Multituberculata
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The Multituberculata are the only major branch of mammals to have become completely extinct, with no living descendants. Rodent-like, they first appeared in the Middle Jurassic, and became extinct in the early Oligocene.
They are named for their teeth, which had many cusps (tubercles) arranged in rows, hence multituberculates.
About 80 genera of multituberculata are known, including Lambdopsalis, Ptilodus and Meniscoessus.
Groups within Multituberculata
In their 2001 study, Kielan-Jaworowska and Hurum found that most multituberculates could be referred to two suborders: "Plagiaulacida" and Cimolodonta. The exception is the genus Arginbaatar, which shares characteristics with both groups.
"Plagiaulacida" is paraphyletic; it is an informal suborder which doesn't satisfy the cladistic criterion of consisting of an ancestor and all of its descendants. Its members are the more basal multis. Chronologically, they ranged from perhaps the Middle Jurassic (unnamed material), until the Lower Cretaceous. This group is further subdivided into three informal groupings: the Allodontid line, the Paulchoffatiid line ,and the Plagiaulacid line.
Cimolodonta is apparently a natural (monophyletic) suborder. This includes the more derived multis, which have been identified from the Lower Cretaceous to the Eocene. Recognized are the superfamilies Djadochtatherioidea, Taeniolabidoidea, Ptilodontoidea and the Paracimexomys group.
Additionally, there are the families Cimolomyidae, Boffiidae, Eucosmodontidae, Kogaionidae, Microcosmodontidae and the two genera Uzbekbaatar and Viridomys. More precise placement of these types awaits further discoveries and analysis.
Geographic distribution
With the possible exception of some poorly preserved South American material, multis are only known from the northern hemisphere. A southern grouping, Gondwanatheria, has in the past been referred to the order, though this placement currently has little support.
Reference
Kielan-Jaworowska Z & Hurum JH (2001), Phylogeny and Systematics of multituberculate mammals. Paleontology 44, p.389-429.
(This information has been derived from [1] Multituberculata Cope, 1884. As that's my webpage, there are no issues of copyright. Trevor Dykes)