Why were prehistoric era animals so gigantic in comparison to animals from the “modern era”

Question

Why were prehistoric era animals so gigantic in comparison to animals from the “modern era”

In: 1281

Answer 1

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The higher 02 atmosphere was only important for the massive invertebrates during the carboniferous.

The reason that dinosaurs were so big was mainly because they had hollow bones and better breathing systems, they had many airsacs and could get O2 both when breathing in and out.

For things like mammoths its mainly because we killed all of the big animals.