Scientists don’t know for certain why some animals survived the time of great extinction at the end of the Cretaceous period.
Whatever the cause, it was a time of rapid change in the environment. It is possible that mammals could adapt to the new environment quickly enough to survive.
At this time, mammals were small nocturnal (nighttime) creatures. Their size meant that they didn’t need a lot of food for survival.
The fact that they came out at night and remained in underground burrows during the day protected them from the Sun’s ultraviolet rays. These two factors might have kept them from dying out with the dinosaurs.