Multituberculata (commonly known as multituberculates, named for the multiple tubercles of their teeth) is an extinct order of rodent-like mammals with a fossil record spanning over 130 million years. They first appeared in the Middle Jurassic, and reached a peak diversity during the Late Cretaceous and Paleocene. They declined from the mid-Paleocene onwards, finally going extinct in the late Eocene. They are the most diverse order of Mesozoic mammals with more than 200 species known, ranging from mouse-sized to beaver-sized. These species occupied a diversity of ecological niches, ranging from burrow-dwelling to squirrel-like arborealism to jerboa-like hoppers. Multituberculates are usually placed as crown mammals outside either of the two main groups of living mammals, Theria — placentals and marsupials — and Monotremata, but usually as closer to Theria than to monotremes. They are considered to be closely related to Euharamiyida and Gondwanatheria as part of Allotheria.
Description
The multituberculates had a cranial and dental anatomy superficially similar to rodents such as mice and rats, 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; the exact homology of these cusps to therian ones is still a matter of debate. Unlike rodents, which have ever-growing teeth, multituberculates underwent dental replacement patterns typical of most mammals (though in at least some species the lower incisors continued to erupt long after the root's closure). Multituberculates are notable for the presence of a massive fourth lower premolar, the plagiaulacoid; other mammals, like Plesiadapiformes and diprotodontian marsupials, also have similar premolars in both upper and lower jaws, but in multituberculates this tooth is massive and the upper premolars are not modified this way. In basal multituberculates all three lower premolars were plagiaulacoids, increasing in size posteriorly, but in Cimolodonta only the fourth lower premolar remained, with the third one remaining only as a vestigial peg-like tooth, and in several taxa like taeniolabidoideans, the plagiaulacoid disappeared entirely or was reconverted into a molariform tooth.
Unlike rodents and similar therians, multituberculates had a palinal jaw stroke (front-to-back), instead of a propalinal (back-to-front) or transverse (side-to-side) one; as a consequence, their jaw musculature and cusp orientation is radically different. Palinal jaw strokes are almost entirely absent in modern mammals (with the possible exception of the dugong), but are also present in haramiyidans, argyrolagoideans and tritylodontids, the former historically united with multituberculates on that basis. Multituberculate mastication is thought to have operated in a two stroke cycle: first, food held in place by the last upper premolar was sliced by the bladelike lower pre-molars as the dentary moved orthally (upward). Then the lower jaw moved palinally, grinding the food between the molar cusp rows.
The structure of the pelvis in the Multituberculata suggests that they gave birth to tiny helpless, underdeveloped young, similar to modern marsupials, such as kangaroos. However, a 2022 study reveals that they might actually have had long gestation periods like placentals. However, in 2024, all Allotheria (including multituberculates) fell outside the crown group of Mammalia, implying that cimolodonts developed placental-like gestation (and viviparity in general) independently, rather than multituberculates and therians having a common viviparous ancestor. At least two lineages developed hypsodonty, in which tooth enamel extends beyond the gumline: lambdopsalid taeniolabidoideans and sudamericid gondwanatheres. Studies published in 2018 demonstrated that multituberculates had relatively complex brains, some braincase regions even absent in therian mammals.
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