Most probably due to earth’s crustal shift. When they used water cannons for mining gold in AAlaska’s gold fields as they cut deep into permafrost they uncovered frozen tropical forests and the remains of sabre toothed tigers and many other temperate/tropical lifeforms. Scientists claimed that this was the result and approximate 90° shift in the Earth’s crust that was so violent that it tore trees and lifeforms apart and resulted in quick freezing those forms that shifted to the Arctic.
You’re quite correct.
You might want to take a trip to Drumheller, Alberta and visit the Tyrell Museum. You might be able to see much of it on the net between their site and YouTubers presentations
There are many geological markers.
Most noticable was the immediate extinction of the dinosaurs. I’ll bet that singular impact affected the existing polarity of the planet.
There have been several impacts, and then the Earth’s own need to restore balance and equilibrium complete with plates shifting.