Multituberculata

The Multituberculata were a group of rodent-like mammals that existed for approximately one hundred and twenty million years&mdash;the longest fossil history of any mammal lineage&mdash;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&mdash;Theria, including placentals and marsupials, and Monotremata &mdash;but closer to Theria than to monotremes.

History
The multituberculates existed for about 120 million years, and are often considered the most successful, diversified, and long-lasting mammals in natural history. They first appeared in the 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.

Geographic distribution
Multituberculates are mostly known from the northern continents (Laurasia), but there are some records, many of which are controversial, from the southern continents (Gondwana). The group Gondwanatheria, known from Argentina, Antarctica, Madagascar, India, and possibly Tanzania, has been referred to the order in the past, but this placement is currently controversial. Two genera, Hahnodon and Denisodon, are known from the Early Cretaceous of Morocco, but they may be haramiyidans instead. Multituberculates have also been recorded from the Late Cretaceous of Madagascar and Argentina, but this material has not been described in detail. An Australian multituberculate, Corriebaatar, is known from a single tooth.

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 Cretaceous–Paleogene 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.

Biology
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 cusps (or tubercles, hence the name) that operated against similar rows in the teeth of the jaw. As 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 supplemented their diet with insects, worms, and fruits.

A ptilodont that thrived in North America was Ptilodus. Thanks to the well-preserved Ptilodus 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&mdash;the Kogaionidae, 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 revealed a smaller number of dental cusps and a retained fifth premolar&mdash;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.

The structure of the pelvis in the Multituberculata suggests that they gave birth to tiny helpless young, similar to modern marsupials.

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.

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, representing the more primitive evolutionary grade. 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.

Taxonomy
Order †Multituberculata Cope, 1884
 * Suborder †Plagiaulacida Simpson 1925
 * Family Incertae sedis
 * Genus †Glirodon Engelmann & Callison, 2001
 * Family †Paulchoffatiidae Hahn, 1969
 * Subfamily †Paulchoffatiinae Hahn, 1971
 * Subfamily †Kuehneodontinae Hahn, 1971
 * Family †Hahnodontidae Sigogneau-Russell, 1991
 * Family †Pinheirodontidae Hahn & Hahn, 1999
 * Family †Allodontidae Marsh, 1889
 * Family †Zofiabaataridae Bakker, 1992
 * Family †Plagiaulacidae Gill, 1872
 * Family †Eobaataridae Kielan-Jaworowska, Dashzeveg & Trofimov, 1987
 * Family †Albionbaataridae Kielan-Jaworowska & Ensom, 1994
 * Suborder Incertae sedis
 * Family †Arginbaataridae Hahn & Hahn, 1983
 * Suborder †Cimolodonta McKenna, 1975
 * Superfamily Incertae sedis
 * Family Incertae sedis
 * Genus? †Ameribaatar Eaton & Cifelli, 2001
 * Genus †Ptilodus Cope, 1881
 * Genus? †Uzbekbaatar Kielan-Jaworowska & Nesov, 1992
 * Paracimexomys group Archibald, 1982
 * Family †Boffidae Hahn & Hahn, 1983
 * Family †Cimolomyidae Marsh, 1889
 * Superfamily †Ptilodontoidea Cope, 1887
 * Family Incertae sedis
 * Genus Neoliotomus Jepsen, 1930
 * Family †Cimolodontidae Marsh, 1889
 * Family †Neoplagiaulacidae Ameghino, 1890
 * Family †Ptilodontidae Cope, 1887
 * Family †Eucosmodontidae Jepsen, 1940
 * Family †Microcosmodontidae Holtzman & Wolberg, 1977
 * Superfamily †Djadochtatherioidea Kielan-Jaworowska & Hurum, 1997
 * Family Incertae sedis
 * Genus? †Bulganbaatar Kielan-Jaworowska, 1974
 * Genus? †Chulsanbaatar Kielan-Jaworowska, 1974
 * Genus †Nemegtbaatar Kielan-Jaworowska, 1974
 * Family †Sloanbaataridae Kielan-Jaworowska, 1974
 * Family †Djadochtatheriidae Kielan-Jaworowska & Hurum, 1997
 * Superfamily †Taeniolabidoidea Granger & Simpson, 1929
 * Family †Taeniolabididae Granger & Simpson, 1929