The correct answer is - extinction.
An asteroid impact, as it happened some 66 million years ago, can have a huge effect on the change of the environment, thus the living conditions. Those changes will happen very quickly, in matter of minutes, hours, or days, but will last for a prolonged period of time. Such a rapid change is something to which most of the animals can not adapt that quickly, especially the bigger and more specialized animals, like the mosasaurs for example.
The end result of such a dramatic change in the environment is a large scale extinction of the species, thus opening a space for the smaller, more opportunistic animals to take over in the future.