Diminishing dung fungus...

and increasing amounts of pollen from broad-leaved floras reveal the timing of the die off of North American megafaunas according to a new paleoecological study.

Grazing patterns of mastodons, camels and giant sloths that modified the habitat, coupled with human hunting, may have caused the extinction of these large animals, and not climate change nor a comet impact.

Or, maybe a combination of all these factors is the cause as one of the study's authors states:

“In North America, there’s a lot of confusion because everything was happening all at once.”