[marktwain]

Why aquatic dinosaurs all died off, but fishes, squids, sharks, etc all lived.

Of course, one large difference is that aquatic dinosaurs breath air. Fishes don't.

[Verginius Rufus]

Avian dinosaurs are still with us. Three Major League Baseball teams are named for specific species.

[PapaBear3625]

Of course, one large difference is that aquatic dinosaurs breath air. Fishes don't.

Which brings us back to my original speculation from #6, that perhaps the event lowered the atmospheric oxygen level below what dinosaurs could survive on.