[SandwicheGuy]

I think it was just coincidence that every large (food-bearing) large mammal in North America vanished at the same time the new (culturally-correct, pristine and non-threatening) 2-footed North American mammals arrived om scene.

That is sarcasm, I think, but you know, sometimes a coincidences is just a coincidence. It is quite possible that a population moved in after some event caused the collape of the large mammal population. It may have been a meteor.

[Drammach]

It is quite possible that a population moved in after some event caused the collapse of the large mammal population. It may have been a meteor.

Meteor Crater, AZ ( also known as Barringer Crater..) approx. 50,000 years ago...
Soon thereafter, there was a decided climatic change event, from the previously cool and damp savannah to hot, dry desert..

When you start to throw meteor impact, climate change, probable flooding events beginning due to glacier melts, earthquake, and disease among the animal populations, it is easy to see the fauna suffering a massive decline without any human intervention whatsoever..