I heard this when I was young but never really believed it, but I guess they would need two hearts to be so freaking huge! And if it is true, what evidence do paleontologists/scientists have to support this claim?
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I would certainly doubt it. Birds don't have two hearts. The reasoning would be that some dinosaurs had their heads so far from the heart that auxiliary pumping would be needed to keep blood going up to the head. But there's no fossil evidence for it.
You may also have heard "two brains" -- bogus.
You may also have heard "two brains" -- bogus.