Recovery From the Great Dying
A recent 2016 Stanford University study reported in the article "Oxygen-starved oceans held back life's recovery after the 'Great Dying'", suggests that


  Analysis of ancient seabed rocks from disparate locations reveal that life did not rebound until anoxia had fully ebbed.


Specifically, the researchers found evidence from a lot of places around the world that, according to the researchers, point to a possible conclusion that globally


  anoxic conditions reduced seawater oxygen levels by 100-fold at the onset of the mass extinction. Oxygen levels then slowly rose, only returning to pre-extinction levels after 5 million years, corresponding to when the climate became more stable and life regained its former diversity.


This research paints a picture that the mass extinction event resulted in a global anoxic event, that took the Earth a considerable amount of time to recover from.