I am a senior research associate at the University of Bristol on the NERC funded BETR project examining the recovery of tetrapods after the end-Permian extinction.
At the end of the Permian Period (252 million years ago) there was the largest mass extinction known with 96% of marine species and 70% of life on land going extinct. Killing that much life is not an easy thing to do, and this extinction was so large because there were numerous causes: massive volcanism in Siberia, formation of the supercontinent Pangaea, lack of oxygen in the oceans, and extremely high temperatures among others.
Despite this culmination of events life made it through, yet recovery after the extinction took several million years. How rapid this recovery was and how different vertebrate ecosystems were after the extinction event are the questions of my current postdoc.