It was erected as a stem-based taxon by Hopson and Kitching (2001) and defined as the most inclusive clade containing Mammalia and excluding Procynosuchus, a Late Permian genus that is one of the most basal cynodonts.
Hopson and Kitching (2001) created a taxonomic classification based on stem-lineage, defining it as the most inclusive group containing Mammalia and excluding Procynosuchus, a primitive cynodonts species from the Late Permian era.