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jim2 says:
You can see a CO2 spike ~ 66 million years ago in this stomata study. This study shows CO2 much higher in the past than now.
http://www.sonoma.edu/users/c/crocker/516-2011/300myr%20profile.pdf
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Micula says:
A layer rich in carbon was recorded from the classic K/T (K/Pg) site at Stevn’s Klint in Denmark. It was referred to by Hans Joergen Hansen (Univ. Copenhagen) as the “grey chalk” and has been used by some researchers as evidence of the post impact fire storm. Detailed sedimentological analysis shows that the carbon content includes 1-2 micro diameter hollow graphite spheres, known only from volcanic glasses. The depositional period of the “grey chalk” has been calculated by H.J. Hansen as approximately 600 thousand years. As he once said on a field excursion, “If we could find the wood that burns that long, then the energy problems of the world would be solved.”
I think another factor in the mass extinction could have been destruction of the ozone layer and radiation burning and cancer to animals without thick fur/feathers, or nocturnal or underground habits. Note that the reptile survivors were snakes, alligators, turtles, etc. which all tend to use burrows or are small and can hide. Birds and mammals, feathers, fur, live in trees or jungles, or hide in burrows or were small.