When American ornithologist Alexander Wetmore described this species from subfossil remains which were found at Prosperous Bay, Saint Helena he classified it into the new genus Aphanocrex. However, in 1973 American paleontologist Storrs Olson synonymised this genus with the genus Atlantisia which other representative is the Inaccessible Island Rail (Atlantisia rogersi). While Olson had considered it as congenor of the Inaccessible Island Rail other scientists regarded it not even as close relative and so it was renamed into Aphanocrex.
Since Saint Helena was predator free until the sixteenth century, the swamphen had lost its ability to fly but it wings were better developed like the wings of the rails from Inaccessible Island and Ascension Island. Furthermore it had strong toes with long claws which give that species a good ability to climb and flutter up the steep valley walls. It fed probably on the eggs and the juveniles of several St. Helena terrestrial and pelagic bird species and on snails.