 Therefore, "Herefordopterus" is placed as the sister taxon (closest relative) of the clade of Slimonidae and Pterygotidae and a more derived form than "Hughmilleria".The cladogram presented below, derived from a 2007 study by researcher O. Erik Tetlie, showcases the interrelationships between the pterygotioid eurypterids.The Late Silurian of Herefordshire was home to a wide array of different eurypterids, including species of "Erettopterus", "Eurypterus", "Nanahughmilleria", "Marsupipterus", "Salteropterus" and potentially "Slimonia" (depending on the identity of "S. stylops").