When mass extinctions occur many species that compete for food, resources and die off, leaving the survivors a better habitat and stuff. When all the Antarctic fishies died, the Notothenioid's survived because of their weird mutant powers. (LIKE SUPERMAN)
Before dinosaurs died out, mammals were small rodent-like animals trying to avoid getting stepped on long enough to pass on their genes. If the dino's hadn't died, I wouldn't be typing this right now.