About as big as a chipmunk, Rugosodon eurasiaticus was an extremely versatile rodent-type mammal that lived through the Cretaceous and died out around 35 million years ago. The oldest R. eurasiaticus fossil ever discovered dates back 160 million years ago to the Jurassic Period.
When scientists first reported the discovery of the fossil in 2013, they had determined that R. eurasiaticus paved the way for other mammals that chowed down on plants and lived in trees. These so-called multituberculates flourished during the Cretaceous. They could jump, dig tunnels and climb trees. These rodent-like mammals first appeared during the Jurassic Period but managed to survive for more than million years. Some were as small as a mouse, others the size of beavers [sources: Science, Yandell, AAAS].