Fossil range: Middle Jurassic-Oligocene, 160–35 Ma
|Skull of Ptilodus|
The multituberculata (multituberculates) are a branch of rodent-like mammals that existed for a 100 millions years — the longest fossil history of any mammal linage — but eventually were outcompeted by modern or "true" rodents and finally went extinct during the early Oligocene. At least 200 species are known, ranging in size from tiny mice to beavers; some dwelling in burrows others climbing trees like squirrels.  Not truly therian mammals, the multituberculates were, in biological terms, perhaps more closely related to living monotremes than to placental mammals and marsupials. 
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.
The multituberculates had a cranial and dental anatomy similar to rodents with cheek-teeth separated from the chisel-like front teeth by a wide tooth-less gap (the diasteme). Each cheek-tooth displayed several rows of small cups (or tubercles, hence the name) that operated against similar rows in the teeth of the jaw. Like in modern rodents, this masticatory apparatus formed an efficient chopping device. 
During the Cretaceous and Paleocene the multituberculates radiated into a wide variety of morphotypes, including the squirrel-like arboreal ptilodonts. The peculiar shape of their last lower premolar is their most outstanding feature. These teeth were larger and more elongated than the other cheek-teeth and had an occlusive surface forming a serrated slicing blade. Though it can be assumed that this was used for crushing seeds and nuts, it is believed that most small multituberculates also suplemented their diet with insects, worms, and fruits. 
A ptilodont that successfully strived in North America was the ptilodus. Thanks to the well-preserved ptliodus specimens found in the Bighorn Basin, Wyoming, we know that these multituberculates were able to abduct and adduct their big toes, and thus that their foot mobility was similar to that of modern squirrels which descend trees head first. 
In Europe another family of multituberculates were equally successful — the koganoids, first discovered in Haţeg, Romania. They also developed an enlarged blade-like lower premolar and the Hainina, the most successful genus, was originally believed to be a ptilodont. However, more detailed analysis of this genus reviled a smaller number of dental cusps and a retained fifth premolar — a unique combination of primitive and advanced features indicating that Hainina were related to some Jurassic genera and that enlarged, blade-like premolar were acquired independently in Europe and North America. 
Another group of multituberculates, the taeniolabids, were heavier and more massively built and could reach the size of a modern beaver; indicating they lived a fully terrestrial life. They reached their highest diversity in Asia during the late Cretaceous and Paleocene, which suggests they originated from there. 
About 80 genera of Multituberculata are known, including Lambdopsalis, Ptilodus and Meniscoessus. In the northern hemisphere during the late Cretaceous, more than half of typical land mammalian species were multituberculates.
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 does not satisfy the cladistic criterion of consisting of an ancestor and all of its descendants. Its members are the more basal Multituberculata. 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 Multituberculata, 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.
Redirecting to Multituberculata