Why are all the ‘prehistoric’ versions of animals so massive?

Question

We always hear about how big ancient sharks, alligators, birds were. What was the advantage of being such a massive size and what caused them to become smaller? To my understanding its best to be smaller because of the square-cube law, then why are there exceptions like whales?

In: 3

Answer 1

Oxygen levels were much higher then, so more energy can be gained to evolve bigger. As animals got bigger, the animals that ate those animals got bigger, and so on as evolution favoured being bigger to survive.

But after the mass extinction event 65 Million years ago, the environment now favoured being smaller; less energy required to survive. And here we are.