Question: Why exactly did the KT extinction wipe out the marine dinosaurs? Surely they would have been able to take cover in some underwater cave?

Hugh Roderick answered on 17 Jun 2010:

No t my area so I can’t say for sure, but my understanding is that the probable cause of the extinction is a meteor strike potentially combined with massive volcanic activity in what is now India. And that the effect of the impact and volcanism would have been to throw a lot of debris into the atmosphere that would have blocked out the sun for many months. This would have caused plants on the land to die and algae in the seas to die, so the animals and dinosaurs on the land and in the sea that fed on the plants or algae (the herbivores) would have died and the dinosaurs that fed on the herbivores would have died. So even if the big dinosaurs survived the impact, which most of them probably did, they would have dies out any way because they had no food to eat.