Some of them must have been fairly long lived - they couldn't have grown that large in just a few years. Several species of whale live over a 100 years. And then we have the Greenland shark living possibly up to 500 years and although not massive, their ancestors must have done something right as their descendants are still here over 400 million years later. Or perhaps they were just very good at hiding.

CanadaPlus

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I mean, I don't see why it would be any worse than the average mammal life.

averyminya

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Or a crocodile

CanadaPlus

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Or a modern bird, at that rate.