Predator and prey perhaps?
Obviously not all dinosaurs were carnivores but I think it's safe to assume that most mammals must have been sitting lower on the food chain. Well, it's commonly known that predators keep other species in check, which in their turn often give birth to more siblings in order to increase their chances of survival.
Take the predator out of such an equation and you'll risk overpopulation because the hunted species can now reproduce pretty much unlimitedly, something which has happened quite a few times when people decided to remove a certain animal or place another outside of its natural habitat.
So I can't help wonder if the same could have applied here as well. With the removal of a lot of predators the mammals could much more easily reproduce and survive.
Biting the hand that feeds IT
