Trituberculata
Encyclopedia
Trituberculata is an extinct group of animals existing in the fossil record from about 215 – 85 MYA. It contains the ancestors of Placentalia and Marsupialia; all modern mammals except Monotremata are descended from trituberculates. It is named for the three tubercles (cusps) of the molar teeth (not to be confused with Triconodonta
Triconodonta
Triconodonta is the generic name for a group of early mammals which were close relatives of the ancestors of all present-day mammals. Triconodonts lived between the Triassic and the Cretaceous. They are one of the groups that can be classified as mammals by any definition...

). The clade Trituberculata is not always regarded as a valid one, and they probably don't form a monophyletic group. Instead, some of them may be "true" basal mammals (although not always closest related to each other), while others (such as the symmetrodonts) should rather be treated as advanced mammaliaforms
Mammaliaformes
Mammaliaformes is a clade that contains the mammals and their closest extinct relatives. Phylogenetically, it is defined as a clade including the most recent common ancestor of Sinoconodon, morganuconodonts, docodonts, Monotremata, Marsupialia, Placentalia, extinct members of this clade, and all...

just outside Mammalia proper.
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