The stem species concept is a modified method of phylogenetic systematics which uses not only apomorphies of taxa but also plesiomorphic character states for the assessment of phylogenetic interrelations. In a theorectical section the concept is outlined, and differentiated criteria on the homology of character states are presented on a background of conceptual models on epigenetic control of phenotypical characters. The method is exemplified with families of the early-derivative Poronota of oribatid mites. The superfamily Oribatelloidea is placed in the early-derivative Poronota, for which group a phylogenetic cladogram is proposed. As a result of the analyses, the problem of family diagnoses is discussed, resulting from the issue that classical diagnostic characters are evolved partly within the evolution of the included genera and are not stem species characters of the family.