From the report: O2_Diving_Towards_Danger_Point.
I got the idea, that dinosaurs might have died, because they gobbled so much greenery, that the oxygen balance of the atmosphere collapsed.
We do not know, how the oxygen balance function, but it might be some sort of automatic system, and such systems might close down caused by too big overload as e.g. in hacker attacks. It happens suddenly, and the automatic system goes back to ordinary function as soon as the overload disappear.
This could explain the sudden extinction of the dinosaurs, but later survival of small species.
In Danish. Se: Iltsvind