In reality, the Megalodon was around the same size as the mosasaurus, but they never lived at the same time. The Meg in Ark is way too small, probably because they didn’t want two massive ocean predators, because that would make the oceans almost impossible to traverse. Therefore, they made they Megalodon a smaller surface hunter, and put the Mosasaur in the place of a deep ocean apex predator, even though it was a reptile and had to breathe air.