[null and void]

Was the atmosphere more dense as well?

Dunno. 65 MYA a 10 km wide bolide hit the earth. The impact was large enough to blast enough rock debris into the space to set essentially the entire planet ablaze as it fell back.

Question 1: How much air was blasted permanently off the planet by the strike itself?

Question 2: As luck would have it the strike was in shallow water on the continental shelf. How much hypersonic steam do you blast into space cooling off a 200 km wide crater gouged down to magma?

Question 2a: How much atmosphere gets entrained on the edge of a 200km wide hypersonic jet of steam?

Enough to drop sea level atmospheric density by, pick a number, 20%? 50%? 70%?

[aruanan]

Was the atmosphere more dense as well?

I asked a graduate student in Ecology and Evolution here at the University of Chicago if the present day atmosphere had enough oxygen to be able to support the really huge dinosaurs or whether the atmosphere of their time had a significantly higher oxygen content. He seemed to think that the current atmosphere would have been insufficient to support the metabolisms of the very large dinosaurs.

[gleeaikin]

The likelihood that the heat of the impact would have burned most of the earth seems to have been too much for the imagination of the author. If the richer atmosphere made the huge dinosaurs and airborn creatures possible, and the oxygen was consumed by the vast burning as well as the other factors you mention, then that would account for their extinction, and the fact that the survivors were much smaller types of critters. I also suspect that the ozone layer would have been destroyed, which would explain why the survivors were nocturnal, feathered or hairy, or likely to hibernate in mud or hollows in embankments.