Y'know, the more I think about it, I could really care less if the whole durned area was clear cut, nuked, and napalmed in the early 1900's.
Today, there's plenty of cover to support many large animals. How they got there? At this point, who cares? Maybe they road trains from Illinois? Maybe they tunneled from the Himilayas. Maybe a giant bird dropped them from the sky.
If Bipto's crew can settle this mystery once and for all, we'll worry about that detail (and about a million others) later.
Let's not put the cart before the horse here.