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The most successful mammal in history, unearthed

Like most early nocturnal mammals, Rugosodon eurasiaticus was active at night. This reconstruction shows Rugosodon searching for food among ferns and cycads on the lake shores in the darkness.
[Illustration by April Isch, University of Chicago]

Researchers in China have discovered the 160 million-year-old fossil of an extinct rodent-like creature, which looked a bit like a small rat or a chipmunk. They are calling this newly discovered species Rugosodon eurasiaticus, and they say that it represents the oldest ancestor of a group of mammals called multituberculates, which lived on Earth for 100 million years before more modern rodents replaced them.

The multituberculates are known as the most evolutionary successful mammals in Earth's fossil record and—much like today's rodents—its members dwelled in a wide variety of habitats. Some of them lived on the ground. Others lived below the ground or in the trees. By the end of their time on the planet, most multituberculates had developed teeth that allowed them to eat plants and movements that allowed them to climb trees easily.

But, this new fossil of R. eurasiaticus already had teeth that were appropriate for gnawing on both plants and animals alike, according to Chong-Xi Yuan and the other researchers that analyzed it. And despite the fact that this very early multituberculate probably lived on the ground, it had ankle joints that were already very good at rotating—a feature that is normally reserved for tree-dwelling creatures—they say.

Based on their findings, the researchers suggest that R. eurasiaticus paved the way for later plant-eating and tree-dwelling mammals—and the diverse rodents that eventually appeared on the planet. The tooth and ankle adaptations seen in this fossil must have arisen very early in the evolution of multituberculates, according to the researchers.