[Sapwolf]

So how did the asteroid know to only kill the dinosaurs but not the mammals.

Pretty fishy man...Oh wait...what about fish? How’d they survive?

[entropy12]

The asteroid did not kill dinosaurs directly. Instead the dust and debris kicked into atmosphere created prolonged blocking out the sun, accompanied by excessive cooling of atmosphere. The cold blooded Dino’s perished but the warm blooded small mammals survived.

[gnarledmaw]

Many didn’t. There was a major extinction of many aquatic lifeforms from this event also likely due to acidification.