Multituberculata

Multituberculata (commonly known as multituberculates, named for the multiple tubercles of their teeth) is an extinct taxon of rodent-like allotherian mammals that existed for approximately 166 million years, the longest fossil history of any mammal lineage. They eventually declined from the late Palaeocene onwards, disappearing in the late Eocene, though they might have lived even longer into the Miocene, if gondwanatheres are part of this group. More than 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 to jerboa-like hoppers. Multituberculates are usually placed as crown mammals outside either of the two main groups of living mammals—Theria, including placentals and marsupials, and Monotremata—but closer to Theria than to monotremes. 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 to 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 aren't 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 gondwanatherians and 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. At least two lineages developed hypsodonty, in which tooth enamel extends up beyond the gumline: lambdopsalid taeniolabidoideans and sudamericid gondwanatheres. The latter, having been around already during the Cretaceous, are the earliest known lineage of grazing mammals. A species from the Katsuyama Dinosaur Forest Geopark may offer an even earlier example of grass-eating adaptations as it dates from the Lower Cretaceous at about 120 million years. Multituberculates first appear in the fossil record during the Jurassic period, and then survived and even dominated for over one hundred million years, longer than any other order of mammaliforms, including placental mammals. The earliest known example is Rugosodon, a rodent-like omnivore living 160 million years ago in what would someday be China. During the Cretaceous, the multituberculates radiated into a wide variety of morphotypes, including the squirrel-like arboreal ptilodonts. Like all categories of life on earth, most species of multituberculata appear to have been wiped out during the K-T event (the extinction of the dinosaurs), but they seem to have been among the first to recover and diversify again. 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. Tooth marks attributed to multituberculates are known on Champsosaurus fossils, indicating that at least some of these mammals were scavengers. A ptilodont that throve 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.