A cladistic analysis of the major groups of early metatherian mammals shows that the Deltatheroida and South American dog-like Borhyaenoidea are closely related and are placed in a new supercohort Deltatheralia. while all other metatherians are placed in the supercohort Marsupialia. A reassessment of Early Cretaceous mammals with tribosphenic molars shows that metatherians and eutherians apparently evolved independently from a peramuran-like ancestor, and an entoconid. distinctly basined talonid and large protocone were acquired separately in each group. Consequently. the Tribosphenida (sensu McKenna) is apparently paraphyletic. Aegialodontia are regarded as structurally ancestral to Metatheria but not Eutheria. The known fossil record supports the view that differentation of the stocks which gave rise to metatherians and eutherians apparently occurred in Late Jurassic or Early Cretaceous time.