It's the kind of sound that would easily be heard by other animals through a thick rain forest. The scientists believe the sounds would have been distinctive enough that the animals could have identified individuals by their voices. Because paleontologists currently do not know whether Parasaurolophus had vocal cords, the scientists simulated the sounds the dinosaur could have made both with and without vocal cords. The first set of sounds are without vocal cords, the third set are with vocal cords. I don't know about the second set.
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