[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.

[marktwain]

that perhaps the event lowered the atmospheric oxygen level below what dinosaurs could survive on.

That seems plausible to me.

I have not seen any good research on oxygen levels at that period, but it seems plausible it was higher than it is now.

The level of CO2 could have been higher as well, which would promote more plant growth.