Old proof of fast change
Frozen mammoths, undigested food in their stomachs and so on, found in good shape. They have been finding them for centuries now, and still find them. It just doesn't get any better than that to prove that rapid severe climate changes can occur. They went from living in a temperate, good enough for large grazing animals climate to frozen solid not thawed out for thousands of years. It apparently doesn't take much either, a few degrees here and there change, jet streams shifting, etc, and perhaps the poles moving are a major contributing factor.
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