Specimens of trematopid amphibians from the Asturian (Upper Carboniferous) of Nýřany, Czech Republic, are redescribed as two taxa, namely Mordex calliprepes Steen and Mattauschia (gen. nov) laticeps Fritsch. Mordex calliprepes is represented by a single post-metamorphic specimen and has the diagnostic trematopid characters of the nasal region. Mattauschia laticeps is represented by one adult partial skull and mandible plus some fragments and two small post-metamorphic specimens including the species name-bearer. It has the trematopid-type modified lacrimal and a large but oval naris and appears to be the most primitive trematopid yet described. The stratigraphically sequential large trematopids Mattauschia, Fedexia, Ecolsonia and Acheloma show progressive acquisition of the derived features that characterise the terminal form Acheloma.
Mordex has a combination of primitive and derived characters and its position within the family is less clear. The many ‘branchiosaurs' in the Nýřany assemblage include specimens that could be larvae of both Mordex and Mattauschia but certain attribution is not possible and they are assigned to Olsoniformes incertae sedis. Mordex and Mattauschia appear to be terrestrial exotic elements in the Nýřany tetrapod assemblage, but with possible larvae in the lake assemblage. Representatives of at least four Palaeozoic dissorophoid families were present in late Middle Pennsylvanian/Asturian strata implying diversification of the Dissorophoidea prior to this time.