I think it's possible for the megolodon to still be alive. I didn't read all the posts but megolodon was a pelagic fish so it could be living in the open ocean or in deep parts of the ocean. I could be wrong but I think a while back, teeth from megolodon washed up and appeared fresh. If megolodon isn't alive, could there be a larger sub species of the great white shark?
Rule number one: Always research a fish before you buy it.
Rule number two: Always cycle your tank.