The Multituberculata were a group of rodent-like mammals that existed for approximately one hundred million years—the longest fossil history of any mammal lineage—but were eventually outcompeted by rodents, becoming extinct during the early Oligocene. At least 200 species are known, ranging from mouse-sized to beaver-sized. These species occupied a diversity of ecological niches, ranging from burrow-dwelling to squirrel-like arborealism. Multituberculates are usually placed outside either of the two main groups of living mammals—Theria, including placentals and marsupials, and Monotremata—but some cladistic analyses put them closer to Theria than to monotremes.
The multituberculates existed for over 100 million years, and are often considered the most successful, diversified, and long-lasting mammals in natural history. They first appeared in the early Jurassic, or perhaps even the Triassic, survived the mass extinction in the Cretaceous, and became extinct in the early Oligocene epoch, some 35 million years ago. 
With the possible exception of some poorly preserved South American material, multituberculates 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.
In the late Cretaceous multituberculates were widespread and diverse in the northern hemisphere, making up more than half of the mammal species of typical faunas. Although some lineages became extinct during the faunal turnover at the end of the Cretaceous, multituberculates managed very successfully to cross the K/T boundary and reached their peak of diversity during the Paleocene. They were an important component of nearly all Paleocene faunas of Europe and North America, and of some late Paleocene faunas of Asia. Multituberculates also were most diverse in size during the Paleocene, ranging from the size of a very small mouse to that of a beaver.
Full article ▸