Pentastomida, tongue worms, are a taxon of about 130 species of parasites, living exclusively in the respiratory tracts of vertebrates. Three-dimensionally preserved Upper Cambrian larvae already demonstrate a high degree of adaptation to parasitism, striking morphological conservatism, and a high diversification by the Late Cambrian, thereby suggesting a likewise diversified host group. Not least due to their highly modified morphology, the systematic affinities of pentastomids remain controversial. The two major alternatives place the group as either close to branchiuran crustaceans or as stem-lineage derivatives of the Euarthropoda. To this set of Cambrian fossil representatives of the pentastomids we can add a new form from Lower Ordovician boundary beds from Sweden, most likely reworked from Upper Cambrian horizons. Based on this new species, named Aengapentastomum andresi gen. et sp. nov., and the available information about fossil and Recent pentastomids, we review the diverging ideas on the systematic position of this fully parasitic taxon.
