[2][3] Tribosphenid mammals were originally grouped on the basis of triangular or V-shaped (tribosphenic) molars.

[3] However, a clade between the aforementioned groups, the "true Tribosphenida" or Boreosphenida, is still identifiable, united by characteristics such as the lack of a mesial cingulid and of a triangulated trigonid on the last premolar.

[2][3] Below is a cladogram showing one hypothesis of mammal relationships based on Rowe (1988) and McKenna and Bell (1997):[5][6] †Morganucodontidae Prototheria→Monotremata †Allotheria→†Multituberculata †Triconodonta †Kuehneotheria †Symmetrodonta †Dryolestoidea †Peramuridae †Aegialodontia Metatheria→Marsupialia Eutheria→Placentalia Boreosphenida (from boreas, "northern wind" and sphen, "wedge") were early mammals that originated in the Northern Hemisphere and had tribosphenic molars (three-cusped cheek teeth).

In boreosphenidans, the mandibular angle is placed posteriorly and the primitive postdentary trough (hole in the mandible) is absent (in contrast to Kuehneotheriidae, Eupantotheria, and Australosphenida.)

[3] An alternative hypothesis is offered by Flannery and colleagues, who view part of australosphenidans as members of Tribosphenida (excluding monotremes and others australosphenidans with Steropodon, which they consider to be an unrelated group), with Barremian-Aptian Australian Bisphopidae as the sister taxon to all Northern Hemisphere Theria (Boreosphenida).