The Multituberculata (commonly known as multituberculates, named for the multiple tubercles of their teeth) were a group of rodent-like mammals that existed for approximately one hundred and twenty million years —the second longest fossil history of any mammal lineage, but eventually declined from the late Palaeocene onwards, disappearing in 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 closer to Theria than to monotremes.
In The Land Before Time
Skip from the episode, Return to Hanging Rock, is a Multituberculate.