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

While there are many causes almost every one can be traced back to extreme levels of oxygen in the air. You can’t grow very large if your cells start dying due to lack of oxygen, when there’s more oxygen in the air it’s easier to get bigger