>>17797719
Anon, Mississipian cultural complex fell apart prior to the arrival of Europeans to NA. Reasons unknown/theorized but the people that one Spanish expedition encountered in the mid 1500s were already a unicorn within the context of the ex-mound building area.
European plagues and colonisation can only be blamed for making sure that sedentary cultures didn't reappear after the conditions that caused the destruction of mound builders went away.