A new species Chaetauchenium loki, is described from Parque Oncol in Chile. This species, C. convexipennis (Fairmaire) and Parhypates (Antarctobium) magellanicum (Fairmaire) share a combination of features distinct from all other species attributed to Parhypates and from all other species of South American Pterostichini. To reflect this, Chaetauchenium is elevated as a distinct genus and Antarctobium is considered congeneric. Individuals of these species have highly variable and unusual patterns of supernumerary or reduced numbers of setae and variation in secondary sexual characters. Males have denticulate and falciform aedeagal parameres and females have a digitiform diverticula at the base of the spermatheca. These genital and reproductive tract features are not known from any South American pterostichine species, but are found in many Australian, New Zealand and New Caledonian species.