New fossil sheds light on one long-lasting group of mammals.

National Geographic explains why multituberculates – flexible, cute little pre-rodents – are so important. Before vanishing 35 million years ago, they’d been around for 130 million years, scampering alongside dinosaurs and surviving the cataclysm that killed them:

Dubbed Rugosodon eurasiaticus, the creature bore a superficial resemblance to a small rat or a chipmunk and was an early member of the group of mammals known as multituberculates.

Its nearly complete skeleton was found in the province of Liaoning in northeastern China….

Based on a study of its teeth, scientists proved that Rugosodon, in particular, was also an omnivore. Its remains were found near the edge of an ancient lake, where the creature could have dined on plants such as ferns, as well as insects and a clam-like crustacean called Conchostraca, [University of Chicago researcher Zhe-Xi] Luo said.

What surprised Luo and his team, however, were Rugosodon‘s ankles, which appear to have been capable of “hyper-back-rotation.” Such highly mobile ankle joints are normally associated with the foot functions of animals that are exclusively tree-dwellers—those that navigate uneven surfaces.

“If your ankle gets twisted 90 degrees, you’d suffer pain,” Luo said.

“[Rugosodon] could twist its ankle for the joy of its life.”

The discovery of hyper-back-rotation ankles in such an early multituberculate supports the idea that this was a key reason they were so successful, Luo said.

“Essentially, from the very beginning of their whole lineage, multituberculates were already well equipped to be capable of such a versatile locomotor adaptation,” he added.

They’re not quite sure why rodents beat them out in the long run. But it wasn’t because our distant ancestors were more flexible….