I would say the mid-Miocene from about 10 million years ago. You had some scary amphibious predators like Pontolis; a walrus the size of an elephant seal the size of an elephant seal. And the 11 meter long marine crocodile RhamphosuchusBut as scary as these creatures may have been they would have been preyed on by the two most powerful predators that Ever lived. These being the 60 foot shark megalodon and it's competitor Livyatan (Leviathan) a prehistoric sperm whale as voracious as the modern orca.
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*the size of an elephant seal and as ferocious as a leopard seal.
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