Dave notes that mass extinctions only account for a small fraction of all the extinctions that have taken place over time. Most extinctions take place during the quiet, uneventful times , i.e. background extinctions. This says that even though mass extinctions account only for a small fraction of all the known extinctions, they provide manifestly different results from normal background extinctions. Mass extinction events act as catalysts for more radical changes. Thus, mass extinctions play a very important evolutionary role.
Mass extinctions mediate the major evolutionary changes.
The Cretaceous-Tertiary is most interesting to Dave because this has been studied globally and it is thought that extinction happened in lock step around the world. Some real puzzles will occupy Dave during the coming years. It is obvious that there are winners and losers among the creatures that survive mass extinctions, but not all of the winners explode to fill vacant ecological niches. Testing ideas on the reasons behind the different fates of the survivors will be an interesting challenge. This turns out to be true for reef systems only.