This isn’t true: https://en.wikipedia.org/wiki/Hoatzin

… Since Archaeopteryx had three functional claws on each wing, some earlier systematists speculated that the hoatzin was descended from it, because nestling hoatzins have two functional claws on each wing. Modern researchers, however, hypothesize that the young hoatzin’s claws are of more recent origin… A similar trait is seen in turacos, whose nestlings use claws on their wings to climb in trees.