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) established a stem-based taxonomic classification, defining it as the most inclusive group encompassing Mammalia while excluding Procynosuchus, a Late Permian cynodonts genus considered one of the earliest members of the lineage.