish, etc. all were nearly devastated. About 96% of all marine species and 70% of terrestrial vertebrate species on Earth died out (Benton, 2005).  The figure on the left hand side presents the world at the Permian time. 

For a clearer state of extinction losses, here are one table and one figure that both show the comparison between the diversity of marine life before and after the P-Tr mass extinction from Erwin (1989).

         

Figure on the left shows the sea-bed that has richness of reef life and the burrowing infauna before the P-Tr, and the right side's figure with an absence of such species gives a strong impact compare to the left one. 

Why did it happen?! Why this event was so severe?!

Here is a video from youtube which gives an explanation of the "Great Dying".

Consequently, this mass extinction event was caused by a combination of causes:

severe climate and environmental changes, contemporary volcanic activities, methane hydrate gasification and maybe the asteroid impact and some other reasons. They formed these sequence of catastrophes and each one worse than the previous that resulted in this greatest mass extinction over the time.  

And as the "Great Dying" event did so great the damage to Earth, it demanded about up to 10 million years for recovery. According to Chen,et al. (2009), this also change fundamentally the ecosystem of Earth and after this period,   the modern-type biota and ecosystem formed in the Triassic time when the dinosaurs appeared.

Reference:

Benton M J (2005). When life nearly died: the greatest mass extinction of all time. London: Thames & Hudson.

Erwin, D.H. (1989) The end-Permian mass extinction – what really happened and did it matter? Trends Ecol.

Z.Q.Chen et.al (2009). Permian-Triassic mass extinction and subsequent recovery: an update. Journal of the Geological Society of Australia: Taylor & Francis