I'm not really looking for a "master key" exactly, but in this particular case, I don't understand why a comet could wipe out camels and horses, but spare the buffalo.
I could see how hunting by humans might put enough extra pressure on a particular species - one that is already on the brink - to wipe them out; but the Pleistocene extinctions wiped out hundreds of species. I don't think humans hunted the short-faced skunk to extinction, for example.
There are no close relatives in between.
The answer to that little problem is called DISTEMPER. A specific virus can wipe out an entire species of canid in short order.
http://www.thunderridgebison.com/bison_history.htm ~ this guy has your answer. North American bison are descended from the European bison ~ which is a smaller animal. With the destruction of the large cats the buffalo simply “evolved’ into the large grazing animal niche and became much larger. Did you know these guys can jump 6 foot straight up? This is like your car jumping 6 ft in the air ~ makes it hard to hunt them efficiently without the use of horses or helicopters!