April Isch/The University of Chicago

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ScienceShot: Meet ‘Wrinkly Tooth,’ the Earliest Rodentlike Creature

Rodents are among the most successful animals on Earth, making up some 40% of all mammalian species. But long before there were rats and mice, there were equally successful mammals that looked a lot like them. These were the multituberculates, so-called because their cheek teeth featured multiple cusps, or tubercles. Reporting online today in Science, researchers reveal what they claim is the earliest known multituberculate, a nearly complete skeleton dated to 160 million years ago, from the fossil-rich Tiaojishan Formation in Northeast China. The team has named the new critter (artist’s reconstruction above) Rugosodon eurasiaticus. (Rugosodon in Latin means “wrinkly tooth,” referring to the highly wrinkled and creased surfaces of its molars, and eurasiaticus refers to the team’s conclusions that the specimen resembles somewhat younger ones found in both Europe and Asia.) It is about 17 centimeters long, the size of a rat. It also sports numerous features that explain how multituberculates, many of which ate both plants and crawly things such as worms and spiders—and which were capable of burrowing, jumping, and climbing trees—got off to such a good evolutionary start. Not only did Rugosodon have teeth that would have been very good at shearing plant material, but its ankles were also capable of rotating in wide angles, making it very agile. These features probably contributed to the more than 100-million-year-existence of multituberculates, a success story that ended only after true rodents independently arose about 60 million years ago and apparently outcompeted them to extinction.