A large temnospondyl mandible from the Lystrosaurus Assemblage Zone (Early Triassic) of South Africa is referred to the higher-level taxon Trematosauridae. The mandible is remarkably similar to that described for Trematosaurus from the Middle Buntsandstein of Germany, a genus closely related to the South African Trematosuchus, for which the mandible is unknown. However, the mandible cannot be referred unequivocally to either of these taxa. Trematosuchus is considered to be restricted to the lowermost subzone of the overlying Cynognathus Assemblage Zone, which, based principally on the temnospondyl and therapsid fauna, is considered to be of Upper Olenekian age. The mandible described here extends back the range of the Trematosauridae to the underlying Lystrosaurus Assemblage Zone and argues against the long-held notion of a sharp palaeontological break between the faunas of the Lystrosaurus and Cynognathus Assemblage zones. It also supports the hypothesis that the Lystrosaurus Assemblage Zone extended up to the Upper Olenekian.