What is the X in X-mas?
any member of an extinct group of small, superficially rodentlike mammals that existed from about 178 million to 50 million years ago (that is, from the middle of the Jurassic Period until the early Eocene Epoch). During most of this span, they were the most common mammals. Adult multituberculates were usually the size of mice, though the largest species approached the size of beavers. They were dominantly herbivorous and granivorous. The distinguishing characteristic of multituberculates is the construction of their molars, with two or three longitudinal rows of cusps. In fossils of more primitive forms, there are five or six cusps, whereas up to 30 cusps are present in advanced genera. Multituberculates had a single pair of long lower incisors and possibly one to three pairs of upper incisors. In most genera, the anterior lower premolars were large shearing teeth.