I'd like to comment on the "dumb luck" matters idea. If nothing else (besides the absolute horror and terrible loss of humanity), the recent tsunami should show the presence of dumb luck in a disaster. You had people, side-by-side, similar ages and physical conditions - one dies and the other lives. Just dumb (bad!) luck. You had people swept out to sea who survived for days hanging onto uprooting, floating palm trees; and others who were killed when the room they were in was squashed by the mud and water, several miles inland.
And yet, there was absolutely no filtering: _all_ species survived!
IMHO, the variations in which terrestrial animals survived and which animals
didn't - which correspond with the extinction of sea creatures - large and
small (note that some obvious exceptions exist in this realm as well
[sharks, etc.]) - point to some sort of global scale disaster. Dumb luck
matters.
I don't know why. Most extinctions for which we have actual _evidence_ point to species' interactions.
I'm not sure that we _know_ of other terrestrial genera that were affected.
Curry and others have commented on the _lack_ of obvious differences from one horizon to the next, i.e., K--->T