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

Beginning a few million years ago, these were drastic drying and cooling trends across the globe, causing the decline and extinction of many species of megafauna. On the tail of this, a species of funny hairless ape learnt how to make pointy sticks and throw rocks, and then spread across the globe, causing mass extinctions among the already fragile ecosystems.

What we see today are the animals still surviving from those ongoing extinction events, which are generally far smaller and more generalist that the fauna which went extinct.

Also, people keep claiming it was the high oxygen which allowed large dinosaurs to survive, but this is incorrect. Much of the mesozoic had lower or comparable oxygen levels to today, yet these massive dinosaurs still existed. Sauropods were the largest group of land animals to ever exist, but the groups in 2nd and 3rd place are both mammals. Ancient rhinos and elephants got to larger sizes than any theropods or hadrosaurs.