Low metabolism and they live in fresh water, shielding them from the heating impulse after the impact. Fresh water ecosystems fed by detritus from dead plants could survive the post-impact period of darkness as well.
Don't have an answer other than to say their lineage is sturdy enough to have pulled it off twice - crocodylomorpha were just about the only survivors of their giant and dominant clade in the end-Triassic extinction and, of course, they made it past the asteroid 135 million years later.
"Gee, I don't know. Maybe deep down I'm afraid of any apex predator that lived through the K-T extinction. Physically unchanged for a hundred million years, because it's the perfect killing machine. A half ton of cold-blooded fury, the bite force of 20,000 Newtons, and stomach acid so strong it can dissolve bones and hoofs."
